Automatic detection of evaporation reducing monolayers
Mono-molecular layers, or monolayers, on the surface of water can reduce evaporation by up to 40% on small storages. Effective reduction requires the maintenance of coverage by dispensers, which must be informed by sensors capable of detecting the presence/absence of the monolayer. However, no system has yet been developed that can reliably automatically detect the presence of monolayer. Numerous detection techniques for potential field deployment, including optical and infrared reflectance, measurement of surface tension and wave damping methods were investigated. A wind assisted surface probe (WASP), based on the temperature differential between the sub-surface and volume of a small sample of periodically-entrapped water was subsequently developed. In small scale testing it has shown to be a reliable indicator of monolayer in all but very high humidities.
Understanding the role of monolayers in retarding evaporation from water storage bodies
Retardation of evaporation by monomolecular films by a 'barrier model' does not explain the effect of air velocity on relative evaporation rates in the presence and absence of such films. An alternative mechanism for retardation of evaporation attributes reduced evaporation to a reduction of surface roughness, which in turn increases the effective vapour pressure of water above the surface. Evaporation suppression effectiveness under field conditions should be predictable from measurements of the surface dilational modulus of mono layers and research directed to optimising this mechanism should be more fruitful than research aimed at optimising a monolayer to provide an impermeable barrier.