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Fig. 2. Comparisons between colloid distributions in pendular rings on hydrophilic sand grains (a) without and (b) with evaporation. The larger pictures represent enlargements of the boxed region in inserts. Air–water, water–solid, and air–solid interfaces are denoted by AW, WS, and AS, respectively. When local equilibrium exists between capillary or adsorbed water and water vapor in the soil gas phase, no net water loss (evaporation) occurs along AW interfaces. This dynamic equilibrium between evaporation and condensation, depicted by equal number of blue arrows leaving and entering the AW interface in (a) and dry (less than monolayer coverage) AS interface in (b). Both AS interfaces and the AWS contact lines exist only in systems with net evaporation. While film straining of occurs where pendular rings transition into adsorbed water films (a), net water flow (denoted by v) and colloid transport to AWS contact lines occur only in evaporating systems (b) where the film straining effect is greatly amplified.
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