Session QQ04: V: Active Materials

Self-assembly of binary rotors

Rotors are common in various aspects of life - from rotating proteins in a membrane to point vortices in superfluids. They often form distinct structures. Yet, little is known about the behavior of rotors in viscous fluids when the Reynolds number is small but not negligible, and complexities arise due to inertial effects. We experimentally study the nonequilibrium self-assembly of a binary mixture of rotors, in which some rotate clockwise and others counter-clockwise. We engineered small rotating cylindrical motors that float on the surface of a viscous fluid. Critically, our project utilizes mechanical rotors that enable us to control spin direction and eliminate interference from inner forces such as electromagnetic effects. We find that rotors with the same rotation repel and orbit each other, while oppositely spinning rotors attract and propagate together as a bound pair. Starting from a random organization, with half of the motors spinning clockwise and half spinning counter-clockwise, all with the same angular velocity, the rotors rearrange and create long chains driven solely by fluid motion and steric interactions. Our findings indicate that these chains are the stable form of the system.   

Re-programmable Combinatorial Acoustic Metamaterials

In this work, we study a combinatorial metamaterial composed of asymmetric pillars that can manipulate acoustic waves based on their angular orientation. The proposed metamaterial can produce different attenuation frequency regions (i.e., bandgaps) that can be altered in real-time by means of active control. We utilize a combinatorial design approach to characterize our metamaterials numerically and experimentally for different unit cells in both 1 and 2D configurations. Combinatorial design thus opens up a new avenue towards mechanical metamaterials with unusual order and machine-like functionalities.   

Electrotaxis of artificial swimmers in microchannels

Classically, electrotaxis refers to the directed motion of biological cells or motile organisms, like paramecium, under an applied electric field. Here, we experimentally demonstrate that self-propelled artificial microswimmers also exhibit autonomous changes in their motility to undergo electrotaxis, by considering active droplets as a model system. When a uniform electric field is applied along a microchannel, these self-propelled droplet microswimmers alter their trajectory to swim along the direction of the applied field thereby exhibiting negative electrotaxis. Previously, we demonstrated that an active droplet also autonomously navigates upstream of an external flow (i.e. exhibits positive rheotaxis) in a microchannel in an oscillatory trajectory. Here, we show that on application of an electric field during rheotaxis, the droplet microswimmer attains a steady trajectory along the microchannel center-line, instead of a stable oscillatory trajectory. Finally, we demonstrate that the essential features of the electrotactic and electro-rheotactic behaviour can be explained using a far-field hydrodynamic model considering the electrical response and hydrodynamic interactions of the active microswimmers. Interestingly, the electro-rheotactic behaviour of the microswimmer can be understood as a Hopf bifurcation from a dynamical system perspective. Such electrical sensing of self-propelled artifical microswimmers can be judiciously manipulated for targeted cargo delivery.    

Wave speed in an active elastic solid

This work numerically calculates the wave speed of a pulse propagating on an active elastic solid (defined as a one-dimensional network of active stochastic particles interacting by nonlinear hard springs) under tension. Notice that in this model, the rotational dynamics of each particle  is stochastic and independent from the others. By proposing a discrete analysis, it is found that the wave speed in active solids is higher than in a passive one, and that it depends on the amplitude of the pulse. Surprisingly, it is found that at steady state, tension along the active solid is not uniform; in other words, the separation distance between the active beads is not even, thus deviating from equilibrium mechanics  in which  a passive solid is seen to have an even tension and an even separation between beads.