Confined and active liquids: a computational investigation

• Main Author: Yang, Jing
• Other Authors: Massimo Pica Ciamarra
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/165050
• Institution: Nanyang Technological University

Disordered many-particle systems have remarkable mechanical and transport properties, whose theoretical description is made difficult by the absence of crystalline-like periodicity. In this thesis, I investigate two aspects of the physics of these systems. In the first part of the thesis, I consider how confinement influences the physical properties of many-particle systems to rationalise the crossover from a two- to three-dimensional physics. Previous works have shown that confinement may induce layering, a slowdown of the dynamics and a series of ordering transition. These confinement induced effects made difficult to rationalize how a three-dimensional system becomes effectively two-dimensional as the confinement increases. I demonstrated that the confinement dependence of the dynamical properties clearly reveals the dimensionality crossover. This result builds on recent works that have shown that Mermin-Wagner long-wavelength fluctuations influences the dynamical properties of two-dimensional solids and liquids and make them distinct from those of three-dimensional systems. I developed a theoretical model to rationalise the confinement dependence of the long-wavelength fluctuations, and used it to predict the confinement dependence of the dynamics. This model allowed me to make predictions on the dimensionality crossover, which I have verified via detailed numerical simulations. In the second part of the thesis, I consider disordered many-particle systems of repulsive particles that move as able to self-propel rather than because of thermal effects. Self-propelling particles are attracting much interest in the literature as model biological systems and as a playground to explore the physics of driven non-equilibrium systems. The prototypical motility induced out-of-equilibrium effect is a phase coexistence of a liquid- and a gas-like phase (MIPS), which occurs at high enough density and motile forces dominating over other stochastic forces possibly acting on the particles and promoting an homogeneous phase. Besides MIPS, on increasing the density systems of active particles also present a phase transition from a flowing to an arrested state, or jamming transitions. In my thesis, I considered the interplay between these two out-of-equilibrium transitions, MIPS and jamming. Via extensive numerical simulations I have shown that these two transitions, believed to be unrelated in the literature, do actually occur together in the limit of small self-propelled forces in systems of persistent active particles, each self-propelling in a fixed direction.