Indentation-based characterization of the mechanical and adhesion properties of soft materials

Characterization of the mechanical and surface properties of soft materials is important in a number of applications, including microtransfer printing, characterization of biomaterials, and traction force microscopy used in cell mechanics studies. While instrumented indentation is a commonly used tool for measuring mechanical properties of soft materials, the elastic properties obtained using traditional indentation measurements are coupled and thus individual properties (e.g., Young\u27s modulus and Poisson\u27s ratio) cannot be obtained. In the present work, defocused imaging of fluorescent particles embedded in soft materials was used to track in-plane and out-of-plane displacements at discrete points along the surface of bulk materials and thin films during flat punch indentation tests. A technique for tracking 3D particle displacement was developed and the resolution of the technique was quantified. Numerical algorithms, which combine mechanics models and optimization methods, were developed to extract both the Young\u27s modulus and Poisson\u27s ratio from the particle displacements and forces measured during indentation tests. Experiments on PDMS in both bulk and thin-film forms were completed to demonstrate and validate the technique. While friction and adhesion did not significantly affect bulk material measurements, it was demonstrated that they must be accounted for when characterizing thin films. Instrumented indentation is also used to measure the adhesion properties of soft materials. While advancing and receding contact adhesion properties have been shown to differ, current indentation techniques are limited to characterizing receding contact due to mechanical and material instabilities between the indenter and the specimen. To address this, a modified nanoindentation method was developed to collect force and displacement measurements during both advancing and receding contact while avoiding instability. A finite element-based inverse method was developed to extract elastic and adhesion properties from spherical indentation tests on a thin film. Nanoindentation experiments on PDMS thin films were completed at various displacement rates and a finite element based-inverse analysis was used to extract the elastic and adhesion properties. In contrast with previous indentation adhesion studies which only characterized adhesion during separation, this work demonstrated that the rate dependence of both advancing and receding contact adhesion can be quantified using nanoindentation