Sorting of solid and soft objects in vortices driven by surface acoustic waves

A challenging topic of the lab-on-a-chip research is to implement sorting mechanisms on low cost disposable chips. In many applications, surface acoustic waves (SAW) have recently proven to be a versatile and efficient technique for microfluidic actuation. A SAW is excited by applying a high frequency signal to a piezoelectric substrate. When the wave hits the solid/liquid interface it transmits its acoustic energy into the liquid and a local pressure gradient emerges, leading to surface acoustic streaming. Experiments can be performed directly on the piezoelectric substrate or on a separate glass slide positioned on top of the SAW source. We developed a technique for the accumulation of solid and soft objects in SAW generated microvortices in microfluidic channels. For this purpose, the corner of a rectangular microchannel is irradiated by a wide SAW beam. There, the SAW excites sound waves in the fluid producing a typical acoustic streaming flow pattern which typically exhibits two vortices. Particles injected into the flow are accumulated and dynamically trapped in one of these vortices. After the flow is stopped, the collected particles stay in the position of the vortex. In our experiments, we use open microfluidic channels with functionalized hydrophilic-hydrophobic surfaces on glass substrates as well as closed channels build with the elastomer PDMS via soft lithography. We find that the accumulation efficiency for particles is strongly size dependent. Below a critical radius of 500 nm, particles tend to flow through the vortex and are not captured in the corner. Generally, larger particles can be collected at more moderate SAW power levels compared to smaller particles. Therefore, by adjusting the SAW power level, one is able to collect particles above a designated size. This concept is not limited to solid particles but can also be applied to soft objects like cells