Droplet formation in microfluidic T-junction generators operating in the transitional regime. I. Experimental observations

Abstract

This is the first part of a two-part study on the generation of droplets at a microfluidic T-junction operating in the transition regime where confinement of the droplet creates a large squeezing pressure that influences droplet formation. In this regime, the operation of the T-junction depends on the geometry of the intersection (height-to-width ratio, inlet width ratio), capillary number, flow ratio, and viscosity ratio of the two phases. Here in paper I we presented our experimental observations through the analysis of high-speed videos of the droplet formation process. Various parameters are tracked during the formation cycle such as the shape of the droplet (penetration depth and neck), interdroplet spacing, production rate, and flow of both phases across several T-junction designs and flow conditions. Generally, the formation process is defined by a two-stage model consisting of an initial filling stage followed by a necking stage. However, video evidence suggests the inclusion of a third stage, which we term the lag stage, at the beginning of the formation process that accounts for the retraction of the interface back into the injection channel after detachment. Based on the observations made in this paper, a model is developed to describe the formation process in paper II, which can be used to understand the design and operation of T-junction generators in the transition regime.

Droplet formation in microfluidic T-junction generators operating in the transitional regime. II. Modeling

“…Experimental observations showed that L lag is a complicated relationship between the flow conditions and geometry [1]. An adequate description is not available at the moment for L lag ; thus, in the present model, L lag is taken directly from experimental measurements and substituted into Eq.…”

“…The increase in size of the droplet during this stage is related to the time that the neck remains open and the flow rate of the dispersed phase. Only a portion of the continuous phase flow collapses the neck and this depends on this correlates with the fraction of the flow that is blocked by the interface [1].…”

“…Here the assumption is made that 1 − A * bp /h * represents the fraction of the continuous phase actually squeezing the neck and not bypassing it through the gap [1].…”

“…In the preceding paper we presented our experimental observations on the operation of T-junction generators in the transitional regime [1]. The T-junction configuration is one of the most versatile designs for generating droplets in microfluidic devices [2].…”

Effects of Baffle Configuration on Mixing in a T-Shaped Micro-Channel

“…One is to improve mixing in microchannels as in this paper [9]. Another important application is the generation of droplets and/or bubbles in two-phase systems [14][15][16].…”