Indenting fractal-edged elastic materials

Surface roughness plays a crucial role in the accuracy of indentation experiments used to measure the elastic properties of materials. In this study, we present a computational analysis of how surface roughness, represented explicitly by fractal geometry, influences the mechanical properties of soft materials. We model two-dimensional elastic samples with a Koch fractal bottom surface, grown upward or downward to the fourth generation, referred to as fractal down and fractal up, respectively. The elastodynamics equations are solved numerically while a rigid punch indents the elastic sample from the top surface. By applying the Hertz model for mechanical contact, we determine Young’s modulus of the materials. Our findings reveal that fractal surfaces, especially those with dimensions comparable to the sample size, can significantly alter experimental measurement outcomes. In particular, the roughness of the substrate profoundly affects the measured elastic properties, as seen in scenarios involving cell elasticity. For instance, in the down fractal scenario, reductions in the measured elastic modulus range from 2% to 4%, while increases reach up to 40% in the up fractal scenario. These results underscore the importance of incorporating fractal geometry into the design and analysis of indentation experiments. This approach could significantly enhance our understanding and application of material characterization and mechanical testing, leading to more accurate and reliable results. Our numerical findings align with experimental trends, with reduced elastic modulus observed in the down scenario, similar to surfaces treated with abrasives, and increased stiffness in the up scenario, akin to bonded materials with induced roughness.