Macroscopically flat and smooth superhydrophobic surfaces: heating induced wetting transitions up to the Leidenfrost temperature.

We present an investigation of the change in wettability of water droplets on 3 different flat, smooth substrates with an elevation in temperature. Two methods were employed. In the first method the droplet was placed on the substrate before it was heated and in the second method the droplets were induced to fall onto a preheated substrate. We find that the intrinsic wettability of the surface is important and that fundamentally different behavior is observed on a hydrophobic surface relative to hydrophilic surfaces. For the hydrophobic surface and employing the first method, we have observed three different regimes over the temperature range of 65 degrees C to 270 degrees C. In regime I (65 degrees C to 110 degrees C), the contact angle of water droplets exhibit a slight decrease from 108 degrees to 105 degrees and an accompanying significant decrease in droplet lifetime (tau) from approximately 111 s to approximately 30 s is observed. In regime II (120 degrees C to 190 degrees C), tau remains constant at approximately 20 s however the contact angle significantly increases from 127 degrees to 158 degrees--that is we enter a superhydrophobic regime on a flat surface. In this regime the droplet remains stationary on the surface. Regime III (210 degrees C to 270 degrees C), is the Leidenfrost regime in which the water droplet exhibits a rapid motion on the solid surface with a contact angle higher than 160 degrees. In comparison, the wetting behavior of a water droplet on two relatively hydrophilic surfaces (Au and GaAs) have also been investigated as a function of temperature. Here no wetting transition is observed from 65 degrees C up to 365 degrees C. In the second method, the wetting behavior on the hydrophobic surface is similar to that observed in the first method for temperatures below the Leidenfrost temperature and the water droplet rebounds from the solid surface at higher temperatures. Additionally, the Leidenfrost phenomenon can be observed above 280 degrees C for the hydrophilic surfaces.