Topological valley phononic crystals in surface acoustic wave microfluidics

Recent years have witnessed the surge of topological wave phenomena as a versatile platform to engineer exotic wave energy transport which is robust to defects and disorders. Most demonstrations in acoustics remain in a single phase of matter such as solid or air. Here, we introduce the realization of valley phononic crystals for surface acoustic waves and their interaction with fluids in an acoustofluidic setup. It is shown that the interplay between megahertz elastic waves and hydrodynamics where two phases of materials are involved offers rich physics and new engineering potentials of topological matter. By electroplating hexagonal copper pillars on a lithium niobate substrate and adding a liquid layer on top of it, the excited elastic valley spin is transferred at the interface of solid-fluid domains. The interactions lead to valley streaming vortices in the fluid domain that support backward-immune particle transport. In addition, it is found that pressure wells are formed around the small pillars, which enable the concentration of DNA molecules in the nm size range. The studies may open new avenues for applying topological acoustic waves in particle manipulation and life sciences.