Fluid Dynamic Study of Bubble Entrapment during Encapsulation
In the context of cell encapsulation, spherical and smooth capsules are desired for optimum bio-performance. However, morphological defects can form during the encapsulation process. One important defect is associated with bubble entrapment on the capsule surface. The present study systematically examines this phenomenon, in the physical plane, through experimental fluid dynamic approaches. In the first instance, the classical problem of bubble entrapment during the impact of an inviscid drop onto a pool of the same liquid has been extended to the low-viscosity regime, where viscous damping effects have been studied. Bubble entrapment here is initiated during crater collapse and depends on the timely arrival of a capillary ripple at the crater bottom. The entrapment boundaries have been delineated in terms of the impact Weber and Froude numbers. The peak size of the entrapped bubble decreases with viscosity, and this effect is well-captured by the impact capillary number. The measured critical crater cone angle weakly increases with the capillary number. In the second instance, bubble entrapment during the impact of a viscous drop onto an inviscid pool has been examined. Here, there is no capillary wave involvement, but bubble entrapment still occurs, during crater shape recovery, due to bulk flow focusing. In the third instance, the phenomenon of bubble entrapment during the impact of a viscous reacting anion drop into an inviscid cation pool, as in encapsulation, has been examined. Here, entrapment occurs over a wide parameter range with no upper Weber number limit. Following impact, the drop shape is quickly frozen by the gelling reaction and the crater bottom is pinned to it. During crater rebound, bubble entrapment is caused by focusing of the bulk flow above the drop. For moderate impact Weber numbers, the focusing flow is resisted by capillary effects, and this is seen in the bubble size, which increases with Weber number, eventually reaching a plateau. The transition point coincides with singularity conical flow developing around the impact crater. A simple one dimensional dynamic model has been adapted to examine the free surface dynamics of the bubble pinching process and the predictions are consistent with experimental observations.