Dynamics of flowing 2D skyrmions

TL;DR

This study numerically explores how external flows influence the shape and dynamics of 2D liquid crystal skyrmions, revealing a first-order transition between flow-aligned and perpendicular stretching regimes.

Contribution

It introduces a simplified model combining Ericksen-Leslie theory with numerical methods to analyze flow-induced skyrmion shape transitions and interactions.

Findings

Skyrmions stretch perpendicular to flow at high velocities.

A first-order transition separates weak and strong flow regimes.

Skyrmion interactions affect shape evolution.

Abstract

We investigate, numerically, the effects of externally imposed material flows on the structure and temporal evolution of liquid crystal skyrmions. The dynamics of a 2D system of skyrmions is modeled using the Ericksen-Leslie theory, which is based on two coupled equations, one for material flow and the other for the director field. As the time scales of the velocity and director fields differ by several orders of magnitude for realistic values of the system parameters, we have simplified the calculations by assuming that the velocity relaxes instantaneously when compared to the relaxation of the director field. Thus, we have used a finite-differences method known as artificial compressibility with adaptive time step to solve the velocity field and a fourth-order Runge-Kutta method for the director field. We characterized the skyrmion shape or configuration as a function of the time and the average velocity of the flow field. We found that for velocities above a certain threshold, the skyrmions stretch in the direction perpendicular to the flow, by contrast to the regime of weak flows where the skyrmions stretch along the streamlines of the flow field. These two regimes are separated by an abrupt (first-order) dynamical transition, which is robust with respect to e.g., the liquid crystal elastic anisotropy. Additionally, we have found how the presence of a second skyrmion affects the evolution of the shape of the skyrmions, by comparing the evolution of pairs of skyrmions to the evolution of a single-skyrmion.