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The influence of mechanical deformations on surface force measurements

Experimental investigations of surface forces generally involve two solid bodies of simple and well-defined geometry interacting across a medium. Direct measurement of their surface interaction can be interpreted to reveal fundamental physics in confinement, i.e. independent of the particular geometry. However real solids are deformable - they can change shape due to their mutual interaction - and this can influence force measurements. These aspects are frequently not considered, and remain poorly understood. We have performed experiments in a dry atmosphere and across an ionic liquid with a Surface Force Balance (SFB), combining measurement of the surface interactions and simultaneous in-situ characterization of the geometry. First we find that, whilst the variation of the contact radius with the force across dry nitrogen can be interpreted by the Johnson-Kendall-Roberts (JKR) model, for the (ionic) liquid it is well described only by the Derjaguin-Muller-Toporov (DMT) model. Secondly, we find that mica does not only bend but also experiences a compression. By performing experiments with substantially thicker mica than usual we were able to investigate this with high precision, and find compression of order 1 nm with 7 um mica. These findings imply that, in some cases structural forces measured across nanoconfined liquids must be interpreted as a convolution of the surface forces across the liquid and the mechanical response of the confining solids. We discuss the influence of mica thickness, and propose a scaling criterion to distinguish situations where the solid deformation is negligible and when it is dominant.