Abstract
Rheological responses are the most relevant features to describe soft
matter. So far, such constitutive relations are still not well
understood in terms of small scale properties, although this knowledge
would help the design of synthetic and biomaterials. Here, we
investigate, computational and analytically, how mesoscopic-scale
interactions influence the macroscopic behavior of viscoelastic
materials. We design a coarse-grained approach where the local elastic
and viscous contributions can be controlled. Applying molecular dynamics
simulations, we mimic real indentation assays. When elastic forces are
dominant, our model reproduces the Hertzian behavior of contact.
However, when friction increases, it restores the standard linear solid
model. We show how the response parameters depend on the microscopic
elastic and viscous contributions. Besides, we show that the contact
Hertz model is equivalent to the virial stress, an atomistic approach of
the continuum Cauchy stress tensor. In addition, relaxation experiments
were performed on polyacrylamide gels with an atomic force microscopic
in order to validate our numerical relaxation curves. The bisacrylamide
concentration in the gel has a clear impact on the equivalent elastic
and viscous contributions of the sample. Moreover, our findings also
suggest that the relaxation times, obtained in relaxation and
oscillatory experiments, obey a universal behavior in viscoelastic
materials.
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