Collective forces and torques in active matter : the role of anisotropy
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Abstract
Statistical mechanics played a key role in the development of modern physics. Going beyond equilibrium systems, the statistical description of non-equilibrium systems has gained significant attention. In particular, active matter has emerged as a paradigm to study the broader class of systems driven out of equilibrium. Motile active matter is composed of autonomous active ``particles'' which, unlike their passive counterparts, have a tendency to move persistently along the direction of preceding displacements. Ranging from nanomachines carrying cargo within our cells to human beings, such matter is ubiquitous in the natural world. Facilitated by advancement in fabrication techniques and computational capability, the past two decades have witnessed an enormous interest in understanding, engineering and controlling active matter.
This thesis explores the role of collective forces and torques in active matter with a focus on the effect of geometric anisotropy. In bulk, these forces and torques that result from a combination of propulsion and inter-particle interactions determine the collective behavior of active particles. Moreover, these can also be relayed to boundaries or objects suspended in active matter. The suspended object undergoes linear or angular propulsion depending on its shape. Microscopic engines that rotate in bacterial baths, for example, have been realized based on this principle. In this thesis, first, we study how the manifestation of collective forces and torques shapes the emergent phase behavior of ellipsoidal active particles. The resulting macroscopic structure is determined by how anisotropic the constituent particles are. Secondly, we probe into how an active fluid conveys forces onto passive objects immersed in it. We show that in a periodic system, the force on the object can be related to the vorticity of the polarization of the surrounding active fluid. Finally, we relate the origin of the force on an optically trapped probe in a confined active fluid to the microstructure of its neighborhood.