Quantifying the Role of Transport by Acoustic Streaming in MHz Focused-Ultrasound-Based Surface Sampling

We have developed an ultrasound-based surface sampling method permitting surface studies for liquid immersed samples. The method employs high-intensity focused ultrasound, which can remove material from predetermined areas and induce acoustic streaming that causes the immersion liquid to flow. In this study, we studied several conditions of acoustic streaming, which can affect particle transport away from the sampled surface. First, we explored suitable acoustic streaming conditions by finite element modelling. Next, we measured the induced streaming fields by particle image velocimetry. This study comprised cases, when a high-intensity focused ultrasound beam encountered a solid surface at different transducer-surface distances. A change in direction of streaming occurred when a focusing transducer was moved from -2 lambda defocus to -4 lambda defocus (towards the surface). Thus, we found suitable conditions for an upwards directing acoustic streaming field. This kind of defocus condition can be coupled to the surface sampling process allowing efficient particle transport for subsequent chemical analysis.