Drop Deformation and Burst in Two-Dimensional Flows

Citation

Bentley, Barry Jerome (1985) Drop Deformation and Burst in Two-Dimensional Flows. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2axg-ce38. https://resolver.caltech.edu/CaltechETD:etd-04112008-112307

Abstract

The deformation and burst of small fluid droplets suspended in a second immiscible fluid undergoing a steady linear shearing motion are discussed. The effects of Capillary number, ratio of fluid viscosities, and flow type are considered both experimentally and theoretically.

The experiments are unique in that a spectrum of strong two-dimensional flows (those with the magnitude of the strain rate exceeding that of the vorticity) was considered. These flows were generated in a four roll mill specifically designed for the experiments. Previous investigations had been limited to one specific strong flow owing to the difficulty in holding drops stationary at the stagnation point in such flows. We overcame this obstacle by using a computer interfaced to a digital video camera to locate the drops in the flow field and adjust the roller speeds to effect an inferential feedback control scheme. It is believed that the control system implemented for the present experiments could be adapted to a variety of fluid flow experiments with similar control problems.

Drop deformation and burst experiments were performed for viscosity ratios ranging from 0.001 to 27., and flows with ratio of vorticity to strain rate ranging from zero to 0.667. In a typical experiment a drop went through a succession of increasingly deformed steady shapes as the Capillary number was slowly increased with the flow type constant. The appearance of the drop was recorded photographically. In most cases, a Capillary number was reached where no steady shape was possible, and this was recorded as the critical Capillary number for drop burst. In a few cases with high viscosity ratio and large vorticity to strain rate ratio, drop burst was impossible. In cases where burst occurred, the transient motion of the drops at the critical Capillary number was observed. The drops continued to deform, but did not break into fragments until the flow was turned off. They then either fragmented or returned to the spherical shape through a complex interfacial tension driven motion.

The experimental deformation and burst observations were compared to the predictions of several available theories. Separate theories apply to cases where the deformation is small (nearly spherical drops) or large (threadlike drops). Comparisons to existing numerical results for one particular viscosity ratio are also included. Agreement between the experimental observations and the predictions of the theories was very good.

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