Electrical Switching of Wetting States on Superhydrophobic Surfaces: A Route Towards Reversible Cassie-to-Wenzel Transitions

We demonstrate that the equilibrium shape of the composite interface between superhydrophobic surfaces and drops in the superhydrophobic Cassie state under electrowetting is determined by the balance of the Maxwell stress and the Laplace pressure. Energy barriers due to pinning of contact lines at the edges of the hydrophobic pillars control the transition from the Cassie to the Wenzel state. Barriers due to the narrow gap between adjacent pillars control the lateral propagation of the Wenzel state. We demonstrate how reversible switching between the two wetting states can be achieved locally using suitable surface and electrode geometries.

Figure 1

System configurations. (a) Scanning electron microscope image of Teflon coated microstructure. (b) Electrowetting setup: a voltage $U$ is applied between a droplet and an electrode covered with a dielectric micropatterned surface. The contact area is monitored with an inverted microscope from the bottom through the superhydrophobic substrate. (c) The unit cell of microstructure. (d) Magnified sketch of the solid-liquid interaction area. Dark grey (blue) arrows: optical rays leading to interference. Light grey (red) arrows: electric field distribution (shown only close to contact line).

Figure 2

Spreading of droplet on the superhydrophobic surface by ramping the voltage ($d=5\mu \mathrm{m}$). Voltage increases from left to right: (a) 0 V—the Cassie state, the entire contact area is shown: the droplet touches the tops of the microscopic pillars and rests on a cushion of air. (b)–(g) Magnified views of the black rectangle region in (a) at $U=0–250\mathrm{V}$ for every 50 V, respectively. Black dots appeared near the contact line in (d)–(g), representing the area in the Wenzel state. Inset: magnified view of the solid-liquid contact area at maximal deflection of menisci.

Figure 3

Normalized deflection versus normalized electrostatic force (see text for details). Symbols: experimental data for variable post spacing $d=10\mu \mathrm{m}$ [(red) squares], 8 (black circles), 5 [(blue) up triangles], and $3\mu \mathrm{m}$ [(green) down triangles]. Symbols with lines: numerical calculations. The last symbols on the lines in model calculations, around $\Lambda \approx 0,4$, correspond to ${\alpha }_c=110°$. Light grey dashed lines: numerically stable continuation of the solution. Inset (left): same experimental data in dimensional form ${\zeta }_0$ versus $U$. Error bars represent the typical standard deviation over $3\times 3$ unit cells. Inset (right): Zoomed view of main panel.

Figure 4

(a) Snapshots of the liquid interface for single-stripe electrode (vertical) under two grooves (horizontal) at zero (top) and at maximum (bottom) voltage. (b) Surface with an array of parallel grooves (horizontal) and electrodes (vertical) with and without applied voltage. (c) Sketch of liquid vapor interface along a groove. Dashed line: zero voltage; solid: maximum voltage.