8th Thermal and Fluids Engineering Conference (TFEC)
EFFECT OF THERMOPHYSICAL AND DIELECTRIC PROPERTIES OF A LIQUID DROPLET ON CONTINUOUS MOTION IN AN ELECTRIC FIELD
Electrically actuated droplets find utility in micro-fluidic applications. The present study investigates the role of thermophysical and electrical properties of a glycerin droplet on its continuous motion over a PDMS coated electrode with water as a reference. Droplet motion is achieved in an electric field over an active electrode when a horizontal ground wire is placed above in an open-EWOD device. A 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 positions. The dynamic contact angle of the drop is determined from the tangent drawn over the air-liquid interface. Droplet volumes studied are in the range of 2-10 µl with voltages within 170-270VDC. Simulations have been carried out in 2D Cartesian coordinate system using COMSOL Multiphysics® software. The drop is taken to spread immediately after application of voltage as per the Young-Lippmann equation and is followed by continuous motion. The interfacial forces arising from the electric field are calculated in terms of the Maxwell's stress tensor (MST). Electrostatic force is introduced in the Navier-Stokes equations leading to a fully coupled electro-hydrodynamic simulation. A dynamic contact angle model that includes hysteresis and friction is introduced. Simulations of interface shapes are seen to be in good agreement with experiments. In glycerin, high viscosity and low permittivity increase wall resistance and lowers bulk velocity and increases deformation, in comparison to water.