Trapping microfluidic drops in wells of surface energy

A 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 _γ equal to the energy gradient, which is estimated from geometric arguments. We use the drag F_d from an outer flow to probe the trapping mechanism. When F_d<F_γ, 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 F_d=F_γ. The measured anchoring force agrees with the geometric model.