Vortex Dynamics During Pinch-off of Micro-Droplets

TL;DR

This study investigates vortex formation during micro-droplet pinch-off in a microfluidic device, revealing flow structures, stresses, and proposing scaling laws through experimental visualization with PIV.

Contribution

It introduces new insights into vortex dynamics during droplet pinch-off and develops scaling laws for internal vortical flow in microfluidic droplets.

Findings

Vortices form in droplets during pinch-off due to rapid fluid acceleration.

Scaling laws predict maximum circulation within droplets post pinch-off.

Stress fields are characterized during the transient vortex formation.

Abstract

Micro droplets are extensively used in chemical, biological, and medical research, primarily for conducting various tests on samples, including living organisms, using a microfluidic framework. Recent studies have shown that the physiology of bacteria can be significantly altered when subjected to shear and/or extensional stresses. With this motivation, we perform experiments to understand the vortex dynamics involved during the pinch-off process in a cross flow droplet generator, using particle image velocimetry (PIV) to visualize the vortical structures and to quantitatively measure the associated stresses developed inside droplets. The process of pinching off inherently leads to bi-directional acceleration of fluid in the rapidly thinning capillary bridge, resulting in a vortex in the separated droplet as well as in the retracting ligament. We propose scaling laws for the vortical flow inside the droplet post pinch-off and predict the maximum circulation production inside droplet. Further, we discuss the vortex dynamics inside the droplet, the retracting ligament and the advancing ligament and examine the stress fields associated with this transient phenomenon.