Measuring failure pressure of porous superhydrophobic coatings via microfluidic method

Abstract

The transition of critical wetting conditions on superhydrophobic surfaces, that is, from Cassie state to Wenzel state, often indicates the failure of super-hydrophobicity on the surface. Recognizing the critical hydrostatic pressure at which a superhydrophobic surface starts departing from the Cassie state is important in the application of surfaces used under water. However, the measurement of the critical pressure for a superhydrophobic surface often involves complicated methods and systems such as optical spectroscopy and pressure vessels. This paper reports a microfluidic method for the measurement of critical failure pressure of superhydrophobic porous coatings by allowing water to penetrate the coatings from cross-sectional direction in microchannels. Two types of superhydrophobic porous coatings with different hydrophobicities, namely, titanium oxide (TiO2) coatings and TiO2/single-walled carbon nanotube (SWNT) coatings, were fabricated inside microchannels by liquid-phase deposition and lift-off process. Experimental results show that the critical failure pressures of TiO2 coatings decrease from 36.6 to 28.0 mBar as the porosity increases. TiO2/SWNT coatings with porosity of 19% show a higher failure pressure of 46.1 mBar due to its higher contact angle and lower hysteresis in comparison with TiO2 coatings. The method presented here provides a simple way to measure the critical failure pressure of superhydrophobic coatings. Although only TiO2 and TiO2/SWNT coatings were tested, any porous coating material which is compatible with lift-off process can be measured by this microfluidic method.