Direct measurement of surface forces: Recent advances and insights

The direct measurement of forces between atomically smooth mica surfaces down to sub-nanometer separation was pioneered over 50 years ago and has yielded deep understanding of a range of interfacial effects, not least the forces that determine colloidal stability and self-assembly, the properties of highly confined fluids, and the molecular origin of friction and lubrication. Here, we describe recent advances, including the use of substrates other than mica, probing the shear properties of highly confined fluids including hydration layers, and the modulation of surface forces by surface-attached macromolecules and amphiphiles, together with microscopic imaging of the surface morphology. These advances enabled novel features such as external potential control of the interacting surfaces, new understanding of lubrication in aqueous and biological systems, the design of novel nanoparticles and surface assemblies for modulating frictional dissipation, and insight into the nature of long-ranged attraction between surfactant-hydrophobized surfaces. We conclude by briefly outlining future challenges and opportunities provided by such direct surface forces studies.