Comparison of contact angle measurements by three methods
The objective of this study was to compare the accuracy and precision of interfacial contact angle measurements estimated by three methods, two manual and one instrumental. Contact angles were measured at the liquid/liquid/solid and liquid/air/solid interfaces using either a platinum or a Teflon plate. The apparatus used for the manual methods consisted of an optical cell and a Wilhelmy plate. Photographs of the interface were employed to estimate the contact angle. The tangent of the liquid interface at the point of contact with the plate was estimated by two methods. Method I involved subjective determination of the position of the tangent. Method II employed an additional step of completing the circle formed by the arc of the liquid interface contacting the plate to aid placement of the tangent. The optical cell, a Wilhelmy plate, a motorized sample holder and a Cahn 2000 recording microbalance were used for the instrumental method (III). The microbalance was used to measure tension values from which the contact angles were calculated. Similar estimates of contact angle were obtained by each method for the majority of the interfaces studied. In a few cases the manual methods were less accurate than the dynamic technique. For example, the contact angle measured at the chloroform/water interface using method I (photograph A) was 51.8 ± 8.23. The same contact angle measured using method II was 44.9± 11.8 and using method III it was 38.9 ± 0.00. The instrumental method was the most reproducible (standard deviation < 0.231). In all cases the manual methods were less precise (standard deviation 7.37–12.3) than the dynamic technique.