Theoretical Condensed Matter Physics
A common way to lubricate surfaces is to coat them with liquids which are able to keep them apart and reduce the friction. There are two common types of liquid lubrication, hydrodynamic and boundary lubrication. In hydrodynamic lubrication, a sufficiently thick layer of liquid is maintained between two surfaces by hydrodynamic forces as they slide (for example by having structures on the surfaces known as hydrodynamic bearings, which enhance the hydrodynamic forces holding the surfaces apart). The resulting friction between the surfaces will be viscous friction due to the shearing of the lubricant, which is generally much lower than the friction between bare solid surfaces. Hydrodynamic lubrication will also occur for an axle turning in a bearing cage, where the rotation of the axle tends to result in the lubricant being pushed under the axle, so that the axle is supported by hydrodynamic forces. For slow speed sliding, however, we must rely on boundary lubrication, in which the surfaces are separated by a layer of lubricant, which is typically not sufficiently thick to keep the highest points on the two surfaces apart, since in this case the hydrodynamic forces are not sufficiently strong to maintain a sufficiently thick liquid layer. Many liquids, including water, could serve as excellent boundary lubricants if we had a way to hold them in place as the surfaces are slid relative to each other. Human and animal joints are very effectively lubricated and are able to bear large loads at both slow and fast sliding speeds by holding a sufficiently thick lubricating layer of water in place. Three possible mechanisms for lubrication in living beings are being studied. Two of these three mechanisms are lubrication by polymer brushes and lubrication by hydrogels. The third is the ability of cartilage to cushion loads with little dissipation. This is classified as a form of lubrication because it protects the bone surfaces of the joints and reduces dissipation during motion, which is one of the functions of a lubricant. In all three of these mechanisms, it is argued that the primary mechanism for support of the load is osmotic pressure due to the counterions associated with charged polymers. The goals of this project are to provide microscopic models for these lubrication mechanisms in living creatures with the possibility that similar mechanisms can be used in other lubrication applications, including, but not restricted to, prosthetic devices.
J. B. Sokoloff, “Friction between Neutral Polymer Brushes,” Macromolecules 40, 4053-4058 (2007).
J. B. Sokoloff, “Superlubricity for Incommensurate, Crystalline and Disordered Interfaces,” a chapter in a book, “Superlubricity,” by A. Erdemir and J.-M. Martin (Elsevier, Boston, 2007).
J. B. Sokoloff, “Theory of the Observed Untra-Low Friction between Sliding Poly-Electrolyte Brushes,” Journal of Chemical Physics 120, 014901 (2008).
J. B. Sokoloff, “Static and Dry Friction due to Multiscale Surface Roughness,” Physical Review E 78, 036111 (2008).
J. B. Sokoloff, “Theory of Hydrostatic Lubrication for Two Like-Charge Polymer Hydrogel Coated Surfaces,” Soft Matter 6, 3856 – 3862 (2010).
J. B. Sokoloff, “Theory of Hydrostatic Lubrication for Polymer Hydrogel Coated Surfaces with Excess Salt,” Journal of Physical Chemistry B 115 (12),2709–2716 (2011).
J. W. Ruberti and J. B. Sokoloff, “Theory of the Short Time Mechanical Relaxation in Articular Cartilage,” submitted to the Journal of Biomechanical Engineering.
J. B. Sokoloff, “Theory of Fluid Lubrication of Hydrogels and Articular Cartilage during Compression Under an Applied Load,” submitted to the Journal of Biomechanical Engineering.
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