Investigation of actuated droplet motion on smooth and superhydrophobic surfaces
Abstract
Efficient droplet transport is critical in microassays, microfluidic devices, and a range of heat transfer applications. The main advantages of miniaturizing assays and bioanalytical tools include improved performance and speed, reduced cost, and the ability to perform parallel and integrated analysis. A careful study of the electrically (and gravitationally) actuated droplet motion on hydrophobic surfaces is essential to understanding and improving performance in these applications. The first part of the thesis focuses on understanding the physics of droplet motion under gravitational actuation. Investigation of droplet actuation and motion under the action of electrical forces is conducted in the remaining part of the thesis. The physics of droplet motion on a smooth surface before rolling off and at terminal velocity are studied under gravitational actuation. An experimentally validated model based on the Volume of Fluid - Continuous Surface Force (VOF-CSF) framework with varying contact angles along the triple contact line is developed to predict droplet statics and dynamics on an incline. The model is successfully used to predict critical inclination angle and the terminal velocity of the droplet beyond the critical inclination angle. The effect of contact angle models on the terminal velocity prediction is investigated. The physics of droplet motion, including the internal fluid motion, is explained in detail. The effect of electrowetting is incorporated into the VOF-CSF framework for droplets on smooth surfaces. The droplet motion is shown to originate from the contact line. Contact line friction is shown to be the dominant damping force. An approximate mathematical model is successfully developed to predict the overall contact line motion of the droplet. The numerical model is extended to include a treatment of superhydrophobic surfaces through geometrical modeling of the microstructured surface with full fidelity. The model accurately predicts the droplet shapes, apparent contact angle and the voltage required to induce Cassie-Wenzel transition on two different surface morphologies. The transient features of the Cassie-Wenzel transition are explained through the analysis of the transient surface energy and contact line lengths. The effective contact line friction coefficient on surfaces is predicted using the approximated mathematical model developed for smooth surfaces.
Degree
Ph.D.
Advisors
Murthy, Purdue University.
Subject Area
Mechanical engineering
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