Graduation Year

2016

Document Type

Thesis

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Nathan Crane, Ph.D.

Committee Member

Nathan Gallant, Ph.D.

Committee Member

Rasim Guldiken, Ph.D.

Committee Member

Julie Harmon, Ph.D.

Committee Member

Andrés Tejada-Martinez, Ph.D.

Keywords

EWOD, Fluid bearing, Surface tension, Micro actuator, Droplet force

Abstract

The goal of this work is to quantify the key design parameters such as the load capacity, actuation force, positioning repeatability, and reliability for droplet-based electrowetting actuators. Due to the fact that surface tension dominates gravity at both the mesoscale and microscale, droplet-based actuators can provide adequate force in manipulation tasks at those scales. Electrowetting, which uses an electric field to modulate the apparent surface tension of the liquid-ambient, provides a method to actuate droplets, which in turn transports the object carried by the droplet.

Most previous electrowetting actuation efforts have concentrated on manipulating droplets in a closed two-plate configuration. In these configurations, a voltage potential is applied between a series of electrodes. The droplets can merge, split, and mix with only a voltage input, and without any external machinery. While some mechanical actuation demonstrations have been done, limited studies have been performed to investigate the key actuation performance characteristics of droplet-based actuators carrying solid objects. Design criteria for using droplets to carry solid components are still not well defined.

The first part of this work provides fundamental understanding of the forces in electrowetting-based droplet actuation. The actuation force during electrowetting was experimentally validated according to the governing relation (Young–Lippmann equation) on a custom-designed testing apparatus. The results from the experiments show that the electrowetting actuation force is independent of surface tension below saturation, but the peak force is proportional to surface tension. Higher surface/interfacial tension would increase the actuation force in the horizontal direction, as well as the speed of the actuator.

The second part of the dissertation demonstrates two actuation configurations based on electrowetting. The first actuator uses a droplet to carry a solid object and can be actuated in discrete steps to function as a micro-stepping linear motor. By implementing a leaky dielectric coating, the droplet/substrate contact area acts as an electrical diode. By varying the duty cycle of a square waveform, a range of droplet/part equilibrium position combinations are established. The underlying actuation mechanism was investigated and the position versus duty cycle relation was shown to be symmetrical but non–linear around the center of the electrodes. In contrast to the conventional electrowetting control scheme, the proposed actuation method required no feedback control loop while achieving a repeatability of less than 0.8% of the droplet diameter. Positioning matched a theoretical model based on idealized electrical elements to within 2.5% of the droplet diameter.

The second type of electrowetting actuation uses metal-semiconductor diodes (Schottky diodes) in place of electrochemical diodes. This configuration uses only one pair of electrodes to actuate the droplet over a large distance (5X or more the droplet diameter). While the actuation concept had been previously demonstrated, the reliability of the diodes were shown to be insufficient. The new diodes actuated without degradation under repeated actuation (2000 cycles). Comparing this to electrochemical diodes, a 50% reduction in actuation voltage was also accomplished by Schottky diodes. The measured maximum speed also increased from 32 mm/s (electrochemical diodes) to 240 mm/s (Schottky), a 7.5 fold improvement.

The last part of this dissertation used numerical simulations to investigate the load bearing capability and the stiffness variation of droplet-based actuators. The vertical force and stiffness - which are the primary figure of merit in designing droplet-based actuators are quantified. Three types of loading conditions were analyzed using simulation software and a simple analytical equation is shown to provide a useful approximation of the droplet force and stiffness. The results were further used in various case studies to demonstrate the optimal design strategy when using an electrowetting driven droplet as a fluidic bearing.

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