EFFECT OF THERMOPHYSICAL AND DIELECTRIC PROPERTIES OF A LIQUID DROPLET ON CONTINUOUS MOTION IN AN ELECTRIC FIELD

Abstract

The present study investigates the role of thermophysical and electrical properties of various liquid drops on their continuous motion over a Polydimethylsiloxane (PDMS) coated electrode with DI water as a reference. Droplet motion is achieved in an electric field around an active electrode when a ground wire is placed horizontally in an open-electrowetting-on-dielectric (EWOD) device. A charged-coupled device (CCD) camera is used to record the drop shapes and displacement of the moving droplet at 120 fps. Using image processing tools, the velocity of the droplet is determined from a time sequence of its centroid position. The dynamic contact angle of the drop is determined from the tangent drawn over the air-liquid interface. Liquids of interest include ferrofluid and a surfactant solution in water, as well as glycerin for droplet volumes in the range of 2-10 μl with voltages within 170-270V_DC. Simulations are carried out in a 2D Cartesian coordinate system within COMSOL Multiphysics^® software. The drop is taken to spread immediately after application of voltage following the Young-Lippmann equation and is accompanied by continuous motion. The interfacial forces arising from the electric field are calculated in terms of the Maxwell's stress tensor (MST). The electrostatic force is a source term in the Navier-Stokes equations treated using a fully coupled approach. Interface shapes of ferrofluid and surfactant droplets do not show significant departure from moving water droplets. As the concentration of the ferrofluid increases, surface tension decreases, and the droplet speed increases. The extent of spreading of a surfactant solution is higher, thus generating a higher interfacial area for the electric field, leading to a higher droplet velocity. In glycerine, high viscosity and low permittivity increase wall resistance, which is an important factor in reducing the velocity attained during continuous motion. In comparison with a water droplet, the glycerin drop moves slower, surfactant solution is faster, and the ferrofluid drop has characteristics similar to water. Droplets of glycerin, aqueous solutions containing surfactant, and ferrofluid show the greatest deformation.