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|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