Systems of motile interacting units (active matter) can exhibit fascinating emergent phenomena such as self-organization and directed motion at large scales. Familiar examples are flocks of birds, schools of fish, and bacterial biofilms. While most attention has been focused on the collective motion of translating units such as bacteria (at the microscale) and birds (at the macroscale), the recent discovery of rotating bacteria Thiovulum majus prompted interest in their distinct collective dynamics. Our group is was one of the first to examine the collective dynamics of these so called microrotors (Yeo et al, PRL 2015) and highlight the importance of hydrodynamic (fluid flow mediated) interactions in the formation of the collective states of these systems.
We have several projects aimed at harnessing the dynamic self-assembly of active particles for advanced materials. In contrast to the living word, there are not many experimental realizations of self-propelled particles at the microscale. A popular design relies on chemical reactions using hydrogen peroxide as a suspending medium, which severely limits practical applications. Our goal is to design microparticles with facile actuation and tunable motility. We have made progress in this direction using the Quincke rotation to power particle motion. The Quincke effect is an electrohydrodynamic instability that gives rise to a torque on a dielectric particle in a uniform DC electric field. A sphere initially resting on the electrode rolls with steady velocity. However, we discovered another regime, in which the rotating sphere levitates in the space between the electrodes. We are currently exploring the collective dynamics of the hovering Quincke rotors.
My group has been able to program the Quincke roller to reproduce the run-and-tumble and Levy trajectories common to many swimming and swarming bacteria (Karani et al, PRL 2019. Our strategy enables to design the sequence of repeated “runs” (nearly constant-speed straight-line translation) and “tumbles” (seemingly erratic turn) to emulate any random walk. Our experiments show that a population of these random walkers exhibit behaviors reminiscent of bacterial suspensions such as dynamic clusters and mesoscale turbulent-like flows. I envision this Quincke random walker as a new paradigm for active locomotion at the microscale that opens new opportunities for experimental explorations of the collective dynamics emerging in living fluids.
Quincke random walkers
Reference: Karani, G. Pradillo, P. M. Vlahovska “Tuning the Random Walk of Active Colloids: From Individual Run-and-Tumble to Dynamic Clustering”, Physical Review Letters, 123: 208002 (2019) (Editor’ suggestion, Featured in Physics) DOI: 10.1103/PhysRevLett.123.208002