Collective chemomechanical oscillations in active hydrogels.

We report on the collective response of an assembly of chemomechanical Belousov-Zhabotinsky (BZ) hydrogel beads. We first demonstrate that a single isolated spherical BZ hydrogel bead with a radius below a critical value does not oscillate, whereas an assembly of the same BZ hydrogel beads presents chemical oscillation. A BZ chemical model with an additional flux of chemicals out of the BZ hydrogel captures the experimentally observed transition from oxidized nonoscillating to oscillating BZ hydrogels and shows this transition is due to a flux of inhibitors out of the BZ hydrogel. The model also captures the role of neighboring BZ hydrogel beads in decreasing the critical size for an assembly of BZ hydrogel beads to oscillate. We finally leverage the quorum sensing behavior of the collective to trigger their chemomechanical oscillation and discuss how this collective effect can be used to enhance the oscillatory strain of these active BZ hydrogels. These findings could help guide the eventual fabrication of a swarm of autonomous, communicating, and motile hydrogels.

First Order Alignment Transition in an Interfaced Active Nematic Fluid.

We investigate experimentally the dynamic phase transition of a two-dimensional active nematic layer interfaced with a passive liquid crystal. Under a temperature ramp that leads to the transition of the passive liquid into a highly anisotropic lamellar smectic-A phase, and in the presence of a magnetic field, the coupled active nematic reorganizes its flow and orientational patterns from the turbulent into a quasilaminar regime aligned perpendicularly to the field. Remarkably, while the phase transition of the passive fluid is known to be continuous, or second order, our observations reveal intermittent dynamics of the order parameter and the coexistence of aligned and turbulent regions in the active nematic, a signature of discontinuous, or first order, phase transitions, similar to what is known to occur in relation to flocking in dry active matter. Our results suggest that alignment transitions in active systems are intrinsically discontinuous, regardless of the symmetry and momentum-damping mechanisms.

Traveling fronts in vibrated polar disks: At the crossroad between polar ordering and jamming.

We investigate experimentally the collective motion of polar vibrated disks in an annular geometry, varying both the packing fraction and the amplitude of the angular noise. For low enough noise and large enough density, an overall collective motion takes place along the tangential direction. The spatial organization of the flow reveals the presence of polar bands of large density, as expected from the commonly accepted picture of the transition to collective motion in systems of aligning polar active particles. However, in our case, the low density phase is also polar, consistent with what is observed when jamming takes place in a very high density flock. Interestingly, while in that case the particles in the high density bands are arrested, resulting in an upstream propagation at a constant speed, in our case the bands travel downstream with a density-dependent speed. We demonstrate from local measurements of the packing fraction, alignment, and flow speeds that the bands observed here result both from a polar ordering process and a motility induced phase separation mechanism.