The effect of contact line dynamics and drop formation on measured values of receding contact angle at very low capillary numbers

Owing to contact angle hysteresis, the contact angle that a sessile drop adopts on a surface can be quite different depending on the process of drop formation. This is particularly noticeable for receding contact angle because it is most susceptible to the details of the measuring method. We performed low-rate dynamic contact angle experiments with steadily moving contact lines and millimeter-size drops in the very low-capillary number regime. We employed a quadratic volumetric flow rate (V alpha t(3)) to achieve the steady motion of contact lines such as happens in the Wilhelmy balance. The values of receding contact angle provided with the quadratic flow rate were stable and time-independent over a larger area of the polymer surfaces studied. Next, we monitored receding sessile drops with equal initial volume but different static contact angle on the same surface. This procedure allowed us to scan the contact angle hysteresis range up to the minimum observable value of contact angle. We probed with distilled water the different response of surfaces of polymer (PMMA, PC and PTFE) and metal oxide (titanium). We found two behaviors according to the substrate employed: a constant value of receding contact angle regardless of the static contact angle and a decrease of receding contact angle as the static contact angle decreased. (C) 2012 Elsevier B.V. All rights reserved.