  Eukaryotic cell motility is crucial during development, wound healing, the
immune response, and cancer metastasis. Some eukaryotic cells can swim, but
cells more commonly adhere to and crawl along the extracellular matrix. We
study the relationship between hydrodynamics and adhesion that describe whether
a cell is swimming, crawling, or combining these motions. Our simple model of a
cell, based on the three-sphere swimmer, is capable of both swimming and
crawling. As cell-matrix adhesion strength increases, the influence of
hydrodynamics on migration diminish. Cells with significant adhesion can crawl
with speeds much larger than their nonadherent, swimming counterparts. We
predict that, while most eukaryotic cells are in the strong-adhesion limit,
increasing environment viscosity or decreasing cell-matrix adhesion could lead
to significant hydrodynamic effects even in crawling cells. Signatures of
hydrodynamic effects include dependence of cell speed on the medium viscosity
or the presence of a nearby substrate and the presence of interactions between
noncontacting cells. These signatures will be suppressed at large adhesion
strengths, but even strongly adherent cells will generate relevant fluid flows
that will advect nearby passive particles and swimmers.
