Static contact angle hysteresis on smooth, homogeneous solid substrates

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 contact angle θ ≠ 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 = θ a or θ r = θ, 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 pressure isotherm. It is shown that an advancing contact angle of a droplet on a solid substrate depends on the drop volume and is not a unique characteristic of the liquid–solid system. The suggested mechanism of contact angle hysteresis has direct experimental confirmation.