Droplet retention and release mechanisms in microfluidic trap geometries

The present work examines the fundamental hydrodynamic and interfacial mechanisms that govern the droplet retention and escape in the pillar-type microfluidic traps. Four different trap geometries—straight, convex, narrow-concave, and concave traps—were systematically designed to study the influence of the design curvature, trap orientation, and constriction on the trapping stability affected by flow and pressure. To investigate the effectiveness of droplet trapping, two-phase flow simulations are performed for a wide range of capillary numbers from 0.01 to 1.0, minimum gap parameter gmin = dh/a: 0.1–1.0; trap orientation θ: 10°–80°; confinement ratio Wc: 0.2–0.35. The droplet stability in the trap was studied based on non-dimensional numbers of flow and geometrical parameters. Results showed that the balance between the capillary force and the hydrodynamic pressure force dictates the droplet trapping success. In the concave traps, due to the symmetric pressure distribution and reduced axial pressure gradient across the droplet, trapping was observed for the entire tested range of capillary numbers (0.01–1.0) for gmin up to 0.7 and at different orientation angles where the convex trap fails beyond Ca > 0.4. Under certain geometric non-dimensional numbers. On the contrary, when the geometry-generated axial pressure gradients exceeded the Laplace pressure barrier, droplet escape was observed. For the concave trap design, the numerical predictions in terms of non-dimensional numbers showed a good match with the experimental data (R2=0.926). The results provide a quantitative framework to better understand the forces of the hydrodynamic pressure and capillary pressure in designing microfluidic geometric traps of two-phase systems.