Droplet and Contact Line Friction
A droplet on a perfectly smooth and chemically homogenous surface has a unique equilibrium contact angle. If its volume increases or decreases its contact radius smoothly increases or decreases to maintain the equilibrium contact angle. However practical surfaces have roughness and chemical heterogeneity and so a pinning force has to be overcome before the contact line can move. This results in contact angle hysteresis (CAH) – the difference between the maximum contact angle before a contact line advances and the minimum contact angle before the contact line recedes. In the liquid-on-solid Amontons’ laws, the coefficient of static friction is proportional to the CAH. The force to move a droplet is proportional to the product of the CAH and the normal component of the surface tension force.
The pinning force on a droplet of water on a Slippery Omniphobic Covalently-Attached Liquid-like (SOCAL) surface is easily overcome to set the droplet into motion. Amazingly, although droplets are easy to set into motion on this surface they move slowly and have a high coefficient of kinetic contact line friction, as seen in the video below (where the drop is moving, but very slowly).
The SOCAL coating is a hydroxy-terminated PDMS surface, and a simple molecular capping to convert to a methyl-terminated PDMS surface results in a dramatic reduction in kinetic contact line friction. Drops move far more quickly on this surface, as seen in this video:
Publications
- Contact-angle hysteresis and contact-line friction on slippery liquid-like surfaces H. Barrio-Zhang, É. Ruiz-Gutiérrez, S. Armstrong, G. McHale, G. G. Wells and R. Ledesma‐Aguilar, Langmuir 36 (2020), 15094–15101
- Molecularly capped omniphobic polydimethylsiloxane brushes with ultra‐fast contact line dynamics B. Khatir, Z. Azimi Dijvejin, P. Serles, T. Filleter and K. Golovin, Small 19 (2023), 2301142