hysteresis
Contact Angle Hystersis is what explains the common phenomenon of water droplets sticking to a vertical surface despite gravity pulling them down. The manipulation of advancing and receding contact angles on a given surface is a key element of surface texture manipulation, especially in relation to icephobic and superhydrophobic surfaces. The affects have driven research towards specific nano-structures and nano-textures that use contact angle hysteresis to their advantage to remain water repellent and resistant towards ice formation.
Why Contact angle hysteresis is important
For research on icephobicity and hydrophobicity, contact angle hysteresis is the most crucial concept when constructing surfaces and finding the most productive level of roughness or surface texture. Contact angle hysteresis (CAH) refers to the phenomenon of a water droplet stickning to a surface despite the surface being at an angle, which is a reflection of the difference between advancing and receding contact angles. Lower CAH would correlate to higher hydrophobicity. Think of CAH as the level of velcro a water droplet has to a surface, high CAH results in the ability for a surface to be at a sharp angle while the drop still maintains its position. For obvious reasons, this would not be ideal for hydrophobicity (as it would increase wetting) or icephobicity (as it would increase the likelihood of a phase change and ice formation). To connect to the Lotus Effect which was explained on the Superhydrophobicity page, the dominance of the Cassie state and the large contact angle, which is the prime example of low CAH, is practically the key characteristics of the Lotus Effect which is why the Lotus leaf remains the go-to example in nature for hydrophobicity and icephobicity surface construction.
For certain contact angles, the CAH effect can greatly affect the adhesion on a surface. Thinking of practical applications, high adhesion can inhibit the self-cleaning properties of surfaces; if liquid sticks to a surface for longer, it’s harder to clean. Since superhydrophobic surfaces are often associated with their ability to stay clean (low wettability and high water repellency), a high adhesion associated with CAH that takes away the cleanliness of the surface is therefore considered a worse superhydrophobic surface.