Session J49: Focus Session: Active Living Matter II

Active Microrheology in Active Matter Systems: Mobility, Intermittency and Avalanches

We examine the mobility and velocity fluctuations of a driven particle moving through an active matter bath of self-mobile particles for varied system densities and activities. The driven particle mobility is strongly non-monotonic and is correlated with distinct spatial-temporal structures that arise in the active media. We identify an activity-induced crystallization regime that is distinct from the higher activity-induced phase-separated cluster regime. The probe particle velocity fluctuation distributions exhibit specific features in the different dynamic regimes. In the cluster phase, we observe telegraph noise, while in the denser active jamming regimes, the probe particle moves in intermittent jumps or avalanches of power-law distributed sizes.   

Using artificial microswimmers for controlling the motion of passive colloidal particles in straight and asymmetric channels

Artificial self-propelled microswimmers capable of autonomous navigation through complex environments provide appealing opportunities for localization, pick-up and delivery of micro-and nanoscopic objects. Such self-driven microswimmers show not only the ability to navigate through the environment but also modify the environment. Using numerical simulations, we investigate active Brownian motion of self-propelled overdamped microswimmers, i.e., Janus spheres illuminated by light, in straight and corrugated channels. We demonstrated that a small fraction of active microswimmers injected in a system of passive colloids are capable of rectifying the passive species (i.e., in asymmetric channels [1]) or separating various species (i.e., in a mixture of passive species [2]). We analyze the effect of autonomous pumping of passive species by active microswimmers in various corrugated channels. \\[4pt] [1] Pulak K. Ghosh, Vyacheslav R. Misko, Fabio Marchesoni, and Franco Nori, Phys. Rev. Lett. {\bf 110}, 268301 (2013).\\[0pt] [2] W.~Yang, V.R.~Misko, K.~Nelissen, M.~Kong, and F.M.~Peeters, Soft Matter {\bf 8}, 5175 (2012).   

Visco-elastic Dynamics of an Active Polar Dynamic System

We study the dynamics of systems with a polar dynamic preferred direction that are embedded in visco-elastic media. Examples include the pattern-forming growth of bacteria and molecular motors. Because the ordered state only exists dynamically, but not statically, the macroscopic variable of choice is the velocity of the active units. The passive visco-elastic medium is described by a relaxing strain tensor. We derive the macroscopic equations for such a system and discuss novel static, reversible and irreversible cross-couplings connected to this two-fluid (two-velocity) system. The dynamics is rather different compared to the case of passive, static polar order. In particular, we find a complicated normal mode structure that reflects the broken time-reversal symmetry due to the non-equilibrium situation, anisotropy of first sound and a possible second sound excitation due to the active velocity, and various manifestations of the visco-elastic relaxation. We discuss critically the role of the so-called active term in the stress tensor as well as the thermodynamically correct description of the hydrodynamic transport velocities.   

Long range self-organization in bacterial swarms

When grown on air-semisolid interface, many bacteria are able to move in groups and expand rapidly, in a manner called swarming. Bacteria swarming displays rich collective behavior. In this work, we focus on the interaction between swarm cells of E. coli and their fluid environment. Using novel tracers, we discovered large scale self-organization in E. coli swarming colonies that spans a distance of milimeters. This long range self-organization most likely results from local interactions. The results provide new insights into the collective behavior in active matter systems.   

Dynamic Clustering in Suspension of Motile Bacteria

Bacteria suspension exhibits a wide range of collective phenomena arising from interactions between individual cells. Here we investigate dynamic clusters of motile bacteria near an air-liquid interface. Cell in a cluster orient its flagella perpendicular to the interface and generate attractive radial fluid flow that leads to cluster formation. Rotating cell also creates tangential forces on neighbors that sets clusters into counter-clockwise rotation. We construct a numerical model of self-propelled particles that interact via pair-wise forces extracted from hydrodynamic calculations; such a model reproduces many properties of observed cluster dynamics.   

Dynamics and elasticity of fire ant aggregations

Fire ants, \textit{Solenopsis invicta}, form aggregations that are able to drip and spread like simple liquids, but that can also store energy and maintain a shape like elastic solids. They are an active material where the constituent particles constantly transform chemical energy into work. We find that fire ant aggregations shear thin and exhibit a stress cutoff below which they are able to oppose the applied stress. In the linear regime, the dynamics is fractal-like with both storage and shear moduli that overlap for over three orders of magnitude and that are power law with frequency. This dynamic behavior, characteristic of polymer gels and the gelation point, gives way to a predominantly elastic regime at higher ant densities. In comparison, dead ants are always solid-like.   

Colony Rheology: Active Arthropods Generate Flows

Hydrodynamic-like flows are observed in biological systems as varied as bacteria, insects, birds, fish, and mammals. Both the phenomenology (e.g. front instabilities, milling motions) and the interaction types (hydrodynamic, direct contact, psychological, excluded-volume) strongly vary between systems, but a question common to all of them is to understand the role of particle-scale fluctuations in controlling large-scale rheological behaviors. We will address these questions through experiments on a new system, {\itshape Tyrolichus casei} (cheese mites), which live in dense, self-mixing colonies composed of a mixture of living mites and inert flour/detritus. In experiments performed in a Hele-Shaw geometry, we observe that the rheology of a colony is strongly dependent on the relative concentration of active and inactive particles. In addition to spreading flows, we also observe that the system can generate convective circulation and auto-compaction.   

Interactions of colloidal particles in an active medium

An individual colloidal particle that asymmetrically catalyzes chemical reactions generates a chemical concentration gradient, and thus moves directionally along the gradient by self-diffusiophoresis. Symmetrical particles may also move via their interactions with the chemical gradients generated by other active colloidal particles. We develop and analytically solve a model for self-diffusiophoresis in a medium that both produces and destroys a chemical that interacts with the colloidal particle. This alters the scaling of the self-diffusiophoretic velocity from the case of the inert medium and screens the interaction between colloidal particles. We numerically simulate multiple catalytic particles in the active medium and extract the basic rules for their interactions. Finally, we propose that this mechanism could be used for biological processes involving dynamic self-organization, such as chromosomal locus positioning.   

Microswimmers enhance the rotational diffusivity of colloids.

The random fluid flows generated within a suspension of swimming microorganisms have been shown to affect the translational diffusivity of suspended passive particles. Here we report results from experiments and simulations which show that a dramatic enhancement is found as well for the rotational diffusivity. The consequences of this effect when the passive tracers are replaced with small active particles will be discussed.   

Diffusion of an ellipsoid in a quasi-2D bacterial suspension

Enhanced translational diffusion of tracer particles in a suspension of micro-swimmers has been established as a distinct feature of active fluids. Here, instead of spherical tracers, we study the diffusion of ellipsoidal particles of various aspect ratios in a free-standing film of E. coli. Using high-speed digital video microscopy, we measured the mean-square displacements and calculated the translational and rotational diffusion coefficients of elliptical tracer particles. We found that both the translational and rotational diffusion of the particles are dramatically enhanced by the motion of bacteria. At low concentrations, this enhanced diffusion arises from random scatterings of bacteria, whereas at high concentrations it is attributed to the collective swarming of bacteria. Through a detailed analysis of the coupling between translational and rotational diffusion and theoretical modeling, we explored the origin of enhanced diffusion in translational and rotational degrees of freedom.   

Tunable long range forces mediated by self-propelled colloidal hard spheres

Most colloidal interactions can be tuned by changing properties of the medium. Here we show that activating colloidal particles with random self-propulsion can induce giant effective interactions between large objects immersed in such a suspension. Using Brownian dynamics simulations we find that the effective force between two hard walls in a 2D suspension of self-propelled (active) colloidal hard spheres can be tuned from a long range repulsion into a long range attraction by changing the active particle density. At relatively high densities, the active hard spheres can form a dynamic crystalline bridge, which induces a strong oscillating long range dynamic wetting repulsion between the walls. With decreasing density, the dynamic bridge gradually breaks, and an intriguing long range dynamic depletion attraction arises. A similar effect occurs in a quasi-2D suspension of self-propelled colloidal hard spheres by changing the height of the confinement. Our results open up new possibilities to manipulate the motion and assembly of microscopic objects by using active matter.