Superhydrophobic Breakdown of Nanostructured Surfaces

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Superhydrophobic breakdown on nanostructured surfaces characterized by in-situ ATR-FTIR Nandi Vrancken, Stefanie Sergeant, Guy Vereecke, Geert Doumen, Frank Holsteyns, Herman Terryn, Stefan De Gendt, and XiuMei Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04471 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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Superhydrophobic breakdown of nanostructured surfaces characterized by in-situ ATR-FTIR Nandi Vrancken1,2*, Stefanie Sergeant3†, Guy Vereecke2, Geert Doumen2, Frank Holsteyns2, Herman Terryn1, Stefan De Gendt2, XiuMei Xu2**. 1

Vrije Universiteit Brussel, Pleinlaan 2, 1050 Elsene, Belgium

2

Imec, Kapeldreef 75, 3001 Leuven, Belgium

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UC Leuven-Limburg, Herestraat 49, 3000 Leuven, Belgium

KEYWORDS ATR-FTIR, nanoscale wetting characterization, superhydrophobic breakdown, wetting hysteresis

ABSTRACT In situ characterization of the underwater stability of superhydrophobic micro- and nanostructured surfaces is important for the development of self-cleaning and anti-fouling materials. In this work, we demonstrate a novel ATR-FTIR (Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy) based method for large-area wetting characterization of silicon nanopillars. When air is present in between the structures, as is characteristic of the

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Cassie-Baxter state, the relative intensities of the water bands in the absorption spectrum change due to the wavelength-dependent attenuation of the evanescent wave. This phenomenon enables unambiguous identification of the wetting state and assessment of liquid impalement. Using mixtures of isopropanol and water with different concentrations,

the breakdown of

superhydrophobic states as well as wetting hysteresis effects are systematically studied on uniform arrays of silicon nanopillars. A transition from Cassie-Baxter to Wenzel state is observed when the isopropanol concentration exceeds 2.8 mol%, corresponding to a critical surface tension of 39 mN/m. Spontaneous dewetting does not occur upon decreasing the isopropanol concentration and pure water can be obtained in a stable Wenzel state on the originally superhydrophobic substrates. The developed ATR-FTIR method can be promising for real-time monitoring of wetting kinetics on nanostructured surfaces.

Introduction When a liquid is brought into contact with a structured surface, the liquid may either fully or partially wet the surface depending on the interfacial energies. Two models are generally referred to when describing the wetting state, as was proposed almost a century ago by Wenzel1 and Cassie and Baxter2 and elaborately described in recent reviews3–5. In the Wenzel model the liquid fully penetrates the surface structures, and the apparent contact angle measured on the ∗ rough surface (
 ) is given by: ∗  cos
 = cos
 1

with θ the contact angle on a flat surface of the same material and r the roughness factor, defined as the ratio of the true and projected surface areas, r = Areal/Aprojected. The Cassie-Baxter model describes a superhydrophobic state with the liquid residing on top of the structures and an air

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layer trapped in the surface cavities. The apparent contact angle of a droplet in Cassie-Baxter ∗  is given by: state (
 ∗  cos
 =  cos
 − 1 +  2

in which fs represents the solid surface fraction. Cassie-Baxter states are generally characterized by very high contact angles and low contact angle hysteresis. Various plant species exploit this property to remove particles and dirt from their leaves, as shown by Barthlott.6 Inspired by this observation in nature, superhydrophobic surfaces for self-cleaning and anti-fouling applications have already been demonstrated.7–9 However, breakdown of the superhydrophobicity is generally irreversible10,11 and limits the lifetime of these surfaces. On the other hand, complete wetting of nanostructured surfaces is desired for applications from wet processing in IC fabrication12–15 to single molecule detection in nanofluidics16,17. Consequently, establishing an in-depth understanding of the wetting behavior on nanostructured surfaces is critical. The most commonly used method to study solid-liquid interactions and superhydrophobic breakdown is based on goniometric techniques, i.e. macroscopic contact angle measurements.18–24 This technique yields a good estimation of the wetting state in most cases. However, contact angles are dictated by the area in the vicinity of the triple-phase line, and do not provide information on the liquid behavior away from the drop edge.25 Therefore close attention should be paid when interpreting data from heterogeneous surfaces, mixed wetting states or wetting transitions. Moreover, contact angle measurements can be affected by experimental parameters, e.g. drop size, dispensing rate, applied pressure, etc. Various other techniques have been developed in order to obtain more detailed information on the liquid-solid interactions at the micro- or nanoscale. The high number of very recent

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publications13,26–31 illustrates the relevance and interest in this field. Techniques for wetting state characterization can be classified according to the length scale of the surface roughness, i.e. microstructured or nanostructured surfaces (characteristic dimension below optical diffraction limit). Analysis of wetting states on microstructures is commonly based on optical methods. Conventional microscopy allows for differentiation between Wenzel and Cassie-Baxter states by evaluation of droplet shape and image focus.32,33 Recent works28,31,34–36 reported application of in-situ confocal microscopy to visualize the liquid interface on micropillars and inside micropores. Additionally, accurate 3D-imaging of the liquid interface and assessment of local liquid impalement is feasible with interference microscopy.37 Lei et al.38 demonstrated the use of diffraction patterns of a submerged superhydrophobic PDMS grating to characterize the wetting state and transitions on micropillars, although only qualitative information was obtained. More quantitative measurements of wetting depth and wetting transitions on microstructures have been reported by Saad39 and Dufour40 by analysis of the reflection of high-frequency acoustic waves at the solid/liquid interface. Wetting characterization of nanostructures is still under exploration. Conventional optical methods fail at the nanoscale due to resolution limitations, and imaging of the liquid-solid interface at the nanoscale typically requires environmental29 or cryo-based30 SEM. Several techniques have been recently developed, based on indirect measurements of a physical property that can be correlated to the wetting state. Using ordered arrays of silicon nanopillars, we have demonstrated that nanoscale liquid penetration depths can be accurately determined from optical reflectance spectroscopy.13,41 Wetting transitions and gas trapping on the same nanopillars with superhydrophobic coatings have also been quantitatively characterized by an acoustic method26 and by an ATR-FTIR based technique to detect trapping of gaseous carbon dioxide in between the pillars.27 Most of the above mentioned techniques can be used to probe a

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small local area, but for material characterizations, a method suitable for in-situ, large-area wetting characterization is still needed.

ATR-FTIR is a well-known surface characterization technique to identify substances and reactions near the surface. Its high surface sensitivity and non-destructive character renders it especially popular for analysis of biological samples.42,43 ATR-FTIR relies on total internal reflection of an infrared beam inside an ATR-crystal. An evanescent wave is emitted every time when the beam is reflected, and penetrates outside the crystal with an electric field amplitude that decays in an exponential manner.44–46 Molecules in the sample absorb the evanescent waves with wavelengths corresponding to the excitation energies of their vibration modes, resulting in characteristic absorption bands in the spectrum. Sensitivity and measurement area can be increased by multiple reflections of the IR beam inside the crystal.45,47 A detailed mathematical study on the nature and physical interpretation of the evanescent wave has recently been discussed in the work of Milosevic46. In order to assess the decay of the evanescent wave, Harrick and duPre48 defined the ‘penetration depth’ as the sample depth for which the electric field amplitude of the evanescent wave, E, decays to a value of E0 e-1 (with E0 the value of the electric field amplitude at the surface). Penetration depths of IR radiation are generally on the order of few hundreds of nm, rendering ATR-FTIR very well suited to explore the liquid-solid and air-solid interactions at the nanoscale. For example, Öhman49,50 and Taheri51,52 applied ATRFTIR for in-situ analysis of water penetration through organic coatings on top of a metal layer. Exploitation of the highly surface-sensitive character of this method enabled them to monitor various surface and interface phenomena, including oxide formation, evolution of the OHtermination, degradation of interfacial bonding and water diffusion towards the surface.

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In this work we demonstrate a novel ATR-FTIR based method for in-situ analysis of the wetting state of silicon nanopillars. At first, contact angle measurements are used to measure the surface wettability by isopropanol (IPA) and water mixtures. Then a detailed nanoscale wetting characterization is carried out using ATR-FTIR. The wetting stability and wetting hysteresis are discussed at the end.

Experiments Surface treatments. Dense arrays of silicon nanopillars (height 260+9 nm, diameter 40+3 nm, pitch 90 nm) are fabricated using deep UV immersion lithography and plasma etching technology as described in previous works12,13,16,26,27,53. The dimensions and the mechanical stability of the structures are assessed with SEM. The pillars are sufficiently robust to withstand capillary force-induced aggregation or collapse upon wetting and drying, as illustrated in Figure S5. The hydrophobicity of the surface is tuned through dry surface modification techniques, i.e. UV/O3-treatment and coating with 1H,1H,2H,2H-perfluorodecyl-trichlorosilane (FDTS). Experimental details are given in the supporting information. UV/O3-treatment renders the surface hydrophilic (contact angle