How Confined Lubricants Diffuse During Shear

Abstract

The translational diffusion of a fluorescent dye embedded at a dilute concentration in a confined fluid was compared at rest and during shear. The fluid, octamethylcyclotetrasiloxane (OMCTS), was confined between step-free muscovite mica to thickness 3-4 layers. Fluorescence correlation spectroscopy showed that the time scales of intensity-intensity autocorrelation functions were essentially the same during shear and at rest, except they were faster during shear by a factor of 2 to 5. This dynamical probe of how liquids order in molecularly thin films fails to support the hypothesis that shear produced a melting transition. DOI: 10.1103/PhysRevLett.93.236105 PACS numbers: 68.35.Af, 68.08.-p A curious fact about liquids is that they wrap themselves around solid surfaces to form layers; the simple reason is that space must be filled. Those molecules closest to the surface form the best-defined layer but the number of layers is believed to extend several molecular dimensions, to a distance normal to the surface approximately the correlation length of the bulk fluid. The layered structures on opposed surfaces interfere with one another when the spacing between two solids is less than twice this length [1]. Questions about confined fluid structure involve deep scientific puzzles. On the applied side, they also pertain directly to understanding the physics of porous media, of dense suspensions of colloidal particles in fluids, and of friction and lubrication.Here we employ a fluorescence-based method to contrast the rate of Brownian diffusion during shear and at rest. Previously we introduced a method of few-molecule fluorescence to study confined fluids [2,3] but that study was restricted to fluids at rest. Here we describe what happens while sliding one surface past the other. The premise was that if the hypothesis of shear melting held, shear should induce speed up of the translational diffusion of a nonadsorbing fluorescent dye.The fluid, containing dilute dye, was confined to a thickness of 3-4 molecular dimensions between parallel single crystals of muscovite mica. A large amount of prior study probed the friction of such systems. The hypothesis of a shear-induced phase transition, from solid to fluid, has been advanced based on the observation that, in most such experiments, static friction gives way to kinetic friction in molecularly thin liquid films [4]. The generality of this interpretation has been cast into doubt by the observation that fluids confined between rough metal surfaces behave similarly [5] and especially by recent experiments showing that the solidity of molecularly thin films in mica-based experiments depends on a particular method of surface preparation [6,7]. As arguments about experimental protocols are surely of no interest outside a specialized community, it is opportune to address the important question of order in confined fluids by an independent technique. Fluorescence correlation spectroscopy (FCS) is usually used to study molecules dissolved in bulk solution [8,9]. As implement...

The unique properties of the solid-like confined liquid films: A large scale molecular dynamics simulation approach

“…The diffusion coefficient decreases exponentially from the edges towards the center of the contact area. These results associated with the collective motion of the liquid molecules indicate the lateral ordering of the confined liquid [9] . The mechanism of the slip at the liquid-solid interface and between the layers of the confined liquid is of particular importance for the understanding of the shear response of the confined ultrathin films.…”

“…films [9] . The activation volume for diffusion exceeds that of the bulk liquid by 3 orders of magnitude.…”

“…As a consequence, the properties of the confined liquid become dramatically different from those of the bulk state. The new dynamic behaviors of the confined liquid include: the density oscillations near the solid walls [3,4] , the enhanced effective shear viscosity [5,6] , the prolonged relaxation time [2] , the transition to solidification [7,8] , the reduced diffusion coefficient [9] , etc. These solid-like properties cannot be simply understood according to the classical theory of the bulk liquid.…”

Frontiers of fundamental tribological research

“…This has allowed interferometry to be used to precisely measure film thicknesses as a function of applied load and sliding speed and to provide an experimental verification of the equations describing EHD lubrication. More recent advances have considerably improved the precision of such interferometric measurements potentially allowing much thinner boundary layers to be investigated [12][13][14][15][16]. In addition, optically based spectroscopies [17], for example, laser Raman spectroscopy, have exploited this strategy to provide chemical information at the tribological interface [18].…”

“…While the friction of lubricated surfaces in now reasonably well understood [10] and has benefited from in-situ interferometric [12,16] and spectroscopic measurements [17,18] of the elastohydrodynamic interface, our understanding of energy dissipation within the boundary layer regime, or at the solid-solid interface is much less well developed. This is partly due to the problems outlined above in defining contact areas.…”