Swimming of Buoyant Bacteria in Quiescent Medium and Shear Flows

Gravity
has an unavoidable effect on all living organisms inhabiting
fluidic surroundings. To investigate the spatial distribution of bacteria
in quiescent fluids and their rheotactic behavior in shear flows under
buoyancy, we adjust the buoyant force to regulate bacterial swimming
in a microfluidic channel. It is found that swimming bacteria of Escherichia coli exhibit an obvious vertical separation
when exposed to a medium with high density and gradually gather close
to the up wall within minutes. The bacterial population presents a
net upward number flux, which enhances the trapping of motile bacteria
onto the up surface as a result of buoyancy force apart from the hydrodynamic
and kinematic interactions in quiescent fluids. When flow is imposed
into the channel, the buoyancy effect is however significantly suppressed.
Additionally, the drift velocity perpendicular to the buoyancy vector
as a result of chirality-induced transverse swimming decreases with
buoyancy force. However, this transverse drift capability is recovered
after excluding the intrinsic swimming motility in a quiescent medium.
Failing to escape from the trapping as a result of buoyant force allows
for a facile separation of bacteria along the vertical direction.
The findings also offer a controllable way to redisperse and homogenize
the bacteria distribution close to walls by imposing a weak shear
flow.