Determination of the receding contact angle of sessile drops on polymer surfaces by evaporation

The receding contact angles of water drops on PMMA and PET surfaces were determined by using video microscopy to follow the time-dependent evaporation of sessile drops. Depending on the initial drop size, receding angles of theta(r) = 54-64 degrees for PMMA and theta(r) = 64-66 degrees for PET were found with an average hysteresis of 23.5 +/- 1.5 and 19.5 +/- 1.5 degrees, respectively. Advancing and receding angles, theta(a) and theta(r), were also determined by the needle-syringe and the inclined plane methods for comparison. The discrepancies from the mean of the maximum and minimum contact angle results of both the needle-syringe and the inclined plane methods were larger than expected for all the polymer surfaces. A general trend was seen with samples giving a larger hysteresis also producing a larger discrepancy for all the samples. The major cause of this discrepancy is the variation of the rate of liquid introduction and withdrawal with the syringe in the needle-syringe method and the drop size effect in the inclined plane method. In this respect the drop evaporation method allows a rate of liquid withdrawal which minimizes (or standardizes) the linear rate of retreat effect on receding contact angle measurement. The literature values are also given for comparison. A discrepancy of 11-15% from the mean for theta(a) and 27-32% for theta(r) is reported in the literature for these polymers. This is approximately five times more than the previously claimed 2-3% deviation from the mean for theta(a) on the same clean homopolymer samples.