Drop motion through a confining orifice

The motion of gravity-driven deformable drops (Bond number, Bo ∼0:8-11) through a circular confining orifice (ratio of orifice diameter to drop diameter, d/D < 1) was studied using high-speed imaging. Drops of water/glycerin, surrounded by silicone oil, fall toward and encounter the orifice plate after reaching terminal speed. The effects of surface wettability were investigated for both round-edged and sharp-edged orifices. For the round-edged case, a thin film of surrounding oil prevents the drop fluid from contacting the orifice surface, such that the flow outcomes of the drops are independent of surface wettability. For d/D < 0.8, the boundary between drop capture and release depends on a modified Bond number relating drop gravitational time scale to orifice surface tension time scale and is independent of viscosity ratio. Drops that release break into multiple fragments for larger Bo and smaller d/D. For the sharp-edged case, a contact is initiated at the orifice edge immediately upon impact, such that surface wettability influences the drop outcome. When the surface is hydrophobic, the contact line motion through the orifice enhances penetration of the drop fluid, but the trailing interface becomes pinned at the orifice edge, inhibiting drop release. When the surface is hydrophilic, a fraction of the drop fluid is always captured because the drop fluid spreads on both the upper and lower plate surfaces.