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Show 170 A WATER NANODROPLET SKATING ON ICE? Anthony Oyler (Valeria Molinero) Department of Chemistry University of Utah RESEARCH POSTERS ON THE HILL SPRING 2012 Despite the importance of water, little is known on the structure of the first layers of water molecules in the surface of materials. These first layers of water are important for the chemical reactivity of the surface and also for physical properties such as friction and wetting. In this work, we use molecular simulations to investigate the ordering and phase transformations of water deposited on flat surfaces with different strength of the water-surface attraction, where it forms an ice bi-layer. The question we address in this work is whether liquid water wets the ice bi-layer or is it repelled and forms a droplet on top of it? A water nanodroplet skating on ice? Anthony Fratto Oyler and Valeria Molinero DEPARTMENT OF CHEMISTRY Student Photo Logo For Student Support We used molecular dynamics simulations to characterize the phases of interfacial water as a function of the amount of water at the surface, the temperature, and the attraction between the water molecules and the surface. We deposited water on flat surfaces with water-surface attraction ε between 1.5 and 2.5 kcal/mol at temperatures between 200 and 270 K, -99.4 and 26.6 °F. We found that the interplay between the water-water interactions and the water-surface interactions results in six distinct structures that include a bi-layer crystal tiled with flat hexagons, a thin liquid film, and unique structures in which a liquid nanodroplet skates on the surface of the bi-layer ice crystal. Phase Diagram of interfacial water deposited to a density of 23.4 molecules/nm2. Blue and red circles indicate the conditions in which a liquid nanodroplet skates on an ice bi-layer. Deposition of water on surface with interaction strength 2.2 kcal/mol, at 220 K. Surface densities are 3.1-11.7-23.4 water molecules/nm2. Ice bi-layer shown in black, and droplet in red. Surface is not shown. Shown above are different scenarios for water on a surface. The droplet is sketched in red, bi-layer ice in black, and surface in yellow Conclusions 1 - Interfacial water has a rich phase behavior that includes at least six distinct structures. 2 - The liquid droplet partially wets the ice bi-layer, but does not spread completely. The ice bi-layer is mildly hydrophilic. 3 - Crystallization of the droplet can be attained by i. Cooling the droplet at constant size, or by ii. Increasing the size of the droplet, through deposition of water, at a constant temperature. 4 - The melting of the ice bi-layer and the water droplet can be controlled independently: the melting temperature of the ice bi-layer increases with the water-surface attraction. The melting temperature of the droplet on top of the bi-layer increases with the size of the droplet. 5 - A nanodroplet skating on a bi-layer ice surface can be attained for strong water-surface interactions and surfaces from 220 K up to room temperature. At the lowest temperatures, the droplet is supported on top of the ice bi-layer and at the highest temperatures the liquid droplet wets the surface. Anthony Oyler Valeria Molinero The growth of the droplet by deposition under constant temperature leads to crystallization because the melting temperature increases with the droplet radius Ice bi-layer is shown in black, the droplet in red, and surface is not shown B-N D-X F-X ε=1.8 kcal/mol, T=210 K B-N D-L F-L ε=1.8 kcal/mol, T=220 K B-X D-T F-T ε=1.9 kcal/mol, T=220 K B-X D-L F-L ε=2.4 kcal/mol, T=240 K B-X D-L F-X ε=2.3 kcal/mol, T=220 K B-X D-X F-X ε=2.3 kcal/mol, T=230 K The six distinct structures of water deposition found. The bottom two depict a droplet skating on ice http://en.wikipedia.org/wiki/File:Ion-mask_water_droplet_material_surface_2.jpg Addition of more water B-N D-X F-X ε=1.8 kcal/mol 210 K Hydrophobic surface B-X D-L F-X ε=2.3 kcal/mol 230 K Hydrophilic surface 6.3 molecules/nm2 12.5 molecules/nm2 23.4 molecules/nm2 27.3 molecules/nm2 19.5 molecules/nm2 30.5 molecules/nm2 Complete wetting Partial wetting Non-wetting on a bi-layer Non-wetting on a surface Water droplet on a hydrophobic surface Red represents a liquid droplet on a crystal bi-layer while blue is a liquid droplet on a locally melted bi-layer. Despite the importance of water, little is known on the structure of the rst layers of water mol ecules in the surface of materials. These rst lay ers of water are important for the chemical reactivity of the surface and also for physical properties such as friction and wetting. In this work, we use molecular simulations to investigate the ordering and phase transformations of water deposited on at surfaces with di erent strength of the water-surface attraction. The primary ques tion of this work is whether the rst layers of water deposited on a surface may make the surface less attractive to further addition of water, i.e. hydrophobic. We used molecular dynamics simula-tions to characterize the phases of interfacial water as a function of the amount of water at the surface, the temperature and the attraction between the water molecules and the surface. We found that the interplay between the water-water interactions and the water-surface interactions results in six distinct structures that include a bilayer crystal tiled with at hexagons, a thin liquid lm and unique structures in which a liquid nanodroplet skates on the surface of the bilayer ice crystal. The shape of the skating nanodroplet suggests that the surface of the bilayer ice is not very hydrophilic as the droplet does not spread on the ice surface. An increase in the size of the droplet, by further deposition of water, results in its crystallization to form a droplet of ordinary ice. Di erent from our everyday experience of making ice by cooling water, this process of crystallization occurs at constant temperature and is due to the increase in the melting tem-perature of the nanodroplet with its size. |
