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Liquid flow along a solid surface reversibly alters interfacial chemistry

Science  06 Jun 2014:
Vol. 344, Issue 6188, pp. 1138-1142
DOI: 10.1126/science.1253793

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Monitoring water interfaces in motion

Water behaves differently at interfaces—where it meets the air, or a solid surface—than it does in the middle of the liquid. Past laboratory studies of this phenomenon have mainly focused on still samples, despite the fact that in natural settings such as rivers and rain, the water moves along the surfaces. Lis et al. used a microfluidics apparatus and a spectroscopy technique called sum frequency generation to study the effects of flow on aqueous chemistry at silica and fluorite surfaces (see the Perspective by Waychunas). The flow of fresh water along the surfaces disrupts the equilibrium of dissolved ions, substantially changing the surface charge and the molecular orientation of the water at the interface.

Science, this issue p. 1138; see also p. 1094

Abstract

In nature, aqueous solutions often move collectively along solid surfaces (for example, raindrops falling on the ground and rivers flowing through riverbeds). However, the influence of such motion on water-surface interfacial chemistry is unclear. In this work, we combine surface-specific sum frequency generation spectroscopy and microfluidics to show that at immersed calcium fluoride and fused silica surfaces, flow leads to a reversible modification of the surface charge and subsequent realignment of the interfacial water molecules. Obtaining equivalent effects under static conditions requires a substantial change in bulk solution pH (up to 2 pH units), demonstrating the coupling between flow and chemistry. These marked flow-induced variations in interfacial chemistry should substantially affect our understanding and modeling of chemical processes at immersed surfaces.

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