Transient Analysis of a Droplet Approaching an Interface in Microfluidics
A lot of experiments and simulations have been performed by various research groups
in the area of collision of a liquid drop with liquid or solid surface, but most of the
studies have been performed for an open environment (external coalescence). In the
present work, we have numerically studied the transient dynamics of a droplet
coalescence with a liquid-liquid interface in a microfluidic device. This study will help
us in understanding the more complex problem of particle formation due to
polymerization in a microfluidic device. Microparticle generation has an important
application in drug delivery.
In this study, we modeled the coalescence of a liquid droplet with the liquid-liquid
interface in Comsol, which is set up in a microfluidic device. When the drop reaches
the interface, coalescence does not occur immediately. A thin layer of fluid exists
between drop and surface. The time taken to drain the fluid between interface and drop
and give way to the next stage is called residence time. The film thickness becomes so
thin that a hole is generated due to Van der Waals forces and after that rupture between
the pool of liquid and drop occurs. This is the first step of the coalescence process.
After that interface changes shapes, and finally, it gets stable, which is called the
complete coalescence process. We have used the two-phase laminar flow and assumed
immiscible, incompressible Newtonian fluids, neglecting inertia, and gravity (the flow
is horizontal). Phase-field method is used to study the interfacial motion and
coalescence of the droplet (Figure 1) in the two-phase flow. Initial conditions such as
flow rates, pressure, and wall conditions have been selected. We have used the 2D
axisymmetric plane for the geometry (Figure 2). In our study we investigated the
coalescence process and evaluated the effect of parameters such as viscosity of drop
and fluid, the diameter of the drop, flow rate of both the fluids and interfacial tension,
on the coalescence process. The observed coalescence phenomena are interpreted with
the help of dimensionless parameters such as Weber number (We), Reynolds number
(Re), Ohnesorge number(Oh) and Capillary number (Ca), and compared with existing
external droplet – interface coalescence works.
