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February 2007

Environmental influences on drop size / contact angle relationship
Over the years a number of studies have attempted to determine the relationship between environmental factors and the drop size / contact angle relationship. Good and Koo1, for example, concluded that in general the contact angle decreased as the drop diameter decreased -- at least with water and ethylene glycol. With n-decane, however, there was no change. In their study, temperature was ambient while the gas phase was air-saturated vapor.

In a separate study Ponter and Boyes2 examined the contact angles and drop diameters of water and benzine on copper and Teflon solids. Interestingly their results reflect an opposing phenomenon: as the drop diameter decreased, the contact angle increased. In their experiments the gaseous phase was a pure vapor at the boiling point over a range of pressures.

In an attempt to understand the reason for these opposing conclusions, it's beneficial to look at some environmental factors to better understand their influences on the contact angle as the drop size changes. Some have concluded that the decrease in contact angle as the drop volume and drop diameter increase can be explained as a function of increased gravitational forces. Good and Koo1 refuted this theory.

Bernet and Zisman3 studied the effects of relative humidity (RH) on critical surface tension. They found that extreme changes in RH, from 1% to 95% had some impact on the contact angle of water on a dozen different metals. They observed as RH increased the contact angle would increase, the rate of spreading would decrease, and the critical surface tension would decrease.

Ponter and Yekta-Fard4 attempted to better understand the effects of humidity, pressure, temperature, and a change in the gas phase on the drop size / contact angle relationship. They measured contact angles of water on Teflon, copper, stainless steel, and PMMA as well as n-decane on Teflon -- all with a range of drop sizes. Measurements were taken at 25° C in an air or nitrogen-saturated atmosphere and then compared with measurements taken at boiling point. They observed a decrease in contact angle as the drop size decreased for water on Teflon and PMMA while noting an increase in contact angle as drop size decreased at the boiling point in a pure vapor atmosphere. This study indicates that the effect of the equilibrium spreading pressure at the solid-vapor interface may have an influence toward higher contact angles as the drop volume decreases.

1 Good RJ, Koo MN (1979), Journal of Colloid Interface Science, 71:283
2 Boyes AP, Ponter AB (1974), Journal of Chemical Engineering Japan, 7:314
3 Bernett MK, Zisman WA (1968), Journal of Colloid Interface Science, 28:243
4 Ponter AB, Yekta-Fard M (1985), Journal of Colloid & Polymer Science, 263:673-681

Tools of the Trade
It is worthy to point out that all of the above studies can be carried out using ramé-hart instruments and accessories. Contact angle can be measured using Models 100, 120, 200, 250, 300, and 500. Critical Surface Tension can be measured with Models 200 and up. Studies conducted in a humidity and temperature controlled environment can be accomplished with Model 300 or 500 and our optional Environmental Chamber with Humidity (p/n 100-07-H). Captive bubble studies can be accomplished with Model 200 and up and the optional Environmental Fixture (p/n 100-07-60) at ambient temperatures or with our Environmental Chamber (p/n 100-07), Quartz Cell (p/n 100-07-50), and Proportional Temperature Controller (100-50) in temperature controlled environments up to 300° C. Advancing and receding studies can be carried out very precisely with Model 250 or 500 and our Automated Dispensing System (p/n 100-22-100).

Please contact us for more information on any of the above products or for a quotation.



Carl Clegg
Director of Sales
Phone 973-448-0305
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