Session G26: Active and Externally Driven Granular Matter I

Hydrodynamics of polarized crowds

Modeling crowd motion is central to situations as diverse as risk prevention in mass events and visual effects rendering in the motion picture industry. The difficulty of performing quantitative measurements in model experiments has limited our ability to model pedestrian flows. We use tens of thousands of road-race participants in starting corrals to elucidate the flowing behavior of polarized crowds by probing its response to boundary motion. We establish that speed information propagates over system-spanning scales through polarized crowds, whereas orientational fluctuations are locally suppressed. Building on these observations, we lay out a hydrodynamic theory of runenr crowds and demonstrate its predictive power.  I will close my talks by the discussion on the robusteness of of ourperimental and theoreticla findings and discuss the dynamics of ultra-dense polarized crowds.    

Acoustically levitated lock and key grains

The self-assembly of materials with complex structure requires mechanisms for programmable, specific bonds between subunits. Such mechanisms have been achieved in micrometer size systems, using colloids bonded with complementary DNA strands or shape-dependent depletion forces. However, equivalent schemes for out-of-equilibrium directed assembly are just beginning to be explored.  Here we show that acoustic levitation offers one possibility for the generation of shape-dependent, specific bonds between pairs of millimeter scale particles.  We levitate particles in an ultrasonic standing wave, allowing for substrate-free assembly. Secondary scattering generates shape-dependent attractive forces between particles, while driving the acoustic trap above its resonance frequency produces active fluctuations that mimic an effective temperature. We 3D print planar particles, and show that the local curvature of their binding sites tunes the energy landscape along the particle perimeter, in turn controlling the selectivity and bound-state lifetime for attaching a matching particle to the binding site. We show that these principles can be used to design and assemble particles into complex structures.    

Competition between local activity, gravity, and boundary shear in active-passive granular mixtures

Organisms living within granular materials disturb their local environment as they move, sometimes creating bulk flows. To quantitatively examine these effects, we mix flour beetle larvae with natural grains of various sizes and examine the resulting flows. We characterize the rate of particle-scale rearrangements due to fluctuations by diffusing wave spectroscopy, the macroscopic flow rate via observing the angle of a relaxing sandpile, and the effective shear viscosity via a rheometer. We find that both the rates of rearrangements and the macroscopic flows depend on both the percentage of larvae, and the grain type of the mixture. Increasing the percentage of larvae increases the rearrangement rate and the flow rate; as such, we make an analogy to a fluid reducing its viscosity at higher temperatures. Additionally, we find that the rheology of the mixtures becomes insensitive to the fluctuations of the larvae when the mixtures are subjected to the shear force of the rheometer. By comparing the results across the experiments, we conclude that the rheology of the active-passive mixtures depends on four key factors: the fluctuation rate due to the active matter, the grain inertia, the cohesive force between the grains, and the driving shear.   

Do brittle-to-ductile transitions occur in dense active matter?

The nature of the yielding transition in sheared amorphous solids is highly dependent on material preparation. Well-annealed, highly stable packings yield in a brittle fashion accompanied by the formation of system-spanning shear bands, while poorly annealed system yield in a ductile manner. Recent works have shown that there is a direct link between the dynamics of sheared and dense active matter in the pre-yielding regime. Therefore, an interesting open question is whether this brittle-to-ductile paradigm for yielding still holds in dense active matter. Here we study the yielding transition for active matter in the limit of infinite persistence and quasi-static driving, and find that the magnitude of the observed stress drops, a measure of brittleness, decreases systematically with the spatial correlation length of the applied active field. We then study the spatial organization of the plasticity that occurs during yielding, and use extensions of the Radon transform to demonstrate that putative shear bands still form in active matter, but their lengthscale also decreases with decreasing input correlation length. This suggests that the brittle-to-ductile transition is governed not only by material preparation, but also by the symmetry and correlation length of the driving field   

Granular Matter in Acoustic Traps: The Wave-Driven Oscillator

A focused acoustic standing wave creates a Hookean potential well for a small sphere and can levitate it stably against gravity. Exposing the trapped sphere to a second transverse travelling sound wave imposes an additional acoustic force that drives the sphere away from its mechanical equilibrium. The driving force is shaped by interference between the standing trapping wave and the traveling driving wave. If, furthermore, the traveling wave is detuned from the standing wave, the driving force oscillates at the difference frequency. Far from behaving like a textbook driven harmonic oscillator, however, the wave-driven harmonic oscillator instead exhibits a remarkably rich variety of dynamical behaviors. These include oscillations at both harmonics and subharmonics of the driving frequency, period-doubling routes to chaos and Fibonacci cascades. This model system therefore illustrates opportunities for dynamic acoustic manipulation based on spectral, rather than spatial, control of the sound field.   

Collective Dynamics in Chiral Granular Gases

We discuss the dynamics of chiral granular matter activated by mechanical vibrations on an oscillated substrate. Each specimen is disk shaped with tilted legs which are shown to induce clockwise or anticlockwise rotation when viewed from above depending on their tilt angles and the vibration strength. The grains are 3D printed and identical to within manufacturing errors.  Global rotation states are observed with angular frequencies which depend on the system size and the area fraction of the near identical granular matter below the maximum packing fraction. Above a critical area fraction, the individual grains orbit about the center in a circular arena while preserving their relative radial position. Whereas transitions between well-defined orbits are observed below this critical fraction. The orbital speed is also observed to change with area fraction with linear dependence with radial distance at higher area fractions, and sub-linear increase at lower area fractions. We shall discuss the observed decreasing orbital speeds with system size in terms of a minimal model which accounts for the pair-wise interaction of the grains during collisions. The effect of changing friction between grains and boundary walls will be further discussed.      

Freezing an acoustically levitated granular liquid

Phase transitions far from equilibrium are behind many diverse and intriguing phenomena in both nature and the laboratory. Here we present a study of an out-of-equilibrium phase transition for a membrane of acoustically levitated particles, 30 microns in diameter. This quasi-2D membrane is a dissipative but driven granular system; the energy density of the acoustic cavity plays the role of an effective temperature and determines the fluctuations in the system. Furthermore, the scattered sound between such small particles establishes interactions that are repulsive at a close approach but become attractive upon contact. Underdamped, collisional dynamics in this complex potential landscape give rise to two kinetically distinct states, which we call “liquid” and “solid”. In the “liquid” state, particles freely diffuse throughout the membrane. In the “solid” state, particles interconnect to form a kinetically arrested chain-like structure. The structures formed after the transition can be tuned by adjusting the rate of change of the effective temperature: as the effective temperature is decreased more slowly, particles assemble into a lacey structure consisting of many open loops.   

Activating granular creep with vibrations and heat

Under low stresses or prolonged shear, granular materials become jammed, where we expect motion to cease. We have recently observed persistent, indefinite deformation in an experimental granular heap under only the action of gravity. These dynamics showed hallmarks of of creep in a relaxing glass: power-law aging, quadrupolar spatial correlations, and heterogeneous zones of deformation. This hints at a regime in which activated creeping motion may be sustained below the global yield. Motivated by similarity of relaxing sandpile and relaxing glass, we seek to activate creep by external disturbances and observe the resultant states. To do so, we introduce controlled vibrations and heat to an otherwise quiescent granular heap, and measure the resulting creep strains with spatially-resolved diffusive wave spectroscopy (DWS). In response to gentle taps, creep deformation is distributed throughout the pile, manifest as discrete zones of plastic activity. As time progresses, deformation within the bulk diminishes and becomes confined to a thin layer at the free surface. Increasing the vibration strength causes this layer to span the system size, at which point a steady hydrodynamic-like flow appears at the free surface. Heating the grains produces small displacements due to thermal expansion and contraction. In this case, the phenomenology is similar to vibrations, albeit with a clear distinction: elastic strains are observed in the compacted core of the pile. We identify critical amplitudes of vibration required for system-spanning failure that vary as a function of the age of the pile. These results are geophysically relevant, as soils are heated and cooled daily, and experience ambient vibrations provided by the environment.   

Collective behaviour of self-propelled elliptical particles

Ensembles of anisotropic self-propelled particles exhibit a rich variety of emergent phases. A combination of short-ranged excluded volume interactions, which induce inter-particle forces and torques, and self-propulsion determines the resulting macroscopic structure. Starting from a point in parameter-space which displays motility-induced phase separation (MIPS) for isotropic particles, we systematically increase the aspect ratio of the constituent ellipses. On doing so, first, MIPS breaks down paving way to a spatially homogeneous state comprising polar domains. Secondly, at sufficiently large aspect ratios, particles aggregate into polar bands. We rationalize these observations from simulations by extracting two effective parameters, viz., the force imbalance coefficient and the coupling to the local polarization, that enter the mean-field description of the system.   

Magnetically driven granular gas in low gravity

Experimental investigation of granular gas systems in three dimensions desire the removal of the weight of constituent particles. In a sounding rocket experiment module, such condition is realized and maintained for 6 minutes, allowing 4 rounds of particle excitation and cooling. Varying magnetic force from 8 magnets surrounding the sample cell is adopted as the bulk-driving mechanism to enable a homogeneous spatial distribution of around 3000 particles. Soft ferromagnetic particles are chosen to minimize the long-range interactions between particles once the excitation finishes. The ensuing cooling behavior is thus mainly governed by energy-dissipative particle collisions and the kinetic theory of granular gases becomes comparable with the experimental results. From such comparisons, the prediction of non-Maxwellian velocity distribution is confirmed, while the Haff's cooling law, although qualitatively verified, quantitatively overestimates the cooling time scale significantly.   

Motility-driven segregation, self-trapping and interfaces in active granular matter

We report unexpected states of organization in experiments and simulations on mixtures of motile fore-aft asymmetric rods and spherical beads. When the sum of the packing fractions of beads and rods exceeds about 0.7 the system spontaneously demixes into rod-rich and bead-rich regions. Crucially, the polar rods first flock without demixing, and then the aligned rods segregate from the beads. We present a coarse-grained theory of the instability leading to phase separation, through the interplay of the active forcing of the beads by the rods and the asymmetry of the steric interaction of the rod with the beads. We show further in experiments and simulations that in a suitably engineered channel geometry a small minority of motile rods can sequester and immobilize a bead-rich domain, and we present a theory of such self-jamming. In fully two-dimensional experiments at high total area fraction we show that the interface between bead-rich and bead-poor regions is decorated by rods pointing towards high bead density, and we measure the spatial power spectrum of interface height fluctuations.    

Chirality in a system of active air-driven spinners

We present an analysis (based on kinetic theory) of the rich phase behavior and dynamics that we have

observed in experiments with a fluid of disk-shaped rotors. All rotors feature the same chiral geometry.

Contrary to current knowledge, our results show that this chiral fluid displays different flows whose chirality

is not necessarily analogous to the chirality of its constituent particles. We will show that these transitions are

controlled by particle spin and translational velocity correlations and heat dissipation at the boundaries.

Most notably, we will show that particle spin synchronization within the fluid leads to an inversion of the flow chirality, this

inversion resulting from a continuous transition where inverse chiral vortexes appear gradually.

We finally describe a mechanisms through which a set of active particles can control their global rotation state

and hence their transport properties. Our results are relevant for a variety of important biological processes   

Acoustically Induced Superthickening and Enhanced Elasticity in Shear Jamming Dense Colloidal Suspensions

Acoustic perturbation can dramatically enhance the solid properties of a shear jammed suspension.

The application of a transient acoustic perturbation (training) prior to shear jamming was shown to result in the suspension remaining shear jammed when later exposed to a perturbation (testing) that would have otherwise resulted in unjamming. The state of the suspension prior to testing was studied via rate cessation and small amplitude oscillatory shear (SAOS). It was found that the stress relaxation timescale increases with the power applied during training. Strikingly, beyond a critical training power, the stress increases instead of decaying during cessation. SAOS experiments show that the storage modulus immediately before testing increases with training power. As SAOS is applied, the storage modulus decays and reaches a stable value which can be orders of magnitude larger for large training powers when compared to an untrained suspension.

Training was also found to significantly reduce the time taken for shear jamming to occur, promoting what would have been a discontinuously shear thickening phase to a shear jamming phase. These results demonstrate the tremendous potential acoustic perturbation has for controlling the flow and elasticity of dense colloidal suspensions.