Elsevier

Applied Surface Science

Volume 256, Issue 16, 1 June 2010, Pages 4930-4933
Applied Surface Science

Water condensation on zinc surfaces treated by chemical bath deposition

https://doi.org/10.1016/j.apsusc.2010.03.004Get rights and content

Abstract

Water condensation, a complex and challenging process, is investigated on a metallic (Zn) surface, regularly used as anticorrosive surface. The Zn surface is coated with hydroxide zinc carbonate by chemical bath deposition, a very simple, low-cost and easily applicable process. As the deposition time increases, the surface roughness augments and the contact angle with water can be varied from 75° to 150°, corresponding to changing the surface properties from hydrophobic to ultrahydrophobic and superhydrophobic. During the condensation process, the droplet growth laws and surface coverage are found similar to what is found on smooth surfaces, with a transition from Cassie-Baxter to Wenzel wetting states at long times. In particular, it is noticeable in view of corrosion effects that the water surface coverage remains on order of 55%.

Introduction

Wettability of liquid on solid is extremely important in our daily life and also in industry and agriculture. One can cite, for instance, the cases of wetting of water on glass windows, painting on different kinds of surfaces and spreading of pesticide on plants. In the wetting process, the chemical composition and the geometric structure of the surface play a key role. Completely wetting surfaces are frequently used in antifogging windows or bacteria resistance coating [1] while extremely non-wetting surfaces (i.e. superhydrophobic surfaces where the contact angle is larger than 150°) are mostly used in self-cleaning surfaces [2], tunable lenses [3], micro-fluidic systems [4] and non-adhesive coating [5], [6]. Since the last two decades, the wetting phenomena on superhydrophobic surface has become a subject of fundamental research in physics [7], [8], [9], chemistry [10], biology [11], [12] and materials science [13], [14]. In material science, many metallic superhydrophobic surfaces are mostly used as anticorrosive surfaces. In particular, zinc and zinc oxide surfaces are particularly interesting because of their special electrical, optical and switchable wetting properties.

Superhydrophobic surfaces are in principle rough surfaces. A water drop deposited on such surfaces can rest in a Wenzel state [15] or a Cassie-Baxter state [16], the selection depending on the most stable situation. In the Wenzel state, water fills the microstructure and the apparent contact angle θ* of a deposited drop is given bycosθ*=rcosθ.Here r is the surface roughness defined as the ratio of the actual surface area to the projected surface area and θ is the equilibrium contact angle. In the Cassie-Baxter state, the rough surface is composed of solid and air. A water drop sits partially on the solid surface and partially on the air trapped in the microstructure. In such a situation, the apparent contact angle θ* is given bycosθ*=ϕs[cosθ+1]1.

The angle θ* depends on the area fraction of the liquid–solid interface (Φs), the liquid–air interface (1  Φs) and θ. For a given set of r, θ, and Φs values, the equilibrium state of the drop will depend on whether the minimum energy corresponds to a Wenzel or a Cassie-Baxter type. The critical contact angle of a drop that determines the wetting situation is given by [17]θc=cos1[(ϕs1)/(rϕs)].

When θ > θc, the most stable state is of Cassie-Baxter type; when θ < θc, the most stable state is of Wenzel type. Furthermore, a transition from a high energy state to a low energy state is always possible if the drop gained the required energy to overcome the transition barrier (e.g. by pushing a deposited drop with a syringe to go from a Cassie-Baxter type to a Wenzel type).

Numerous correlations between superhydrophobic surface configurations and liquid wetting properties in case of deposited (projected) drops have been reported in many experimental and theoretical studies. In contrast, and although this process is of major importance for the evaluation of corrosion, the studies of condensation-induced wetting on superhydrophobic surface and their corresponding wetting properties (self-cleanliness, different wetting states), are much less documented [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. The aim of the present work is thus to study condensation of water on a multi-scale rough superhydrophobic surface where the contact angle is varied in a wide range (70–150°).

Section snippets

Experimental

In the present study, we use a zinc substrate whose surface wetting properties can be easily tuned. A plain zinc surface is hydrophobic and, when coated with hydroxide zinc carbonate (HZC) by chemical bath deposition, depending on the deposition time, it can become either ultra hydrophobic or superhydrophobic. The substrate that we used is a 1 mm thick zinc sheet (99.97%). The zinc substrate is provided by Goodfellow (U.K). Analytical grade N-N-dimethylformamide (C3H7NO) (DMF) is supplied by

Results and discussion

During condensation, we observed in general the following growth stages.

Initial stage. At the beginning, typically t < 3 min, very small drops nucleate on the rough surface. The atomic composition and hydrophilicity of HZC film contributes to the final surface wettability and hence facilitates the nucleation of the drops [30]. The drop surface coverage ɛ2 (ratio of area covered by the drops to the total surface area) is low and only a very few number of drop coalescence takes place. Then the drops

Concluding remarks

The study of the condensation process on Zn-modified surfaces by chemical bath deposition technique – a very simple and powerful method to modify the surface roughness and wettability, from hydrophobic to superhydrophobic – shows that, on such surfaces, water condensation, although a complex and challenging process, is similar to smooth planar surfaces. As the nucleation events occur at a much smaller length scale than the surface texture scales, the surface chemistry dominates the texture

Acknowledgments

This work was partly supported by the Spanish MEC (Grant n. FIS2008-01126) and by Departamento de Educación (Gobierno de Navarra). R.D.N. acknowledges the support of Marie Curie International Incoming Fellowship (MCIIF) within the 7th European Community Framework Program. We thank Prof. Jordana for taking SEM images of the substrate.

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