Wetting of superhydrophobic and superoleophobic surfaces

Wettability can be defined as the property of a material to be wetted by a liquid. Controlling the wettability of a material is a fundamental issue for a large number of applications. In addition to chemistry, the adjustment of surface texture is crucial to wettability control. This texture can be used, for example, to shape droplets, direct wetting, and increase or decrease wettability.

The remarkable superhydrophobic properties of lotus leaf, linked to surface microtexture by Neinhuis and Barthlott in the late 1990s, served as a reminder of the extreme non-wetting properties that can be achieved by texturing a surface. Since then, numerous efforts have been made to reproduce such superhydrophobic surfaces, culminating in superoleophobic textures that are not wetted by even low surface tension liquids such as oils.

Progress in the development of non-wetting surfaces has been such that the droplet technique, traditionally used to characterize the wettability of a surface, is no longer applicable to these surfaces, as the wetting angles achieved are very high and the anchoring of the liquid extremely weak or non-existent. What's more, some of the applications promised by these new materials in many cases require characterization of the stability of the air trapped between the material and the liquid, which is responsible for the non-wetting properties. As a result, it soon became necessary to propose alternatives to the drop-on-demand technique for further characterizing the wettability of these surfaces.

The aim of this article is to present techniques for characterizing the wettability of textured surfaces with superhydrophobic and superoleophobic properties, in addition to the classic drop-on-drop technique. It begins with a review of wettability, discussing in particular contact angle and wetting hysteresis, which are the basic experimental parameters for characterizing wettability, and used to qualify non-wetting behavior. It then goes on to describe the different wetting regimes of a textured surface, one of which is responsible for superhydrophobic and superoleophobic behavior. The aim is to enable the reader to differentiate between the different behaviors that can be observed, possibly on the same textured surface. In this context, a reminder is also given of wetting transitions, which demonstrate the instability of the non-wetting state (i.e. the instability of the trapped air at the origin of this behavior), and whose consideration in an experimental approach can make it possible, as we shall see, to qualify the stability of the non-wetting state under different conditions.

Superhydrophobic and superoleophobic surfaces are then defined, introducing historical concepts and presenting examples found in nature,...