Molecular Investigations of Nanocolloid Rheology
Sundaravadivelu Devarajan, Dinesh
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In recent years, the nanocolloidal suspensions have attracted much attention for their wide range of applications in electronics, pharmaceutical, and coating and lubricant industries. The knowledge of the structure-flow-end use property relationships is essential for designing and innovating the practical applications of the nanocolloidal suspensions. The flow properties of the nanocolloidal suspension systems are governed by the specific chemical interactions in these systems, the role of which is often difficult to delineate in experiments. Molecular dynamics (MD) simulations can provide direct insights into the role of the detailed interactions on the rheological behavior of the nanocolloidal systems. In the first part of this work, an explicit particulate solvent model of the nanocolloidal suspensions is developed that has the ability to capture the effect of the colloid-colloid, colloid-solvent, and solvent-solvent interactions on the rheological behavior of the nanocolloidal systems. The trends of zero shear viscosity and dynamic viscosity as a function of volume fraction for the nanocolloidal systems obtained from the molecular simulations are quantitatively consistent with the literature experimental, simulation, and theoretical studies. It is shown that the frequency range accessible to the MD simulations can be extended by more than four decades by applying the time-concentration superposition (TCS) principle to the simulated viscoelastic modulus values. The ability of the molecular simulations to account for the specific chemical interactions was further demonstrated by investigating the molecular mechanisms underpinning the rheology of graphene oxide/poly (vinyl alcohol) suspensions. In the second part of this work, the applicability of the probe rheology molecular simulation technique for investigating the nanoscale rheology of colloidal suspensions is studied. Both the active and the passive modes of probe rheology are employed in conjunction with the continuum analysis framework to establish a connection between the probe motion and the linear viscoelastic properties of the suspension systems. The viscoelastic properties so-obtained were found to be in good agreement with those from the bulk rheology simulations and the theoretical predictions. A methodology to account for the artificial probe hydrodynamic interactions is applied that leads to an improved quantitative accuracy in the viscoelastic modulus values. The role of probe-colloid size ratio in determining the linear viscoelastic modulus is discussed in the context of the probe-colloid interactions. This study demonstrates that the probe rheology simulation technique can be used by the rheologists to investigate the nanoscale rheology of complex fluids. On the application side, the ability of the probe rheology molecular simulation technique to determine the local viscoelasticity of nanocolloidal systems is also discussed.