Durability of Submerged Hydrophobic Surfaces
Hydrophobic and Superhydrophobic surfaces have gained wide popularity due to its potential in various areas like self-cleaning and anti-fouling materials, drag reduction and microfluidics. However, for all practical applications, the long term durability of these surfaces is extremely important, yet not often investigated. Of particular interest is the long term durability of soft hydrophobic surfaces that remain submerged underwater for a prolonged duration. In this article, we explore how the chemical durability of flat and patterned crosslinked PDMS surfaces (polydimethylsiloxane, a preferred material for microfabrication) change as a function of time when submerged in acidic, basic and neutral mediums for different durations over a prolonged period of time. Based on contact angle measurements, atomic force microscopy, confocal microscopy and SEM analysis of the surfaces, we checked if there is any change in morphology of the surface due to deposition or etching. We created biomimetic positive replica of lotus leaf that exhibit superhydrophobicity and Cassie state of wetting with static water contact angle (θ) > 150˚, and compared the degradation with negative replica of lotus leaf (θ ~ 127˚), grating patterned surfaces that exhibit Wenzel state of wetting (θ ~ 110˚) and flat crosslinked PDMS surfaces (θ ~ 105˚). The positive replica maintained reasonable hydrophobicity (θ > 90˚) up to a month but lost superhydrophobic property. The surface-wettability degraded maximum in case of basic solution due to deposition.