  We consider the dynamics of monodisperse bubbly fluid confined by two plane
solid walls and subjected to small-amplitude high-frequency transversal
oscillations. The frequency these oscillations is assumed to be high in
comparison with typical relaxation times for a single bubble, but comparable
with the eigenfrequency of volume oscillations. A time-averaged description
accounting for mutual coupling of the phases and the diffusivity of bubbles is
applied. We find nonuniform steady states with the liquid quiescent on average.
At relatively low frequencies accumulation of bubbles either at the walls or in
planes oriented parallel to the walls is detected. These one-dimensional states
are shown to be unstable. At relatively high frequencies the bubbles accumulate
at the central plane and the solution is stable.
