In-situ ATR-FTIR for dynamic analysis of superhydrophobic breakdown on nanostructured silicon surfaces

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.


Effect of in-situ IPA concentration changes on the relative peak intensity ratio of the water bands
The relative peak intensity ratio of the water bands corresponding to a typical concentration profile for wetting kinetics tests is plotted in figure S1. The data shown is this figure is recorded on a FDTScoated flat silicon crystal. Similar results are obtained on hydrophilic patterned samples (data not shown). Although the hydroxyl group present in IPA contributes to the OH-stretching band, this does not significantly affect the relative peak intensity ratio IOH-stretch/IOH-bend in the concentration range covered by the experiments (0-4 mol%). A wider concentration range has already been assessed exsitu in our previous work 1 , where significant increase of this ratio has only been observed for IPA concentrations above 20 mol%. Thus, observed variations in the relative peak intensity ratio of the water bands can be directly correlated to changes in the wetting state. Figure S1. Variation of the relative peak intensity ratio of the water stretching and bending bands upon changing the IPA concentration on a FDTS-coated flat silicon sample. 3

Determination of the transition criterion with ATR-FTIR
A first estimate of the transition criterion for superhydrophobic breakdown is obtained from contact angle measurements. This value corresponds to a concentration that leads to full wetting in a very short timescale. ATR-FTIR can measure over longer timescales and can therefore yield a more accurate value for the onset of the wetting transition. IPA/water mixtures with a concentration well below the critical concentration found with contact angle measurements are injected in the liquid cell.
Then, the IPA concentration is slowly increased by flowing 35 ccm of N2 gas saturated with IPA through the cell. The real-time concentration near the surface and relative peak intensity ratio of the water bands are monitored in situ. The obtained wetting curve is depicted in figure S2 for the 78 nm

Assessment of the wetted area fraction
Assessment of the total wetted area fraction is feasible based on the obtained relative peak intensity ratio of the water bands and the wetting spectra corresponding to pure Wenzel and Cassie-Baxter states on the same surface. This approach yields an average wetting state of the surface, although no information is obtained locally.
Consider the case of a partially wetted crystal. The absolute peak intensities of the OH-stretching and OH-bending bands can be calculated as the superposition of the contributions from the wetted and the non-wetted areas: Here, x is the area fraction in Wenzel state and (1-x) is the area fraction in Cassie-Baxter state. The subscripts W and CB are used to denote the absolute peak intensities of reference spectra corresponding to the pure Wenzel and Cassie-Baxter states. Thus, the relative intensity ratio of the water peaks can be written as: 6

Wetting kinetics: extraction of the front velocity
Wetting on microstructures has been shown to follow a stepwise mechanism with a constant front velocity for solid/liquid couples near the critical transition criterion. 2 In case of a single infiltration point on the ATR-crystal, the total wetted area is calculated as Awet = (vf t) 2 with vf the front velocity and t the elapsed time. Figure S4 depicts the wetted area fraction (Awet/Atotal) as a function of t 2 for the four experimental sets discussed in the main text. Three different wetting regimes can be observed: (1) initiation stage, (2)

Wetting mechanisms: wicking test
In order to separate the contributions from wicking and vertical depinning, a superhydrophobic ATRcrystal was partially stripped to create a hydrophilic reservoir. Part of the crystal is covered with a protective layer and subsequently the crystal is subjected to O2-plasma treatment at 100 W for 6 seconds. The uncovered side is rendered hydrophilic by O2-stripping, whereas the covered side remains superhydrophobic. A visual illustration of this behavior is given in figure S5 for FDTS-coated flat silicon and a patterned sample. 8 Figure S6. Intensity of the OH-bending band and IPA-concentration as a function of time, measured on a partially FDTScoated crystal with liquid injected on the hydrophilic side.