Interactions between swimming cells and surfaces are essential to many
microbiological processes, from bacterial biofilm formation to human
fertilization. However, in spite of their fundamental importance, relatively
little is known about the physical mechanisms that govern the scattering of
flagellated or ciliated cells from solid surfaces. A more detailed
understanding of these interactions promises not only new biological insights
into structure and dynamics of flagella and cilia, but may also lead to new
microfluidic techniques for controlling cell motility and microbial locomotion,
with potential applications ranging from diagnostic tools to therapeutic
protein synthesis and photosynthetic biofuel production. Due to fundamental
differences in physiology and swimming strategies, it is an open question
whether microfluidic transport and rectification schemes that have recently
been demonstrated for pusher-type microswimmers such as bacteria and sperm
cells, can be transferred to puller-type algae and other motile eukaryotes, as
it is not known whether long-range hydrodynamic or short-range mechanical
forces dominate the surface interactions of these microorganisms. Here, using
high-speed microscopic imaging, we present direct experimental evidence that
the surface scattering of both mammalian sperm cells and unicellular green
algae is primarily governed by direct ciliary contact interactions. Building on
this insight, we predict and verify experimentally the existence of optimal
microfluidic ratchets that maximize rectification of initially uniform
Chlamydomonas reinhardtii suspensions. Since mechano-elastic properties of
cilia are conserved across eukaryotic species, we expect that our results apply
to a wide range of swimming microorganisms.Comment: Preprint as accepted for publication in PNAS, for published journal
version (open access) and Supporting Information see
http://dx.doi.org/10.1073/pnas.121054811
