Relaxation and creep responses of biological materials under spherical indentation considering surface tension

For soft biological materials, indentation has been extensively utilized to evaluate their viscoelastic properties. Nevertheless, the influence of surface tension has been scarcely considered in the analysis, which might lead to a misestimation of the viscoelastic properties. In this work, for the relaxation and creep of viscoelastic solids under spherical indentation, the impact of surface tension is investigated, and the explicit relation between load and indentation depth with surface tension is established. It is revealed that the existence of surface tension leads the apparent relaxation modulus to be higher than the real one for relaxation experiments, but the contrary tendency is observed for creep compliance in creep experiments. Moreover, the relationship between indentation depth and contact radius depends also on the material properties, distinct from the conventional viscoelastic models. Furthermore, a novel avenue is proposed to determine the viscoelastic properties and surface tension simultaneously. Our results are beneficial for understanding of surface tension on relaxation and creep responses, and greatly enhance the accuracy of evaluation on the mechanical properties of soft biological materials through indentations.