Understanding the voltage-induced electrowetting and microfluidic droplet movement phenomena on a Teflon-on-flexible (TOF) substrate

The current work focuses on the basic principle of voltage-induced electrowetting and relevant movement of the microfluidic droplets. The prototype of microfluidic devices are fabricated on the Teflon-on-flexible substrate. Three different liquid droplets, namely, the de-ionized (DI) water, sucrose (aq.) solution, and eugenol, have been studied for such purpose within the voltage range of 1-16 V. Electrowetting and subsequent changes in contact angle are extensively investigated with the modification of "work of adhesion" and "work of cohesion" upon application of external voltage. The liquid droplet is positioned on the dielectric-hydrophobic layer which also separates it from the metal electrodes. Eugenol exhibits more susceptibility to electrowetting compared to sucrose solution and DI water. Consequently, sucrose (aq.) solution and DI water show comparatively more droplet displacement. The "work of spreading" for the liquids under test on Teflon surface is obtained. The spreading of eugenol starts at relatively low voltages than sucrose (aq.) solution and DI water. Eugenol follows the Young-Lippmann equation, i.e., linear relation between {cos(theta(v)) - cos(theta(0))} with voltage(2) (V-2); however, sucrose (aq.) solution and DI water deviate from such nature. Here, theta(0) and theta(v) are the initial and voltage modified contact angles, respectively. Thus, the current study provides an accurate approach to analyze the interaction of solid-liquid surfaces and its consequent effect upon application of external voltages.