Air-water surface tensions

The practical importance of monolayer formation is generally because of its relationship to reduction of surface tension. Air—water surface tension can affect such important phenomena as contact angle with a solid surface (affecting flotation), rate of wetting of a solid, or foaming (with applications in enhanced oil recovery or fire extinguishers), just to name a few. Reduction of air—water surface tension could, for example, cause a liquid to spread on a solid instead of beading up on it. 

Finally, having determined a, by the geometrical parameters obtained by DI we may calculate the air/water surface tension by... 

The air-water surface tension decreases with increasing surfactant concentration below the CMC and... 

Thermal thickness fluctuations in pseudoemulsion films mean that the amounts of adsorbed oil will vary with film thickness as a consequence of different rates of transport of oil molecules arising from differences in concentration gradient. Thinner parts of the film will therefore experience a transient lowering of air-water surface tension relative to adjacent thicker parts of the film. The resultant surface tension gradient will drive liquid away from the thinner parts of the film to the thicker parts of the film, which will increase the concentration gradient. Thickness fluctuations will therefore be amplified and will tend to destabilize the film. Amplification of such thickness fluctuations is opposed by both disjoining pressures, if dfl woCWd < 0, and capillarity. 

Even on micellar solutions of this compound, the PDMS oil forms lenses with zero spreading pressure. Similar behavior is seen with submicellar solutions even up to air-water surface tensions of 45 mN m. Since the oil spreads on water, this suggests a possible first-order wetting transition between partial wetting and pseudo-partial wetting at even higher air-water surface tensions. Rather surprisingly then, this means that no penetration of the monolayer of this silicone surfactant by the PDMS must occur. 

An example of these simulations is illustrated in Figure 4.26a, which gives two different views of an oil drop trapped in a Plateau border for which the ratio of oil-water to air-water surface tensions is a realistic 0.1 and the ratio of the spherical equivalent drop radius to the reciprocal Plateau border curvature is unity. The equivalent radius of the drop is therefore almost an order of magnitude greater than the minimum radius We see from the figure that the drop is indeed markedly deformed as a result of the high capillary pressure due in turn to the relatively high air-water surface tension. The drop shape in cross-section appears to closely conform to the... 

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