Monitoring microfluidic interfacial flows using impedance spectroscopy

Microfluidic platforms capable of complex on-chip processing and liquid handling enable a wide variety of sensing, cellular, and material-related applications across a spectrum of disciplines in engineering and biology. However, there is a general lack of available microscale sensors capable of non-optically monitoring and quantifying on-chip fluid motion. Hence, many microfluidic systems are confined to the laboratory because their use requires optical microscopy. Here, we present a method for dynamically tracking laminar interfacial flows in microfluidic channels non-optically using impedance spectroscopy. Using a microfluidic T-channel, we generate a liquid interface by co-flowing two different electrolyte streams side-by-side. The interface is driven through an array of “displacement” electrodes where it is electrokinetically deflected across the microchannel. The interfacial flow is monitored downstream using an array of interdigitated “impedance” electrodes which dynamically measure the electrochemical impedance near the surface of the flow channel. We demonstrate that the impedance spectrum is sensitively influenced by the position of the deflected interface. While laminar fluid interfaces are ubiquitous to microfluidic flows and used extensively in rheology and biomolecular detection, it is currently difficult to measure their position non-optically. The sensing method presented here enables the interfacial position to be dynamically determined without a microscope and provides a new tool for lowering the barriers to operating microfluidic devices outside the confines of a traditional laboratory.