Leveraging microchannel cross-sectional geometry for acoustophoretic manipulation of submicron particles

SAW (surface acoustic wave)-based microfluidics is fast becoming a recognised method for isolating and concentrating particles given its capacity for safe and label-free particle manipulation. However, widespread adoption of acoustofluidics for clinical and industrial applications is hindered by its limited capability handling submicron particles. Smaller particles, which are primarily influenced by the acoustic streaming effect, can be captured and enriched by streaming-induced vortices. This study investigated the role of microchannel cross-sectional geometry on the streaming patterns in an acoustically actuated volume and the behaviour of particles in it, in an effort to address the size limitation. Different regimes of particle trapping behaviour were observed and identified, and its dynamics explained using the competing outward centrifugal and inward inertial lift forces. These observations led to a proposed model of particle behavior in a vortex. The study found sloped sidewalls intensify streaming flows and concentration effect. Additionally, the individual vortices were observed becoming more/less affinitive to particles of a certain size, i.e. particles of different sizes were observed settling in distinct streaming vortices. This type of enhanced size-selective capturing behaviour can enable enrichment and binary separation of submicron particle mixtures, with a greater yield and purity than conventional rectangular microchannels.