Verification Of The Influencing Factors To The Anti-icing/Icephobic Properties Of Superhydrophobic Surfaces In A Condensate Environment

Many flora and fauna surfaces in nature exhibit superhydrophobic and self-cleaning properties and lotus leaf is the most typical representative. The phenomenon that water droplets bead up on the surface and drip off with dust rapidly is called "lotus effect". Inspired by this phenomenon, researchers have recently made significant progress in fabrication of superhydrophobic surfaces with a water contact angle (CA) greater than 150° and a sliding angle (SA) less than 10°. One attractive application of superhydrophobic surfaces is based on their speculated icephobic or anti-ice capability. An ideal anti-ice surface should possess the follow two characteristics. One is that overcooled water droplets could roll off the surface rapidly before ice formation. The other is that the ice adhesion should be weak when ice accumulated on the surface. However, most superhydrophobic surfaces lose part or all of their superhydrophobicity under an overcooled condition, such as in the weather of freezing rain and wet snow. This leads to a dramatic decrease in CA and a significant increase in SA.Although the icephobic properties of some superhydrophobic surfaces were verified by experimental results, some other researchers still doubt about the practical application of superhydrophobic surfaces under condensate environment (low temperature and high humidity). They claimed that the superhydrophobic surfaces would lose their entire hydrophobicity since the cavities on the rough surface could be wetted by the moisture from the environment upon freezing. Moreover, the condensed water in the cavities could turn to ice roots to anchor the ice layer into the rough surface structure so that the ice adhesion would increase with surface roughness. Apart from experimental method diversity, sample difference is the primary cause for this controversy. They simply noticed the deterioration of superhydrophobicity but ignored that this anchoring effect can be prevented when the surface become hydrophobic enough to prohibit water entering the cavity through wetting process or even condensation. In this thesis, the active anti-icing/icephobic properties were firstly verified through systematic simulated experiments under controllable condensate environment. After that, the influence of different chemical modifications on the icephobic properties of superhydrophobic surfaces was discussed. The main achievements and innovations of this thesis are:(1) Four aluminum surfaces with wettability varied from superhydrophilic to superhydrophobic were prepared by combining an etching and a coating process. The surface wettability was checked in terms of water contact angle (CA) and sliding angle (SA) under different controllable condensate environment. High-speed photography was applied to study water droplet impact dynamics on these surfaces. It was found that the retraction process of impact droplets could mainly reflect the interaction between water droplet and surface. The results indicated that the SA of superhydrophobic increased from 1 to 22 after condensation occurred (like lotus leaf). Besides, single and successive water droplets could rebound on the superhydrophobic surface and roll off at a tilt angle larger than 30° under an extremely condensing weather condition (-10 ℃ and relative humidity of 85-90%). In addition, the superhydrophobic surface showed a strong icephobic property, the ice adhesion on this surface was only 13% of that on the superhydrophilic surface without "anchoring effect". The active anti-icing/icephobic properties of PTES coated superhydrophobic surfaces was verified under condensate environment. This feature was owed to the stable preservation of "air cushion" between the surface and water and this conclusion would be conclusive to the relative scientists on the research focus.(2) Three superhydrophobic surfaces have been prepared on aluminum substrate, which was roughened by acid etching to form a nano-/micro-topological surface structure, and then the surface was modified by coating a PTES (a fluorinated coupling agent), TTPS (a siloxane coupling agent) or PA (an aliphatic coupling agent) layer. Their sliding angle (SA), reduction of ice adhesion and water condensing dynamic were studied under condensate environment. The results indicated some superhydrophobic surface can maintain an excellent sliding and rebounding ability of water droplet even under an extremely condensate condition while others changed to those with adhesion. The surface free energy and geometries of the chemical modification had significant influence on the growth and spontaneous removal of condensate micro-droplets on superhydrophobic surface. A higher jumping scale and frequency on PTES surface leads to a better water-repellency and icephobicity under extremely condensate conditions. The more important is that the continuously jumping behavior could be seen macroscopically which provided a simple intuitive method to filter anti-icing/icephobic materials.(3) The icephobicities of hydrophobic and superhydrophobic polysiloxane surfaces were compared under condensate environment. The results suggested that hydrophobic surfaces showed a better icephobic performance when superhydrophobic surface lost all their water-repellency under condensate environment.(4) The icephobic durability of superhydrophobic surface upon two multiple de-icing (ice-breaking and ice-melting) processes was studied. Besides, the aging resistance of superhydrophobic surface by air exposure and seawater immersing were also discussed.