Controlled Oil Entrapment through Photo-Patterned Obstacles

There is a growing interest in trapping liquid inside a microchannel to create isolated nanolitre chambers, much like a multi-well plate system, for applications in diagnostics, drug discovery, reaction engineering and material synthesis. The compartmentalization of the liquid chambers at small length scale enables large number of parallel experiments in a high concentration environment. In this work, we develop a novel method to photopattern obstacles with controlled size, shape and wetting properties. We demonstrate oil entrapment around oleophilic obstacles by sequentially injecting oil and water in a microfluidic channel. We present a simple geometrical approach to explain the experimentally observed evolution of water/oil interface as it flows over the obstacle. Our model provides an analytic solution of the interface evolution, and successfully predicts the dependence of pinching off position and the amount of trapped oil with the obstacle contact angle. We also show that our model can be extended to two posts where we are able to successfully predict the critical distance for the formation of a liquid bridge. We further demonstrate effects such as interface pinning by using obstacles of different shapes.