Abstract
Superhydrophobic surfaces are highly promising for self-cleaning, anti-fouling and anti-corrosion applications. However, accurate assessment of the lifetime and sustainability of super-hydrophobic materials is hindered by the lack of large area characterization of superhydrophobic breakdown. In this work, attenuated total reflectance−Fourier transform infrared spectroscopy (ATR-FTIR) is explored for a dynamic study of wetting transitions on immersed superhydrophobic arrays of silicon nanopillars. Spontaneous breakdown of the superhydrophobic state is triggered by in-situ modulation of the liquid surface tension. The high surface sensitivity of ATR-FTIR allows for accurate detection of local liquid infiltration. Experimentally determined wetting transition criteria show significant deviations from predictions by classical wetting models. Breakdown kinetics is found to slow down dramatically when the liquid surface tension approaches the transition criterion, which clearly underlines the importance of more accurate wetting analysis on large-area surfaces. Precise actuation of the superhydrophobic breakdown process is demonstrated for the first time through careful modulation of the liquid surface tension around the transition criterion. The developed ATR-FTIR method can be a promising technique to study wetting transitions and associated dynamics on various types of superhydrophobic surfaces.
Highlights
Inspired by nature, numerous biomimetic functional materials with controlled wettability have been designed
ATR-FTIR is used for real time monitoring of wetting states and wetting transitions on immersed superhydrophobic nanopatterned surfaces
Differentiation between Wenzel, Cassie-Baxter and mixed wetting states is based on the relative intensity ratio of the water bands, which can be very accurately predicted by finite-difference time-domain (FDTD) simulations
Summary
Numerous biomimetic functional materials with controlled wettability have been designed. Sustainability of superhydrophobic surfaces has become a critical issue that limits the application and lifetime of these functional materials and devices. Classical models predict that structured surfaces made from inherently hydrophilic materials (intrinsic contact angle on flat surfaces θ < 90°) will be fully wetted, whereas surface patterning may turn hydrophobic materials (θ > 90°) superhydrophobic. ΘcCB−W is the critical contact angle as measured on a flat surface of the same material below which a transition to the Wenzel state will occur. Cassie-Baxter states have been reported even for intrinsically hydrophilic materials with contact angles in the range of 70–90°37–41. These discrepancies hamper accurate prediction of the performance and lifetime of superhydrophobic surfaces in real applications
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