Dynamic behavior of confined branched hydrocarbon lubricant fluids under shear

We have measured the friction forces between molecularly smooth mica surfaces confining thin films of different branched hydrocarbons, using the surface forces apparatus (SFA). The evolution of the systems to steady-state sliding from rest or after a change in sliding velocity was thoroughly studied, and the presence of different length and time scales was observed. Using a new "extended bimorph slider" which allows continuous shearing for distances well beyond the contact diameter, we show that the evolution to steady-state sliding in these films is governed by the distance the surfaces are sheared rather than the time. From these results it is clear that both time and distance of sliding have to be considered in order to fully describe the dynamic response of lubricants and complex fluids under shear.