Archives: Events

Quasi-long-range polar order and KT-like transition in dry active matter

All Magnetic Active Matter

We will describe a system of artificial nanoswimmers that are powered by small external magnetic fields, and whose direction of motion is governed by orientational diffusion and interactions with each other. This all magnetic active matter can be seamlessly converted into a driven system and provide a promising platform to test and investigate various collective phenomena. In particular, we will describe experiments to study this novel system under strong confinement that is different from typical hard wall interactions.

Pattern Recognition with Active Matter: Designing “Materials that Compute”

Using theoretical and computational modeling, we design an active materials system that can autonomously transduce chemical, mechanical and electrical energy to perform a computational task in a self-organized manner, without the need for external electrical power sources. Each unit in this system integrates a self-oscillating gel, which undergoes the Belousov-Zhabotinsky (BZ) reaction, with an overlaying piezoelectric (PZ) cantilever. The chemo-mechanical oscillations of the BZ gels deflect the piezoelectric layer, which consequently generates a voltage across the material. When these BZ-PZ units are connected in series by electrical wires, the oscillations of these units become synchronized across the network, with the mode of synchronization depending on the polarity of the piezoelectric. Taking advantage of this synchronization behavior, we show that the network of coupled BZ-PZ oscillators can perform pattern recognition tasks. We define the “stored” pattern as a set of polarities of the individual BZ-PZ units, and the “input” patterns are coded through the initial phase of the oscillations imposed on these units. The results of the computational modeling show that the “input” pattern closest to the “stored” pattern exhibits the fastest convergence time to the stable synchronization behavior. In this way, the networks of coupled BZ-PZ oscillators achieve pattern recognition. Further, we show that the convergence time to the stable synchronization provides a robust measure of the degree of match between the input and stored patterns. Through these studies, we establish experimentally realizable design rules for creating “materials that compute”.

Active matter with intent: clog control in excavating collectives

Ensembles of self-propelled systems can spontaneously form clusters, clogs and jams. However, in crowded biological and robotic swarms, clog mitigation is important for collective task completion. To discover principles by which active materials can ensure high task performance under severe constraints, we studied workforce organization in fire ant tunnel excavation, examining the biological strategies in theoretical, computational and robophysical models. Workload inequality coupled with selective retreats led to high performance excavation, despite the lack of centralized guidance. Tools from the study of dense particulate ensembles elucidated how the seemingly counterintuitive strategies of idleness and retreating lead to optimum traffic conditions: idleness reduces the frequency of flow-stopping clogs, and selective retreating reduces clog dissolution time for the rare clogs that still occur. Our results point to strategies by which active materials can become task-capable without sophisticated sensing, planning and global control of the collective.

Suspensions of Active Particles

We study the behaviour of dense active suspensions of self-propelled colloids. At intermediate densities we observe the formation of clusters, resulting from a permanent dynamical merging and separation of active colloids. We have characterised in depth their kinetics of formation and their dynamics which shows vivid translational and rotational motions. These experimental results are discussed in the framework of a simple statistical model that captures quantitatively the measured dynamics. This sheds some new light on the internal organisation of the clusters and on the mechanisms underlying their formation.

In the dense regime, characterizing the dynamics as a function of the activity we have evidenced an unexpected
behavior.

Self-organization of micro-swimmers in confined spaces

The intricate self-organized dynamics of micro-swimmers is shown to result from a complex interplay of the individuals with the surrounding fluid and environment. I will present simulations that resolve these interactions to look at motion of swimmers in circular drops, racetracks or moving domains. Experiments with Bacillus subtilis bacteria confirm the predictions of a stable swirling vortex in circular drops or a persistent unidirectional stream in racetracks. The similarities and differences between the behavior of bacteria, algae and spermatozoa will be discussed.

Persistent limit of an active particle in a nonconvex potential

The effective potential approach has brought much recent progress to the theory of simple self-propelled particles. Gaussian colored noise models, in particular, allow one to systematically and explicitely map an active system onto an equilibrium one. The method, however, fails when the activity is persistent and the potential is not convex. Unfortunately, and somewhat expectedly, that is also where some the most interesting nonequilibrium physics tends to happen. I will discuss one of the simplest realizations of this situation: a single self-propelled particle in a 1D non convex potential in the persistent limit.

Pattern formation using light-actived bacterial swimmers

We have created E. coli bacteria that only swim when illuminated by light, and have \’tuned\’ the genetics to obtain the shortest possible \’stopping distance\’ when illumination ceases. I will describe protocols for creating bespoke patterns using these bacteria by applying static and dynamic light fields, and explain the physical bases underlying these protocols. The same bacteria also allow us to prove a theorem for active particles, viz., that the product of their velocity and density should be constant in a region where both quantities are spatially dependent.