Hysteresis of Contact Angle of Sessile Droplets

Department of Chemical Engineering, Loughborough University, LE11 3TU, UK.

^⋆ Corresponding author. E-mail: V.M.Starov@lboro.ac.uk

Abstract

A theory of contact angle hysteresis on smooth, homogeneous solid substrates is developed in terms of shape of disjoining/conjoining pressure isotherm and quasi-equilibrium phenomena. It is shown that all contact angles, θ, in the range θ_r<θ<θ_a, which are different from the unique equilibrium value θ_e, correspond to the state of slow “microscopic” advancing or receding motion of the liquid if θ_e<θ<θ_a or θ_r<θ<θ_e, respectively. This “microscopic” motion almost abruptly becomes fast “macroscopic” advancing or receding motion after the contact angle reaches the critical values θ_a or θ_r, correspondingly. The values of the static receding, θ_r, and static advancing,θ_a, contact angles in cylindrical capillaries were calculated earlier, based on the shape of disjoining/conjoining pressure isotherm. It is shown that both advancing contact and receding contact angles of a droplet on a solid substrate depends on the drop volume and are not a unique characteristic of the liquid-solid system. The suggested mechanism of the contact angle hysteresis of droplets has direct experimental confirmation.