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Contact Charge Electrophoresis: An Electrostatic Motor for Microfluidics and Active Matter Systems
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Cartier, Charles Antoine
Doctor of Philosophy
Date of Defense:
March 02, 2017
Kyle Bishop, Dissertation Advisor
Kyle Bishop, Committee Chair
Darrell Velegol, Committee Member
Scott Milner, Committee Member
Ayusman Sen, Outside Member
Contact Charge Electrophoresis (CCEP) refers to the rapid continuous oscillation of a conductive body (either solid particle or droplet) between electrodes in the presence of a DC electric field. This behavior has been observed to be an efficient, low power method to convert electrical energy into mechanical motion at small scales, effectively acting as a microscale motor. Despite the appeal of these advantages, CCEP has received relatively little attention in literature. There remain numerous open questions about the charging mechanism, only a handful of successful strategies for CCEP-based applications, and design challenges such as the spontaneous short circuiting of electrodes in multiparticle CCEP systems. One of the most recent developments, ratcheted CCEP, drives particle transport by incorporating a periodic array of insulating ramps on the electrode surface to redirect or ratchet the oscillatory motion of a particle undergoing CCEP downstream. This demonstration of coupling CCEP to additional mechanisms to drive particle or droplet transport has significant value to the fields of microfluidics and active matter. This dissertation presents systems which modify either the particle or encompassing geometry within a CCEP system to successfully design microfluidic devices and study multiparticle systems for active matter applications. Chapters 1 and 2 provide background on CCEP. Chapter 1 provides a brief historic perspective and the overarching motivation for the development of miniaturized devices. Detailed background is provided specifically for microfluidic and active matter systems with a focus on the sub-fields of microfluidic mixing, droplet microfluidics, and synchronization. Chapter 2 focuses on the relevant background for CCEP, covering the mechanism, physics, application strategies in literature, and observations of collective behavior in multiparticle systems. Chapters 3 and 4 focus on implementing ratcheted CCEP for microfluidic applications. In Chapter 3, ratcheted CCEP is used to develop a microfluidic mixer. Dielectric ramps are used to trap a particle in a small region of a channel preventing it from drifting downstream while the oscillations homogenize fluid flowing past. The effect of particle trajectory, particle size, fluid flow rate, and fluid properties are investigated and discussed for optimal mixing. Chapter 4 presents a microfluidic device containing an electrostatic droplet generator coupled with ratcheted geometry. An applied DC electric field generates and transports droplets through the device independent of bulk fluid flow. The effects of experimental parameters, such as system geometry, fluid properties, and voltage are used to validate scaling analyses for droplet diameter, generation frequency, and transport speed, and are ultimately used to find the optimal system performance. Appendix D and chapter 5 focus on modifying CCEP for the design and study of active matter systems. In appendix D, CCEP of heterogeneous particles (metallodielectric Janus particles) are shown to undergo directed motion between parallel plates perpendicular to the electric field in the absence of external biasing forces. Both experiment and model confirm that this translational motion is due to the rotation of Janus particles near the electrodes after charge transfer. Notably, when Janus particles oscillate near one another, their trajectories coordinate with one another as the two oscillators couple together. Chapter 5 expands on interactions between CCEP oscillators discussed in chapter 4 and appendix D by designing systems which physically constrain particle trajectories to parallel tracks. As the particles undergo CCEP they are observed to synchronize with one another and form traveling waves with the wavelength controlled by the system geometry. As traveling waves are commonly used as a propulsion mechanism at the microscale, chapter 5 explores appealing analogies between the coordinated motion of CCEP motors and artificial "muscle." Finally, Chapter 6 summarizes new questions and proposes applications motivated by this dissertation.
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