Simulations of Three-dimensional Nematic Guidance of Microswimmers

TL;DR

This paper develops a hybrid simulation method to study 3D nematic liquid crystal environments guiding microswimmer dynamics, revealing how defect structures and activity influence microswimmer behavior and defect evolution.

Contribution

It introduces a novel 3D simulation approach coupling active Brownian dynamics with nematic hydrodynamics, extending understanding of microswimmer behavior in complex liquid crystal environments.

Findings

Defect dynamics depend on activity, size, and initial distribution.

Winding profile influences loop defect behavior.

Microswimmers can be guided by defect structures and nematic patterns.

Abstract

It has been shown that an anisotropic liquid crystalline (LC) environment can be used to guide the self-propulsion dynamics of dispersed microswimmers, such as bacteria. This type of composite system is named "living nematic" (LN). In the dilute limit, bacteria are found to mainly follow the local director field. Beyond the dilute limit, however, they exhibit novel dynamical behaviors, from swirling around a spiral +1 defect pattern to forming undulating waves, and to active turbulence. Our current knowledge of how these different behaviors emerge at different population densities remains limited. Here we develop a hybrid method to simulate the dynamics of microswimmers dispersed in a nematic LC. Specifically, we model the microswimmers using active Brownian dynamics method, which is coupled to a hydrodynamic model of nematic LCs to describe the evolution of the flow field and the LC structure. Our method is validated by comparing to existing quasi-two-dimensional (2D) experiments, including undulated swirling around a spiral pattern and stabilized undulated jets on a periodic C-pattern. We further extend our method to three-dimensional (3D) systems by examining loop-defect dynamics. We find that the morphodynamics and destiny of a loop defect not only depend on the activity (self-propulsion velocity), effective size, and the initial distribution of the swimmers, but also rely on its winding profile. Specifically, +1/2 wedge and radial twist winding can dictate the dynamics of loop defects. By varying the characteristic reversal time, we predict that microswimmers not necessarily accumulate in splay regions. Taken together, our hybrid method provides a faithful tool to explain and guide the experiments of LNs in both 2D and 3D, sheds light on the interplay between microswimmer distribution and defect dynamics, and unravels the design principles of using LCs to control active matter.