Confinement Effects on Drop Coalescence: An Experimental Study Using Hele-Shaw Cell

When an initial contact occurs between two drops, a liquid bridge is formed between them and grows rapidly. Although the evolution of the liquid bridge is driven by surface tension during the coalescence process, there exist regimes dominated by viscous or inertial force, which are identified by scaling relations of the temporal growth of the bridge. These scaling relations have been obtained by studying unconfined drops (i.e., spherical drops) or partially confined drops (i.e., drops resting on a plane). Considering drop coalescence can happen in a narrow confinement, we experimentally investigated drop coalescence in Hele-Shaw cell devices, which were formed by two parallel hydrophobic surfaces with controllable spacing. The growing bridge of drops coalescing in the Hele-Shaw cell was captured using high-speed video microscopy, and the temporal evolution of the bridge width was measured by using image processing. Obtained scaling relations of the bridge width showed noticeable differences from the unconfined and partially confined cases, which is expected to contribute to unveiling effects of the confinement on drop coalescence.