Nanoscale mapping of the directional flow patterns at liquid-solid interfaces

The nanoscale behavior of liquid molecules and solutes along the interface with solids controls many processes such as molecular exchanges, wetting, electrochemistry, nanofluidics, biomolecular function, and lubrication. Experimentally, several techniques can explore the equilibrium molecular arrangement of liquids near the surface of immersed solids but quantifying the nanoscale flow patterns naturally adopted by this interfacial liquid remains a considerable challenge. Here we describe an approach based on atomic force microscopy, and able to quantify the flow direction preferentially adopted by liquids along interfaces with nanoscale precision. The approach, called vortex dissipation microscopy (VDM), uses high-frequency directional oscillations to derive local flow information around each location of the interface probed. VDM effectively derives nanoscale flow charts of the interfacial liquid parallel to a solid and can operate over a broad range of soft and hard interfaces. To illustrate its capabilities, we quantify the dynamics of aqueous solutions containing KCl or MgCl2 along the surface of a same graphene oxide flake. We show that dissolved K+ ions can move evenly in all directions along the interface whereas Mg2+ ions tend to move in registry with the underlying lattice due to enthalpic effects. The results provide in situ nanoscale insights into the ion-specific sieving properties of graphene oxide membranes.