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Lab Description

A major theme of our research work is the development of reliable constitutive models for describing the dynamics and equilibrium (i.e., no imposed flow) and flow behavior of complex polymeric fluids. At equilibrium, we derive and solve theories of polymer dynamics using simple molecular models of different polymeric systems (e.g., ring polymers). The aim is to understand the properties of various polymeric systems and to provide the avenue to design tailor-made systems with improved properties. In this case, we consider simple models that can be solved analytically or numerically, however, as we mention below, we strive to always compare the predictions of these theories with (equilibrium) Molecular Dynamics (MD) simulations. On the other end, under the imposition of a flow field, we rely on the use of non-equilibrium thermodynamics (NET), in particular the Generalized Bracket and GENERIC formalisms, for developing closed-form balance equations for the fundamental hydrodynamic fields. No matter what the system is (say biological or chemical), it must obey the laws of thermodynamics. In particular, when the system is beyond equilibrium (e.g., under the influence of a flow field), its time evolution must be dictated by the laws of NET. This is exactly the reason for employing NET: by construction, the new constitutive models obey the laws of thermodynamics. In our models, the underlying microstructure of the complex fluid is described by using structural variables, such as the conformation tensor for polymer chains (describing their average conformation), which are hydrodynamically coupled with the imposed flow field. The relation between microstructure (structural variables) and macroscopic observables (material functions) takes eventually the form of a stress tensor equation. So far, we have developed generalized constitutive models for polymer melts, polymer solutions, and polymer nanocomposites. Currently, we are using NET to develop constitutive models for biomolecular fluids, such as blood.

 

In most cases, the resulting constitutive equations contain parameters whose values are not known. To overcome this, we resort to atomistic simulations, both (equilibrium) MD and non-equilibrium MD (NEMD). This allows me to develop interconnections between three different levels of system description: the atomistic or microscopic, the mesoscopic, and the macroscopic. As one moves from the atomistic to the macroscopic level (coarse-graining), the degrees of freedom of the system are significantly reduced, which results in a dramatic reduction in computational demands. However, coarse-graining must be done carefully to avoid the loss of important information. Our work connects the three levels through the development of scale-bridging methodologies, and the outcome is a set of, closed-form, constitutive equations for the time evolution of the structural and hydrodynamic fields selected to describe the system. Overall, the building blocks of our bridging methodology are the following:

1)      At the atomistic level: We execute atomistic MD and NEMD simulations to simulate the actual chemical and biological system under exact processing conditions to obtain the values of important parameters entering the description of the system at the mesoscopic level.

2)      At the mesoscopic level: We design coarse-grained simulations (e.g., Brownian dynamics (BD) simulations, Dissipative Particle Dynamics (DPD), coarse-grained MD) which provide information about the evolution of the system for much larger time spans than what is addressed by atomistic simulations.

3)      At the macroscopic level: We use NET to derive generalized constitutive models for complex systems whose parameters are evaluated from the previous levels.

 

More recently, we are also involved in developing modeling approaches for biological systems, such as predicting the deposition of a substance in various organs in our body and describing the process used by specific organisms to produce important products, i.e., bioreactors. 

HIGHLIGHTS

My research concerning the non-equilibrium thermodynamics description of polymer nanocomposites provide new insights into understanding the interrelation of nanostructure, phase behavior (miscibility) and rheology in these materials. This research project received the «Cyprus Research Award–Young Researcher 2015» (Thematic area: Physical Sciences and Engineering).

The rheological behavior of drillings fluids is usually described by the Casson or the Herschel- Bulkley models. Despite the overwhelming data highlighting the significance of their use in numerous fields, they fail to produce normal stresses, whose importance in drilling operations has only recently attracted attention. We herein introduce a continuum model for predicting the rheological behavior of drilling fluids with plate-like suspensions that bears the unique capability to accurately describe the rheological behavior of these systems, for both shear viscosity and normal stresses. 

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Polymers continue to fascinate people with their intriguing properties and continuous application in new fields in all aspects of our life. Most of these properties have their origin in the macromolecular nature of the constituent molecules. Chain connectivity and chain uncrossability give rise to the development of topological constraints in macromolecular systems, collectively known as entanglements, which govern to a large extent the relation between structure, properties, processing, and performance of the corresponding materials. Recognizing the importance of theory and simulations in understanding the properties of polymers across scales and under a variety of conditions, this Special Issue of Polymers invites contributions addressing several aspects of entangled macromolecular systems, such as the formulation of new constitutive modelling, the study of entanglement dynamics under flow, the development of new hierarchical or multi-scale strategies to address more complicated systems than pure homopolymers, such as nanocomposites, associating polymers, and self-assembled systems, nonequilibrium simulation methodologies satisfying the fundamental laws of nonequilibrium thermodynamics and statistical mechanics, well-founded coarse-graining schemes for speeding up the simulations under both equilibrium and nonequilibrium conditions, new theoretical developments and simulations advancing our knowledge of ring polymers, new methods for computing rare events, approaches for predicting chain organization and morphology or self-assembly in nanostructured polymers, etc.. The above list is only indicative and by no means exhaustive; any original theoretical or simulation work or review article on the role of entanglements in polymer dynamics is welcome.

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Recognizing the significance of understanding the rheological properties of polymeric systems, across scales and under a variety of conditions, we launch this Special Issue of Polymers entitled “Rheological Properties of Polymers and Polymer Composites", and invite the submission of papers that address several rheological aspects of macromolecular systems via experiments, theory, and simulations. Submissions may address the following topics:

  1. Formulation of new constitutive modeling;

  2. The study of entanglement dynamics under flow;

  3. The development of new hierarchical or multi-scale strategies, the linear and nonlinear rheology of ring polymer nanocomposites, associating polymers, and self-assembled systems;

  4. Non-equilibrium simulation methodologies, well-founded coarse-graining schemes for speeding up the simulations, Brownian or slip-link simulations, numerical simulations, and novel theoretical developments;

  5. Polymer-filler materials;

  6. Rheology, the production of neat polymers and nanocomposites;

  7. How rheology can be useful for mixing and compounding processes.

  8. Rheological equipment and new developments

 

The above list is only indicative and by no means exhaustive; any original theoretical or simulation work or review article on the rheological properties of polymers and polymer nanocomposites will be highly welcome! We hope that these contributions will also address a variety of other systems, including linear and nonlinear polymer architectures, polymer solutions, polymer blends, copolymers, semi-conductive conjugate polymers, multicomponent polymeric systems, and polymers for biological or medical applications

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