The influence of liquid flow on interfacial chemistry

The natural world is full of materials being weathered or dissolved from their contact with water—by its percolating through pores, say, or its flowing over a riverbed. But distinguishing the influence of such dynamic flows from the liquid’s mere presence has proven difficult. As a material dissolves, its surface becomes charged as ions from solution bind with it, a process that both aligns the dipole moments of nearby water molecules and attracts oppositely charged ions from solution to the interface. Dan Lis (University of Namur, in Belgium), Mischa Bonn (Max Planck Institute for Polymer Research, in Germany), and their colleagues have now found that the flow of an aqueous solvent over a material can exert a dramatic influence on the charge structure. To reach that conclusion they used a nonlinear optical technique known as sum frequency generation (SFG), whose signal is exquisitely sensitive to the extent of the water molecules’ dipole alignment on two different minerals, calcium fluoride and fused silica, each held in a microfluidic channel. From the change in SFG spectra when the channel’s flow was turned on and off, the researchers determined the change in surface charge and compared it with the charge they predicted would develop during dissolution. For both minerals, the shear from a rapid laminar flow modified the charge by clearing away products of dissolution near the interface. The effect on the dissolution rate for CaF₂ was roughly equivalent to increasing the acidity of a free-standing solution by two units on the pH scale. (D. Lis et al., Science 344, 1138, 2014 doi:10.1126/science.1253793 .)