Microstreaming effects on particle concentration in an ultrasonic standing wave

It is shown that the magnitude of Rayleigh microstreaming convective drag on microparticles in water in a 3.2‐MHz ultrasonic standing wave can be comparable to the lateral direct radiation force in the nodal plane (DRF 1 ) and can significantly influence the microparticle aggregation. The transducer of a single half‐wavelength chamber was excited to give a single particle aggregate. The estimated sound pressure amplitude was 0.5 MPa. Particle image velocimetry (PIV) measurements gave the average microstreaming velocity in the nodal plane as 450 μm·s −1 , which is comparable to the 340‐μm·s −1 value calculated from Rayleigh's theory. Movement of 25‐μm latex particles was primarily determined by DRF 1 , while that of smaller 1.0 μm, particles was determined by Rayleigh microstreaming. A 15‐μm latex particle velocity map, simulated from microstreaming data, the measured velocity map of 25‐μm particles, and the cube‐dependent relationship between DRF 1 's on particles of different sizes, was in reasonable agreement with a measured velocity map. Further evidence for the importance of microstreaming came from the result that velocities for 1‐ and 25‐μm particles were of similar magnitude, but were opposite in direction.