(735e) A Simple Model for Understanding Friction between Biomaterial Surfaces | AIChE

(735e) A Simple Model for Understanding Friction between Biomaterial Surfaces

Authors 

He, Y. - Presenter, Zhejiang University (Yuquan Campus)
Xu, N., Zhejiang University
Tan, S., Zhejiang University
Xia, T., Zhejiang University
Biomaterials posses many distinct features in virtue of the confined effects. Two surfaces covered with zwitterionic membranes in the confinement of water layers, like the mammalian synovial joints, exhibit ultralow friction properties while subjected to high loads. Hydration repulsion is considered to be partially responsible for the pressure bearing and ultralow friction features. Studying the interactions between surfaces and the dynamic properties of confined fluids at the molecular level is still trapped by how to establish well-hydrated systems. The ideal method is to balance the chemical potential of the confined fluid with that of the bulk fluid, which is considered to obtain reasonable hydration state and therefore accurate hydration repulsion. However, the calculation of chemical potentials involved in their methods requires modifications to the universal Molecular dynamics(MD) packages and seems to spend more time compared to the classical molecular dynamics. A simple but effective method is introduced as an alternative in our work. Two phosphorylcholine self-assembled monolayer (PC-SAM) surfaces are in the confinement of water layers which are connected with two adjacent water bulks, while two fictitious sheets outside squeeze inward and produce a push of the atmospheric pressure. After equilibrium, the confined systems could reach well hydrated statuses. It is found that pressures between solvated surfaces follow an exponential-type decline with the increase of separation distances Ds, and Ds =20 Å demarks a crossover. For larger distance, the central water slab is bulk-like. For smaller distance, two opposing hydration layers interpenetrate and compete with each other for control the interstitial water orientations, driving the behavior of central water away from bulk-like. These results are found to be in good agreement with previous researches. In addition, hydrogen bonds are found in our work. For small distance, water molecules will in turn affect the orientation of lipid headgroups to form hydrogen bonds and make the lipids swelled. For larger distance, the opposing hydration layers no longer overlaps and the break of lipid-water-lipid hydrogen bonding network may eliminate the hydrogen bonds between lipid and water. Friction properties are also studied after the systems get equilibrated. Linear friction patterns, which mean that friction force is proportional to the shearing velocity, is found in our work. The selection of forcefield parameters for the fictitious sheets outside is proved to have little effect on the results. Our method is expected to have a bright prospect in other confined systems and build bridges between experiments and MD simulations.

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