Surface acoustic wave activated droplet sorting in microfluidics using absorbance.*

Microfluidic sorting is a powerful high-throughput screening tool in biotechnologies, including directed evolution, drug screening and single cell analysis. Drops can be sorted sorted in many ways, such as electric or magnetic fields. We use interdigitated transducers to create acoustic streaming in microfluidic channels to deflect drops. The generated travelling surface acoustic wave changes the flow streaming, and sorts subpopulations. Populations can be sorted independent of an object's properties and responsive particles, or dielectric components. The decision to select a subpopulation can be based on various criteria. Fluorescent detection is widely used, yet it requires labelling. Here we provide a label free technique using absorbance. Absorbance detection is often used to analyse biological samples, provides quantitative data and is particularly useful to sort drops. Based on Beer-Lamberts law, monodisperse drops have a fixed optical pathlength to measure absorbance. We have integrated label free absorbance detection and acoustically activated drop deflection with kHz sorting rates.