  Particles such as microplastics and phytoplankton suspended in the water
column in the natural environment are often subject to the action of surface
gravity waves. By modelling such anisotropic particles as small spheroids that
slowly settle (or rise) in a wavy environment, we consider how the particle
shape and buoyancy couple to the background wave-driven flow to influence the
particle orientation, drift, and dispersion. A multiscale expansion allows the
wave-induced oscillations to be separated from the wave-averaged particle
motion. Using the wave-averaged equations of particle motion, we demonstrate
that spheroidal particles have a wave-induced preferential orientation with
different stable solutions for prolate and oblate particles. The resulting
preferential orientation positions particles with their longest axis pointing
in the direction of wave propagation and upwards against gravity. The angle at
which the longest axis points upwards is a function of particle aspect ratio.
In this orientation, particles drift in the direction opposite to wave
propagation, weakening and potentially even reversing their Stokes drift. The
wave-induced stable orientation also results in a reduced settling velocity
relative to a random (isotropic) orientation. The dispersion of a particle
cloud is controlled by the distribution of orientations. For a cloud of
particles released together with random (isotropic) orientation, the initial
cloud growth rate is ballistic in all directions. Wave action acts to suppress
the vertical dispersion, but enhances horizontal dispersion into a
super-ballistic state when the Stokes drift shear acts on a particle cloud that
has expanded in the vertical direction.
