An Empirically Validated Analytical Model of Droplet Dynamics in Electrowetting on Dielectric Devices

Abstract

Explicit analytical models that describe the capillary force on confined droplets actuated in electrowetting on dielectric devices and the reduction in that force by contact angle hysteresis as a function of the three-dimensional shape of the droplet interface are presented. These models are used to develop an analytical model for the transient position and velocity of the droplet. An order of magnitude analysis showed that droplet motion could be modeled using the driving capillary force opposed by contact angle hysteresis, wall shear, and contact line friction. Droplet dynamics were found to be a function of gap height, droplet radius, surface tension, fluid density, the initial and deformed contact angles, contact angle hysteresis, and friction coefficients pertaining to viscous wall friction and contact line friction. The first four parameters describe the device geometry and fluid properties; the remaining parameters were determined experimentally. Images of the droplet during motion were used to determine the evolution of the shape, position, and velocity of the droplet with time. Comparisons between the measured and predicted results show that the proposed model provides good accuracy over a range of practical voltages and droplet aspect ratios.

Electrowetting Effect: Theory, Modeling, and Applications

“…It is also useful for defining the curvature of a fluid in a lab on a chip device with a top-plate. In this case, the left and right surface may have a different curvature during actuation (50).…”

Digital Microfluidic System with Vertical Functionality

“…Liquid droplets flowing through a capillary tube will experience resistive forces that arise from contact angle hysteresis, contact line friction, and viscous shear forces from the wall and the surrounding medium [57]. The correct mechanisms and modeling for these drag forces in both static and dynamic conditions are not fully understood or agreed upon [8].…”

“…In this scenario, the droplet has been positioned over the channel via electrode actuation, and as the droplet comes to rest, the velocity, and therefore the dynamic viscosity, becomes zero. Second, empirical data quantifying the viscous drag forces have been found to be from one to several orders of magnitude smaller than both the contact angle hysteresis effect and the driving capillary forces during actuation [57]. Once the capillary forces are large enough to drive the droplet into the channel, and the velocity is greater than zero, the droplet immediately comes in contact with the hydrophilic glass walls and generates a capillary force far greater than the viscous drag forces that arise during the dynamic episode of droplet insertion.…”

Optimization and limit of a tilt manipulation stage based on the electrowetting-on-dielectric principle

“…[6][7][8] Among these methods, EWOD has become a popular topic and a useful technology in academic research worldwide relative to other driving forces or control methods due to special advantages, such as straightforward fabrication, low cost, compatibility with conductive or polar fluids, and convenient programmable control. The effect changes the electron distribution in droplets, and the force of static electricity changes the contact angle of droplets, enabling diverse applications, such as micro-valves, 9 focal lenses, [10][11][12][13] fibers, 14,15 screens, 16,17 transport, [18][19][20][21][22][23][24] printing, 25 transistors, 26 electrical switches, 27,28 thermal control 29,30 and thermal management. 31 An increasing number of studies have applied the advantage of EWOD to be amplified in micro-systems, particularly for micro-optical devices, because droplets can adjust the focal length by changing the radius of curvature, using the properties of droplet flexibility with an applied voltage.…”