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Anisotropic materials have directionally dependent properties and are gaining attention for their applications in aerospace, sensing, soft robotics, and tissue engineering. In this thesis, we focus on soft anisotropic materials consisting of units with rod-like or disk-like shapes. We perform particle-based simulations complemented by theoretical analysis to study the structure and dynamics of passive and active anisotropic materials, including liquid crystals (LC) and bacteria-based active solids.

In the first system, we used the coarse-grained Gay–Berne (GB) model to study topological defects and LC–solvent interfaces. We started with examining passive LCs in terms of defect structure and dynamics. We proposed two independent methods to directly measure the ratio of splay and bend moduli without performing any mechanical tests, and these two methods show good agreement with each other and are also consistent with reported values in the literature. Next, we simulated the annihilation process of a pair of nearby ±1/2 defects. By performing NV E ensemble simulation of the above defect annihilation process, we were able to estimate the elastic constant magnitude based on the energy conservation law. Lastly, we examined how temperature gradient can impact defect motion. We observed that +1/2 defects tend to migrate toward hotter areas.

We further developed the interaction potential between GB and Lennard–Jones (LJ) XIX solvents to realize two types of anchoring conditions (planar and homeotropic) for a free LC–solvent interface. This allowed us to analyze the shape and defect structure of nematic droplets dispersed in an isotropic solvent as a function of droplet size. We constructed phase diagrams for LC droplets with the two anchoring conditions, considering the droplet size and temperature, revealing a shift in the transition temperature compared to the bulk system. Additionally, we compared the aspect ratio of the tactoids observed in our simulations with the predicted aspect ratio from the theoretical model, and we found a good agreement between the two. Next, we investigated the diffusion of LC droplets, focusing on translational and rotational persistence time. By combining the orientation of molecules and the droplet itself, our approach successfully achieved a good agreement with the simulation results. This finding supports the notion that the orientation of the LC droplet is indeed influenced by both the shape of the droplet and the internal molecules within it. Furthermore, we investigated the coalescence behavior of tactoids and found that droplet size, anchoring strength, and solvent viscosity were key parameters that influenced the orientation of the final state. In the second system, we used an active lattice spring model to study the elastic modes in a bacteria-based active solid. Our theoretical calculation and simulations reveal two physical modes, one of which is the oscillatory translation and the other is the oscillatory rotation. The two physical modes can transit to each other with a steplike frequency change by manipulating the self-propelled force or the elasticity in the model. We analyzed the lowest-order elastic oscillations in a two-dimensional disk and also performed simulation-informed theoretical analysis and related the ratio of two modes’ frequency to the Poisson’s ratio of the active solid.

Taken together, our particle-based simulations shed light on microscopic details of various dynamical phenomena in passive and active soft anisotropic materials, paving the way toward their future applications. XX Chapter 1 Introduction 1.1 Overview of Particle-Based Simulation and Anisotrop