Focused Liquid Pinching in Coaxial Drop Capsule Generation

The pinching dynamics of an inviscid inner drop in a coaxial pendant drop structure have been investigated here both experimentally and numerically. The thinning rate of the inner drop, when it pinches in an inertial regime in this configuration, is found to be faster than that of a single drop pinching in a liquid medium. This is attributed to a focusing of the flow between the inner and outer drop interfaces induced by the contraction of the outer drop during its own pinching process. Our results reveal that this focusing effect increases dramatically when the ratio of the inner to outer nozzle radii, R̃, in a coaxial nozzle configuration exceeds 0.67. Beyond this value, the thinning rate becomes dependent on the outer nozzle size. Furthermore, the difference between the minimum neck radii of the inner and outer drop, denoted as Δh_min, serves as a reliable predictor of the inner drop's thinning rate, even for different outer liquid viscosities, and helps identify the conditions under which the satellite droplets form. Satellite droplets are observed when the outer drop squeezes the inner filament above its minimum neck location, which occurs when Δh_min ranges between 0.3% and 1% of the capillary length.