Wetting and Roughness: Part 2

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 $\theta^{*}$ 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.

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Fig. 5

Micro/Nanotexture Inspirations from Nature

As early as AD 77, Pliny the Elder reported the beading of water drops on woolly plant leaves, the first reported observation of superhydrophobicity. Truly systematic studies of natural microtextures, however, have only happened in the last decade.

Plant leaves often features bumps on the scale of 10-50 $\mu$m, and some, including the now famous lotus, also have a finer 100 nm scale of features (Fig. 6b). The fractal geometry appears to contribute to the superhydrophobicity and self-cleaning ability of the leaf. The mechanism of the fractal surface is debated still, but phenomenologically it provides both a high contact angle and a low hysteresis. Thus drops on the leaves have very high mobility and roll off with ease. Remarkably, the rice leaf has an anisotropic arrangement of papillae that direct the flow of water along preferred directions. Other noteworthy example of superhydrophobic surfaces in nature are the feathers of pigeons and ducks; cicada, butterflies; and the leg setae of water striders in Fig. 6c that rest on trapped air (c.f., Leidenfrost effect) as they travel on water. Mosquitoes' eyes are completely drying due to a pattern of 100 nm bumps (Fig. 6d).

Trying to Synthesize

Quere notes that we can make a superhydrophobic surface by a very crude technique in the garage: take a piece of glass to a sooty flame, and the dark soot coating will provide microroughness and plenty of carbon to repel water. Obviously most of today's research uses sophisticated techniques to create surface patterns.

Hemiwicking

Quere introduces the term "hemiwicking" to describe an imbibition phenomena in rough surfaces that is similar but different from classical wicking. As a liquid film progresses through surface micro/nanostructures such as those shown in Figure 7, at least one additional side is exposed to air. Thus, we have another liquid-air interface. This is different from the Wenzel model, in which the cavities of the surface are filled as though they are capillary tubes while areas not under the drop remain dry.

The simplest example of a hemiwicking microtexture is a groove, shown in Figure 8.

1.