Reducing Water Evaporation Using Nanoparticles

Abstract

<p>Water evaporation rates can be reduced by covering the surface of the water resource with a monomolecular layer of material. This surface layer is a one molecule layer at the air/water interface, and effective monomolecular layers form from amphiphilic long-chain carbon molecules with a polar head group and long-chain hydrocarbon tail. Such surface-active molecules self-organise on the water's surface with the polar head group adsorbed to the surface of the water and the non-polar tail extending away from the surface. Previous studies have proposed that these systems operate via a barrier mechanism, i.e., a hydrophobic barrier to diffusion that limits evaporation by slowing the movement of molecules away from the surface. Evaporation reduction is correlated to wave damping capacity and an alternative mechanism proposed is that wave suppression reduces the circulation of air on the surface of the water, and hence the rate of removal of water-saturated air away from the boundary region. The hypothesis of this work is that wave suppression is the main mechanism by which monomolecular surface layers work. Commercial hollow polystyrene nanoparticles (Ropaque) were applied to test this hypothesis. Ropaque nanoparticles are round, rigid, and of approximately 400 nm diameter, and so they should not be able to form a continuous barrier on the surface because they are around the shape. Thus, they are not able to provide an impermeable barrier at the surface and cannot be acting by the barrier mechanism. This research evaluated the evaporation reduction and wave damping capacity of Ropaque nanoparticles under different conditions and compared the behaviour with hexadecanol and octadecanol monolayers. The detailed results of evaporation reduction tests with different wind speeds of pure water both in the water tank and with the dam water in water pans showed that the Ropaque exhibited a greater increase in saving water through the prevention of evaporation than the hexadecanol and octadecanol monolayers. In practical terms, the Ropaque nanoparticles gave better evaporation control than the chemical monolayers investigated in this study.</p> <p>It was determined that Ropaque nanoparticles were less easily dispersed by high winds speeds and remained effective for longer. Furthermore, in wave damping studies, Ropaque nanoparticles increased the wave damping capacity at the frequency range between 1 and 2 Hz, which is most important for damping capillary waves to a greater extent than hexadecanol and octadecanol.</p>