Dynamics and breakup of deformable drops in constricted microchannels

Droplet transport through confined geometries is central to microfluidic technologies for cell sorting, emulsification, and microreactors. We study the motion of deformable droplets through constricted microchannels in pressure-driven flow at low Reynolds numbers using a 3D boundary integral method with a dynamically adaptive computational cell, allowing for efficient resolution of drop-wall hydrodynamic interactions. In channel geometries with obstructions, sharp corners fail to provide adequate lubrication, leading to droplet piercing. Thus, we replace sharp corners with smooth arcs. Simulations reveal a range of outcomes — including passing (small drops) or potentially trapping (large drops) without breakup at low capillary numbers (Ca) due to low viscous deformation, breakup due to significant stretching in the constriction or pinch-off during post-constriction retraction for medium to large drops at intermediate Ca, and again, passing without breakup at high Ca due to increased deformability. Experimental observations with aqueous glycerol droplets in castor oil are compared with numerical results to validate predicted outcomes. We also see that the transit time of a drop to travel through the constricted channel decreases with increasing Ca, increases with drop size at high drop-to-bulk fluid viscosity ratios, and decreases with drop size at low viscosity ratios. The results provide insight into drop breakup modes with implications for droplet manipulation in confined flows.