  Swimming microbes, such as bacteria and algae, live in diverse habitats
including soil, ocean and human body which are characterized by structural
boundaries and heterogeneous fluid flows. Although much progress has been made
in understanding the Brownian ratchet motions of microbes and their
hydrodynamic interactions with the wall over the last decades, the complex
interplay between the structural and fluidic environment and the
self-propelling microbial motions still remains elusive. Here, we developed a
Langevin model to simulate and investigate the transport and dispersion of
microbes in periodic pillar arrays. By tracing the spatial-temporal evolution
of microbial trajectories, we show that the no-slip pillar surface induces
local fluid shear which redirects microbial movements. In the vicinity of
pillars, looping trajectories and slowly moving speed lead to the transient
accumulation and the sluggish transport of microbes. Comprehensive microscopic
motions including the swinging, zigzag and adhesive motions are observed. In
the pillar array of asymmetric pillar arrangements, the adjacent downstream
pillars provide geometric guidance such that the microbial population has a
deterministic shift perpendicular to the flow direction. Moreover, effects of
the topology of the pillar array, fluid flowing properties and microbial
properties on the microbial advection and dispersion in pillar arrays are
quantitatively analyzed. These results highlight the importance of structures
and flows on the microbial transport and distribution which should be carefully
considered in the study of microbial processes.
