Such synergy between the nanoscale and microscale surfaces was unexpected, Koratkar says. His team discovered, though, that the dynamics of bubble generation and release differ fundamentally between a conventional surface and one with the nanorods. Pockets of air trapped within the rods feed nanobubbles into the microscale cavities of the vessel surface to help prevent those cavities from becoming flooded with water, he explains. “We observed a 30-fold increase in active bubble nucleation site density — a fancy term for the number of bubbles created — on the surface treated with copper nanotubes over the nontreated surface.” This spurred to a six-to-ten times greater amount of vapor at the heating surface, he adds.
Tests were conducted with 1-in. × 1-in. wafer covered with nanorods via oblique-angle deposition. The nanorods’ height was about 450 nm and the tip-to-tip spacing between them was around 50 nm.
While the technique ultimately may lead to better energy efficiency in industrial heat exchangers, Koratkar foresees the most immediate potential for taking care of hot spots on microelectronic circuits. He’s already talking to some chipmakers and hopes to start tests within the next year. He also aims to explore whether a thin hydrophobic layer on the nanorods will provide even greater bubble density and to investigate the effect of rod height, spacing and orientation.
Robustness of the rods may need attention, he admits. In the tests after five to 10 cycles, some aging or deterioration occurred. So, he also plans to explore alternatives to create more robust rods.
