Student Poster Abstracts

Please note that while there is not one required or mandatory size, all posters should fit in a space no longer than 36” (91 cm) wide and preferably be no more than about 48” (122 cm) in height. Arrangements can be made for other sizes as necessary.

If you are not able to get your poster printed before you leave home (or are unable to easily transport), you may get posters printed at FedEx Office (

Pratik Prashant Aghor
University of New Hampshire

Invariant Subspaces of Channel Flow

It is important to study the symmetries of a system because they influence dynamics and determine kinds of allowable solutions. We consider plane Poiseuille flow (PPF) driven by a constant bulk velocity constraint. We show that half-box-shifts in the periodic streamwise and spanwise directions are important. Therefore, instead of considering infinitesimal translations in the periodic directions, we focus on the half-box-shifts. The full symmetry group then reduces to a discrete Abelian group generated only by reflections and half-box-shifts. We then define an equivalence relation using quarter-box-shifts and partition the subgroups in conjugacy classes under quarter-box-shift conjugation. Taking into account only one representative per conjugacy class, we list the independent invariant subspaces of PPF. We also compute traveling waves in a few of the listed invariant subspaces, using the knowledge about symmetries to simplify numerical computations.

Samar Alqatari
University of Chicago

Confined Rayleigh-Taylor Instability

The interface between two stratified layers of fluids is stable when the denser fluid is on the bottom. The inverted situation, with the denser fluid on top, gives rise to the Rayleigh-Taylor instability. Setting up the unstable situation with clean initial conditions can be experimentally challenging. Here we take advantage of the inherent stratification of fluids formed by a different instability – the viscous fingering of miscible fluids inside a thin gap – to achieve controlled initial conditions for the Rayleigh-Taylor instability. By varying the thickness of the gap between two plates, b, we find that the instability is completely suppressed below a critical gap bc. As b is increased, the instability changes from this mass-diffusion dominated regime of stability, through an intermediate regime where the dynamics are set strictly by the gap, λ ∼ b, to the limit where the boundaries no longer affect the dynamics. In this last regime, the instability is governed by a competition between momentum diffusion and buoyancy as predicted by theory in the unconstrained limit. DOI: 10.1126/sciadv.abd6605

Ryo Araki
Ecole Centrale de Lyon

See topics list below

  1. Intrinsic Periodicity of Turbulence and Its Minimal Modeling
    In box turbulence with a steady forcing, we find a quasi-cyclic temporal behavior between the large scale (energy input rate) and the small scale (energy dissipation rate). This phenomenon represents the imbalance among scales due to the energy cascade; thus, it is an intrinsic property of turbulence. We propose a minimum three-equation model to reproduce this temporal behavior.
    Ref: arxiv.2112.03417
  2. Turbulence without Spatially Non-Local Interactions
    The scale locality is the foundation of the Kolmogorov 1941 theory and the energy cascade process. On the other hand, the space locality has been gathering much less attention. We define the space-local variant of the vorticity equation by eliminating the space non-local nonlinear interactions to investigate how it affects the dynamics and the statistics of turbulence.
  3. Information-Thermodynamic Description of Energy Cascade
    We consider how the information is transferred in the scale space of the flow by applying the information thermodynamics to the stochastic shell model of turbulence. We find the “information transfer” from large to small scales along with the energy cascade. In another way, small-scale eddies are “learning” about large-scale eddies while receiving energy from large scales.

Madhuvanthi Guruprasad Athani
Johns Hopkins University

Studying the active collective motions of flexible filaments using Brownian Dynamics simulations

Ordered, collective motions commonly arise spontaneously in systems of many interacting, active particles, such as self-propelled colloids. Examples of active phases range from the flocking behavior seen in Active Brownian particles to anisotropic particle systems of active nematics where liquid crystalline order emerges. To understand the active phases formed in these anisotropic particle systems, understanding collective motion of active flexible chains is necessary. We use Brownian Dynamics simulations to study the collective motions of flexible chains that can ‘self-propel’. Depending on the energy cost of overlap of these chains and their flexibility, our model predicts that the active global state transitions between an active polar state and an active nematic state. Our model also suggests that the active global nematic state is only transient and eventually the system orders into an active global polar state. These global active states can be realized in gliding microtubule assay experiments.

Srivatsa Badariprasad
University of Newcastle upon Tyne

Vortex Lattice Nucleation in Dipolar Bose-Einstein Condensates

Degenerate quantum gases with strong permanent dipole moments are a robust platform for studying anisotropic and long-ranged phenomena in strongly correlated quantum systems. When subjected to a rotating magnetic field, the resulting precession of the dipole moments of a magnetic dipolar Bose-Einstein condensate (dBEC) imparts angular momentum to the system. Due to the superfluidity of an interacting BEC, this has the consequence of quantum vortices forming in a hitherto vorticity-free fluid. Our recent work focusses on theoretically tracking the evolution of a dBEC as the magnetic field rotation frequency is slowly accelerated from zero until the vortices have formed, and then observing the relaxation of the system to its final state at a fixed rotation frequency. We find that the dBEC closely follows pre-existing semi-analytical predictions until the onset of vorticity, and that the vortex-filled states are characterised by background density striping and tilting. After sufficiently long hold durations, the vortices relax into an Abrikosov lattice with the lattice and background dBEC density profile approaching our predictions of the expected ground state.

Subhadip Biswas
The University of Sheffield

Thermodynamics of microdroplet phase in elastic phase separation

We study the thermodynamics of binary mixtures with the volume fraction of the minority component less than the amount required to form a flat interface and show that the surface tension dominated equilibrium phase of the mixture forms a single macroscopic droplet. Elastic interactions in gel-polymer mixtures stabilize a phase with multiple droplets. Using a mean-field free energy we compute the droplet size as a function of the interfacial tension, Flory parameter, and elastic moduli of the gel. Our results illustrate the role of elastic interactions in dictating the phase behaviour of biopolymers undergoing liquid-liquid phase separation.

Christopher Browne
Princeton University

Elastic turbulence in 3D porous media

Many energy, environmental, industrial, and microfluidic processes rely on the flow of polymer solutions through porous media. Unexpectedly, the macroscopic flow resistance often increases above a threshold flow rate in a porous medium—but not in bulk solution. The reason why has been a puzzle for over half a century. Here, by directly visualizing flow in a transparent 3D porous medium, we demonstrate that this anomalous increase is due to the onset of an elastic instability in which the flow exhibits strong spatio-temporal fluctuations reminiscent of inertial turbulence, despite the small Reynolds number. Our measurements enable us to quantitatively establish that the energy dissipated by pore-scale fluctuations generates the anomalous increase in the overall flow resistance. Because the macroscopic resistance is one of the most fundamental descriptors of fluid flow, our results both help deepen understanding of complex fluid flows, and provide guidelines to inform a broad range of applications.

Freya Bull
University of Edinburgh

A model for the infection dynamics of a urinary catheter

Catheter associated urinary tract infections (CAUTI) constitute up to 40% of hospital acquired infections, yet there is still limited understanding of how CAUTI develops. By combining population dynamics and fluid dynamics, I construct a mathematical model to find new insight. Within this model, bacteria spread as a wave on the external surface of the catheter. On reaching the bladder they proliferate within the urine, before being swept down the inside of the catheter by the urine flow. Some bacteria stick to the inside surface of the catheter, where they grow just as on the outside. From this model, the rate of urine production by the kidneys emerges as a critical parameter, governing a pseudo second-order phase transition. Low urine production rates give rise to an infected bladder state, transitioning, as the rate increases, into a washed-out state where the catheter can still be infected but there is no detectable growth within the urine.

Mingyun Cao
University of California, San Diego

Instability and turbulent relaxation in a stochastic magnetic field

An analysis of instability dynamics in a stochastic magnetic field is presented for the tractable case of the resistive interchange. Externally prescribed static magnetic perturbations convert the eigenmode problem to a stochastic differential equation, which is solved by the method of averaging. The dynamics are rendered multi-scale, due to the size disparity between the test mode and magnetic perturbations. Maintaining quasi-neutrality at all orders requires that small-scale convective cell turbulence be driven by disparate scale interaction. The cells in turn produce turbulent mixing of vorticity and pressure, which is calculated by fluctuation-dissipation type analyses, and are relevant to pump-out phenomena. The development of correlation between the ambient magnetic perturbations and the cells is demonstrated, showing that turbulence will `lock on’ to ambient stochasticity. Magnetic perturbations are shown to produce a magnetic braking effect on vorticity generation at large scale. Detailed testable predictions are presented. The relations of these findings to the results of available simulations and recent experiments are discussed.

Pierre Chantelot
University of Twente

Leidenfrost effect as a directed percolation phase transition

Volatile drops deposited on a hot solid can levitate on a cushion of their own vapor, without contacting the surface. We propose to understand the onset of this so-called Leidenfrost effect through an analogy to non-equilibrium systems exhibiting a directed percolation phase transition. When performing impacts on superheated solids, we observe a regime of spatiotemporal intermittency in which localized wet patches coexist with dry regions on the substrate. We report a critical surface temperature, which marks the upper bound of a large range of temperatures in which levitation and contact coexist. In this range, with decreasing temperature, the equilibrium wet fraction increases continuously from zero to one. Also, the statistical properties of the spatio-temporally intermittent regime are in agreement with that of the directed percolation universality class. This analogy allows us to redefine the Leidenfrost temperature and shed light on the physical mechanisms governing the transition to the Leidenfrost state.

Sihan Chen
Rice University

Motor-free Contractility in Active Gels

Animal cells form contractile structures to promote various functions, from cell motility to cell division. Force generation in these structures is often due to molecular motors such as myosin that require polar substrates for their function. Here, we propose a motor-free mechanism that can generate contraction in biopolymer networks without the need for polarity. This mechanism is based on active binding/unbinding of crosslinkers that breaks the principle of detailed balance, together with the asymmetric force-extension response of semiflexible biopolymers. We find that these two ingredients can generate steady state contraction via a non-thermal, ratchet-like process. We calculate the resulting force-velocity relation using both coarse-grained and microscopic models.

Yves-Marie Ducimetière
École Polytechnique Fédérale de Lausanne

Weakly nonlinear evolution of stochastically driven nonnormal systems

We consider nonlinear dynamical systems driven by a stochastic forcing. It has been largely evidenced in the literature that the linear response of nonnormal systems may exhibit a large variance amplification even in a linearly stable regime. It is typically the case for some parallel or nonparallel fluid flows, governed by the Navier-Stokes equations [1,2,3]. However, the linear response is relevant only in the limit of vanishing forcing intensity. As the latter increases, the spectrum and the variance of the response may be strongly modified, owing to nonlinear effects [4]. To go beyond this limitation, we propose a theoretical approach to derive an amplitude equations governing the weakly nonlinear evolution of these systems. This approach, contrarily to classical ones, does not rely on the presence of an eigenvalue close to the neutral axis, and the Fourier components of the response are allowed to be arbitrarily different from any eigenmode. It applies to any sufficiently nonnormal operator, and reconciles the nonnormal nature of the growth mechanisms with the need for a center manifold to project the leading-order dynamics. The methodology is outlined for a generic nonlinear dynamical system, and the application case highlights a common [2] nonnormal mechanism in hydrodynamics: the convective nonnormal amplification in the flow past a backward-facing step. At low numerical cost, the amplitude equation quantifies the respective contribution of each dominant nonlinear interaction, thus bringing insight on the saturation mechanisms of the stochastic amplification.

[1] Farrell and Ioannou, Phys. Fluids A 5, 11 (1993).
[2] Dergham et al., J. Fluid Mech. 719, (2013).
[3] Fontane et al., J. Fluid Mech. 613, (2008).
[4] Mantic-Lugo et al., Phys. Rev. Fluids 1, (2016)

Kirsten Daniela Endresen
Johns Hopkins University

Inducing Topological Defects in Cell Monolayers Using Topography

We investigate the effects of the long-range orientational order that arrises from anisotropically-shaped cells aligning with their neighbors, as a nematic liquid crystal. A consequence of their liquid crystal order is the presence of highly disordered regions known as topological defects, where stresses are concentrated. There is growing evidence that in living systems, the presence of topological defects impacts the behavior of cells. We induce the formation of topological defects with +1 and -1 topological charges in monolayers of 3T6 fibroblasts and EpH-4 epithelial cells using topographically patterned substrates. We study the density and dynamics of the cells in these regions, and we examine the effects of varying the height of ridges, which breaks up the monolayer by creating barriers between groups of cells.

Hao Fu
Stanford University

The small-amplitude dynamics of spontaneous tropical cyclogenesis

Spontaneous tropical cyclogenesis over a uniform sea surface temperature is an idealized setup for studying the formation of tropical cyclones. While the longwave radiation feedback and surface flux feedback have been identified as the main mechanisms for the growth of the perturbation at the early stage, there is no theoretical model for the growth rate and length scale of the perturbation. In this paper, we develop a theoretical framework based on Fourier analysis to understand the small-amplitude dynamics revealed in the cloud-permitting simulations. Initially, the random stretching of planetary vorticity by the stochastic convection produces an initial vorticity perturbation that is proportional to the square root of the domain-averaged accumulated rainfall. The perturbation then kicks off a mesoscale hydrodynamic instability that features an exponential growth. The mesoscale instability has a clear most unstable mode, which is theoretically shown to be below the Rossby deformation radius and above a convective spreading length scale. This convective spreading length scale is considered to depend on the spread of convective activity by cold pools and the nonlocal longwave radiative heating by the anvil clouds. These hypotheses are confirmed by cloud-permitting simulations that modify the sub-cloud rain evaporation rate and horizontally filter the longwave radiative heating tendency respectively.

Julia Ann Giannini
Syracuse University

Searching for structural predictors of plasticity in dense active packings

In amorphous solids subject to shear or thermal excitation, so-called structural indicators have been developed that predict locations of future plasticity or particle rearrangements. An open question is whether similar tools can be used in dense active materials, but a challenge is that under most circumstances, active systems do not possess well-defined solid reference configurations. We develop a computational model for a dense active crowd attracted to a point of interest, which does permit a mechanically stable reference state in the limit of infinitely persistent motion. Previous work on a similar system suggested that the collective motion of crowds could be predicted by inverting a matrix of time averaged two-particle correlation functions. Seeking a first-principles understanding of this result, we demonstrate that this active matter system maps directly onto a granular packing in the presence of an external potential, and extend an existing structural indicator based on linear response to predict plasticity in the presence of noisy dynamics. We find that the strong pressure gradient necessitated by the directed activity, as well as a self-generated free boundary, strongly impact the linear response of the system. In low-pressure regions the linear-response-based indicator is predictive, but it does not work well in the high-pressure interior of our active packings. Our findings motivate and inform future work that could better formulate structure dynamics predictions in systems with strong pressure gradients.

Sébastien Gomé

Wavelength selection in transitional turbulence

The transition to turbulence in plane shear flows is characterized by coexisiting turbulent and laminar zones. Unlike in the case of pipe flow, these zones appear in the form of regular patterns, oblique in the streamwise-spanwise plane. The mechanism explaining how those statistically steady structures arise from uniform turbulence is still unclear: is it due to a random nucleation of laminar gaps; or to a long-wavelength instability of the uniform turbulent flow?

We first address this question by using periodic domains of restricted lengths and fixed tilt, where patterns of fixed wavelengths can exist, over a limited range in Reynolds number. The stability of such fixed-wavelength patterns is studied as a function of Reynolds number, showing that there is a prefered wavelength (around 40 half-gaps) for which the pattern is most stable.

We secondly study the impact of the mean flow along those turbulent bands. We therefore introduce a Modified Plane Couette flow, in which the large-scale spanwise velocity is suppressed, while still allowing modulations of the streamwise velocity, necessary for the existence of turbulent bands. We show that in the absence of large-scale spanwise flow, laminar gaps appear at random locations in space and time (like the puffs in pipe flow). This suggests that the regular patterned appearance of gaps is related to large-scale spanwise flow but not essential to the evolution of bands in Plane Couette Flow.

Kyung Ha

Using micro particles to generate microdroplets

Droplet-based microfluidics technology is important for bioengineering research and design. These microdroplets are commonly generated by using microchannels or electrowetting devices. Recently our team proposed a new method using amphiphilic microparticles. These particles are observed to hold nearly equal volumes of aqueous liquid when dispersed in an oil-water mixture. We develop a theory behind this behavior by building a model based on energy minimizing surfaces and random pairwise interactions. By analyzing this model, we demonstrate that the key properties of the particle guarantee the formation of small droplets within a volume range unique to the particle.

Louise Head
University of Edinburgh

Transport of a passive rod in an active nematic film

Suspended particles offers a means to measure the transport and rheological properties in many turbulent flows in industrial and natural environments. A class of internally-driven, biologically inspired fluids, called active nematics, demonstrate large scale turbulent-like flows at low Reynolds numbers. Motivated by experimental work, we study the transport of passive colloidal rods suspended in water above an active nematic film via a coarse-grained MPCD algorithm. We find that the rods respond to the active flows to preferentially advance towards stagnation points in the flow field. Further, the rods orientation couples to the hydrodynamic stresses of the film, to maintain alignment with the underlying local director. These results may shed light of the interplay between passive and active constituents in hybrid systems.

Arthur Hernandez
University of California, Santa Barbara

2D vertex model cellular tissue mechanics

Vertex models, such as those used to describe cellular tissue, have an energy controlled by deviations of each cell area and perimeter from target values. The constrained nonlinear relation between area and perimeter leads to new mechanical response. Here we provide a mean-field treatment of a highly simplified model: a uniform network of regular polygons with no topological rearrangements. Since all polygons deform in the same way, we only need to analyze the ground states and the response to deformations of a single polygon (cell). The model exhibits the known transition between a fluid/compatible state, where the cell can accommodate both target area and perimeter, and a rigid/incompatible state. We calculate and measure the mechanical resistance to various deformation protocols and discover that at the onset of rigidity, where a single zero-energy ground state exists, linear elasticity fails to describe the mechanical response to even infinitesimal deformations. In particular, we identify a breakdown of reciprocity expressed via different moduli for compressive and tensile loads, implying nonanalyticity of the energy functional. We give a pictorial representation in configuration space that reveals that the complex elastic response of the vertex model arises from the presence of two distinct sets of reference states (associated with target area and target perimeter). Our results on the critically compatible tissue provide a new route for the design of mechanical metamaterials that violate or extend classical elasticity.

Aaron Hui
Ohio State University

Noise thermometry in electron hydrodynamics

Johnson noise thermometry, based on the Johnson-Nyquist theorem, offers a powerful primary thermometry technique to access the electron temperature at the nanoscale. In practical situations, one needs to generalize the Johnson-Nyquist theorem to handle spatially inhomogenous temperature profiles. This was previously done for Wiedemann-Franz-obeying ohmic devices, where it was found that Joule heating leads to a geometry-independent increase in Johnson noise. However, there has been great recent interest in strongly-interacting electron hydrodynamic systems which do not admit a local conductivity nor obey the Wiedemann-Franz law, signatures of which have even been observed in previous thermometry experiments. In this paper, we study low-frequency Johnson noise in the hydrodynamic setting for a rectangular geometry. As opposed to the ohmic setting, we find that the Johnson noise is no longer geometry-independent due to non-local viscous gradients. Despite this, ignoring the geometric correction only leads to an error of at most 40% as compared to naively using the ohmic result.

Narges (Gess) Kelly
Brandeis University

Modeling the Phase Transition Between Localized and Extended Deposition of Particles in Porous Media

Improving filtration efficiencies and lifetime of filters relies on an accurate prediction of the progression of clogging in porous media. More specifically, we try to explore how system properties, such as the applied pressure and network structure, affect the deposition of colloidal particles in disordered packings of glass beads. Past experiments have shown that depending on the applied pressure across such systems, we may expect either localized deposition (at lower pressure) or extended deposition (at higher pressure). We develop a mathematical model and use agent-based simulations to capture the results previously observed in experiments. To do so, we apply previously formulated deposition and erosion laws that lead to these two different behaviors and discuss the existence of a phase transition theoretically.

Tali Khain
University of Chicago

Stokes flows in three-dimensional fluids with odd and parity-violating viscosities

The Stokes equation describes the motion of fluids when inertial forces are negligible compared to viscous forces. In this presentation, we explore the consequence of parity-violating and non-dissipative (i.e. odd) viscosities on Stokes flows in three dimensions. Parity-violating viscosities are coefficients of the viscosity tensor that are not invariant under mirror reflections of space, while odd viscosities are those which do not contribute to dissipation of mechanical energy. These viscosities can occur in systems ranging from synthetic and biological active fluids to magnetized and rotating fluids. We first systematically enumerate all possible parity-violating viscosities compatible with cylindrical symmetry, highlighting their connection to potential microscopic realizations. Then, using a combination of analytical and numerical methods, we analyze the effects of parity-violating viscosities on the Stokeslet solution, on the flow past a sphere or a bubble, and on many-particle sedimentation. In all the cases we analyze, parity-violating viscosities give rise to an azimuthal flow even when the driving force is parallel to the axis of cylindrical symmetry. For a few sedimenting particles, the azimuthal flow bends the trajectories compared to a traditional Stokes flow. For a cloud of particles, the azimuthal flow impedes the transformation of the spherical cloud into a torus and the subsequent breakup into smaller parts that would otherwise occur. The presence of azimuthal flows in cylindrically symmetric systems (sphere, bubble, cloud of particles) can serve as a probe for parity-violating viscosities in experimental systems.

Egor Kiselev

Hydrodynamic Floquet Plasmons

Periodic driving can be used to manipulate the band structure of electrons in solids. We propose to use this effect to tune the dispersion relation of plasmons in two dimensions. In particular, we show that a slow modulation of the driving amplitude can induce parametric instabilities and provides a highly efficient plasmon source.

Jeremiah Steven Lane
Air Force Institute of Technology

Steady State Thermal Blooming with Convection

The modeling, simulation, and analysis of high energy laser propagation remains a research topic of pertinent interest to the defense community. In particular, steady-state thermal blooming is detrimental to the propagation of lasers over long distances and through atmospheric conditions with significant aerosol presence. The simulation of thermal blooming in the present literature relies on wave optics models and a priori averaging of aerosol effects. Since thermal blooming is a product of natural convection, however, there is a need for simulating this coupled fluid-beam effect using a first principles approach. We introduce a coupled method to solve the steady-state Boussinesq Navier–Stokes equations for fluid behavior with the paraxial equation for beam propagation. We consider three distinct schemes for solving the forced Boussninesq equations in stream function-vorticity variables in two dimensions: a fixed-point approach, a perturbation series expansion, and a Pade approximant. The functions governing fluid behavior at 2D slices along the propagation path are then used to determine index of refraction fluctuations as a result of laser-driven temperature fluctuations. Pertinent results include a proof of the existence of steady solutions for small laser intensities through the fixed-point method along with a convergence proof for the perturbation series of solutions in a laser intensity parameter. The Pade approximant is used to arrive at higher laser intensity solutions, and we discuss the analysis of the complex pole distribution of rational approximants to the fluid functions.

Wei-Ting Lin
Northwestern University

Pair Fluctuation Correction to the Kinetic Theory for Liquid 3He

The fluctuations in a Fermi liquid near the Cooper instability can influence its transport properties. We develop a theory to include the fluctuation-induced correction in the Boltzmann-Landau equation using the Keldysh formalism. In particular, a collision integral is obtained from the microscopic theory, which is different from the previous heuristic result. We calculate the change of attenuation and velocity of zero sound in liquid 3He, and compare the theoretical results with experiments.

The zero sound attenuation calculated from our theory is consistent with the experiment well, while the velocity is different from experimental data. Our results thus raise new questions and should motivate future research directions.

Chloe Lindeman
University of Chicago

How does a material make memories?

Jammed systems which are cyclically sheared will often find periodic orbits after just a few cycles, thus encoding a memory of their drive amplitude. Aspects of these periodic orbits can be captured by models of interacting hysteretic regions (“hysterons”) that represent particle rearrangements in the jammed packings. However, there remains a gap between hysterons as a model and the rearrangements they aim to describe. Here, we study small jammed systems — on the order of 10 particles — to probe the nature of rearrangements. For packings with one pair of rearrangements (one hysteron) in their periodic orbit, we examine the two-state nature of the strain region between the rearrangements and begin to paint a more robust picture of the extent to which these systems behave like model hysterons.

Chang Liu
University of California, Berkeley

Staircase solutions and stability in bounded salt-finger convection

This work performs bifurcation analysis of bounded salt-finger convection using single-mode equations obtained from a severely truncated Fourier expansion in the horizontal. We find staircase-like solutions having respectively one, two, and three well-mixed mean salinity regions (S1, S2, and S3) in the vertical direction. Tilted fingers (TF1) and traveling waves (TW1) break horizontal reflection symmetry with spontaneous formations of large-scale shear. Solutions that break vertical reflection symmetry are also found. S1 solution shows a trend closely matching the DNS results for Sherwood number, mean salinity and temperature profiles over density ratio. The single-mode solutions close to high wavenumber onset are in excellent agreement with 2D DNS within a small horizontal domain.

Pawel Matus
Max Planck Institute for the Physics of Complex Systems

Anomaly-induced transport regime in Weyl semimetals

We study propagation of an oscillatory electromagnetic field inside a Weyl semimetal. In conventional conductors, the motion of the charge carriers in the skin layer near the surface can be diffusive, ballistic, or hydrodynamic. We show that the presence of chiral anomalies, intrinsic to the massless Weyl particles, leads to a hitherto neglected transport regime, in which the relation between the current and the electric field becomes nonlocal as a result of the diffusion of the valley charge imbalance into the bulk of the material. We propose to use this novel regime as a diagnostic of the presence of chiral anomalies in optical conductivity measurements. These results are obtained from a generalized kinetic theory which includes various relaxation mechanisms, allowing us to investigate different transport regimes of Weyl semimetals.

Siddhartha Mukherjee
Tata Institute of Fundamental Research

Anton Peshkov
University of Rochester / CSU Fullerton

Metachronal waves in swarms of nematode Turbatrix aceti

There is a recent surge of interest in the behavior of active particles that can at the same time align their direction of movement and synchronize their oscillations, known as swarmalators. I will present an experimental investigation of the collective motion of the nematode Turbatrix aceti, which self-propel by body undulation. I will show that under favorable conditions these nematodes can synchronize their body oscillations, forming striking traveling metachronal waves that, similar to the case of beating cilia, produce strong fluid flows.

I will demonstrate that the location and strength of this collective state can be controlled through the shape of the confining structure; in our case the contact angle of a droplet. This opens a way for producing controlled work such as on-demand flows or displacement of objects. I will illustrate this by a practical example: showing that the force generated by the collectively moving nematodes is sufficient to change the mode of evaporation of fluid droplets, by counteracting the surface-tension force, which allow us to estimate its strength.

Olivia Pomerenk
Courant Institute of Mathematical Sciences

Hydrodynamics of drinking straws and finite-length pipes

Extensive studies of pipe hydraulics have focused on limiting cases such as well-developed laminar or turbulent flow through long conduits and undeveloped flow through an orifice, for which there exist laws relating pressure drop and flow rate. We carry out experiments for rates and pipe dimensions that interrogate intermediate conditions between the well-studied limits. Organizing this information in terms of dimensionless friction factor, Reynolds number, and pipe aspect ratio yields a surface that is shown to match the three laws associated with developed laminar, developed turbulent, and orifice flows. While each law fails outside of its applicable range of Reynolds number and aspect ratio, we present a hybrid theoretical-empirical model that includes inlet and development effects and that proves accurate to about 10% over wide ranges of Reynolds number and aspect ratio. We also present simple formulas for the boundaries between the three regimes, which intersect at a triple point. Measurements show that sipping through a straw is an everyday example of such intermediate conditions not accounted for by existing laws but accurately described by our model. More generally, our findings provide formulas for identifying regimes and predicting hydraulic resistance for finite-length pipes and intermediate-Reynolds flows.

Calvin Pozderac
Ohio State University

Exact solution for the thermalization transition in a model fracton system

In fracton systems, the dynamics is constrained by higher-order conservation laws, such as the conservation of both charge and dipole moment. In certain situations, these constraints prevent the system from thermalizing by causing a “fragmentation” of the Hilbert space into many dynamically disconnected sectors. In this work we consider a simple one-dimensional lattice of charges that evolve under the influence of random local operators that conserve both charge and dipole moment. This system exhibits a thermalization transition as a function of the total charge, such that only systems with sufficiently large charge density are able to thermalize. We construct an exact solution for this transition by mapping the dynamics to two different problems in combinatorics. Our solution allows us to identify the critical charge density as a function of the gate size, as well as the critical scaling and certain critical exponents.

Bauyrzhan Primkulov
Massachusetts Institute of Technology

Avalanches in Strong Imbibition

Earthquakes, landslides, avalanches, solar flares, and financial markets are examples of self-organized criticality (SOC), a near-equilibrium process where small perturbations lead to scale-free (and thereby essentially unpredictable) consequences. A classic example of SOC is the drainage of a wetting fluid from a porous medium (Furuberg et al., PRL 1988). In drainage, the wetting fluid retreats preferentially from pore bodies as the non-wetting fluid invades. Reversing the flow direction leads instead to imbibition, where the wetting fluid invades by preferentially coating the solid surfaces. Here, we show with experiments and simulations that strong imbibition shares all of the scale-free features of drainage—avalanches, intermittency, and spatiotemporal correlation scaling—a striking finding, given that these two processes occur through radically different displacement mechanisms at the pore scale.

Mehrana Raeisian Nejad
University of Oxford

Active Extensile Stress Promotes 3D Director Orientations and Flows

We use numerical simulations and linear stability analysis to study an active nematic layer where the director is allowed to point out of the plane. Our results highlight the difference between extensile and contractile systems.

Contractile stress suppresses the flows perpendicular to the layer and favors in-plane orientations of the director. By contrast, extensile stress promotes instabilities that can turn the director out of the plane, leaving behind a population of distinct in-plane regions that continually elongate and divide. Our results suggest a mechanism for the initial stages of layer formation in living systems and explain the propensity of dislocation lines in three-dimensional active nematics to be of twist-type in extensile, or wedge-type in contractile, materials.

Shikhar Rai
University of Rochester

Scale of oceanic eddy killing by wind from global satellite observations

Wind provides net energy to the ocean at the surface but not all lengthscales receive energy from the winds. The mesoscale and smaller scales are damped by the wind by the process known as eddy killing. We investigate the lengthscale below which the winds effectively kill eddy, it’s variability and their causes and its importance. We find in the global average eddies of below 260km are killed by wind at the rate of ~50GW, eddy killing has seasonality because of the wind speed and the magnitude of eddy killing is comparable to the magnitude of inverse cascades, PE to KE conversion, in the energetic regions like western boundary currents and Antarctic Circumpolar Current.

Matt Reeves
University of Queensland

Turbulent Relaxation to Equilibrium in a Two-Dimensional Quantum Vortex Gas

We experimentally study the emergence of microcanonical equilibrium states in the turbulent relaxation dynamics of a two-dimensional chiral vortex gas. Same-sign vortices are injected into a quasi-two-dimensional disk-shaped atomic Bose-Einstein condensate using a range of mechanical stirring protocols. The resulting long-time vortex distributions are found to be in excellent agreement with the mean-field Poisson Boltzmann equation for the system describing the microcanonical ensemble at fixed energy H and angular momentum M. The equilibrium states are characterized by the corresponding thermodynamic variables of inverse temperature β and rotation frequency ω. We are able to realize equilibria spanning the full phase diagram of the vortex gas, including on-axis states near zero temperature, infinite temperature, and negative absolute temperatures. At sufficiently high energies, the system exhibits a symmetry-breaking transition, resulting in an off-axis equilibrium phase at negative absolute temperature that no longer shares the symmetry of the container. We introduce a point-vortex model with phenomenological damping and noise that is able to quantitatively reproduce the equilibration dynamics.

Brendan Rhyno
University of Illinois at Urbana-Champaign

Thermodynamics in expanding shell-shaped Bose-Einstein condensates

Inspired by investigations of Bose-Einstein condensates (BECs) produced in the NASA Cold Atom Lab (CAL) aboard the International Space Station, we present a study of thermodynamic properties of shell-shaped BECs. Within the context of a spherically symmetric “bubble trap” potential, we study the evolution of the system from small filled spheres to hollow, large, thin shells via the tuning of trap parameters. We analyze the bubble trap spectrum and states and track the distinct changes in spectra between radial and angular modes across the evolution. Using the spectral data, for a range of trap parameters, we compute the critical temperature for a fixed number of particles to form a BEC. For a set of initial temperatures, we also evaluate the change in temperature that would occur in adiabatic expansion from small filled sphere to large thin shell were the trap to be dynamically tuned. We show that the system cools during this expansion but that the decrease in critical temperature occurs more rapidly, thus resulting in depletion of any initial condensate. Additionally, we present recent observations from CAL of bubbles of ultracold atoms created using a radiofrequency-dressing protocol. We observe bubble configurations of varying size and initial temperature, and explore bubble thermodynamics, demonstrating substantial cooling associated with inflation. The observations are among the first measurements made with ultracold atoms in space, using perpetual freefall to explore quantum systems that are prohibitively difficult to create on Earth. This work heralds future studies (in orbital microgravity) of BEC bubbles, the character of its excitations and the role of topology in its evolution.

Jonas Rønning
University of Oslo

Flow around topological defects in active nematic films

Jonas Rønning1, M. Cristina Marchetti2, Mark J. Bowick3 and Luiza Angheluta1

  1. Njord Centre, Department of Physics, University of Oslo, P. O. Box 1048, 0316 Oslo, Norway
  2. Department of Physics, University of California Santa Barbara , Santa Barbara, CA 93106, USA
  3. Kavli Institute for Theoretical Physics, University of California Santa Barbara, Santa Barbara, CA 93106, USA

The hydrodynamics of active nematics is characterized by an active turbulence regime whereby topological defects proliferate and interact with each other. The nematic state corresponds to orientational order, which is punctuated by half integer disclinations, as lowest energy defects. We study the flow induced by the active stress around isolated ±1/2 defects in an active nematic film in the presence of both viscous dissipation (η) and friction with the substrate (Γ). The interplay between the two dissipation mechanisms gives rise to a length scale〖 l〗_d^2=η/Γ, which sets the scale of the self-propulsion velocity for the +1/2 defect, and the scale for the decay of flow velocity and vorticity. Spanning vortices with alternating vorticity are formed around the defect in an active nematic film confined to a disk. The shape of these vortices, in addition to the self-propulsion of the positive defect, is determined by the relation between the discs radius R and〖 l〗_d. For small systems, in units of l_d, the velocity is proportional to the radius. As the size become larger the velocity become set by the dissipation length and the vortices become elongated. In the limit of R→∞ the vortices closes at infinity.

Linsy Jane Selvin Robert
Jožef Stefan Institute

Liquid crystal colloidal phases

Dense micron-sized colloidal particles of shapes similar to liquid crystal molecules have shown to exhibit interesting liquid crystalline-like phases. Previous works have shown how achiral particles such as bent rods/ banana-shaped particles can self-assemble into smectic and chiral phases in water. For such phases, geometric parameters of the particles such as shape, dimension, aspect ratio, etc. are known to have a strong influence on their phase diagram. In my present work, I study how dense polymeric particles of rod and banana shapes that are 3D printed behave in liquid crystal. My main motivation is to then study the active dynamics and self-assembly behaviour of dense Janus particles (metal-polymeric particles) in liquid crystal.

Pramodh Viduranga Senarath Yapa
University of Alberta

Triangular Pair-Density Wave in Confined Superfluid 3-He

Recent advances in experiment and theory suggest that superfluid 3He under planar confinement may form a pair-density wave (PDW) whereby superfluid and crystalline orders coexist. While a natural candidate for this phase is a unidirectional stripe phase predicted by Vorontsov and Sauls in 2007, recent nuclear magnetic resonance measurements of the superfluid order parameter rather suggest a two-dimensional PDW with noncollinear wavevectors, of possibly square or hexagonal symmetry. In this work, we present a general mechanism by which a PDW with the symmetry of a triangular lattice can be stabilized, based on a superfluid generalization of Landau’s theory of the liquid-solid transition. A soft-mode instability at finite wavevector within the translationally invariant planar-distorted B phase triggers a transition from uniform superfluid to PDW that is first order due to a cubic term generally present in the PDW free-energy functional. This cubic term also lifts the degeneracy of possible PDW states in favor of those for which wavevectors add to zero in triangles, which in two dimensions uniquely selects the triangular lattice.

*P.S.Y. was supported by the Alberta Innovates Graduate Student Scholarship Program. R.B. was supported by Département de physique, Université de Montréal. J.M. was supported by NSERC Discovery Grants Nos. RGPIN-2014-4608, RGPIN-2020-06999, RGPAS-2020-00064; the CRC Program; CIFAR; a Government of Alberta MIF Grant; a Tri-Agency NFRF Grant (Exploration Stream); and the PIMS CRG program.

Adhithiya Sivakumar
University of New Hampshire

Generalized Quasilinear Simulations of 2D Strongly Stratified Kolmogorov Flow

Generalized quasilinear (GQL) theory provides self-consistent approximations for the small-scale dynamics of various flows. The approximation is performed by specifying a cut-off wavenumber Λ that separates state variables into large and small scales in spectral space and then removing select nonlinear interactions. The resulting equations respect the conservation laws of the original PDEs and enable a systematic homotopy between quasilinearity (Λ=0), i.e. the mean field limit, and full nonlinearity (Λ→ ∞), i.e. DNS. When Λ>0, nonlinear interactions among the large scales and small scale energy transfers via interaction with the large scales are captured. These physical processes are particularly important for shear flows with highly anisotropic structures. Here, we investigate the accuracy of the GQL approximation by performing DNS and GQL simulations of 2D, strongly stratified Kolmogorov flow, and comparing dynamics and statistics across a range of cut-off wavenumbers.

Daniel William Swartz
Massachusetts Institute of Technology

Both colonization and competition abilities determine the fitness at the frontier of an expanding population

Bacteria colonize new territory in a process known as range expansion. This expansion is driven by the division of bacteria, allowing for the expanding population to accrue mutations resulting in evolution at the expansion front. In most cases, evolution will select for a faster expansion rate. However, there have been reported cases where a mutant evolves which expands slower than the wildtype and yet is able to out-compete the wildtype strain in coculture. This competition between fast and slow expanders generates dented fronts which we study in this work. We present a theory describing the competition at the boundary of a growing front between two populations. In typical one-dimensional competition, invasion generates a travelling wave which can be either ”pulled” or ”pushed”. We generalize these cases to competition on a growing front in both pulled and pushed regimes. We then demonstrate how coupling competition to the morphology of the expansion can have large effects on both the invasion velocity and the outcome of competition.

Stephen Joseph Thornton
Cornell University

Extracting universal scaling functions of rigidity transitions from an effective medium theory

Rigidity transitions in random systems, such as jamming (J) and rigidity percolation (RP), have long evaded description by the usual framework of critical phenomena. The coherent potential approximation (CPA), a type of effective medium theory, has served in the past as a valuable tool to predict dynamics and transition points in randomly percolated lattices. We leverage the analytically tractable self-consistency equations for the self-energy in the CPA to express physically observable quantities, such as frequency-dependent viscoelastic moduli and correlation functions, in the usual scaling framework of critical phenomena. We find the scaling behavior of these transitions in two spatial dimensions to be modified from that of higher dimensions – including a dangerous irrelevant variable that modifies the low-energy physics and logarithms that appear in the scaling functions.

Adrian van Kan
University of California, Berkeley

Are all two-dimensional turbulent flows born equal? -- A study of two-dimensional turbulence driven by an instability

Instabilities of fluid flows often generate turbulence. Using extensive direct numerical simulations, we study two-dimensional turbulence driven by a wavenumber-localised instability superposed on stochastic forcing, in contrast to previous studies of state-independent forcing. As the instability growth rate increases, the system undergoes two transitions. For growth rates below a first threshold, a regular large-scale vortex condensate forms. Above this first threshold, shielded vortices (SVs) emerge and coexist with the condensate. At a second, larger value of the growth rate, the condensate breaks down, and a gas of weakly interacting vortices with broken symmetry spontaneously emerges, characterised by preponderance of vortices of one sign only and suppressed inverse energy cascade. The number density of SVs in this broken symmetry state slowly increases via a random nucleation process. Bistability is observed between the condensate and mixed SV-condensate states. Our findings provide new evidence for a strong dependence of two-dimensional turbulence phenomenology on the forcing.

Philipp Patrick Vieweg
Technische Universität Ilmenau

Pattern formation in natural convection flows

Turbulent convection, the essential mechanism by which heat is transported in natural flows, manifests often in a hierarchy of structures and flow patterns. In addition to extreme Prandtl and Rayleigh numbers, natural flows exhibit a variety of different boundary conditions for both the velocity and temperature field which can influence these flows. This study aims to improve our understanding of their role using high-resolution spectral element simulations of three-dimensional turbulent Rayleigh-Bénard convection at very large aspect ratio. We find that thermal boundary conditions play a crucial role – once a constant heat flux is prescribed at the top and bottom planes, the gradual aggregation of small convection cells towards a domain-sized superstructure is observed. We study this process using leading Lyapunov vector and spectral transfer analyses. Further, we find that weak rotation around the vertical axis – which is quantified by the Rossby number – limits this gradual aggregation process effectively. Our studies might thus have interesting implications for atmospheric and stellar convection processes where heat fluxes are typically prescribed at the top and bottom boundaries of the convection zone.

Xueying Wang
University of Illinois, Urbana-Champaign

Stochastic model for the laminar-turbulent transition in pipe flow

In transitional pipe turbulence, a sequence of phases is observed experimentally in the range of Reynolds numbers between 1900 and 5000, passing through the laminar-turbulent transition at Re ~ 2040. These phases are characterized by transient decay of puffs (Re < 2040), puff-splitting and propagation (2040 < Re < 2250), expansion of turbulent regions via “weak slugs” (asymmetric upstream and downstream fronts, 2250 < Re < 4500), and via “strong slugs” (symmetric upstream and downstream fronts, Re > 4500). In earlier work, an intrinsically stochastic model for puff-decay and splitting accounted for the corresponding single-puff super-exponential timescales. This model focused on the dynamics and fluctuations within a single puff and described the near-critical behavior of transitional pipe flow. Here we extend this model by considering the previously neglected energy balance and turbulence-shear flow interaction. The resulting model recapitulates the full phase diagram of the transition, successfully capturing the mechanism of puff splitting and pushing and the puff-slug transition. Structural stability analysis on the model also shows the deep connection between the subcritical bifurcation picture and the directed percolation picture of the transition. The model is not restricted to pipe flow geometry and is extendable to other transitional shear flows like quasi-one-dimensional Taylor-Couette flow.

Jiarong Wu
Princeton University

How does the wind amplify ocean surface waves?

Wind excites and amplifies ocean surface waves. These wind waves modulate the mass, momentum and energy transfer between the ocean and the atmosphere. There are still vigorous discussions about the mechanism of wind wave growth despite decades of theoretical, experimental, and numerical studies. We conducted fully-coupled direct numerical simulation to better understand the physics of wind-wave interaction, and to quantify the wind energy input for various wind wave conditions. We verified the pressure forcing assumption with detailed flow field outputs. We also obtained reasonable wave growth rates for different wave ages and wave steepness, and identified potential reasons for discrepancies in previous results.

Weixuan (Rosa) Xu
Brown University

Topological Signature of Stratospheric Poincare -- Gravity Waves

The rotation of the earth breaks time-reversal and parity symmetries in an opposite sense north and south of the equator, leading to a topological origin for certain atmospheric and oceanic equatorial waves. Away from the equator the shallow water and stably-stratified primitive equations show that Poincaré-gravity waves have nontrivial topology as evidenced by a phase singularity in frequency-wavevector space.  This non-trivial topology then predicts, via the principle of bulk-boundary correspondence, the existence of two equatorial waves, the Kelvin and Yanai waves.  To verify the existence of these topological properties, we examine ERA5 reanalysis data to study cross-correlations between the wind velocity and geopotential height in the mid-latitude stratosphere at the 50 hPa level, and find the expected vortex and anti-vortex in the phase of the correlations at the frequencies of the Poincaré-gravity waves. These results demonstrate a new way to deepen understanding of waves in the stratosphere, and this approach provides a new tool for investigating waves in other components of the climate system.

Houssam Yassin
Princeton University

Surface Quasigeostrophic Turbulence in Variable Stratification

Numerical and observational evidence indicates that, in regions where mixed-layer instability is active, the surface geostrophic velocity is largely induced by surface buoyancy anomalies. Yet, in these regions, the observed surface kinetic energy spectrum is steeper than predicted by uniformly stratified surface quasigeostrophic theory. By generalizing surface quasigeostrophic theory to account for variable stratification, we show that surface buoyancy anomalies can generate a variety of dynamical regimes depending on the stratification’s vertical structure. Buoyancy anomalies generate longer range velocity fields over decreasing stratification and shorter range velocity fields over increasing stratification. As a result, the surface kinetic energy spectrum is steeper over decreasing stratification than over increasing stratification. An exception occurs when the near surface stratification is much larger than the deep ocean stratification. In this case, we find an extremely local turbulent regime with surface buoyancy homogenization and a steep surface kinetic energy spectrum, similar to equivalent barotropic turbulence. By applying the variable stratification theory to the wintertime North Atlantic, and assuming that mixed-layer instability acts as a narrowband small-scale surface buoyancy forcing, we obtain a predicted surface kinetic energy spectrum between k−4/3 and k−7/3, which is consistent with the observed wintertime k−2 spectrum. We conclude by suggesting a method of measuring the buoyancy frequency’s vertical structure using satellite observations.

Ziyan Zhu
Harvard University

Topology of rotating stratified fluids with and without background shear flow

Poincaré-gravity modes described by the shallow water equations in a rotating frame have non-trivial topology, providing a new perspective on the origin of equatorially trapped Kelvin and Yanai waves. In this work, we investigate the topology of rotating shallow water equations and continuously stratified primitive equations with and without background sinusoidal shear flow. Background shear flow not only breaks the Hermiticity and homogeneity of the system but also leads to instabilities. By introducing a gauge-invariant winding number, we show that singularities in the phase of the Poincaré waves of the unforced shallow-water equations and primitive equations persist in the presence of shear. By bulk-boundary correspondence, the bulk Poincaré bands have non-trivial topology and we expect and confirm the persistence of the equatorial waves in the presence of shear along the equator where the Coriolis parameter f changes sign.