  Microswimmers exhibit more diverse behavior in quasi-two dimensions than in
three dimensions. Such behavior remains elusive due to the analytical
difficulty of dealing with two parallel solid boundaries. The existence of
additional obstacles in quasi-two dimensional systems further complicates the
analysis. Combining experiments and hydrodynamic simulations, we investigate
how the spatial dimension affects the interactions between microswimmers and
obstacles. We fabricated microscopic pillars in quasi-two dimensions by etching
glass coverslips and observed bacterial swimming among the pillars. Bacteria
got trapped around the circular pillars and the trapping efficiency increased
as the quasi-two-dimensionality was increased or as the curvature of the
pillars was decreased. Numerical simulations of the simplest situation of a
confined squirmer showed anomalous increase of hydrodynamic attractions,
establishing that the enhanced interaction is a universal property of
quasi-two-dimensional microhydrodynamics. We also demonstrated that the local
curvature of the obstacle controls the trapping efficiency by experiments with
elliptic pillars.
