Bridging Tribology and Microrheology of Thin Films
By Christian Clasen, H. Pirouz Kavehpour, Gareth H. McKinley
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 10-7 rad allows
us to reliably measure shear stresses at shear rates up to 104 s-1. 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.
Keywords: microrheology, thin film rheology, tribology, boundary lubrication, sliding plate rheometer, micro gap, FMR