Boulder School 2015: Student Poster Abstracts

Student Poster Sessions

Session 1

Session 2

Session 3

Session I (Thursday, July 9, 2015)

Andrew Balin (Rudolf Peierls Centre for Theoretical Physics)

Title: Entropic orientational bistability in a magnetically confined colloidal gyroscope

Abstract: Understanding the equilibrium thermodynamics of colloidal particles immersed in viscous media is the first step towards engineering microfluidic devices and understanding their driven or collective dynamics. Here we report a novel equilibrium effect in a magnetic nanorod in the presence of an external field. We observe that the field not only traps the azimuthal orientation of the rod as expected, but also induces a polar bistability evident from the ‘hopping’ dynamics of the rod between vertical and horizontal states ––despite the fact that the vertical state comes with a ~4 kT energy cost. We provide a theoretical explanation behind this effect and explore its entropic origin.

Deshpreet Bedi (University of Michigan)

Title: The Stabilizing Effects of Thermal Fluctuations on an Extensible Buckling Rod

Abstract: The classical problem of a rod buckling under a compressive force, “Euler buckling”, has long been studied and solved in physics; however, the classical theory of Euler buckling does not take into account thermal fluctuations. At small enough scales, entropic effects become significant, and a more intricate analysis incorporating thermal fluctuations is needed, for instance, in the study of biopolymers and nanotubes.

In this talk, we discuss buckling of an extensible, semi-flexible rod embedded in two and three dimensions. We systematically examine the problem both analytically, using a momentum-space renormalization procedure, and numerically, using Monte Carlo simulations, to determine the topology of the phase diagram containing the unbuckled and buckled states. We determine that thermal fluctuations tend to stabilize the straight-rod state over the buckled state in both two and three dimensions and that this stabilization increases with temperature. We also analyze the mechanical response of the rod in order to study the differing scaling regimes of the system.

Arman Boromand (Case Western Reserve University)

Title: Self-Assembly of alkathiol molecules on gold surface with non-uniform curvature

Abstract: Emergence of different nanoscopic and microscopic manufacturing techniques facilitates synthesizing nanoparticles with different shapes and sizes, including but not limited to tripod, prism, polyhedral, cubs, and elliptical nanoparticles in nano and micron sizes. Due to the presence of simple entropic forces self- assembly of these building blocks are limited to simple FCC, BCC, diamond crystal structures. Therefore, to obtain more complicated and complex self-assembled structures one requires to introduce specificity and directionality on the surface of nanoparticles in order to induce anisotropic interactions between those constituents. One way to create patchy particles is by spontaneous self-assembly of the physisorbed supramolecules layer on the surface of a nanoparticle. Those nanoparticles protected by the supramolecular shell is known as monolayer protected nanparticles (MPNs). Although there are limited literatures available on the self-assembly of monolayer protected nanoparticles, most of them revolve around surfaces with uniform curvatures, e.g. spheres and cylinders. In this presentation we studied self-assembly of alkathiol surfactants on the surface of gold nanoparticles with spheroid geometries. Owning to the fact that Oblate and Prolate geometries possess non-uniform curvature, phase separation will be altered by the geometry of the substrate which leads to observation of new phases that are absent in the case of systems with uniform curvature. For this purpose we used Dissipative Particle Dynamics (DPD) as a mesoscale simulation technique to study phase separation of alkathiol molecules with different length and miscibility. On the road to understand the physics of phase separation we also address the effect of geometry on the brush-mushroom transition of those mobile surfactants.

Coline Bretz (University of Pennsylvania)

Title: Hyperuniformity of self-assembled colloidal spheres

Abstract: Hyperuniformity characterizes a state of matter for which density fluctuations vanish on large scales. Hyperuniform materials are of technological importance as they exhibit interesting photonic properties. We have shown that such materials can be obtained by assembling spheres into a disordered jammed 2D- packing. To this end, we use a binary mixture of large and small Poly(NIPAM) particles confined between two cover slips. These soft spheres have been chosen for their temperature-sensitive properties . We can locally increase or decrease the volume fraction occupied by the spheres by finely tuning the temperature. By applying various temperature patterns, we are studying the spatial arrangements of the microgels and characterizing their hyperuniform properties through reconstruction and detection algorithms. The 3D properties as well as the structure factor of the packings are investigated through a small-angle static light scattering set-up.

Kui Chen (Johns Hopkins University)

Title: Anisotropic Colloidal Transport and Periodic Stick-Slip Motion in Cholesteric Finger Textures

Abstract: We have investigated the transport of colloidal particles within cholesteric finger textures formed by mixtures of the nematic liquid crystal 4-cyano-4’-pentylbiphenyl (5CB) and the chiral dopant4-(2-methylbutyl)-4-cyanobiphenyl (CB15) with cholesteric pitches between 24 and 114 micrometers. Spherical silica colloids (radius 5-10 micrometers) moving under the force of gravity through the texture translated strictly perpendicular to the cholesteric axis and had no measurable mobility parallel to the axis. Thus, when the applied force was oriented at an oblique angle to the axis, the spheres moved at an angle to the force. Nickel disks, 17 micrometers in radius and 300 nanometers thick, driven by gravity similarly showed no mobility parallel to the cholesteric axis in texture with small pitch. For larger pitch, the disks displayed a periodic stick-slip motion caused by elastic retardation followed by yielding of the finger texture. A simple model considering Stokes drag on the particles and elastic energy cost of deforming the finger texture describes the stick-slip motion accurately. The model assumes an elastic energy cost that depends linearly on the increase in the length of defects in the texture, and the proportionality constant obtained from fitting data is found to match the Frank constants. The effective drag viscosities obtained from the sphere and disk motion are anomalously large compared with those of pure 5CB indicating a large contribution to the dissipation from the motion of disclinations in the texture in the vicinity of the translating particles.

Joel T. Clemmer (Johns Hopkins University)

Title: Critical scaling of stresses and correlations with strain rate in overdamped sheared disordered solids

Abstract: Disordered solids exhibit a power-law distribution of avalanches and other critical behavior when driven slowly. We extend molecular dynamics studies1 of quasistatic shear of 2D and 3D overdamped binary LJ glasses to finite strain rate. As strain rate is increased, the deformation of the system is no longer driven by avalanches but by continuous, localized particle rearrangement. We use finite size scaling to study the critical behavior of various system properties including shear stress and diffusion which is governed by the rise in the dynamic correlation length with decreasing strain rate. 1K. M. Salerno and M. O. Robbins, Phys. Rev. E 88, 062206 (2013).

Matteo Contino (University of Warwick)

Title: Microorgansims motility in 2D porous material

Abstract: We study the interaction of a model microswimmer (Chlamydomonas reinhardtii) against curved solid surfaces in a quasi 2D microfluidic channel. Through simulations and experiments we propose a model to describe the phenomenon and try to use it to finely control the algae spatial distribution.

Natasha Cowley (University of Dundee, Scotland, UK)

Title: Exploring effect of position-dependent curvature on active motion

Abstract: Underlying curvature is expected to non-trivially affect the behaviour of active systems due to the incompatibility of order and curvature. While active systems on a plane have been extensively studied in the past, little work has been done to understand the effects of curvature. There are a wide range of active systems that move on curved surfaces, for example, cells in crypts in the gut, vortex patterns observed in the mammalian corneal epithelium or actively driven microtubule bundles on a droplet.

Following a recent study (R. Sknepnek and S. Henkes, Phys. Rev. E 91, 022306 (2015)) of active swarms on spheres, we aim to expand the model to a range of closed surfaces, with position-dependent Gaussian curvature. We explore how curvature of the surface affects the dynamics of the collective motion of the system, coupled with the effects of alignment timescale and driving velocity. We aim to characterise the different behaviours which emerge and relate particle motion patterns to geodesics on the surface.

Agnese Curatolo (University of Paris)

Title: Phase diagram of multi-lane driven diffusive systems

Abstract: The Asymmetric simple exclusion process (ASEP) is a paradigmatic non-equilibrium system. In this lattice-based model, particles hop on empty neighboring sites at constant rates, with a left-right bias that drives the system out of equilibrium. This model has been largely used to describe a range of systems, from molecular motors to traffic jams. When connected at its ends to particle reservoirs, the ASEP is a prototypical example of one-dimensional boundary driven phase transitions. Realistic examples, however, seldom involve only one lane; microtubules are made of several tubulin tracks while cars and pedestrian seldom move along a single line. In my poster, I explain how one can construct analytically the phase diagram of general multilane systems, in which particles can also hop between parallel lanes. In particular, I show that the “one-dimensional” phase transition seen in the ASEP survives this additional complexity but involves new features such as non-zero steady transverse currents and shear localization.

Tine Curk (University of Cambridge)

Title: Rational Design of Molecularly Imprinted Polymers

Abstract: Molecular imprinting is the process whereby a polymer matrix is cross-linked in the presence of molecules with surface sites that can bind selectively to certain ligands on the polymer. The cross-linking process endows the polymer matrix with a chemical `memory’, such that the target molecules can subsequently be recognised by the matrix. We present a simple model that accounts for the key features of this molecular recognition. Using a combination of analytical calculations and Monte Carlo simulations, we show that the model can account for the binding of rigid particles to an imprinted polymer matrix with valence-limited interactions. We show how the binding multivalency and the polymer material properties affect the efficiency and selectivity of molecular imprinting. Our calculations allow us to formulate design criteria for optimal molecular imprinting.

Michael David Czajkowski

Title: Models for Active Confluent Tissue Dynamics

Abstract: Following the success of recent mechanical models for confluent biological tissues, we treat cells as self-propelled polygons that have a potential energy associated with the polygon shape. Simulations of this model are performed in a strip geometry where cells in a band are allowed to expand into open space, commonly found in wound-healing experiments.  Preliminary simulations suggest that a non-dimensional “shape index” can be used as an order parameter to determine whether cells in the strip can migrate and intercalate. Future work will reveal underlying mechanisms required for collective migrating fronts in wound healing scenarios.   Based on this microscopic model, we also propose a set of phenomenological equations for the hydrodynamics of tissues, based on two macroscopic variables – the shape index and the self-propelled polarization – and analyze the linear stability of the homogeneous states. We develop a phase diagram illustrating the stability of these solutions as a function of the mechanical properties of individual cells and the rate at which the polarization vector decays.

Eleni Degaga (Georgetown University & National Institute of Health)

Title: TFM-TIRF for growth cone motility and biomechanics

Abstract: In the early phase of axon outgrowth, the growth cones at the tips of axonal processes actively exert forces on their environment, thus pulling on the axonal processes and aiding extension. Evidence is accumulating that the biological processes such as growth cone migration, axon extension and the formation and regeneration of neuronal connections during nervous system development are driven in part by mechanical cues and forces.

These dynamic growth cones are responsible for guiding axons to their synaptic target by translating the different cues into signals that regulate the cytoskeleton and thereby determine the rate and direction of axonal outgrowth, but many aspects of this process are not fully understood. Our research is focused on elucidating interactions between mechanical forces and the molecular machinery that enables microtubule-actin coupling and their connections with the dynamics of ECM adhesions.

To study these connections in detail, we are combining high resolution traction force microscopy (TFM) and total internal reflection (TIRF) microscopy methods. TIRF enables us to minimize phototoxicity and photobleaching observed with the conventional TFM and thereby maintain cell viability during high resolution time lapse imaging. The widely used traction force microscopy utilizes acrylamide gel with a low refractive index as a substrate, which makes it unsuitable for TIRF. Polydimethylsiloxane (PDMS) gels were utilized to produce deformable substrates with high refractive indices matching the numerical aperture of TIRF objectives, meeting the conditions for TIRF at the cell substrate interface.

We have implemented the TFM-TIRF hybrid method and are optimizing it for the study of growth cone motility and biomechanics of neuronal cell lines and primary cells. (Authors: Eleni K. Degaga1, Herbert M. Geller2, Jeffrey S. Urbach1. 1) Department of Physics, Institute for Soft Matter Synthesis and Metrology, Georgetown University Washington DC. 2) National Heart, Lung, and Blood Institute, NIH Bethesda, MD. )

Ian Estabrook (University of Sheffield)

Title: A continuum elastic model of cell nuclei deforming in microfabricated channels

Abstract: We present a continuum model of a cell nucleus undergoing deformation. In experiments of cells migrating in confinement, the cell nucleus has been shown to be a rate limiting step for cells encountering constrictions. When forces are applied to a cell, different components of the cell exhibit remarkably different responses. On timescales typical of cell migration, the cell cytoplasm responds to applied forces in a fluid-like manner, while the nucleus demonstrates a more elastic response.

We therefore model the nucleus as a deforming elastic solid, governed by the laws of continuum mechanics. We apply this model to experimental images of DAPI stained dendritic cell nuclei in microfabricated channels containing constrictions of controlled dimensions provide a simplified confined environment mimicking confinement experienced by cells in tissues. The nucleus boundary of consecutive images over time is traced using imageJ software. The deformation field between images is chosen such that the free energy of the deformation is minimised. This deformation field is then used to determine the strain, stress and traction force fields over the nucleus boundary. Our model therefore estimates the forces felt by a nucleus as it deforms.

In future work we plan to use this model to calculate whether these forces alone are sufficient to cause the breakdown of the nuclear membrane that is seen when nuclei migrate through particularly small channels.

Varda Faghir Hagh (Arizona State University) - Poster

Title: Rigidity Transition: from random networks to jamming (In collaboration with Wouter G. Ellenbroek and Martin van Hecke)

Abstract: We study the qualitative differences in the rigidity transition of three types of disordered networks: randomly diluted spring networks (RP), stress-relieved networks obtained by diluting the stressed bonds in spring networks (SR) and disk packing networks (Jamming). Unlike randomly diluted networks that contain both over-constrained (stressed) and under-constrained (floppy) regions at their marginal state, the transition points of stress-relieved and jammed networks are globally isostatic. However they behave very differently when we add/remove one bond to/from their isostatic state [Wouter G. Ellenbroek, Varda F. Hagh, Avishek Kumar, M. F. Thorpe, and Martin van Hecke; Phys. Rev. Lett. 114, 135501].

We introduce two new indices h and s that measure the average fractions of hinges and stressed bonds that appear as we remove or add one bond to the isostatic state of the networks. These indices characterize the high degree of self-organization at the jamming point where global changes occur by adding or removing only one bond. We then incorporate a new condition into stress-relieved networks, where at each site we insist that the Hilbert stability condition is obeyed. The Hilbert condition involves having at least three contacts at each site, with at least one contact in every semicircle. This introduces an isostatic state with a unique structure of Laman sub-graphs identical to the Laman sub-graphs of jamming that has similar h and s indices and exposes the underlying geometrical self-organization of jammed networks. (Authors: Varda F. Hagh1, M.F. Thorpe2. 1 Department of Physics, Arizona State University, Tempe, AZ 85287-1504, USA, 2 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Rd, Oxford OX1 3NP, England)

Emily Gehrels (Harvard University)

Title: From strand displacement to colloidal dancers: using DNA to program novel interactions

Abstract: We present an experimental system of DNA-functionalized colloidal particles which exhibit directed motion (‘dancing’) along patterned substrates in response to temperature cycling. We take advantage of toehold exchange in the design of the DNA sequences that mediate the colloidal interactions to produce broadened, flat, or even re-entrant binding and unbinding transitions between the particles and substrate. Using this new freedom of design, we devise systems where, by thermal ratcheting, we can externally control the direction of motion and sequence of steps of the colloidal dancer.

Omer Gottesman (Harvard University)

Title: Dynamics of crumpling

Abstract: The simple process of crumpling a sheet of paper holds many questions regarding the properties of the resulting complex network of creases. Many aspects of crumpling have been described by modeling thin sheets as purely elastic, and finding the deformations which minimize elastic energy. However, most real materials can deform plastically, and deformations may leave permanent scars in the sheet. Thus, history dependence is introduced into the system. We present work on the dynamics of crumpling at different scales, from the behavior of single defects to the emergent macroscopic phenomena resulting from the complex structure of the crease network and their interaction.

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Session II (Wednesday, July 15, 2015)

Ming Han (Northwestern University)

Title: Programmable Collective Behavior of Self-Propelled Janus Particles

Abstract: We examine the possibility of combining active behavior and self-assembly to achieve various, programmable collective structures. A prototypical design is proposed, and realized in computer simulations and experiments. This design consists of colloidal Janus particles with one metal-coated hemisphere and one dielectric hemisphere, placed in an AC perpendicular electric field. Such particles display well-known swimming behavior, as the field orients the particles and exposes them to an asymmetric ionic flow. We exploit the induced dipole moments and their dependence on field frequency and coating thickness to modulate the colloidal interactions, resulting in the organization of the active Janus swimmers into random arrangements, swarms, clusters, chains, and even streamers. The design of active, non-Brownian agents with multiple interaction sites is generalizable to other nanoparticles and colloids, and potentially to molecules.

Joseph Harder (Columbia University)

Title: Activity induced collapse and re-expansion of rigid polymers

Abstract: We study the elastic properties of a rigid filament in a bath of self-propelled particles.  We find that while fully flexible filaments swell monotonically upon increasing the strength of the propelling force, rigid filaments soften for moderate activities, collapse into metastable hairpins for intermediate strengths, and eventually re-expand  in the large activity limit. This collapse and re-expansion of the filament with the bath activity is reminiscent of the behavior of polyelectrolytes in the presence of different concentrations of multivalent salt.

Kabir Husain (National Centre for Biological Sciences)

Title: Dynamics of Emergent Structures in an Active Polar Fluid

Abstract: Inspired by recent evidence that the actomyosin cortex templates the organisation of certain cell surface proteins, here we construct an active fluid description of actin filaments and myosin motors. We study the nucleation and form of compact structures that arise spontaneously in this description. Elastic forces, self-propulsion and contractility give rise to a phase diagram of various localised defect configurations, some of which are mobile. We explore their kinetics as well as the dynamics of interacting structures. Finally, we discuss some implications for the study of the cell surface.

Uroš Jagodič (Institut Jožef Stefan)

Title: Topological defect production in nematic liquid crystal with non-trivial geometry

Abstract: The formation and the structure of topological defects are of great interest in various soft matter systems [1-3]. Liquid crystals exhibit universal behaviour known from cosmology to superconductivity on a scale which is observable with optical microscopy. In the experiments, we study the formation of topological defects right after sudden temperature driven symmetry breaking phase transitions [1, 4, 5], well described by the Kibble – Zurek mechanism [4, 5].

We study the production of topological defects along and in particles of non-trivial geometry dispersed in nematic and chiral nematic liquid crystal [6]. We use the process of two photon polymerization to create colloidal particles and cavities of various non-trivial geometry. The colloidal particles are placed in nematic liquid crystal with a given orientation of the director dield alon the surfaces, and filled with nematic and chiral nematic liquid crystal. We study the formation of topological defects right after sudden temperature driven symmetry breaking phase transition induced by photothermal effect. We use a modified shadowgraphy experimental set-up to study the dynamics of the system right after the dynamic freezing of the system [7]. We also use polarized fluorescence microscopy to study the stable director field configuration in and along the colloidal particles [8]. We also study the self-assembly of individual colloidal cavities of various non-trivial shape, which can be used as micro resonators for lasing. (Authors: U. Jagodič1, I. Muševič1,2. 1 Institut Jozef Stefan, Jamova 39, 1000 Ljubljana, Slovenija. 2 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia)

Keywords: Liquid crystals, topological defects creation, Kibble – Zurek mechanism, non-trivial geometry, colloidal particles


  1. de Gennes PG, Prost J. The Physics of Liquid Crystals. Oxford: Oxford Science; 1993.
  2. Muševič I. Nematic colloids, topology and photonics. Phil. Tans. R. Soc. A. 2013;371.
  3. Kurik MV, Lavrentovich OD. Defects in Liquid Crystals: homotropy theory and experimental studies. Sov. Phys. Usp. 1988;31(3).
  4. Bradač Z, Kralj S, Žumer S. Early stage domain coarsening of the isotropic-nematic phase transition. The Journal of Chemical Physics. 2011;135(2):024506-024515.
  5. Bradač Z, Kralj S, Žumer S. Molecular dynamics study of the isotropic nematic quench. Physical Review E. 2002;65(2):021705-021715.
  6. Nikkhou M, Škarabot M, Čopar S, Ravnik M, Žumer S, Muševič I. Light-controlled topological charge in a nematic liquid crystal. Nature physics. 2015;11.Gregorčič G, Možina J.
  7. High-speed two-frame shadowgraphy for velocity measurements of laser-induced plasma and shock-wave evolution. Optics letters. 2011;36(15).
  8. Smalyukh II, Senyuk BI, Gu M, Lavrentovich OD. Focused laser beams and liquid crystals: three-dimensional imaging of structures and topological defects. Liquid Crystals: Optics and applications. 2005;5947.

Mathijs Janssen (Utrecht University)

Title: Putting the electric double layer to work - harvesting sustainable energy using variable capacity engines

Abstract: In recent years there has been a growing interest in so-called “blue energy”, energy harvested from the salt concentration differences between sea and river water. 

Next to membrane-based engines, of which a pilot-plant is currently being tested at in the Netherlands, a new capacitive blue engine was proposed recently [D. Brogioli, Phys. Rev. Lett. 103, 058501 (2009)]. This engine operates by cyclically charging and discharging nanoporous supercapacitors immersed in salty and fresh water, respectively. We show that a temperature difference between sea and river water has a dramatic effect on the work performed during operation cycles: it can lead to a threefold increase in the energy harvested if warm (waste-heated) fresh water is mixed with cold sea water. 

The physical mechanisms at work at the core of the process can be easily generalized; I will present two seemingly unrelated capacitive engines and will show analogies among all of them.

Leroy L. Jia (Brown University)

Title: Force vs. extension of colloidal membranes

Abstract: In experiments, disk-shaped colloidal membranes composed of long rod-like viruses will twist into a ribbon shape under the application of a diametric stretching force. We use an effective model valid for membranes with small twist penetration to study this phase transition and calculate the force necessary to stretch the membrane out to a given extension. The model predicts that for small deformations, the force is linear with spring constant depending on the effective edge bending stiffness of the membrane, while for large extensions, the force is found to saturate to a constant value. Surprisingly, the force is not a monotonic function of the extension. Finally, we use simple simulations of a rectangular strip to find a power law that accurately describes the critical stretch at  which the membrane starts to twist, which may be used to estimate the value of unknown constants by comparison with experimental data.

Tal Kachman (MIT)

Title: Emergent Computation in Nonequilibrium Systems

Abstract: For equilibrium systems the Boltzmann distribution gives an elegant and simple relation between the probability of a given microscopic configuration, its energy and system parameters Many of the more interesting results in physics and in nature occur in systems far from thermal equilibrium. In nonequilibrium systems there is a need to resort to other tools such as fluctuation theorems. Perhaps the most well known is the Crooks relation which gives the relation between entropy production and irreversibility. In this work we will build upon this idea and devise a scheme for emergent computation and adaption. We devise a model system that can exhibit adaptation and show the effect of the non equilibrium drive on the manner in which the system shows emergent adaptation

Hridesh Kedia (University of Chicago)

Title: Conservation of helicity in superfluids

Astract: Helicity arises as a special conserved quantity in ideal fluids, in addition to energy, momentum and angular momentum. As a measure of the knottedness of vortex lines, Helicity provides an important tool for addressing a wide variety of physical systems such as plasmas and turbulent fluids. Superfluids flow without resistance just like ideal (Euler) fluids, making it natural to ask whether their knottedness is similarly preserved. We address the conservation of helicity in superfluids theoretically and examine its consequences in numerical simulations.

Katherine Klymko (University of California, Berkeley)

Title: Pattern Formation in Driven Systems

Abstract: We are interested in understanding assembly processes in far-from-equilibrium systems, in particular particle systems under the influence of external driving fields. We have been using theory and computer simulations to study pattern formation in a particular model of driven particles. Our model of interest consists of two types of particles that are pushed in opposite directions by an external AC or DC field and interact with each other through purely repulsive interactions. This model is appealing because it is simple yet provides access to a wide range of patterns in both two and three dimensions which can be accessed by tuning the strength of the external field, the frequency of the field, and the density of the system. For example, this system undergoes a homogeneous to ordered transition as the strength of the external field is increased, characterized by the emergence of dynamic lanes of like particles moving parallel to the field direction. These lanes persist at low field frequencies, but transition to bands perpendicular to the field direction as the frequency increases. Additionally, the bands’ widths can be tuned by adjusting the frequency of the field. We have been working to develop a microscopic understanding of the origins of the different patterns this system supports.

Johannes Knebel (Ludwig-Maximilians-Universität München) - Poster

Title: Evolutionary games of condensates in driven-dissipative systems of non-interacting bosons

Abstract: Condensation is a collective behavior of particles observed in both classical and quantum physics. For example, when an equilibrated, dilute gas of bosonic particles is cooled to a temperature near absolute zero, the ground state becomes macroscopically occupied (Bose-Einstein condensation). Whether novel condensation phenomena occur far from equilibrium is a topic of vivid research.

Only recently has it been proposed that a driven and dissipative gas of bosons can condense not only into a single, but also into multiple non-degenerate states. This phenomenon may occur when a system of non-interacting bosons is weakly coupled to a reservoir and is driven by an external time-periodic force (Floquet system). Coherence becomes negligible and the condensation is described by a Pauli master equation, which also arises in the evolutionary dynamics of classical agents.

In our work, we applied concepts from evolutionary dynamics to determine the states that become condensates. This condensate selection is guided by the vanishing of relative entropy production. We found that the system of condensates never comes to rest: The occupation numbers of condensates oscillate, which we demonstrated for a rock-paper-scissors game of condensates.

Jie Lin (New York University)

Title: Scale-free avalanches below the yield stress

Abstract: In non-equilibrium phase transition, scale-free avalanches only happen, at one single point in the parameter space generally, e.g., only when the driving force is equal to the critical force in the depinning transition, or when the driving force has self-organized to the critical value in the context of self-organized criticality. In this paper, we show that in the yielding transition of amorphous solids, scale-free avalanches can be observed in a large range of stress space below the yield stress, by shearing the system continuously. We prove that this transient criticality is intimately related to the singular distributions of the density of local excitations, shear transformation zones, as function of the distance to the local instability. We point out the connection between the local slop of stress-strain curves to the mean avalanche sizes, and derive the scaling relation, between \theta, df, \tau, characterizing, respectively, the density of local excitations, the fractal dimension of avalanches, and the power law exponent of avalanche size distributions. We test our idea by elasto-plastic model, and find scale-free avalanches below the yield stress associated with a correlation length scale comparable to the system size. Our theoretical arguments are verified nicely and suggest a new type of self-organized criticality.

Sofia Magkiriadou (University of Chicago)

Title: Geometry and Phase Transitions in a Driven Colloidal System

Abstract: Colloidal particles in suspension are excellent model systems for atomic matter. By encoding information in the particles, for instance with chemistry, their interactions can be engineered in a variety of ways, giving rise to rich phase behavior in their aggregates. Here, we ask: what happens when we can tune these interactions dynamically? And can we further control the phase behavior with geometry? We will share our observations on such a colloidal system where boundary effects give rise to intriguing emergent behavior.

Stewart Mallory (Columbia University)

Title: An active approach to engineering the micro-scale

Abstract: Suspensions of bacteria and synthetic active particles offer a novel approach to manipulating matter at the micro-scale. This poster highlights two of the many ways that the unique thermomechanical properties of an active fluid can be exploited to regulate the behavior of microscopic systems. Using a combination of numerical simulations and analytical theory, we illustrate how a large tracer can be  effectively activated when immersed in a low-density suspension of active particles, and propose simple scaling arguments to characterize this induced activity in terms of the curvature of the tracer and the strength of the self-propelling force.  We also demonstrate how active fluids can mediate a new set of effective interactions between passive elements. Our results suggest new ways of manipulating the interactions between passive tracers and controlling their transport  properties.

David Mayett (Syracuse University)

Title: Interacting active elastic dimers: Two cells moving on a rigid track

Abstract: Cell migration in morphogenesis and cancer metastasis typically involves an interplay between different cell types. The rules governing such interplay remain largely unknown, however, a recent experiment studying the interaction between neural crest (NC) cells and placodal cells reveals an example of such rules. The study found that NC cells chase the placodal cells by chemotaxis, while placodal cells run away from NC cells when contacted by them. Motivated by this observation, we construct and study a minimal one-dimensional cell-cell model comprised of two cells with each cell represented by two-beads-connected-by-an-active spring. The active spring for each moving cell models the stress fibers with their myosin-driven contractility (and alpha-actinin extendability), while the friction coefficients of the beads describe the catch/slip bond behavior of the integrins in focal adhesions. We also include a dynamic contact interaction between the two cells, as well as a chemotactic potential, to decipher the chase-and-run dynamics observed in the experiment. We then use our modeling to further generalize the rules governing the interplay between different cell types during collective cell migration. 

Simon Merminod (University of Paris) - Poster

Title: Labyrinthine phase and slow dynamics in a driven granular medium

Abstract: Labyrinthine patterns arise in two-dimensional physical systems submitted to competing interactions, ranging from the fields of solid-state physics to hydrodynamics. Our work reports the observation of a labyrinthine phase in an out-of-equilibrium system constituted of magnetized macroscopic particles. Using accurate particle tracking, we are now able to characterize the appearance of the labyrinthine phase when the interaction strength is increased. Here the physics differs strongly from analogous labyrinthine or stripe phases made of colloidal particles, which were observed in several numerical simulations and in a single experiment. Indeed, by studying aging properties and the response to a magnetic quench, we show that the large-scale disordered labyrinthine phase exhibits a slow dynamics, which occurs typically in out-of-equilibrium disordered systems such as structural glasses.

Lisa Nash (University of Chicago)

Title: Topological mechanics of gyroscopic metamaterials

Abstract: Topologically protected states can arise in electronic systems with broken time-reversal symmetry. We present a classical mechanical model composed of gyroscopes in which broken time-reversal symmetry gives rise to topologically protected edge-modes, analogous to the edge modes in the quantum Hall effect. We will discuss numerical and experimental observations of these chiral edge-modes, their topological characterization, robustness and broader phenomenology. (Authors: Lisa M. Nash, Dustin Kleckner, Alismari Read, Vincenzo Vitelli, William T.M. Irvine)

Thomas C. O’Connor (Johns Hopkins University)

Title: Finite Length Chains and Yield in Polyethylene Crystals

Abstract: Understanding the microscopic mechanisms of yielding in oriented polymer fibers is a long outstanding problem in polymer mechanics. While advances in polymer processing have produced highly ordered polyethylene (PE) fibers with substantial tensile strengths between 4-7 GPa, these values are far less than the theoretical ultimate strength of 25 GPa, where PE chains fail due to C-C bond scission. This reduction in strength is caused by the presence of defects within the crystalline structure of the ordered fiber. The simplest defect that can be considered is the presence of chain ends – i.e., accounting for the finite length of polymer chains. The presence of chain ends allows a polymer fiber to yield by chain slip without the scission of covalent bonds. However, chain ends also provide likely sites of strain localization that may allow for bond scission at applied stresses much lower than the theoretical maximum.

Here we present extensive molecular dynamics simulations of crystalline PE fibers subjected to uniaxial tension. The fibers are fully aligned crystals constructed from chains of finite lengths, which span three orders of magnitude (10^1- 10^4 monomers). To explain the yield of these systems we parameterize an effective Frenkel-Kontorova model for a finite chain in the periodic potential of its neighbors, and apply this model to determine the stability and yield of the isolated chain end defect.

Naomi Oppenheimer (Princeton University)

Title: Effect of hydrodynamic interactions on quasi two-dimensional reaction rates

Abstract: The Brownian motion of two particles in three dimensions serves as a model for predicting the diffusion-limited reaction rate as first discussed by Smoluchwski. Deutch and Felderhof (J. Chem. Phys., 1973) extended the calculation to account for hydrodynamic interactions between the particles and the target. This results in a reduction of the rate coefficient by about half. Many chemical reactions take place in quasi two-dimensional systems (such as on the membrane or surface of a cell). We perform a Smoluchowski-like calculation in a quasi two-dimensional geometry (a membrane surrounded by fluid) and account for hydrodynamic interactions between the particles. We show that rate coefficients are reduced relative to the case of no interactions. The reduction is more pronounced than the three-dimensional case due to the long range nature of two-dimensional flows. 

Giuseppe Passucci (Syracuse University)

Title: Experimental cell migration as a Levy walk

Abstract: Self-propelled particle (SPP) models have been used extensively to study collective cell motion, but they do not always accurately capture the long-time behavior observed in experiments. Furthermore, the equation for polarization in these models is not experimentally well-constrained. Therefore we developed a novel method for quantifying polarization in Hs578T breast carcinoma cells in a wound healing geometry. During cell movement, the nucleus orients toward the anterior of a cell while the Golgi body orients towards the posterior; we simultaneously imaged and tracked the Golgi and nuclei and constructed a polarization vector defined by the Golgi-nuclei axis. We find that cells in the bulk are not highly polarized, while those on the edge are highly polarized outward perpendicular to the wound edge. We incorporate these polarization dynamics into a Levy walk based SPP model and compare long time motility to experiments.

Matthew Pinson (University of Chicago) - Poster

Title: Signal Transmission through Disordered Hypostatic Materials

Abstract: A bag of sand is a slightly hypostatic system: the number of constraints is just a little smaller than the number of degrees of freedom. As a result, several linearly independent modes of motion are available at zero energy cost. The question naturally arises: can we use these modes to transmit information from one side of the system to the other? In this poster, I show why we cannot. Even though each mode considered on its own spans a large portion of the system, combining the modes yields only a few independent long range modes, and many localised modes. Thus the effective number of free modes seen by any small portion of the system is much smaller than we would have guessed based on Maxwell counting. This provides an unexpected limitation on the perturbations that can be applied, and even most of those that are accessible are not transmitted.

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Session III (Wednesday, July 22, 2015)

Ishan Prasad (University of Massachusetts, Amherst)

Title: Chirality Transfer in Block Copolymer Melts

Abstract: Block copolymer systems are known to assemble into rich spectrum of ordered phases. Many complex phases have been observed by introducing asymmetry in copolymer architecture. Despite recent progress, influence of intrinsic chirality on equilibrium mesophase structure of block copolymers is not well understood and largely unexplored. Self consistent field theory has played a major role in prediction of physical properties of polymeric systems. Only recently, a polar orientational self-consistent field (oSCF) approach was adopted to model chiral BCP having a thermodynamic preference for cholesteric ordering in chiral segments. We implement oSCF theory for chiral nematic copolymers, where segment orientations are characterized by quadrupolar chiral interactions, to study thermodynamic stability of bi-continuous chiral network morphologies. Unique photonically-active properties identified in butterfly wings have been attributed to presence of chiral single-gyroid networks, this has made it an attractive target for chiral metamaterial design. 

S.Ganga Prasath (Tata Institute of Fundamental Research)

Title: Non-linear behaviour of elastic filaments under large deformations

Abstract: We perform experiments to investigate the relaxation of a highly deformed elastic filament at a fluid interface. The dissipative forces arising due to elastic energy of filament and viscosity of fluid balance to help the filament reach its equilibrium state. We characterise the dynamics by quantifying the end-to-end distance and elastic energy as a function of time. The time taken for the filament to straighten depends on the viscous drag, bending modulus of the filament. We perform numerical computations with initial condition similar to that of experiments to see if simplified dynamics capture the experiments well.

Elias Putzig (Brandeis University)

Title: Continuum theory of an overdamped active nematic: Instabilities, defects, and defect ordering

Abstract: Active liquid crystals are a high profile topic in the field of active matter because of the novel dynamics seen in these materials.  They are highly nonequilibrium as the ‘active’ forces are generated by the particles themselves, rather than an external field or boundary. We consider a phenomenological continuum theory for an extensile, overdamped active nematic liquid crystal, applicable in the dense regime. Constructed from general principles, the theory is universal, with parameters independent of any particular microscopic realization. We show that it exhibits a bend instability similar to that seen in active suspensions, that lead to the proliferation of defects. Using numerical and analytic tools we find three regimes: a defective nematic regime, which can be turbulent, an undulating nematic regime with no defects, and a regime in which +1/2 disclinations develop polar ordering. We characterize the phenomenology of each of these phases and identify the relationship of this theoretical description to experimental realizations and other theoretical models of active nematics.

Archishman Raju (Cornell University)

Title: Sloppy models and geometry

Abstract: There has been recent work on the behaviour of nonlinear multiparameter models fit to data. These models, called sloppy models, seem to share universal characteristics. The widths of the model manifold have a hierarchy with important ‘stiff’ directions followed by less important ‘sloppy’ directions. These directions are linear combinations of the bare parameters of the theory. Analyzing traditional models in physics shows that the idea of effective theories describing several microscopic models can be described in a similar framework. This hierarchy in parameter space gives a unifying principle for modelling in science. I will present our recent attempts to connect our understanding of sloppiness in biological and multiparameter fits to the tools we use to describe emergent theories in physics.

Sepideh Razavi (City College of New York)

Title: Colloidal Particles and Fluid Interfaces: Approach, Breach and Flow Behavior

Abstract: Recently, a great deal of attention has been focused on colloidal particles at interfaces owing to their substantial desorption energy that can be harnessed to stabilize fluid interfaces. The physical and chemical heterogeneity of the particle surface is known to influence the interaction of colloidal particles with fluid interfaces; hence, to render an interface stable, proper wettability and suitable surface properties are essential. For instance, amphiphilic (Janus) particles are believed to bind particularly strongly to interfaces and form a breathable interfacial skin. The dynamics of such colloidal particles moving towards and onto an interface is therefore of considerable interest. Using digital holography microscopy and molecular dynamics simulations, we have analyzed the motion of a Janus particle adjacent to a liquid interface by monitoring the translational and rotational dynamics as the particle approaches and then binds to the interface. Based on our findings, the particle behavior shows strong orientation dependence both before and after breaching.

In addition to binding dynamics and interfacial configuration of colloidal particles, the flow behavior of colloidal monolayers formed at the interface contributes to stability because in many of the applications that involve particle-laden interfaces, the interface undergoes large deformations. The response of interfacial layers to compression has been examined in the literature and different collapse mechanisms have been reported, including monolayer buckling and expulsion of particles from the interface. Despite the large body of work on particles at interfaces, the key factors governing the mode of collapse have not been clearly identified. To better understand how particle surface properties impact instabilities at fluid interfaces, we have studied interfaces decorated with plain particles of different surface wettabilitiy as well as amphiphilic Janus particles and their response to compression. Our results provide insight on the consequential role of particle-particle and particle-interface interactions in determining the stability and collapse of particle-laden interfaces.

Jason Rocks (University of Pennsylvania)

Title: Cardiac tissue as an electrically and mechanically active medium

Abstract: The heart is an active solid in which energy is injected at the cell scale when cardiomyocytes contract. This energy is transduced up to macroscopic scales, leading to a collective function–the pumping of the heart–in which a wavefront of contraction propagates across the heart from one end to the other. We present results for a model that couples a traditional model for electrical signaling to an overdamped biphasic model for tissue mechanics to look at the competition between mechanical and electrical signaling in the contractile wavefront in the embryonic heart. We speculate on the ramifications of our results for the adult heart, which is conventionally described exclusively in terms of electrical signaling.

Seyed Mahdi Sadjadi (Arizona State University)

Title: Unique Minimal Energy Configuration in Isostatic Networks

Abstract: We present results of energy minimization in two-dimensional (2D) network of corner sharing triangles. These isostatic networks are important in modelling of bilayers of vitreous silica (SiO2) where upper and lower layers consist of tetrahedra joined at the apex. In the 2D monolayer, triangles are formed with the O atoms at the vertices, while Si atoms (after projecting onto the plane) are placed at the center of each triangle.

The isostatic nature of the experimental samples requires a careful choice of boundary conditions as the surface effect is not negligible, even for large samples. We employ anchored boundary conditions, where half the atoms at the surface and pinned, to relax the structure using harmonic potential to produce corner sharing networks with perfect equilateral triangles.

In contrast to zeolites that exhibit a flexibility window, this 2D network shows that minimum energy corresponds to a unique configuration of atoms at a single density. We do find a separate flexibility window in the 2D network, but this window corresponds to unphysical case where triangles are overlapping. Currently, this work is being extended to the bilayer case. (Authors: Mahdi Sadjadi(a) and M.F. Thorpe(a,b). (a) Department of Physics, Arizona State University, Tempe, AZ 85287-1504, USA, (b) Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Rd, Oxford OX1 3NP, England)

Raphael Sarfati (Yale University)

Title: Interaction measurement of particles bound to a lipid membrane

Abstract: The local shape and dynamics of the plasma membrane play important roles in many cellular processes. Local membrane deformations are often mediated by the adsorption of proteins (notably from the BAR family), and their subsequent self-assembly. The emerging hypothesis is that self-assembly arises from long-range interactions of individual proteins through the membrane’s deformation field. We study these interactions in a model system of micron-sized colloidal particles adsorbed onto a lipid bilayer. We use fluorescent microscopy, optical tweezers and particle tracking to measure dissipative and conservative forces as a function of the separation between the particles. We find that particles are driven together with forces of order 100 fN and remain bound in a potential well with a stiffness of order 100 fN/micron.

Sumantra Sarkar (Brandeis University)

Title: Granular materials in flatland: Understanding rigidity through the geometry of the stresses

Abstract: Granular materials, such as sand, have to obey certain constraints. In two dimension we exploit these constraints to make a geometric representation of  the microscopic stresses. This geometric representation, called force tiling, provides a conceptually cleaner description of the jamming transition. In this poster, I’ll apply this technique to experiments on shear jamming, discontinuous shear thickening, and such shear driven phenomena and show that these rigidity transitions correspond to topological and morphological changes in the tiling. Study of these geometric changes provide useful insights into the jamming transition, which can be extended to construct a microscopic theory of any athermal, shear driven, rigidity phenomena.

Pablo Sartori (Max Planck Institute for the Physics of Complex Systems)

Title: How curvature regulates the beat of Chlamydomonas flagella

Abstract: The bending of cilia and flagella is driven by forces generated by dynein motor  proteins. These forces slide adjacent microtubule doublets within the axoneme, the motile  cytoskeletal structure. To create regular, oscillatory beating patterns, the activities of the  axonemal dyneins must be coordinated both spatially and temporally. It is thought that coordination is mediated by stresses or strains, which build up within the moving axoneme,  and somehow regulate dynein activity. Different types of regulation will give raise to different beating patterns. For example, in the case of Chlamydomonas, its cilia beat asymmetrically and show strong wave propagation. Yet there are mutants in which the beat becomes symmetric.

It has been proposed that the mechanics of the cilium together with the regulated dynein activity conform a dynamical system. Within this framework, beat patterns correspond to critical points of a Hopf bifurcation. Here we study how different types of motor regulation give raise to distinct beat patterns, and compare them to experimental data for the beat of Chlamydomonas cilia. To do so we developed a theory for critical asymmetric beats in which motors can be regulated by sliding, curvature, and normal forces. While in an infinite media all types of regulation can produce wave propagation, we
show that in cilia as short as that of Chlamydomonas sliding regulation only produces standing waves. We further show that for symmetric cilia normal forces become a second order effect, and so are unable to regulate the beat. We thus conclude that curvature must be the main element involved in dynein regulation. Indeed, the beat patterns predicted by curvature regulation agree very well to those measured for Chlamydomonas and its mutant.

Robert Schulz (Freie Universität Berlin)

Title: Water diffusion at biological molecules and interfaces: Bridging stochastic and hydrodynamic descriptions

Abstract: The unique properties of liquid water are relevant for a broad range of processes in biology, chemistry, and physics, as well as for technological applications. A prominent goal has been to relate macroscopic properties (among those the notable anomalies and singularities of equilibrium as well as transport properties of water) to the microscopic structure and thus to the hydrogen bonding pattern between individual water molecules.

We consider a Molecular Dynamics simulation based on a 10ns long trajectory of bulk water. Pairs of water within a certain separation length R are considered and analyzed. With the help of a Markov state model, we are able to discern different processes which describe the switching of hydogen bonds between different partners of water molecules. The concept of transition path theory for Markov models allows to reveal competetive reaction pathways when a hydrogen bond is broken and formed.

Toby Searle (University of Edinburgh)

Title: What is purely elastic turbulence? A numerical investigation into a self-sustaining process

Abstract: Turbulence, the seemingly random motion of fluid at high flow rates, is commonplace and yet poorly understood. From the mixing of milk in tea, billowing smoke and the roiling motion of boiling water, turbulence is everywhere. However, we still are unable to predict when the transition to this seemingly disordered motion will occur for a given flow rate, viscosity and geometry. ‘Exact coherent structures’ (a specific kind of long lifetime patterns in the flow) are a new approach that give more accurate predictions of the transition to and behaviour of a turbulent flow. Purely elastic turbulence is a new kind of turbulence due to the elasticity instead of the inertia of a viscoelastic fluid. This kind of turbulence is thought to commonly occur in flows of polymer and biopolymer solutions as well as controlling the rate of production of many plastic products.

I will present some results obtained during the first attempt to find these exact coherent structures in flows of viscoelastic fluids. I will present an analogous self-sustaining process in viscoelastic plane Couette flow to one found in the flow of Newtonian fluids (Waleffe, 1997) and discuss how it might lead to an exact coherent structure.

R. G. Larson. Turbulence without inertia. Nature 405: 27-28, 2000.
F. Waleffe. On a self-sustaining process in shear flows. Phys. Fluids 9: 883-900, 1997.

Nimrod Segall (Tel Aviv University)

Title: Jamming vs Caging in Three Dimensional Jamming Percolation

Abstract: Kinetically-constrained models (KCM) are widely used in the study of jamming and glass transitions in granular and amorphous materials. These lattice gas (or Ising) models are characterized by having a restricting set of rules for particle movement (or spin flip) which depends on the state of a given site’s neighbors. Most KCM become jammed only at the limit of zero temperature (Ising spins) or equivalently full occupation (lattice gas) or when considering finite-sized or confined systems. An interesting exception is the Spiral Model, which has been proven to undergo a directed percolation phase transition at a density 0 < \rho_J < 1, in which both jamming and particle caging occurs.

We have expanded the Spiral Model into a 3D model and investigate it using a method which allows us to avoid the dynamics of the model by looking at the connectivity of jammed clusters.  We show the similarities and differences between the two models, notably, that in the 3D model there are two critical densities, one for jamming and a second for caging.

Suraj Shankar (Syracuse University) - Poster

Title: Confined Nematic Defects As Active Particles

Abstract: Away from equilibrium, as is seen in many active systems, topological defects play an important role in understanding the dynamical behaviour of ordered phases. Focusing on systems with a nematic symmetry, understanding the route to fully developed active turbulence is believed to be directly linked to the dynamic proliferation of disclination defects (in 2 dimensions). Given this to be the case, the first step would be to have a comprehensive description of the motion of a single disclination, by virtue of the viscous flows generated by active stresses. In this case, +1/2 disclinations behave as effective self-propelled particles by virtue of their symmetry. Using just dimensional scaling, we get a naïve velocity scale that diverges like the system size. In order to understand the length scales controlling the divergence, we analyze the motion of defects in a 2D fluid interface bounded by finite bulk fluids. One then finds a frictional screening length induced by the confinement that governs the long distance hydrodynamic behaviour along with a renormalization of the interfacial viscosity. Additionally, the core velocity is found to have an anomalous scaling with the confinement depth, which gives us an explicit length scale controlling the divergence of the defect motion in large systems.

Yu Shi (Johns Hopkins University)

Title: Dissecting Subcellular Actomyosin Mechanics with Magnetically Actuated Micropost Arrays

Abstract: The cellular actomyosin cytoskeleton is widely regarded as an archetypal example of an active matter system. However, the extent to which the wide range of observed cellular motility behaviors arise from active-matter physics is not well understood. Characterizing an active matter system requires simultaneous measurement of the fluctuation spectrum of the internal force generators and also the local viscoelasticity in order to separate the distinct effects of the material’s internal stresses from its viscoelastic response to those stresses. By placing cells on top of micropost array detectors (MPADs) with magnetic nickel nanowires embedded in selected posts, we can actuate local regions of the cells by applying AC magnetic fields to dynamically probe the local viscoelasticity, while simultaneously using the posts as “probe particles” to make passive microrheology measurements of the cytoskeletal force fluctuations. Fabrication of microposts array detectors will be briefly introduced. Results of measurements of the frequency-dependent viscoelasticity as well as the distribution of force fluctuations for different subcellular regions of 3T3 fibroblasts will be presented, and the results compared to simple active material models based on known or predicted behavior of molecular motors in viscoelastic networks.

Karandeep Singh (Institute of Complex Systems and Institute for Advanced Simulation)

Title: Receptor-mediated wrapping of nanoparticles

Abstract: Biological cells internalize cargo using different internalization processes, e. g. endocytosis, phagocytosis, and pinocytosis. In all cases, the cargo is inside a carrier, which interacts with the membrane via its wrapping by the membrane. The complex wrapping process depends on a variety of different parameters, such as shape and size of the carrier [1, 2], the interaction between cell membrane and the carrier, bending rigidity and tension of the membrane, and possibly the cytoskeleton [3]. Hence, understanding the physics of the internalization process is in itself a complex problem. Towards the understanding of the full process, we tackle this problem using a minimalistic model of a spherical nanoparticle with attached ligands, a lipid bilayer membrane, and receptors that mediate the interaction between membrane and nanoparticle.

In our model, we calculate the deformation energy of the fluid membrane using Helfrich Hamiltonian. The receptors on the membrane bind to the ligands on the nanoparticle, and this binding is controlled by an interplay between binding energy per receptor-ligand bond, entropy, and bending energy of the membrane. For a given fraction of the particle being wrapped by the membrane, we obtain an optimum number of bound receptors at equilibrium. The number of bound receptors depends on the overall receptor density on the membrane, the binding energy gain per receptor, and the area of the membrane that is not bound to the particle. For a homogeneous adhesion strength between the particle and a tensionless membrane, a direct transition from an unbound to a completely-wrapped state occurs beyond a threshold adhesion strength [4]. For receptor-mediated adhesion, we find that the transition between unwrapped and completely-wrapped states occurs gradually via partially-wrapped states. We calculate the dependence of the wrapping state of the nanoparticle on particle size and ligand density, receptor density on the membrane, and binding energy per receptor. These partially-wrapped states induce a spontaneous curvature in the membrane, and we quantify its effects on the red blood cell shapes. We further investigate dynamics of the receptor-mediated wrapping of these nanoparticles from a Smoluchowski dynamics approach and compare its results with experiments.

  1. S. Dasgupta, T. Auth, and G. Gompper, Soft Matter 9, 5473 (2013)
  2. S. Dasgupta, T. Auth, and G. Gompper, Nano Lett. 14, 687 (2014)
  3. S. Tollis et al., BMC Systems Biology, 4, 149 (2010)
  4. M. Deserno, Phys. Rev. E 69, 031903 (2003)

Lisa Tran (University of Pennsylvania)

Title: Suspended N-genus handle bodies in nematic liquid crystals

Abstract: Freestanding colloidal handle bodies of genus varying from 1-5 in 5CB have been studied. Within and around these colloids, point and ring defects satisfied the topological requirements of the system in accordance with the Gauss-Bonnet theorem. What happens when the genus of the handle bodies is increased? Here, we probe defect structures formed by 5CB in homeotropic handle bodies with genus of at least 100 by suspending thin plates with arrays of circular holes between glass slides with well-defined anchoring conditions. We find domain walls can stretch across the system and separate different inner director structures within the holes. Remarkably, we find several settings in which the nematic is defect-free within and around the handle bodies. We discuss energetic versus topological requirements of this system. This work improves our fundamental understanding of confined liquid crystals (LCs) and will improve our ability to manipulate defects and director fields for various applications. (Authors: Lisa Tran, Maxim O. Lavrentovich, Randall D. Kamien, Kathleen J. Stebe)

Kazage J Christophe Utuje (Syracuse University)

Title: Mechanical coordination during epithelial expansion

Abstract: Coordinated motion of cell monolayers during epithelial wound healing and tissue morphogenesis involves mechanical stress generation. Here we propose a continuum model of epithelial expansion that couples mechanical deformations in the tissue to contractile activity in the cells. A new ingredient of our model is a feedback between local strain and contractility that naturally yields a mechanism for viscoelasticity and effective inertia in the cell monolayer. Using a combination of analytical and numerical techniques, we demonstrate that our model quantitatively reproduces many experimental findings [Nat. Phys. 8, 628 (2012)], including the build-up of intercellular stresses, and the existence of traveling mechanical waves guiding the monolayer expansion. (Authors: Kazage J.C. Utuje1, Shiladitya Banerjee2 and M. Cristina Marchetti1. 1Physics Department and Syracuse Biomaterials Institute, Syracuse University, Syracuse NY 13244. 2James Franck Institute, The University of Chicago, Chicago IL 60637)

Duanduan Wan (Syracuse University)

Title: Planar and Curved Droplet Networks

Abstract: We study how a two-dimensional square sheet of droplets evolves to the preferred triangular lattice. The transformation can be induced either by an impurity seed for flat sheets or by curving the sheet. We analyze the propagation of elastic waves and the spatial reordering subsequent to the initial perturbation. For the specific case of spherical topology we find that the required topological defects drive a buckling transition from an initial round sphere to a faceted icosahedral shell.

Vikrant Yadav (Clark University)

Title: Experimental Model of Traffic Jams Using Self Propelled Particles

Abstract: We model behavior of traffic using Self Propelled Particles (SPPs). Granular rods with asymmetric mass distribution confined to move in a circular channel on a vibrated substrate and interact with each other through inelastic collision serve as our model vehicle. Motion of a single vehicle is observed to be composed of 2 parts, a linear velocity in the direction of lighter end of particle and a non-Gaussian random velocity. We find that the collective mean speed of the SPPs is constant over a wide range of line densities before decreasing rapidly as the maximum packing is approached indicating the spontaneous formation of Phantom jams. This decrease in speed is observed to be far greater than any small differences in the mean drift speed of individual SPPs , and occurs as the collision frequency between SPPs increase exponentially with line density. We estimated the critical density for decay of velocity by comparing mean step size of a single SPP with mean free path at a given density of SPPs. While the collective motion at low densities is characterized by caravan following behind the slowest particle leading to clustering, at higher densities we see formation of jamming waves travelling in direction opposite to that of motion of particles. The behavior of velocity is modeled using a piecewise continuous functions. The dynamics of jamming waves are studied using Lighthill Whitham Model. (Authors: Vikrant Yadav and Arshad Kudrolli)

Le Yan (New York University)

Title: A model for the erosion onset of a granular bed sheared by a viscous fluid

Abstract: We study theoretically the erosion threshold of a granular bed forced by a viscous fluid. We first introduce a novel model of  interacting particles driven on a rough substrate. It predicts a continuous transition at some threshold forcing $\theta_c$, beyond which the particle current grows linearly $J\sim \theta-\theta_c$, in agreement with experiments. The stationary state is reached after a transient time $t_{\rm conv}$ which diverges near the transition as $t_{\rm conv}\sim |\theta-\theta_c|^{-z}$ with $z\approx 2.5$. The model also makes quantitative testable predictions for the drainage pattern: the distribution $P(\sigma)$ of local current is found to be extremely broad with $P(\sigma)\sim J/\sigma$, spatial correlations for the current are negligible in the direction transverse to forcing, but long-range parallel to it. We explain some of these features using a scaling argument and  a mean-field approximation that builds an analogy with $q$-models. We discuss the relationship between our erosion model and models for the depinning transition of vortex lattices in dirty superconductors, where our results may also apply.

Jie Zhang (University of Illinois, Urbana-Champaign)

Title: Self-organization of active colloidal polymer

Abstract: In order to better understand active polymeric matter, colloidal polymers are imaged, in situ in real time, obtaining not only temporal and spatial information about each “monomer” in these living polymers but also about the time-dependent and orientation-dependent correlations between them. Our reversible colloidal polymer system is assembled from self-propelled monomeric Janus particles with dynamic “plug and play” self-assembly and programmed direction-specific interactions between the particles. Enabling this, AC voltage induces dipoles on the monomeric Janus particles that link them into chains while also generating active phoretic motility. Unique features of this system relative to conventional Brownian polymers are emphasized. Turbulence emerges from dense active solutions. 

Shuang Zhou

Title: Living liquid crystals: single and collective bacterial motion in nematic media

Abstract: As a typical pusher type microswimmer, bacillus subtilis propel their cylindrical body by rotating their helical flagella buddle. It has been widely used as standard model in studies ranging from collision with another bacterium or surface, to collective motions at high concentration. However, most of the studies were done in isotropic environment. We introduce nematic order into the system by replacing the isotropic media with lyotropic chromonic liquid crystals and observe fascinating phenomena due to the interplay between activity and orientational order. At low bacteria concentration, bacteria motion is dominated by the nematic environment and boundary conditions. In a planar cell, where the director of liquid crystal is aligned tangential to the bounding substrates, bacteria swim along the local director and are “domesticated”, showing the ability to transport cargo particles along predetermined trajectories, a feature never seen in their natural habitat (water). In a homeotropic cell with director of liquid crystal normal to the bounding substrates, some of the bacteria can swim parallel to the plane (perpendicular to the director) while others are “pinned” normal to the plane (parallel to the director). Swimming bacteria in a homeotropic cell move in linear or circular motions. They also form “trains” to facilitate efficient swimming. When the bacteria concentration is increased, the activity-triggered flow reorients the director to form first periodic “bend walls”, followed by topological turbulence with steady creation and annihilation of ±1/2 defects. A correlation length ξ characterizing the director distortions is determined by both the elasticity of the chromonic LC and the activity of bacteria. In a freely suspended sessile drop, bacteria concentration required to form collective motion is <10% of the concentration for collective motion in water. The phenomenon is rooted in the long-range elasticity-mediated interactions of the bacteria in a liquid crystal. Our study provides insight to understand hierarchy of spatial scales in active systems with orientational order, and demonstrates new microfluidic concepts with particle transportation possibilities.

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