Self-cleaning surfaces

Article By:

Quéré, David Physique de la Matiere Condensee, College de France, Paris, France.

Last reviewed:2005

DOI:https://doi.org/10.1036/1097-8542.YB050700

Many types of contaminants affect solid surfaces, forcing recurrent cleaning. For example, dust or soot particles or oil aerosols are transported by air until they meet a solid surface on which they will often adhere. Rain may help remove these particles from outdoor surfaces, but it only partially solves the problem. Because solids are imperfect (rough and chemically heterogeneous), the contact angle (measured by depositing a drop and looking at the angle with which it meets its substrate) of water drops can vary in a large interval. The amplitude of this interval is called the contact angle hysteresis. As a consequence, drops can stick to solids, even when the surfaces are inclined, which affects the transparency of the materials (for example, windshields or window panes) or contributes to degrade them (for example, concrete). If the drop is pinned (stuck), the dirt it contains gets concentrated at the contact line (edge), which eventually leads after evaporation to a ring of dirt much more visible than the initial distribution of dust. This is known as the coffee stain effect: as a drop evaporates, a flow takes place, driving matter from the center to the periphery of the drop with higher surface/volume ratio.

The content above is only an excerpt.