An unorthodox method to agitate small amounts of fluids in lab-on-a-chip

applications makes use of the acoustic streaming effect: An acoustic surface wave on a flat substrate generates a stationary streaming pattern in a fluid resting on the surface. The surface wave causes a sound wave in the fluid, which -- by the nonlinearities of the Navier--Stokes equation -- creates this stationary flow pattern. For the practical use of the method for, say, particle transport in the internal streaming, it is of essential importance to know the magnitude of the pressure gradients in comparison with viscous drag. The sound wave generates both of them -- however, the standard perturbation theory of the compressible Navier--Stokes equation does not yield the information on their respective strengths. By a completely different approach, namely by numerically analysing the deformation of free surfaces of droplets, we were able to answer

this question in favour of the pressure field, which is the dominant effect of particle transport close to the source of acoustic streaming. This observation helps understanding particle accumulation patterns observed in experiments.