Xue-Mei Li from Nanjing University of Technology, China, and Mercedes Crego-Calama from the Holst Center, Eindhoven, the Netherlands, share their view on how roughness influences surface wettability

Surfaces that have non-stick, anticontamination, and self-cleaning properties are highly desirable for many industrial and biological applications, such as self-cleaning car windscreens, stain resistant textiles, antisoiling architectural coatings and water-proof or fire-retardant clothes. As a consequence, fabricating surfaces with these kinds of properties is becoming a major focus of research.

The extent to which a liquid can wet a solid surface depends on the properties of the liquid and the surface itself. The wettability of a flat surface can be expressed in terms of the contact angle - the angle at which the liquid meets the surface. Surfaces that have a water contact angle of greater than 90° are considered to be hydrophobic. Surfaces with a water contact angle greater than 150° are known as superhydrophobic. 

The Lotus leaf (Nelumbo nucifera) is a famous example of a naturally occurring superhydrophobic surface. Scanning electron microscope images of the microstructures of the leaf's surface have revealed that its water repellency is mainly caused by tiny crystalline projections of wax. These waxy protrusions, which are typically between one and five micrometres in height, cover the surface in a regular pattern. The resulting rough structure of the lotus leaf is believed to be what's responsible for its superhydrophobic properties.

Inspired by nature, scientists have synthesised many superhydrophobic surfaces, with a great variety of materials, ranging from inorganic nanoparticles to bulk polymeric materials, using a large number of different methods. The methods can be categorized into three groups: top-down approaches, bottom-up approaches, and a combination of both. 

Top-down approaches involve the use of templates, lithographic techniques or plasma treatments, to get the desired patterning effect. For example, a lotus leaf can be used as a template that can be coated with polymer to form a negative replica of the leaf. The negative replica can then be used as a template to create a positive replica, with the same surface structure as the leaf itself, right down to the intricate nanotextures between the minute hills and valleys. This affords the replica the same superhydrophobicity as the natural leaf.

Bottom-up methods involve building larger, more complex objects by integration of smaller building blocks or components. In the preparation of superhydrophobic surfaces, self-assembly and self-organization, such as chemical deposition or colloidal assembly, are often used. Chemical deposition is often used to make films of inorganic materials such as cadmium sulfide or zinc oxide. Depending on the material and the deposition conditions, different surface morphologies, such as nanopins, nanotubes or nanorods have been obtained.

The combination of bottom-up and top-down approaches might have the apparent advantages of both techniques. It often consists of two stages. Typically, the first step is the top-down approach for the creation of a rough surface and the second step is bottom-up process for the creation of the fine roughness. It is especially useful for the creation of architectures with a two-scale roughness, resembling the structure of the lotus leaf.

The choice of approach depends on the material and the desired surface properties. However, the criteria for the preparation of superhydrophobic surfaces are still not clearly defined. 

The accurate prediction of wetting is becoming more and more important in the design of superhydrophobic surfaces. The question of whether there is an optimal surface geometry for wetting often arises. Several models to describe superhydrophobic surfaces have been developed, but these can only give a qualitative prediction of roughness effects. 

Nevertheless, it has been recognised that for the superhydrophobic state, roughness alone is insufficient. Other factors such as asperity slopes, liquid density and surface tension must also be considered.

Read the full critical review 'What do we need for a superhydrophobic surface?' in issue 8, 2007 of  Chemical Society Reviews.