Hydrodynamics in nanoscale confinement: SFA and colloid probe AFM liquid drainage experiments

Abstract

Flow and drainage of very thin liquid films play an important role in mineral recovery, drop coalescence and emulsion stability, as well as lubrication of micromechanical devices. Studies of liquid flow under strong confinement (i.e., film thickness below a few hundred of nanometers and down to a few nanometers) can reveal the limits of applicability of a classical hydrodynamics description, but are very challenging. The Surface Force Apparatus (SFA) technique has enabled studies of drainage at nanoscale separation between atomically smooth mica sheets. The development of the colloid probe Atomic Force Microscope (AFM) as an alternative technique has allowed a significantly wider variety of confining solid surfaces to be studied. Both the SFA and the colloid probe AFM have been adapted to permit the surfaces confining the film to be soft, e.g., the surface of a drop or bubble, and therefore deformable. We present a succinct review of the experimental and theoretical modeling challenges for such studies and critically discuss the outcomes of recent experiments.