Masami Kageshima
Kansai Medical University, Japan
Title: Nonlinear response of hydrated polymer under sub-nanometer oscillatory shear
Biography
Biography: Masami Kageshima
Abstract
Viscoelastic response of hydrated 2-methacryloyloxyethyl phosphorylcholine (MPC), a bio-compatible phospholipid polymer and a potential lubricant for artificial joint, under sub-nanometer oscillatory shear was studied while shear amplitude and normal loading force were varied by using atomic force microscopy (AFM). An AFM cantilever having a borosilicate glass colloidal probe with a diameter of 20 μm was employed. Shear motion with a typical frequency of 30 kHz was induced by using a home-built AFM apparatus via magnetic torque exerted from an electromagnet onto magnetic spheres attached to the cantilever. A Si substrate with physisorbed MPC layer was moved toward the probe in water while the normal force between the probe and the sample was measured through the cantilever’s flexural deflection. The approaching motion was halted several times to sweep the shear oscillation amplitude typically between 0.5 nm and 0.02 nm; although this amplitude range shifted lower as the contact area increased with compression. The relaxation time derived from the viscoelastic response measured during these amplitude sweeps exhibited a characteristic dependence on the shear amplitude; the dependence was not marked in the low amplitude regime, i.e., a quasi-Newtonian behavior, whereas it turned to a negative dependence in high amplitude regime. The boundary between these two regimes became sharper as the probe was compressed harder to the sample, a feature obviously different from the one known for macroscopic non-Newtonian fluid. The present result may provide knowledge for understanding microscopic origin of non-Newtonian dynamics