Contact line friction in aspirated sessile drops*

We study the macroscopic properties of sessile drops deposited on and aspirated from silanized glass slides. By observing the contact angle and drop width simultaneously, we can determine the depinning angle and motion of the contact line as the drop is removed. Young’s formulation and models of contact angle hysteresis both predict a constant contact angle after depinning. In contrast, we observe that the contact angle continues to change during the drop removal process. Our data suggest a contact line friction that opposes the motion of the drop’s edge. As has been seen previously, this force is dependent on the contact line speed [1]. We show that it depends also on the concentration of the solute and the amount of time that the drop has been in contact with the surface.

[1] Gao, N., Geyer, F., Pilat, D. W., Wooh, S., Vollmer, D., Butt, H. J., & Berger, R., Nat. Phys. 14(2), 191-196 (2018).