Fluid Drop Coalescence in a Hele-Shaw Cell

A fluid drop in a Hele-Shaw cell moves due to surface tension driven potential flow. Using equations for the pressure and the Green’s function for the Laplace Equation, we can formulate an integral equation that determines the motion of the boundary of the drop. By discretizing the boundary contour and following the motion of boundary nodes, the time evolution of the drop can be determined from initial conditions. Results of a numerical simulation show the movement of a drop relaxing from coalescence and the motion of a drop undergoing electrowetting

Prediction of Organic Droplet Behavior on a Solid Surface as Influenced by Aqueous Surfactant Solutions

This dissertation presents a model capable of predicting equilibrium oil droplet contact angles on a solid surface immersed in surfactant solution, a thorough discussion of the effects of surfactant concentration and salt addition on contact angles, and an experimental investigation into the impact of voltage application to the solid surface on oil droplet shape in an aqueous/organic/solid system. The work contained in this dissertation resulted in five journal articles and numerous presentations. The model applies current theories of surfactant self-assembly, the quasi-chemical approximation for solid surface adsorption, and various aqueous/organic/solid system properties to determine organic droplet contact angles. The computational methodology employed by the model requires the description of the aqueous/organic/solid system by selected component balances and through numerical techniques determines the equilibrium component distribution and the organic droplet contact angle for the specific system. Results from the model are compared to experimental contact angle data for various surfactants, surfactant concentrations, salt concentrations, and surface materials. The investigation into the effects of low magnitude applied voltage on droplet phenomena and oil removal determined that significant changes in droplet shape and removal efficiency can occur for voltages between ±3.0 volts. These changes in droplet shape where then compared to observed improvements in ultrasonic oil removal from metal surfaces in aqueous solutions. Employing the theoretical understanding of aqueous/organic/solid systems a discussion of controlling phenomena and mechanisms was presented. I have shown that (1) organic droplet contact angles on solid surfaces in aqueous/organic/solid systems are significantly affected by aqueous/ solid interfacial surfactant aggregation, (2) this impact is due to changes in the structure of the surfactant aggregate itself, (3) these changes are heavily impacted by surfactant concentration and the addition of low concentration salt to the aqueous surfactant solution, (4) the type of salt added to the solution is of greater relevance than indicated in the existing literature, and (5) that the application of low voltage applied potentials can significantly effect droplet shape and oil removal efficiency in an aqueous/oil/solid system

Droplet actuation on various electrode shapes in electrowetting-based microfluidics

Electrowetting has been widely used in digital microfluidics as a reliable actuation method due to its considerable advantages such as the absence of heat generation, rapid switching response, flexibility, and low power consumption. Research on the improvement of this method has turned into one of the attractive fields in the design of fluid handling micro devices. This work presents a combined numerical and experimental study to investigate the effect of electrode shape in electrowetting-based microsystems. A new crescent-shaped electrode is proposed which provides a uniform actuation force at the contact line as well as overlap between the adjacent electrodes. The onset of actuation and droplet mobility on electrode arrays have been investigated. The numerical method is based on the Volume of Fluid technique to track the 3-D interface along with the Laplace equation solver to calculate the electric field in the domain. Furthermore, the dynamic behaviour of tri-phase contact line is modeled using the molecular-kinetic theory. Validation experiments were carried out to characterize the droplet actuation on various electrode shapes. The superior performance of the new electrode shape is demonstrated by comparing the droplet velocity and deformation, as well as contact angle distribution with those of a simple flat electrode. Using crescent electrode, the droplet actuation occurs with less deformation and higher velocities. The velocities obtained by using the crescent electrode are up to 4 times at the onset of the actuation and 2 times at the steady state motion of those on the flat electrode. The novel crescent shape can even actuate the droplets which are not placed on the electrod