  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.
