Multiparticle collision dynamics simulations of a squirmer in a nematic fluid

Abstract

We study the dynamics of a squirmer in a nematic liquid crystal using the multiparticle collision dynamics (MPCD) method. A recently developed nematic MPCD method [Phys. Rev. E 99, 063319 (2019)] which employs a tensor order parameter to describe the spatial and temporal variations of the nematic order is used to simulate the suspending anisotropic fluid. Considering both nematodynamic effects (anisotropic viscosity and elasticity) and thermal fluctuations, in the present study, we couple the nematic MPCD algorithm with a molecular dynamics (MD) scheme for the squirmer. A unique feature of the proposed method is that the nematic order, the fluid, and the squirmer are all represented in a particle-based framework. To test the applicability of this nematic MPCD-MD method, we simulate the dynamics of a spherical squirmer with homeotropic surface anchoring conditions in a bulk domain. The importance of anisotropic viscosity and elasticity on the squirmer’s speed and orientation is studied for different values of self-propulsion strength and squirmer type (pusher, puller or neutral). In sharp contrast to Newtonian fluids, the speed of the squirmer in a nematic fluid depends on the squirmer type. Interestingly, the speed of a strong pusher is smaller in the nematic fluid than for the Newtonian case. The orientational dynamics of the squirmer in the nematic fluid also shows a non-trivial dependence on the squirmer type. Our results compare well with existing experimental and numerical data. The full particle-based framework could be easily extended to model the dynamics of multiple squirmers in anisotropic fluids. Graphic abstract

Anisotropic swimming and reorientation of an undulatory microswimmer in liquid-crystalline polymers

“…Several studies (Lintuvuori et al. 2017; Daddi-Moussa-Ider & Menzel 2018; Mandal & Mazza 2021) have found that pusher-type particles with local extensile flow generation tend to align with the director while puller-type particles with contractile flows will swim perpendicular to the director. Indeed, besides the steady-state ‘parallel gait’ discussed above, Lin et al.…”

“…(3.12) Note that such anisotropic swimming behaviours are similar to those of a squirmer, a coarse-grained micromechanical model of spherical active particles with specified slip velocity conditions on the surface (Blake 1971), in nematic fluids. Several studies (Lintuvuori et al 2017;Daddi-Moussa-Ider & Menzel 2018;Mandal & Mazza 2021) have found that pusher-type particles with local extensile flow generation tend to align with the director while puller-type particles with contractile flows will swim perpendicular to the director. Indeed, besides the steady-state 'parallel gait' discussed above, Lin et al (2021) reported a weak contractile flow around an undulatory swimmer that is initially aligned perpendicular to the director.…”

Mesoscopic simulations of active nematics

“…Alternative hybrid finite difference/MPCD approaches have been proposed to simulate nematic liquid crystals (50,51). Nematic MPCD has been used to study nematohydrodynamic fluctuations and correlations (52,53), electroconvection (54), defects around nanocolloids (49,55), and living liquid crystals (56). We now extend the nematic Andersen-thermostatted collision operator to simulate wet active nematics.…”

Mesoscopic Simulations of Active-Nematics

“…Alternative hybrid finite difference/MPCD approaches have been proposed to apply MPCD to nematic liquid crystals [67,68]. Nematic MPCD has been employed to study nematohydrodynamic fluctuations and correlations [69,70], electroconvection [71], defects around nanocolloids [66,72,73] and living liquid crystals [74]. We now extend the nematic Andersen-thermostatted collision operator to simulate wet active nematics.…”