Dynamic Response of Droplets to Single Wave Electrowetting Perturbation

• Author / Creator

  Marin Quintero, Juan S.

• Active control over the spreading of a liquid can be the avenue of endless opportunities. This spreading is dictated by the surface energy of a solid surface in which the wetting of a given liquid is quantified by the Young's contact angle. This quantification is the outcome of the interactions and the magnitudes of the interfacial energies at the solid-liquid, the solid-medium, and the drop-medium interfaces. Electrowetting-on-dielectric (EWOD) is a technique that can permit the control over spreading and its rate without permanently altering the surface energies of these three interfaces. This is the key aspect and the most important advantage of electrowetting. The new equilibrium configuration of the drop, as a response to EWOD by means of an externally applied electric field, can be predicted by the Young-Lippmann's equation. Although electrowetting has been widely studied, there is limited knowledge on the initial response of the drop, mainly after exerting different kinds of electric fields. During the implementation of the electric field a spreading behaviour is triggered, and this initial reaction of the drop to electrowetting was investigated thoroughly in this study. The transient evolution in the drop's shape was analyzed meticulously while the drop acquired the new equilibrium position. If the applied electrical actuation is oscillatory (AC), the number of cycles of this actuation dictated the equilibrium configuration and, in some cases, it deviates from the Young-Lippmann's equation prediction. Moreover, the system properties, i.e., the thermophysical properties of the droplet and the surrounding medium, along with the voltage and the frequency of the applied wave, completely altered the initial drop dynamics that eventually resulted in the corresponding equilibrium of the droplet. It is also observed that the first actuation cycle of AC EWOD had the greatest influence on the drop to achieve the maximum possible spreading. Hence, as an extension of this research, the transient dynamics of a drop submitted to a single electrokinetic excitation is scrutinized. The response time of the drop, defined as the time from the instant that the excitation is introduced until the maximum spreading reached during the single actuation cycle, is directly and inversely proportional to the viscosity of the liquids and the frequency of the actuation force, respectively. Based on the extensive parametric study, a unique definition of time scale is proposed to predict the time to reach the maximum spreading of AC electrowetting. This new time scale was based on the imposed actuation time by the frequency of the wave and on the viscous time scale modified by the electrowetting properties. The aftereffects of the actuation on the drop's motion, particularly at the drop-medium interface, are also investigated to find out the role of this induced perturbation on the equilibration process. The oscillatory motion at the drop-medium interface resembled an inertial capillarity response. An equivalent mass-spring-dampener model is also proposed to analyze these oscillations. Finally, with increased knowledge about the response of a single drop to electrowetting, a new technique for electrocoalescence is proposed. The in-house developed technique circumvented the repulsion at the point of merging between two drops, i.e., at the three-phase contact lines. The coalescence of sessile drops is characterized by the evolution of a film bridge between the drops that represents a mass transfer phenomenon. If a redefined time scale for maximum spreading is considered for the coalescence analysis, initial bridge growth suggested a universal growth irrespective of the system properties.

• Subjects / Keywords

  • Droplet oscillations
  • Electrocoalescence
  • Electrowetting
  • Droplet dynamics
  • Droplet spreading

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-c7a9-mm56

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.