  We calculate the acoustic radiation force from an ultrasound wave on a
compressible, spherical particle suspended in a viscous fluid. Using
Prandtl--Schlichting boundary-layer theory, we include the kinematic viscosity
of the solvent and derive an analytical expression for the resulting radiation
force, which is valid for any particle radius and boundary-layer thickness
provided that both of these length scales are much smaller than the wavelength
of the ultrasound wave (mm in water at MHz frequencies). The acoustophoretic
response of suspended microparticles is predicted and analyzed using parameter
values typically employed in microchannel acoustophoresis.
