Session A20: Active Matter in Complex Environments I

Collective behaviors of motile bacteria

Swimming bacteria are among the best characterized examples of biological active matter. Most early studies of bacterial motility focused on movement of individual bacterial cells at low densities, where interactions among bacteria could be neglected. However, motile bacteria can also exhibit a range of collective behaviors. As shown by the example of the model Escherichia coli, chemical or physical interactions between swimming bacteria can lead to emergence of different collective phenomena. Here we first focus on swirling collective motion at high densities of bacteria in microfluidic channels of different height and show that this motion could be primarily explained by hydrodynamic interactions between swimmers. We further demonstrate that collective swimming impedes bacterial ability to follow chemical gradients. We also investigate heterogeneous mixtures of motile and non-motile bacteria, demonstrating that non-motile bacteria can exhibit non-equilibrium dynamic clustering in presence of even small fraction of motile bacteria in the population.   

Effective temperature and activity induced memory effects of a passive particle in an active bath

We present an overview of the properties of a system of a passive particle immersed in a bath of active particles with translational inertia. In this overview, we not only classify the behavior of the passive particle, but also that of the active bath particles. Previous studies have shown analytically that the temperature of an overdamped active particle scales quadratically with its active velocity in the dilute limit. We analytically extend this theory to show that this scaling law also applies to active particles with translational inertia, though the scaling is now between the active particle temperature and its active force. Through computer simulations, we furthermore verify that this law holds in dense systems. We additionally demonstrate through computer simulations that the temperature of a passive particle immersed in an active bath also scales quadratically with the active force of the bath particles. However, the coefficient of the quadratic term is not the same and, consequently, a passive particle in an active bath will not equilibrate to the same temperature as the active bath particles. In contrast to models proposed in some previous studies, we show that the memory kernel of the passive particle can neither be approximated as Markovian nor as decaying exponentially on one time scale. Instead, at high active forces, the memory kernel develops a negative portion, a possible indication of backflow, which can also be seen in the long time tail of its velocity autocorrelation function.   

Collective Dynamics of Quincke Rollers with Fully Resolved Hydrodynamics

Quincke rollers are the active dielectric particles that run and tumble freely on a flat plate due to a torque generated by a uniform DC electric field applied perpendicular to the plate [1]. It has been reported experimentally that the Quincke rollers show a variety of unique collective dynamics such as disorder gas, polar liquid, and active crystal states [2]; however, the physical mechanisms behind their complex behaviors are poorly understood, particularly in their dense system because of the complicated interactions composed of hydrodynamic and electrostatic forces.

We report our successful simulations on the collective motions of Quincke rollers, which depends on the strength of dipole moment induced on a particle, the area fraction of particles, and the strength of interparticle attraction, with fully resolving the hydrodynamics of the system. In particular, we want to emphasize that the polar liquid is unstabilized by the lubrication effect acting between rotating particles at high densities.

[1] A. Bricard et al. Nature 503.7474 (2013): 95-98.

[2] A. Mauleon-Amieva et al. Phys. Rev. E 102.3 (2020): 032609.   

Dynamics of active rings in porous media

Active organisms living in complex environments generate localized forces along their membrane and effectively navigate through their habitat undergoing temporal shape deformations. To understand the key factors that affect the migration of these organisms, we simulate active ring polymers in two-dimensional disordered porous media. The dynamics of three different systems of active ring polymers such as flexible, inextensible, and semiflexible have been simulated. We find that the activity of individual beads differently affects the dynamical and conformal properties of the ring polymer depending on the pore size of the media, flexibility, and stiffness of the polymer. Flexible and inextensible ring polymers migrate smoothly through the random porous media by expanding and shrinking inside pore space. Whereas semiflexible ring polymers exhibit transient trapping inside the pores. The migration of the active rings in such a complex environment is affected by activity and the architecture of the porous media. We discover that the flexible rings swell, while the inextensible and semiflexible rings shrink in porous media with an increase in activity. Our findings shed light on how the shape deforming organisms navigates through the disordered, porous environments and also how the media affect their dynamics and morphology.   

Phase behaviour of active nanoparticles in block copolymer melts

Block copolymer melts have been largely used to control the position and alignment of anisotropic nanoparticles. The self-assembly properties of block copolymer melts allow to obtain hierarchical systems that inherit the symmetry properties of block copolymers. On the other hand, active particles display a rich phase behaviour in the presence of obstacles or under confinement. In this work we explore the co-assembly of active nanoparticles within block copolymer melts in two- and three-dimensions finding single-particle and collective behaviour due to the interplay between colloidal directed motion and block copolymer morphology. Active particles are found to act not as mere fillers but able to distort the block copolymer morphology and produce novel co-assembled structures, both for the case of isotropic and anisotropic particles. The rich phase behaviour of active Brownian particles within block copolymer matrices highlights the co-assembly possibilities of active particles in complex media.   

Disordered boundaries destroy bulk phase separation in scalar active matter

We show that disordered boundaries destroy bulk phase separation in scalar active systems in dimension d <3. This is in strong contrast with the equilibrium case where boundaries have no impact on the bulk of phase-separated systems. The underlying mechanism is revealed by considering a localized deformation of an otherwise flat wall, from which the case of a disordered boundary can be inferred. We find long-ranged correlations of the density field as well as a cascade of eddies which we show prevent bulk phase separation in low enough dimensions. The results are derived for dilute systems as well as in the presence of interactions, under the sole condition that the density field is the unique hydrodynamic mode.   

Pulsating active particles

What happens when particles in a crowded environment can change their size under the effect of a nonequilibrium drive and size synchronization?

It is known that cells in biological tissues interact and give rise to collective behavior such as size oscillation and wave propagation: we explore the physical mechanisms underlying this collective behavior by introducing a novel class of active matter, where the activity is the ability to change an internal degree of freedom at the single-particle level, which can be eventually related to the particle size. The collective dynamics thus emerges as the interplay of conservative forces, nonequilibrium driving and synchronization. We highlight the minimal ingredients needed for the complex behavior above-mentioned and point out future directions in the emerging field of pulsating active matter.   

Tension-Controlled Collective Dynamics in Active Solids

Navigating amongst diverse functionalities is a major goal of metamaterial design, with applications in various fields, from architecture, to soft robotics. Multifunctional materials are usually actuated from an external source of work, which allows for a good control of the targeted functions. The recent finding of selective and collective actuation in active solids, namely solids embedded with active units, opens the path towards autonomous actuation. This however immediately raises the question of its control. Here we show how mechanical tension can serve as a general mechanism for switching between different collective dynamics in active solids. We combine the experimental study of a centimetric model active solids, numerical study of an agent based model and theoretical arguments to reveal how tension allows for the reversible transition between different actuation regimes. More specifically we discuss the transition from a regime dominated by the presence of infinitesimal zero mode in the vibrational spectrum of the elastic structure, to a regime dominated by tension, with purely harmonic modes. We further demonstrate that for large enough tension and activity, any linear elastic structure favors the same type of actuation regime.   

Dynamics of self-propelled particles over a periodic potential landscape

We report a systematic study of self-propelled particles (SPPs) over a periodic potential landscape. A microgroove-patterned polydimethylsiloxane (PDMS) substrate is constructed using the techniques of photolithography and thermal imprinting to provide a gravitational potential landscape $U_0(x)$ for a monolayer of diffusing SPPs. The effective potential $U(x)$ is obtained from the measured population probability histogram $P(x)$ of the SPPs at varying activity levels. Our work provides a physical understanding on how the propulsion speed of SPPs affects the effective potential using the fixed angle approximation. With this understanding, one can reconstruct the equilibrium potential $U_0(x)$ from the measured $U(x)$. It is found that the histogram of dwell time has an exponential distribution and the mean dwell time follows a Kramers-like equation. This work demonstrates a useful experimental system to quantitatively study SPPs in an external potential field.    

The Anomalous Transport of Tracers in Active Baths

We derive the exact long-time dynamics of a tracer immersed in a one-dimensional active bath. In contrast to previous studies, we find that the damping and noise correlations possess long-time tails with exponents that depend on the tracer symmetry. For an asymmetric tracer, the tails lead to superdiffusion and friction that grows with time when the tracer is dragged at a constant speed. For a symmetric tracer, we recover normal diffusion and finite friction. However, when the symmetric tracer is small compared to the active-particle persistence length, the noise becomes anticorrelated at late times, and the active contribution to the friction becomes negative: active particles then enhance motion rather than opposing it.   

Using Phase Field Models to Simulate the Full Chemohydrodynamics of Passive and Active Colloids

Colloidal particles can migrate in a solution in response to a solute concentration field, a phenomenon known as diffusiophoresis. Chemically active colloids can modify the concentration field of its surrounding, thus harvesting energy from the environment to self-propel or to change the trajectory of neighboring colloids. To date, the most efficient methods to simulate these active systems rely on Green's functions of the Laplace and Stokes operators that are only valid in the steady and dilute limits. However, many active systems of interest display interesting feedback behavior in dense and unsteady systems. We have recently developed a method using phase field models that performs full chemohydrodynamics simulations of such dense and unsteady systems and incorporates colloidal particles as highly viscous fluid phases. We demonstrate the feasibility of this approach by simulating diffusiophoresis of colloidal particles and comparing to known theoretical results. We also demonstrate the capacity of the method to simulate self-diffusiophoresis by adding asymmetric chemical reactions to colloidal systems. Finally, we explore the computational and phenomenological limits of the method by simulating multi-particle phoretic motion coupled to complex concentration fields.