Anti-Ice and Condensation Properties of Microtextured Superhydrophobic Surfaces
Mulroney, Alan, Electrical Engineering - School of Engineering and Applied Science, University of Virginia
Gupta, Mool, EN-Elec/Computer Engr Dept, University of Virginia
Research into the manipulation of surface wetting behavior, condensation characteristics, and ice adhesion pressure has led to surface coatings with increased water repellency, self-cleaning characteristics, reduced surface corrosion and ice accumulation. These improvements result in more efficient thermal exchange in heating, ventilation, and air conditioning (HVAC) systems, more efficient wind and solar power generation systems, anti-icing surfaces for aircraft, and reduced maintenance costs for these and other systems. Most hydrophobic and superhydrophobic coatings available today are based on chemical spray methods, which rely on surface chemistry to repel water. These coatings have poor adhesion, and hence are easily removed, damaged, or degraded. So, there is a need for large area anti-icing and condensation resistance surfaces, which are stable.
To overcome the issue of coating durability and adhesion, an alternative method to achieve superhydrophobic and icephobic properties is by creating surface microtexture. A primary advantage of microtextured surfaces is the mechanical and chemical stability. By producing a superhydrophobic or icephobic layer that is primarily composed of the substrate material, the challenge of coating bonding durability is eliminated. The majority of microtextured superhydrophobic studies have focused on wetting properties, and limited results are available for ice adhesion and condensation properties.
In this research, novel microtextured surfaces were developed and characterized for wetting, ice adhesion, and condensation properties. The microtextured surfaces were fabricated by three different techniques: microparticles, laser ablation, and thermal embossing. A microparticle polytetrafluoroethylene (PTFE) coating was applied to aluminum. This coating improved the wetting from an 83º static contact angle and no roll-off angle to a 129º static contact angle and 18º roll-off angle. The ice adhesion pressure was reduced from 1210 kPa to 185 kPa, and the condensation growth rate in icing conditions was greatly reduced. Laser ablation combined with thermal embossing generated a microtextured polydimethylsiloxane (PDMS) surface. Microtextured PDMS with lines spare 60 µm apart showed static contact and roll-off angles of 160.2º and 1.8º respectively, and a low ice adhesion pressure of 60.1 kPa. Optically transparent microtextured PDMS with a line spacing of 160 µm was also fabricated, and had an optical transmission rate of 71.2%, contact angle of 156.6º, and roll-off angle of 2.5º. Subsequently, the effect of an electric field on wetting, ice adhesion, and condensation was investigated. The addition of a 240 VAC electric field to the 60 μm line space microtextured PDMS surface increased the roll-off angle from 7.6º to 20.4º and ice adhesion pressure from 60.1 kPa to 186.8 kPa. The electric field was also shown to increase the condensation growth. The microparticle microtextured surface was examined in refrigeration system cooling fan applications; where the ice adhesion pressure was decreased by 85%, preventing condensation-based ice buildup on and around the fan assembly.
In summary, the wetting, ice adhesion, and condensation properties of microtextured surfaces prepared by dry microparticle coatings and laser ablation followed by thermal embossing were investigated. Microtextured superhydrophobic surfaces have demonstrated very high contact angles of greater than 155º, low roll-off angle of less than 5º, and low ice adhesion pressure of less than 65 kPa. The effect of an electric field on wetting, ice adhesion, and condensation properties was also investigated. Finally, optically transparent microtextured superhydrophobic surfaces were demonstrated, with low ice adhesion pressure.
PHD (Doctor of Philosophy)
superhydrophobic, icephobic, microtexture, laser
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