Nanoindentation in fluid: A pathway to nanoscale poromechanical materials characterization

Abstract

Accurate poromechanical analysis of hydrogels, tissues, geomaterials and other porous systems requires mechanical deformation under controlled and sustained hydration. Here in this study we show that a straightforward modification of an instrumented indentation platform allows acquisition of nanoscale force-displacement data in liquid media, without artifacts of buoyancy or surface tension. Furthermore, geometric artifacts that may arise from imperfect indenter probe geometries are excluded-by design from our analysis. We demonstrate the validity of nanoindentation in fluid via elastoplastic analysis of relatively stiff (E > 1000 kPa), water-insensitive materials (Borosilicate Glass and Polypropylene). We then consider the viscoelastic response and representative mechanical properties of compliant, synthetic polymeric hydrogels and biological tissues (E < 500 kPa). Examples from indentations on water-saturated synthetic (hydrogels) and natural materials (porcine liver and skin) are presented. The elastic properties of the tested materials are in good agreement with macroscopic data found in the literature, validating the accuracy of the proposed fluid-cell module and demonstrating its ability for nanoscale characterization of hard and soft systems in the kPa to GPa range. It is expected that the developed nanoindentation platform will facilitate the poromechanical characterization of any bicontinuous system composed of a solid matrix and a fluid-saturated porous space and could be exploited for fundamental poromechanics studies as well as for poro- and chemo-mechanical materials characterization.