  In order to probe the dynamics of contact-line motion, we study the
macroscopic properties of sessile drops deposited on and then aspirated from
carefully prepared horizontal surfaces. By measuring the contact angle and drop
width simultaneously during droplet removal, we determine the changes in the
shape of the drop as it depins and recedes. Our data indicate that there is a
force which opposes the motion of the contact line that depends both on the
amount of time that the drop has been in contact with the surface and on the
withdrawal rate. For water on silanized glass, we capture the experimentally
observed behavior with an overdamped dynamical model of contact-line motion in
which the phenomenological drag coefficient and the assumed equilibrium contact
angle are the only inputs. For other liquid/substrate pairs, the observed
contact-line motion suggests that a maximum static friction force is important
in addition to damping. The dependence on time of contact and withdrawal rate,
reminiscent of rate-and-state friction between solid surfaces, is qualitatively
consistent across three substrate-liquid pairs.
