Droplet formation in a T-shaped microfluidic junction

Abstract

Using a phase-field model to describe fluid/fluid interfacial dynamics and a lattice Boltzmann model to address hydrodynamics, two dimensional (2D) numerical simulations have been performed to understand the mechanisms of droplet formation in microfluidic T-juntion. Although 2D simulations may not capture underlying physics quantitatively, our findings will help to clarify controversial experimental observations and identify new physical mechanisms. We have systematically examined the influence of capillary number, flow rate ratio, viscosity ratio, and contact angle in the droplet generation process. We clearly observe that the transition from the squeezing regime to the dripping regime occurs at a critical capillary number of 0.018, which is independent of flow rate ratio, viscosity ratio, and contact angle. In the squeezing regime, the squeezing pressure plays a dominant role in the droplet breakup process, which arises when the emerging interface obstructs the main channel. The droplet size depends on both the capillary number and the flow rate ratio, but is independent of the viscosity ratio under completely hydrophobic wetting conditions. In the dripping regime, the droplet size will be significantly influenced by the viscosity ratio as well as the built-up squeezing pressure. When the capillary number increases, the droplet size becomes less dependent on the flow rate ratio. The contact angle also affects the droplet shape, size, and detachment point, especially at small capillary numbers. More hydrophobic wetting properties are expected to produce smaller droplets. Interestingly, the droplet size is dependent on the viscosity ratio in the squeezing regime for less hydrophobic wetting conditions

Physics of Multiphase Microflows and Microdroplets

“…It clearly shows the plug length depends only on Q. However, Liu and Zhang (2009) suggested that the droplet size also strongly depends on Ca in the squeezing regime, which is consistent with the experimental observations (e.g.…”

“…When Ca is increased to 0.032 and 0.056, the detachment point will move from the T-junction corner to the downstream as Q increases. In addition, the droplet detachment point gradually moves downstream until a stable jet is formed when Ca and Q increase, which was also observed both numerically (De Menech et al (2008); Liu and Zhang (2009)) and experimentally (Christopher et al (2008)). …”

“…However, for larger Q, increasing Ca will force the detachment point to move from the corner to the downstream. Liu and Zhang (2009) showed in Fig. 6 that when Ca is fixed at 0.006, varying Q from 1/8 to 1/2 does not change the detachment point of the droplet.…”

“…The figure is obtained from Liu and Zhang (2009) and copyright is awaited from Journal of Applied Physics.…”

Multiphase lattice Boltzmann simulations for porous media applications

“…The underlying physical properties of lattice Boltzmann schemes are determined via the hydrodynamic moments of the equilibrium distribution functions. The moments of the distribution functions should satisfy [86] (38) where P is the pressure tensor, and is a coefficient which controls the phase interface diffusion and is related to the mobility M of the fluid as follows [86,129],…”