Theoretical Condensed Matter Physics
The mechanisms by which both charged and neutral polymer hydrogels (soft 3D networks that absorb large quantities of water) lubricate and prevent surfaces from adhering to each other are being studied. Such properties of hydrogels play an essential role in virtually all biolubrication. For example, mucous secretions, which are an important lubricant, tend to form gels. Cartilage is also a gel-like material, containing proteoglycan molecules, which are charged, and hence contribute counterions, which tend to keep cartilage swollen with fluid. It will likely also result in a thin liquid layer on the surface of the cartilage, which plays an essential role in lubricating joints during slow speed motion. Lubrication by hydrogels also plays an essential role in various biomedical applications, such as contact lenses, catheters. The recent advent of tough hydrogels will likely be used as artificial cartilage in the near future. This would be an extremely great advance in joint replacement therapy, because the mechanism of lubrication of hydrogels by a water layer at their surface (for polyelectrolyte hydrogels) that will be studied has the potential of preventing, or at least significantly reducing, the generation of wear particles, which is the primary cause of failure of present day artificial joints. These theoretical studies will be performed primarily using modeling and scaling methods, but the theory will be verified by molecular dynamics simulation and computational mechanics.
A theoretical treatment is provided for effects of capillary forces on a hemispherically shaped hydrogel sample pressed against a solid hydrophilic surface. It is pointed out that the adhesion of a hydrogel to a surface resulting from capillary forces is different for hydrogels from that of a nonporous solid because of the porous nature of the hydrogel. Effects of Laplace pressure on the degree of swelling of a hydrogel and its effects on friction between the gel and the solid surface are also studied. It is demonstrated that there are important differences in the effects of capillary forces for neutral and charged hydrogels as a consequence of the fact that for charged hydrogels, there is a competition between the repulsive osmotic pressure provided by the counterions and the attractive Laplace pressure, whereas for neutral gels there are no counterions.
The compression of polyelectrolyte microgel particles in a salt-free highly compressed colloid due to osmotic pressure outside of the particles due to counterions located there is studied for a model based on a quasi-analytic solution of the Poisson-Boltzmann equation and a model for the gel elasticity based on counterion osmotic pressure inside the particles and polymer elasticity (of entropic origin). It is found that for particles of radius of the order of a tenth of a micron, the counterion osmotic pressure should play a significant role in the compression of the particles, especially particles which do not have a corona (i.e., nonlinked polymer chains attached to their surface). The presence of a corona of monomer density smaller than that of the core of the microgel reduces the contribution of the osmotic pressure due to counterions outside of the microgel. It is also demonstrated that counterion osmotic pressure outside the particles can provide a significant contribution to the lubrication of the interface between the particles and a surface along which the compressed colloid is made to slide, for sufficiently slow velocities. In the experiments on Microgel colloids, thermally activated diffusion was observed at volume fractions as high as 2.The activation free energy for particle distortion needed for such hopping appears to be typically of the order of NtotkBT, which is >>kBT, where Ntot is the total number of monomers in a microgel particle, making it difficult to understand how such thermally activated particle motion is. Studies based on vertex models, which were previously used to describe cell motion in lung tissue are being explored.
Y. Ou, J. B. Sokoloff and M. J. Stevens, “Discrete Model Studies of Two Grafted Polyelectric Polymer Hydrogels Pressed in Contact,” Journal of Chemical Physics 139, 144902 (2013).
M. Abdelmoula, J. B. Sokoloff, W.-T. Lu, T. Close, L. Menon and C. Richter, “Optical Properties of Titanium Nanotube Arrays,” Journal of Applied Physics 115, 014306 (2014).
J. B. Sokoloff, “A Multi-Scale Treatment of Theoretical Mechanisms for the Protection of Hydrogel Surfaces from Adhesive Forces,” Physical Review E 90, 32408 (2014).
J.B. Sokoloff, “Compression and Lubrication of Salt Free Polyelectrolyte Microgel Particles in Highly Compressed Suspensions by Counterion Osmotic Pressure,” Journal of Chemical Physics 142, 234903 (2015).
J. B. Sokoloff, “Effects of Capillary Forces on a Hydrogel Sphere Pressed Against a Surface,” submitted to Langmuir.