Hysteresis of Contact Angle of Sessile Droplets

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 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 a or θ r 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.