Single Bubble Dynamics on a Hydrophobic Surface

The growth and departure of single bubbles on two surfaces with very different wettability is studied using highspeed video microscopy. An artificial cavity of approximately 10μm diameter is microfabricated on a bare and a Teflon-coated silicon substrate to serve as a nucleation site. The static contact angle of water is approximately 40° on the bare silicon substrate and approximately 120° on the Teflon-coated substrate. The bubble departure diameter is observed to be almost three times larger and the growth period almost 60 times longer for the hydrophobic surface than for the hydrophilic surface. The waiting period is practically zero for the hydrophobic surface because a small residual bubble nucleus remains on the cavity from the previous ebullition cycle. Bubble nucleation occurs on nominally smooth hydrophobic regions with root mean square roughness below 4 nm even at superheat as small as 4 °C. Liquid subcooling significantly affects bubble growth on the hydrophobic surface due to the increased bubble surface area.