ABSTRACT
A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets. The microfluidic device is used in its native state, which is hydrophilic, or treated with OTS to make it hydrophobic. Having both hydrophilic and hydrophobic surfaces allows for creation of both oil-in-water and water-in-oil emulsions, facilitating a large parameter study of viscosity ratios (droplet fluid/continuous fluid) ranging from 0.05 to 96 and flow rate ratios (droplet fluid/continuous fluid) ranging from 0.01 to 2 in one geometry. The hydrophilic chip provides a partially-wetting surface (contact angle less than 90°) for the inner fluid. This surface, combined with the unusually thin channel height, promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and a stable jet is formed. Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel. Non-wetting jets undergo breakup and produce drops when the jet is comparable to or smaller than the channel thickness. In contrast, partially-wetting jets undergo breakup only when they are much smaller than the channel thickness. Drop sizes are found to scale with a modified capillary number based on the total flow rate regardless of wetting behavior.
ABSTRACT
We have developed a novel flow chamber which imposes a controlled axisymmetric stagnation flow to enable the study of external flow effects on coalescence dynamics. This system allows for the first time the precise positioning of a drop in a three dimensional flow and additionally enforces a highly symmetric flow around the drop. We focus on the study of a single drop approaching a stationary flat plane as this is analogous to two drops approaching each other. A single drop is created and then guided along the unsteady center line of a stagnation flow. The real time computer control algorithm analyzes video images of the drop in two orthogonal planes and manipulates flow restricting valves along the four outlets of the flow. We demonstrate using particle image velocimetry that the computer control not only controls the drop position but also ensures a symmetric flow inside the flow chamber. This chamber will enable a detailed investigation of the drainage of the thin film between the drop and the lower surface in order to probe the effect of external flow on coalescence.
ABSTRACT
The dynamics of polymeric liquids and mixtures spreading on a solid surface have been investigated on completely wetting and partially wetting surfaces. Drops were formed by pushing the test liquid through a hole in the underside of the substrate, and the drop profiles were monitored as the liquid wet the surface. Silicon surfaces coated with diphenyldichlorosilane (DPDCS) and octadecyltrichlorosilane (OTS) were used as wetting and partial wetting surfaces, respectively, for the fluids we investigated. The response under complete and partial wetting conditions for a series of polypropylene glycols (PPG) with different molecular weights and the same surface tension could be collapsed onto a single curve when scaling time based on the fluid viscosity, the liquid-vapor surface tension, and the radius of a spherical drop with equivalent volume. A poly(ethylene glycol) (PEG300) and a series of poly(ethylene oxide-rand-propylene oxide) copolymers did not show the same viscosity scaling when spread on the partially wetting surface. A combined model incorporating hydrodynamic and molecular-kinetic wetting models adequately described the complete wetting results. The assumptions in the hydrodynamic model, however, were not valid under the partial wetting conditions in our work, and the molecular-kinetic model was chosen to describe our results. The friction coefficient used in the molecular-kinetic model exhibited a nonlinear dependence with viscosity for the copolymers, indicating a more complex relationship between the friction coefficient and the fluid viscosity.
