Trapping microfluidic drops in wells of surface energy

International audienceA small hole etched in the top of a wide microchannel creates a well of surface energy for a confined drop. This produces an attractive force F ? equal to the energy gradient, which is estimated from geometric arguments. We use the drag Fd from an outer flow to probe the trapping mechanism. When Fd < F?, the drop deforms but remains anchored to the hole. Its shape provides information about the pressure field. At higher flow velocities, the drop detaches, defining a critical capillary number for which Fd=F?. The measured anchoring force agrees with the geometric model. © 2011 American Physical Society