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.