Experimental and theoretical investigation of contact angle variation for water–ethanol mixture droplets on a low-surface-energy solid

Yonemoto and Kunugi (2016), provided an understanding on the variation of the relationship between contact angle and contact area radius for droplets on a low surface energy solid which is important in industrial applications from engineering perspective. The authors developed an analytical model that can treat sequential change in the contact angle caused by volume reduction which focuses on adsorption and adhesion on solid-liquid interface.

When the droplet volume decreases, the contact line is pinned and the contact angle decreases. After the contact angle reaches a specific value, the contact line de-pines and starts to recede where the contact angle also changes. This kind of complex behavior is still unknown and a definite solution is still missing.

Yonemoto and Kunugi (2016), thereby considered a physicochemical description such as liquid-to-solid surface adsorption to give a consistent interpretation of the complex wetting behavior.

For the wettability phenomena experiment, Yonemoto and Kunugi (2016), used six mixture different surface energy density (s) of binary water-ethanol; one of ultrapure (s_lg = 0.0719 Jm^-2), four different binary liquid mixtures (s_lg = 0.0570 (5 vol%), 0.0380 (25 vol%), 0.0318 (35 vol%) and 0.0248 (75 vol%) Jm^-2) and pure ethanol (0.0211 Jm^-2) 99.5 vol%. To investigate the variation of the contact angle, droplets were deposited on the solid Polytetraflouroetheylene (PTFE) surface using calibrated micropipette. Two experiments are performed: first involved multiple liquid extraction using microsyringe while the second was based on natural evaporation.

From the results, Yonemoto and Kunugi (2016) concluded that complex variation of the contact angle correlates to the liquid molecule adsorption at the solid-liquid interface. In addition to this, amount of adsorbed liquid molecules will depend on solid surface properties.