Local linear and nonlinear viscoelasticity of polymeric system by particle tracking rheology simulation



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Over the last two decades, particle tracking microrheology (PTM) emerged as a rheological characterization tools to capture linear local and bulk linear viscoelastic properties of complex soft materials over a large frequency range – typically Hz which is not achievable by the traditional rheological techniques. Moreover, PTM can also provide a measure of local rheology especially for structurally and dynamically heterogeneous materials which has a lot practical importance like predictions of drug particle motions through complex biological medium, nanoparticle motions through polymer matrix in case of nanocomposites, studying local aging effect of glassy materials, gradient of viscoelasticity across polymer thin film etc. On the contrary, its interpretation as the representative for bulk rheology sometimes become ambiguous due to the interplay of different length scales–from probe size to medium microstructures, medium and particle inertial effects at higher frequencies, probe-to-probe indirect hydrodynamic interactions, local modifications of the medium structures by the probes etc. Moreover these phenomena will be more complicated when one enters the nonlinear regime due to the presence of local unsteadiness and direct material-probe collisional effects which have no bulk rheological analog. In this work, we attack some of the physical aspects of the problems especially focusing on the higher frequency material responses by the probe motions. In our molecular dynamics (MD) simulations, by considering both entangled and unentangled polymer physics explicitly, we elucidate the effect of inertia and the interplay of different length scales at much higher frequencies than a typical microrheology experiment. In this way, we demonstrate that by combining traditional rheology (low frequency), experimental microrheology (intermediate frequency) and our probe nanorheology simulations (highest possible range of frequency), one can capture viscoelasticity of over 13-14 decades which will give one the entire spectrum of polymer relaxation timescales–from the whole chain to monomer length scale motions. Moving forward to the nonlinear regime, we also show the importance and the effects of the local polymer microstructures along with stress components, velocity fields and their asymmetrical behaviors around the probe on probable viscoelastic thinning effects.

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Viscoelasticity, Particle Rheolgoy, Molecular Simulation