EFFECT OF DROP SIZE ON CONTACT-ANGLE

We have established experimentally that, for certain liquid-solid systems, the contact angle formed by a drop of liquid varies with drop size, below a critical diameter which is probably a function of the nonuniformity of the solid surface. This effect has been observed with water on Teflon FEP and on polymethylmethacrylate (PMMA), in regard to both advancing and receding contact angles. The decrease was about 8° in θa and 16° in θr for water on Teflon FEP, between diameters of 4 mm and about 1 mm; and for other systems, a comparable or even greater effect was observed. For water on PMMA, θr decreased from 51° to 26°, between diameters of 8 and 3 mm. With ethylene glycol on Teflon FEP, the decrease in θ with drop size was observed in the retreating angle only. With n-decane on Teflon FEP, the contact angle was independent of drop size, between diameters of 1 and 12 mm. For all liquid-solid systems studied, the limiting contact angles for large drops were in good to excellent agreement with the values obtained by the vertical plate method. Qualitatively, this effect could be explained by hypothesizing the existence of a large, negative line tension. It was found that there was quantitative disagreement between observed results and predictions based on this hypothesis. A theory is proposed to explain the experimental results, based on the known heterogeneity of many polymer surfaces and a previously discussed theory of hysteresis. A negative pseudo-line tension can be used in a phenomenological description of the motion and contortion of the three-phase line. © 1979.