Pressure Drop of Confined Microfluidic Droplets

Microfluidics is the study of fluid manipulation on a micron scale. Microfluidic devices, or "lab-on-a-chip" technologies are used in many areas, from studying cell biology to enhancing chemical synthesis. Microfluidics has become a popular research tool in recent years due to the ease of fabrication, low cost for materials and reagents, and high surface area to volume ratios, which decrease residence times.  Microfluidic devices are commonly made using soft lithography, which requires specialty equipment such as a spin coater and mask aligner. When inadequate equipment is used or unintentional errors are made, they can result in features with non-rectangular channels. For some researchers, it is near impossible for them to create perfectly straight channel walls, due to their equipment limitations. Since microfluidics is a field that leaves no room for error, we want to show what effect these imperfections have on the accuracy of the device. We use a simple, well-established pressure measuring technique called a comparator to measure the pressure drop of a confined microfluidic droplet in a train. We compare the measurements made in devices with either a square cross section, or an intentionally irregular cross-section which we call a "trapezoid". Our results show that at low capillary number (Ca_o<0.03), pressure drop is the same in both square and trapezoid cross sections but at higher capillary numbers (Ca_o>0.03), pressure drop in the trapezoid channel is greater than that in the square channel. This supports our claim that minor variations in channel cross section can affect pressure drop. In the future, we hope to do more of these experiments on other devices made out of a mechanically rigid material in order to determine how much elastic deformation affects pressure drop measurements.