Motion of a microswimmer in a lattice of obstacles: Effect of thermal fluctuations

Microswimmers are often found in heterogeneous or crowded environments and show interesting transport behaviors like trapping or escaping in the presence of hydrodynamic interactions and thermal fluctuations. Studying such behaviors helps us to understand infection spreading and plant-microbial interactions and builds applications like targeted drug delivery. To explain the impact of thermal fluctuations, in terms of Péclet number and packing fraction, on the transport behavior of microswimmers in the presence of obstacles, we consider a rigid spherical squirmer swimming in an infinite matrix of rigid spheres arranged in a body-centered-cubic lattice structure with a multiparticle collision dynamics model for the background quiescent fluid. At a constant Péclet number, we observe ballistic or escape behavior for weak pusher, neutral, and puller types of microswimmers. A confined or trapped behavior is observed for strong pushers. The transition from trap to escape is more sensitive to the packing fraction in the case of low Péclet numbers compared to high Péclet numbers. At constant packing fraction, we observe an increase in the effective diffusion coefficient of the squirmers, which shows pure trap or escape behaviors, as the Péclet number increases. Interestingly, the superdiffusive behavior of strong pullers in the presence of obstacles gives the diffusion coefficient close to the squirmer in the absence of obstacles. For the squirmers transitioning between escape and trapped states, the diffusion coefficient increases when they are in the escape state and decreases when they are in the trapped state with an increase in the Péclet number.