Stretchable hydrophobic surfaces and droplet heating

Silicon elastomer surface is treated towards achieving the hydrophobic state. Functionalized nano-silica units are coated onto elastomer surface and resulting texture characteristics are examined prior to stretching, stretched and after stretching. The droplet heating of the hydrophobic elastomer surface is carried out when the surface is subjected to unstretching, stretching and stretch releasing conditions. The thermal-flow field in the liquid is simulated and validated incorporating high speed recording system. Nano-size silica units coated elastomer surface demonstrates the hydrophobic wetting state. The hydrophobic wetting state changes slightly for stretched and stretched released surface. The contact angle is about 154° ± 2° for unstretched surface while it is 152° ± 2° for the stretched released surface; hence, stretch relaxing provides reversible change of the surface wetting state of the elastomer surface. The contact angle reduces to 142° ± 2° when surface is under stretched, which is related to increased pillar spacing on the surface. The droplet heating results in development of Marangoni current in the fluid, which significantly affects the flow and temperature fields and it becomes more apparent for the large size droplets. The maximum flow velocity increases almost 9% in 45 µL as the surface is stretched. The Nusselt number increases with droplet size and the Bond number has the values less than unity; hence, stretching increases the Nusselt number by 60% for droplet of 45 µL.