Wetting and electrowetting on corrugated substrates

Abstract

Wetting and electrowetting (EW) on corrugated substrates are studied experimentally and theoretically in this paper. On corrugated substrates, because of the anisotropy of surface morphology, the droplet shows an elliptical shape and the spreading velocities in different directions are different. Spreading of a droplet is usually controlled not only by the surface tensions but also by hemi-wicking. Our experimental results indicated that liquids along the grooves propagate much faster than those in the direction vertical to the grooves. However, spreading in both directions obeys the same scaling law of l∼t4∕5. EW on corrugated substrates reveals some differences with that on smooth surfaces. The change of contact angles with an applied voltage follows a linear relationship in two stages instead of the smooth curve on flat surfaces. There exists a critical voltage which divides the two stages. The transition of a droplet from the Cassie state to the Wenzel state on corrugated substrates was also discussed. The extended EW equation was derived with the free energy minimization approach, and the anisotropic factor was introduced. From the extended equation, it is found that EW is affected by the anisotropic factor significantly. For the smooth surfaces, the extended EW equation will degenerate to the classical Lippmann-Young equation. Our research may help us to understand the wetting and EW of droplets on corrugated substrates and assist in their design for practical applications.