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The evaporation of a small droplet of a volatile fluid from a solid surface differs according to whether the fluid wets the surface. When the initial contact angle is less than 90°, the evaporation initially proceeds with an accompanying decrease in the contact angle but no change in the contact radius. This stage of evaporation dominates the time scale and has previously been described by a diffusion model. However, when the initial contact angle is greater than 90°, it is the contact radius that decreases rather than the contact angle. In this work we report detailed measurements of the evaporation of drops of water from Teflon film. On deposition the contact angles rapidly relax from around 112° to 108°, which is the equilibrium value for droplets in a saturated vapor. The angle then remains constant for the majority of the evaporation time. Eventually the system changes to a mode of evaporation in which both contact angle and radius change simultaneously. The data are compared to an extension of the diffusion model, and this provides estimates of the diffusion coefficient. It is suggested that the constant contact angle observed during much of the evaporation is a result of a local saturation of the vapor in the region of the contact line.

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 θr = 54−64° for PMMA and θr = 64−66° for PET were found with an average hysteresis of 23.5 ± 1.5 and 19.5 ± 1.5°, respectively. Advancing and receding angles, θa and θr, 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 θa and 27−32% for θr 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 θa on the same clean homopolymer samples.

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