Restoring Superhydrophobicity of Lotus Leaves with Vibration-Induced Dewetting

A lotus leaf retains water repellency after repeated condensation in nature but becomes sticky to water drops after condensation on a fixed cold plate. Our experiments show that mechanical vibration can be used to overcome the energy barrier for transition from the sticky Wenzel state to the nonsticking Cassie state, and the threshold for the dewetting transition follows a scaling law comparing the kinetic energy imparted to the drop with the work of adhesion. The vibration-induced Wenzel to Cassie transition can be used to achieve antidew superhydrophobicity.

Figure 1

(a) Experimental setup where the cold plate was used only for the condensation experiments; (b) parameters of the drop used in the model; (c)–(f) a hypothetical route for the gradual dewetting during a Wenzel to Cassie transition. Once dewetted, the water drop in (f) bounces on the lotus leaf, and lands in the Cassie state when its kinetic energy dissipates. Schematics not to scale.

Figure 2

(a) Vibration-induced Wenzel to Cassie transition of a drop of $>90\%$ water at a frequency of 80 Hz and a peak-to-peak amplitude of 0.6 mm; (b) No transition was observed for a drop of 67% water and 33% ethanol at 80 Hz and 1.5 mm. (c) Sticky residue was left behind under the same conditions as (a) except for a higher amplitude of 1.5 mm. Time stamps correspond to the time after initiation of vibration.

Figure 3

(a) Condensate of water vapor on a lotus leaf horizontally attached to a fixed cold plate; (b) remaining sticky condensate after the leaf was vertically tilted and returned to the horizontal position; (c) the same lotus leaf after vibration on a speaker.

Figure 4

Water condensate experiencing Wenzel to Cassie transition. Millimetric drops were dewetted, while smaller condensate drops stayed in the Wenzel state during vibration at an amplitude of 1 mm and 80 Hz. See supplemental video S1 in 18.

Figure 5

Threshold peak-to-peak amplitude required for the dewetting transition of a $1.5\mu \mathrm{L}$ Wenzel drop. The local minima at 30 Hz and 100 Hz correspond to the first two resonance modes of the drop. Under vertical external forcing, the 1st and 2nd modes correspond to vertical and horizontal resonant motion of the drop, respectively.

Figure 6

The threshold velocity of vibration as a function of initial drop volume. Each data point was recorded when a marginal Wenzel to Cassie transition was observed while the speaker was driven at the 1st or 2nd resonance frequency. The coefficient of determination ($R^2$) of the linear fit is 0.987.