The extrusion of liquids between highly elastic solids

Abstract

This paper surveys some of our more recent work on the extrusion of liquid films between a glass and a rubber surface. By producing optically smooth spheres and cylinders of rubber it has been possible to study the thickness and contour of the liquid film trapped between the surfaces by optical interferometry. When the surfaces are brought together in normal approach a bell of liquid is trapped at the centre of the contact region and gradually extruded until the surfaces are almost parallel and a film of the order of a few tens of nm is formed. Further extrusion or tangential sliding thus provides a means of determining the viscosity of the liquid film. For films thicker than 50 nm the viscosity is essentially the same as the bulk. If the liquid contains a low concentration of ions the liquid film collapses for thicknesses less than about 40 nm. If, however, the ion concentration is adequate (e.g.an aqueous solution of SDS or KCl) and if the ions are appropriately adsorbed on to the surfaces electrical double layers may be formed which are capable of supporting a contact pressure of the order of 10 to 100 kPa (0.1 to 1 atm). The film thickness is reduced ( a ) if the contact pressure is increased and ( b ) if the ion concentration is increased since in the latter case the ‘centre of gravity’ of the charge is moved closer to the surfaces. At the lower pressure range the film thickness is of order 25 nm (250 Å) but even in this case the viscosity is comparable with the bulk viscosity. Evidently there is little ordering of the liquid by surface forces. Thinner films appear to show an increase in resistance to shear but this may be the result of contact between asperities. Similar observations have been made using synovial fluid and saliva and it is suggested that many body fluids which contain free ions are able to provide modest boundary lubrication by the charged double-layer mechanism. The paper concludes with a discussion of the behaviour of windscreen wiper blades both in the absence and in the presence of liquid films.