Wetting and Roughness: Part 2

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Wetting and Roughness: Part 2

Authors: David Quere

Annu. Rev. Mater. Res. 2008. 38:71–99

Soft matter keywords

microtextures, superhydrophobicity, wicking, slip

By Alex Epstein

Abstract from the original paper

We discuss in this review how the roughness of a solid impacts its wettability. We see in particular that both the apparent contact angle and the contact angle hysteresis can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well-controlled micro- or nanotextures on a solid, these effects are being exploited to induce novel wetting properties, such as spontaneous filmification, superhydrophobicity, superoleophobicity, and interfacial slip, that could not be achieved without roughness.

In Part 2, we look at the sections Microtextured Solids and Hemiwicking 

Soft matters

During the past decade, the use of microtextured solids and more recently nanotexturing has been popular to induce surface wetting properties that cannot otherwise be obtained. Roughness of the surface changes the unique Young angle to a range of possible angles and generates an apparent angle in the surface plane that is different from the local angle at the contact line.

Quere asserts that three factors are responsible for the sudden resurgence of interest in this area.

1. Late 1990s research from the Kao Corporation showing large contact angles of liquids on fluorinated rough surfaces. (Note that this was similar to results reported in the 1940s.)

2. Papers by Neinhuis and Barthlott in Germany reporting the variety of surface features found on hydrophobic plants, such as the lotus. Animal studies followed.

3. Developments in micro/nanofabrication techniques that allowed more sophisticated designs to be studied, as inspired by (1) and (2).

Briefly, the Kao experiment plotted relation of the apparent contact angle <math>\theta^{*}</math> of various liquids on a rough fluorinated surface against the expected Young angle of each liquid on a flat fluorinated surface. The S-shaped curve, seen in Figure 5, describes the amplifying effect of roughness on hydrophilicity and hydrophobicity. The first, steeper slope on the right side follows Wenzel's roughness closely. The second, smaller slope is the superhydrophilic regime, in which Wenzel breaks down because hemiwicking of surrounding surface cavities (as considered below) leads to the droplet sitting on both solid and liquid.

Fig. 5

Microtexture Inspirations from Nature

As early