Dynamic assembly of active matter

• Main Author: Ma, Zhan
• Other Authors: Ni Ran
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Bioengineering
• Online Access: https://hdl.handle.net/10356/151042
• Institution: Nanyang Technological University

Active matter refers to particles that can dissipate energy and drive their own motion. They display intriguing dynamic assembly phenomena as not restricted by equilibrium thermodynamics, including motility-induced phase separation (MIPS), collective motion, giant number fluctuation, and formation of complex patterns, etc. The study of dynamic assembly of active matter can help both understand the emergent behavior in living creatures and design dynamic materials. Multiple models have been proposed to study active matters, such as active Brownian particles (ABPs) and Vicsek-like models. However, most of these models ignore the intrinsic trajectory curvature and chiral swimming pattern, which are widely observed in active matter systems. This thesis focuses on the phase behavior of circle swimmers. We construct the circle active Brownian particles (cABPs) model by introducing the self-propulsion torque. Then we formulate a continuum theory for cABPs, and the fluctuation dispersion relation reveals two types of instabilities of the homogeneous state. Compared with simulations results, we verify that the type I instability leads to MIPS, while the type II instability, also called finite wave-number instability, results in a dynamic clustering state and interrupts the conventional MIPS. Besides, by measuring the self-intermediate scattering function, we reveal the dynamical property of this novel inhomogeneous state. Along with obtaining the full phase diagram of cABPs, we also study the phase transition between homogeneous state and dynamic clustering state in the context of percolation theory. Finally, we find the spontaneous demixing of circle swimmers and passive particles, which may have potential application in collecting and sorting passive colloids.