We present a theory for the acoustic force density acting on inhomogeneous
fluids in acoustic fields on time scales that are slow compared to the acoustic
oscillation period. The acoustic force density depends on gradients in the
density and compressibility of the fluid. For microfluidic systems, the theory
predicts a relocation of the inhomogeneities into stable field-dependent
configurations, which are qualitatively different from the horizontally layered
configurations due to gravity. Experimental validation is obtained by confocal
imaging of aqueous solutions in a glass-silicon microchip.Comment: RevTex, 5 pages, 3 eps figure
