Coating-free Superhydrophobicity and Passive Water Transport on Industrial Metals.

Topographic gradients are an attractive option for promoting passive transport of liquid droplets without gravity assistance. These surfaces can potentially enhance heat transfer coefficients by mitigating condensate flooding and promoting rapid surface rejuvenation which is critical for maintaining high-efficiency dropwise condensation on the airside of heat exchangers. An advantage of these gradients over coatings is that they are a part of the substrate and therefore do not succumb to wear. Despite these benefits, topographic gradients have only been demonstrated on a small number of metals.
This work investigates the use of laser-etched micro- and nanopatterns to achieve extreme water repellency (superhydrophobicity) and passive droplet transport on common industrial metals and heat exchanger substrates including titanium, zinc, brass, stainless steel, and nickel. Successful achievement of superhydrophobicity was demonstrated on most metals with titanium achieving a static contact angle of 172±3° and a contact angle hysteresis of 10±2°. Both titanium and nickel topographic gradients successfully demonstrated spontaneous droplet motion. Titanium gradients exhibited travel distances of 2.1 and 2.8 mm, respectively, in close agreement with model predictions.
These results demonstrate a predictable, coating-free pathway for enhancing water drainage across a diverse range of industrial metals used in heat exchanger systems.