Experimental investigation on the effect of accelerating motion of contact line on the dynamic contact angle

The dynamic contact angle, the angle between the surfaces of the liquid and the solid at a moving contact line, is one of the primary conditions for the dynamics of the liquid surface confined by solid wall. It has been commonly modeled as a function of the velocity of the contact line relative to the solid surface under steady state condition. Some situations, such as the impact of a drop on a solid surface or sudden release of a contact line stuck on an edge of solid, however, involve a transient motion of the contact line, which would lead to a deviation of the dynamic contact angle from the steady one. In this study, dynamic contact angle for an accelerating or decelerating contact line advancing on glass surface is measured in two experimental systems different with other, i.e. one with a rapid meniscus formation after contact of a descending rod with the liquid surface, and the other with accelerating advance of liquid column in a circular tube. The experimental results revealed that the contact angle is increased with the (positive) acceleration of the contact line relative to the solid and vice versa. The critical condition for the appearance of the deviation of the contact angle brought with the acceleration is also discussed.