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 (http://local.fedex.com/co/boulder/office0402/).
Madhuvanthi Guruprasad Athani mathani1@jh.edu Johns Hopkins University 
Studying the active collective motions of flexible filaments using Brownian Dynamics simulationsOrdered, collective motions commonly arise spontaneously in systems of many interacting, active particles, such as selfpropelled 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 ‘selfpropel’. 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 srivatsa.badariprasad@ncl.ac.uk University of Newcastle upon Tyne 
Vortex Lattice Nucleation in Dipolar BoseEinstein CondensatesDegenerate quantum gases with strong permanent dipole moments are a robust platform for studying anisotropic and longranged phenomena in strongly correlated quantum systems. When subjected to a rotating magnetic field, the resulting precession of the dipole moments of a magnetic dipolar BoseEinstein 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 vorticityfree 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 preexisting semianalytical predictions until the onset of vorticity, and that the vortexfilled 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. 

Christopher Browne christopher.a.browne4@gmail.com Princeton University 
Elastic turbulence in 3D porous mediaMany 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 spatiotemporal fluctuations reminiscent of inertial turbulence, despite the small Reynolds number. Our measurements enable us to quantitatively establish that the energy dissipated by porescale 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 freya.bull@ed.ac.uk University of Edinburgh 
A model for the infection dynamics of a urinary catheterCatheter 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 secondorder phase transition. Low urine production rates give rise to an infected bladder state, transitioning, as the rate increases, into a washedout state where the catheter can still be infected but there is no detectable growth within the urine. 

Kirsten Daniela Endresen kendres3@jhu.edu Johns Hopkins University 
Inducing Topological Defects in Cell Monolayers Using TopographyWe investigate the effects of the longrange orientational order that arrises from anisotropicallyshaped 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 EpH4 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. 

Aaron Hui hui.122@osu.edu Ohio State University 
Noise thermometry in electron hydrodynamicsJohnson noise thermometry, based on the JohnsonNyquist theorem, offers a powerful primary thermometry technique to access the electron temperature at the nanoscale. In practical situations, one needs to generalize the JohnsonNyquist theorem to handle spatially inhomogenous temperature profiles. This was previously done for WiedemannFranzobeying ohmic devices, where it was found that Joule heating leads to a geometryindependent increase in Johnson noise. However, there has been great recent interest in stronglyinteracting electron hydrodynamic systems which do not admit a local conductivity nor obey the WiedemannFranz law, signatures of which have even been observed in previous thermometry experiments. In this paper, we study lowfrequency 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 geometryindependent due to nonlocal 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. 

Egor Kiselev j.kiselev@gmail.com Technion 
Hydrodynamic Floquet PlasmonsPeriodic 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. 

Chloe Lindeman cwlindeman@uchicago.edu 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 twostate 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. 

Pawel Matus matus@pks.mpg.de Max Planck Institute for the Physics of Complex Systems 
Anomalyinduced transport regime in Weyl semimetalsWe 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. 

Calvin Pozderac pozderac.16@osu.edu Ohio State University 
Exact solution for the thermalization transition in a model fracton systemIn fracton systems, the dynamics is constrained by higherorder 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 onedimensional 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. 

Shikhar Rai shikhar.rai@rochester.edu University of Rochester 
Scale of oceanic eddy killing by wind from global satellite observationsWind 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. 

Jonas Rønning jonas.ronning@fys.uio.no University of Oslo 
Flow around topological defects in active nematic filmsJonas Rønning^{1}, M. Cristina Marchetti^{2}, Mark J. Bowick^{3} and Luiza Angheluta^{1}
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 selfpropulsion 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 selfpropulsion 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. 

Daniel William Swartz dswartz@mit.edu Massachusetts Institute of Technology 
Both colonization and competition abilities determine the fitness at the frontier of an expanding populationBacteria 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 outcompete 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 onedimensional 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. 

Adrian van Kan avankan@berkeley.edu University of California, Berkeley 
Are all twodimensional turbulent flows born equal?  A study of twodimensional turbulence driven by an instabilityInstabilities of fluid flows often generate turbulence. Using extensive direct numerical simulations, we study twodimensional turbulence driven by a wavenumberlocalised instability superposed on stochastic forcing, in contrast to previous studies of stateindependent forcing. As the instability growth rate increases, the system undergoes two transitions. For growth rates below a first threshold, a regular largescale 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 SVcondensate states. Our findings provide new evidence for a strong dependence of twodimensional turbulence phenomenology on the forcing. 

Jiarong Wu jiarongw@princeton.edu 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 fullycoupled direct numerical simulation to better understand the physics of windwave 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. 

Houssam Yassin hyassin@princeton.edu Princeton University 
Surface Quasigeostrophic Turbulence in Variable StratificationNumerical and observational evidence indicates that, in regions where mixedlayer 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 mixedlayer instability acts as a narrowband smallscale 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. 