Bridging Tribology and Microrheology of Thin Films

An enhanced version of the flexure-based microgap rheometer (FMR) is described which enables rheological measurements in steady state shearing flows of bulk fluid samples of PDMS with an absolute gap separation between the shearing surfaces of 100 nm – 100 μm. Alignment of the shearing surfaces to a parallelism better then 10 rad allows us to reliably measure shear stresses at shear rates up to 10 s. At low rates and for shearing gaps < 5 μm the stress response is dominated by sliding friction between the surfaces that is independent of the viscosity of the fluid and only determined by the residual particulate phase (dust particles) in the fluid. This behaviour is similar to the boundary lubrication regime in tribology. The absolute gap control of the FMR allows us to systematically investigate the flow behaviour at low degrees of confinement that cannot be accessed with conventional (controlled normal load) tribological test protocols.