Electromechanical model for actuating liquids in a two-plate droplet microfluidic device

Abstract

Both conducting and insulating liquids can be actuated in two-plate droplet ("digital") microfluidic devices. Droplet movement is accomplished by applying a voltage across electrodes patterned beneath the dielectric-coated top and bottom plates. This report presents a general electromechanical model for calculating the forces on insulating and conducting liquids in two-plate devices. The devices are modeled as an equivalent circuit in which the dielectric layers and ambient medium (air or oil) are described as capacitors, while the liquid being actuated is described as a resistor and capacitor in parallel. The experimental variables are the thickness and dielectric constant of each layer in the device, the gap between plates, the applied voltage and frequency, and the conductivity of the liquid. The model has been used to calculate the total force acting on droplets of liquids that have been studied experimentally, and to explain the relative ease with which liquids of different conductivities can be actuated. The contributions of the electrowetting (EW) and dielectrophoretic (DEP) forces to droplet actuation have also been calculated. While for conductive liquids the EW force dominates, for dielectric liquids, both DEP and EW contribute, and the DEP force may dominate. The general utility of the model is that it can be used to predict the operating conditions needed to actuate particular liquids in devices of known geometry, and to optimize the design and operating conditions to enable movement of virtually any liquid.

Digital Microfluidic System with Vertical Functionality

“…It is important to note that the addition of multiple droplets is a practical method for increasing the pressure differential to a magnitude capable of driving droplet insertion. If droplet insertion does not routinely enter the final phase, a more efficient design may make use of proper dimensioning given by Equation (11). Qualitatively, our experiments using channels with 1.1, 0.74, and 0.45 mm channel diameters behaved as expected.…”

“…Solving for the height of the spherical cap as a function of the volume requires solving a cubic equation and is not trivial. Therefore, the most straightforward tool for checking this design parameter involves first solving for the minimum channel radius via Equation (11). Upon choosing a channel radius greater than this minimum, one can check the height of the maximum spherical cap via Equation (12).…”

“…To move the droplets, an electrical potential is applied to electrodes adjacent to the target liquid droplet. The resulting electromechanical force drives droplet translation through a combination of electrowetting and dielectrophoretic (DEP) mechanisms, depending on the liquid and the applied frequency [8,10,11]. Translation by electrowetting can be analyzed in terms of an electrostatic force, a surface tension force, and a pressure force [8].…”

A Digital Microfluidic Approach to Proteomic Sample Processing

“…[61][62][63][64] A more generalized approach for estimating the force on a droplet is based on an electromechanical derivation. As seen in Eq.…”

Hydrogel discs for digital microfluidics

“…[35][36][37] In such systems, droplets are controlled by electromechanical forces. 38 DMF enables facile control over many different reagents for multi-step processes, a property that has been useful for enzyme assays 27,[39][40][41] and for applications involving cells. [42][43][44][45][46][47][48][49][50][51] In these initial applications, enzymatic bioreactors and cell culture and assays were implemented in homogeneous aqueous droplets manipulated by DMF.…”