Characterization of watermarks formed in nano-carpet effect

In the spreading of a water droplet on an aligned silicon nanorod array surface, a precursor rim was detected moving ahead of the contact line. In this process, nanorods were bundled by the capillary force to form clusters, and a watermark developed on the surface after water evaporated. The size of the watermark, R(p)max, corresponding to the maximum radius of the precursor rim, followed a simple power law relationship with the volume of water droplet omega, R(p)max proportional to omega(beta). The scaling exponent beta increased when the nanorod height decreased, but all in the vicinity of 1/3. This behavior was attributed to the competition of evaporation and spreading of a water droplet during the spreading process. The size of the bundled nanorod cluster formed by the capillary force not only depended on the nanorod height but also on the location in the watermark. The cluster size almost remained as a constant near the center, and then it decreased with the distance from the center. This phenomenon can be qualitatively interpreted through the change of the total free energy during the precursor invading the nanorod array, by considering the contribution from the mechanical energy change due to the bending and clustering of nanorods.