Abstract
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 $\mathrm{K}\mathrm{Cl}$ or ${\mathrm{Mg}\mathrm{Cl}}_{2}$ along the surface of a same graphene oxide flake. We show that dissolved ${\mathrm{K}}^{+}$ ions can move evenly in all directions along the interface whereas ${\mathrm{Mg}}^{2+}$ 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.
Highlights
In bulk liquids molecules diffuse randomly in all directions unless an external perturbation is imposed; no longrange molecular order is present
When operated with amplitudes comparable to the size of the interface, both approaches have been shown to probe local liquid slippage at the interface [14,23]. In the former, the liquid molecules are pushed indiscriminately as the tip squeezes out the interfacial liquid [14], while in the latter the direction of motion of the liquid molecules is defined, but at the cost of lateral resolution [23]
In order to retain the high-resolution capabilities of dynamic mode atomic force microscopy (AFM) while simultaneously gaining the directional insights from shear force microscopy, a combined vertical and lateral oscillatory motion is applied to the tip
Summary
In bulk liquids molecules diffuse randomly in all directions unless an external perturbation is imposed; no longrange molecular order is present. Averaging approaches can be problematic for irregular interfaces or systems where local contextual information is important This is the case for many biological and electrochemical systems where the efficiency of interfacial processes hinge on particular nanoscale topographic or chemical features at a given location of the solid’s surface. The energy needed to displace the liquid molecules located immediately under the tip is directly related to the local slip length [23], itself determined by the local affinity of the liquid for the solid [26] This relationship quantitatively links the ability of liquid molecules to flow along the interface with the measurable energy needed to move the tip through the interface, offering an opportunity to quantify nanoscale interfacial flow patterns. Mapping the flow patterns forming at the surface of GO in the presence of each type of salt provides molecular-level insights into the molecular mechanisms enabling ion-specific sieving by GO membranes
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