From rectangular to diamond shape: on the three-dimensional and size-dependent transformation of patterns formed by single particles trapped in microfluidic acoustic tweezers.

Generally, the pattern formed by individual particles trapped inside a microfluidic chamber by a two-dimensional standing acoustic wave field has been considered only the result of the acoustic radiation force. Previous studies showed that particles can be trapped at the local minima and maxima of the first-order pressure and velocity fields. Thus, either a rectangular or a diamond pattern can be formed solely depending on the particle size, when the acoustic field is unchanged, and the material properties of the particles and the fluid are fixed. In this paper, we report about the co-existence of different patterns with particles of the same size. The actual shape of the patterns depends mainly on the ratio between particle diameter and wavelength. In addition, particles were found to be trapped at locations that coincide with the position of antinodes, even though the particles have a positive acoustic contrast factor. These phenomena imply that the trapping of individual particles cannot be described by the acoustic radiation force solely. Hence, further research is required, taking the viscous drag force caused by the fluid flow induced by the acoustic streaming effect into account.