Topography Driven Spreading

Roughening a hydrophobic surface enhances its nonwetting properties into superhydrophobicity. For liquids other than water, roughness can induce a complete rollup of a droplet. However, topographic effects can also enhance partial wetting by a given liquid into complete wetting to create superwetting. In this work, a model system of spreading droplets of a nonvolatile liquid on surfaces having lithographically produced pillars is used to show that superwetting also modifies the dynamics of spreading. The edge speed-dynamic contact angle relation is shown to obey a simple power law, and such power laws are shown to apply to naturally occurring surfaces.

Figure 1

Scanning electron microscope images of lithographically structured surfaces showing a square lattice of $15\mu \mathrm{m}$ diameter cylindrical pillars with a $30\mu \mathrm{m}$ lattice parameter: (a) view of a field of pillars and (b) close-up view of the pillars.

Figure 2

A log-lot plot of the edge speed-dynamic contact angle relationship for a polydimethylsiloxane droplet on a surface with $15\mu \mathrm{m}$ diameter pillars of height $45\mu \mathrm{m}$ and lattice parameter $30\mu \mathrm{m}$ (×); the slope of these data during the initial spreading ($p=1.296$) is also shown. The upper solid curve with a slope of $p=3$ is data for a smooth surface (shifted upwards by 0.5 for clarity).

Figure 3

A log-log plot of the dynamic contact angle and time with a fit of $\theta \propto 1/(t+t_0{)}^{0.614}$ for the experiment on the textured surface corresponding to the data in Fig. 2. The percentage change in drop volume is also shown (lower curve and right-hand axis).

Figure 4

Exponents $p$ extracted from the edge speed-dynamic contact angle data (×) for spreading of polydimethylsiloxane oils on textured surfaces; the dotted curve indicates the trend from cubic to linear form with increasing pillar height. Each data point is an average of experiments on several drops and surfaces. The dynamic contact angle-time exponent $n$ variation with pillar height (○) is indicated by the right-hand axis. The inset images show the equilibrium shape of water droplets on the flat surface (${\theta }_e=84°$) and a surface consisting of $52\mu \mathrm{m}$ tall pillars (${\theta }_e=142°$).