Effect of surface adhesion in contact : application of Johnson-Kendall-Roberts model of nanoindentation

Abstract

Finally this work studied the effect of surface adhesion on the mechanical behavior of two soft materials (two kinds of biological cells) subjected to atomic force microscope (AFM) indentation, i.e. pancreatic MIN6 cell and hepatocellular carcinoma which were treated by calcimimetic R568 and fullerenol respectively. They were also indented by different AFM probes: MIN6 cell by spherical indenter, and cancer cell by Vickers indenter. First of all, surface adhesion was manifested by the negative value of indentation force. For MIN6 cells，both JKR and finite element method are used to fit the force-displacement curve obtained by AFM indentation. For hepatocellular carcinoma, the JKR model is adapted for the Vickers indenter, and the “adapted” JKR model is used to fit the force-displacement curve. The results showed that JKR model can best describe the unloading force-displacement behaviors of the indentation curves, where a new parameter, termed work of adhesion in addition to Young’s modulus was extracted. Moreover, the difference between two biological cells and their treated counterparts were detected in terms of the magnitudes of the extracted parameters, i.e. Young’s modulus and work of adhesion. This implies that the study on the surface adhesion has potential significance in terms of medical diagnostics. The main contributions from the present research could be summarized as follows: i. For hard materials, this study presents a systematic investigation on the effect of surface adhesion on the shakedown behavior of two hardening materials, i.e. isotropic and kinematic hardening. The simulative results show that surface adhesion alone can initiate plastic deformation. In non-adhesive repeated contact, only elastic shakedown can occur while in adhesive repeated contact, plastic shakedown can occur, which indicate that surface adhesion force can alter the mechanical response of substrate material subjected to repetitive indentation. ii. For soft materials, this work uses JKR model to fit the force-displacement curve, yielding a new parameter, i.e. work of adhesion, in addition to Young’s modulus. In comparison to the Hertzian contact model, the JKR model provides obviously better fitting to the experimental results, indicating that the adhesion is significant in the cell interaction. Moreover, the difference between various biological cells could be characterized by the magnitude of work of adhesion, which implies that this parameter may also serve medical diagnostics