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Final Activity Report Summary - SIMULA COMPLEX (Simulation of complex fluids including hydrodynamics: membranes and polymeric systems)

The project SIMULACOMPLEX focussed on the study of complex fluids using simulation and experiments. Its principal objectives could be divided in the development of a novel methodology and in applied studies.

The new hybrid molecular dynamics hybridMD methodology dealt with the simulation of multi-scale phenomena in liquid phase. Many different processes crucially depend on a fine interplay between molecular interactions at the nanometre scale and hydrodynamic transport at the mesoscale, up to microns. Some examples of these multi-scale problems are crystal growth from fluid phase, slippage of liquids near surfaces in microdevices, macromolecule dynamics under solvent flow, such as resonance and long-time dynamics, wetting and many other examples involving surfaces and interfaces in complex fluids.

In this project, we developed the hybridMD method which treated the important nanoscopic region with molecular accuracy and dynamically coupled this region with a coarse grained hydrodynamic description of the surrounding liquid. The coupling was based on the exchange of fluxes of conserved quantities, namely mass, momentum and energy. In this way, we were able to connect processes at both microscopic and mesoscopic scales. The important contributions of hybridMD to the art of multi-scale modelling of liquids were:
1. the use of chemically accurate interatomic potentials (NAMD) in the open Molecular dynamics (MD) domain;
2. the consideration of hydrodynamic fluctuations with thermodynamic consistency; and
3. the generalisation of the method to sound waves and energy transport, which previous methods only considered as shear flows.

During the applied studies we examined two different applied problems, namely the dynamics of single polymer chains under flow and the structure of polymers in lamellar phases formed by water and surfactants. In terms of the first problem, we studied the long-time dynamics of a single end-tethered chain under shear flow using MD, hybrid MD-continuum and Brownian dynamics' simulations. As observed in previous experiments with tethered Deoxyribonucleic acid (DNA), refer to Physical Review Letters 84, 4769, 2000, the chain performed a cyclic motion. But, contrary to what was suggested, we observed a well defined characteristic period, which was clearly revealed in the cross-spectra of the chain extension along flow and gradient directions. The main cycling time scales, like the time needed to stretch the polymer by convection, were about ten times the longest relaxation time of the chain in flow. The result was quite general and did not depend on the polymer length or on the presence of hydrodynamic interactions between monomers. This coherent recursive motion introduced long memory in the fluid and suggested resonance effects under periodic external forcing. This work was published in Physical Review Letters 96, 088303, in 2006.

Concerning the other objective, we studied the supramolecular assemblies formed by surfactant molecules which were ideal media to conduct chemistry under conditions of confinement at the tens of nanometres level. Of these assemblies, the liquid crystalline mesophases were the most promising, and we used these liquid crystalline media in two ways:
1. to determine the confinement conditions for a polymer as a function of the molecular weight, as described in the Journal of Physical Chemistry B 109, 23896-23904 in 2005; and
2. to analyse the incorporation of an acrylamide family with different substituents varying their hydrophilic degree, namely dimethyl, diethyl, isopropyl, dimethylmethacryl and acryloylmorpholine. This analysis was included in a paper submitted in the Journal of Colloid and Interface Science, which was in press in 2006, by the time of the project completion.

In both cases, the liquid crystalline medium was the lamellar phase of AOT and water. We selected AOT because it was a cheap and environmental friendly surfactant and polyacrylamides because they were water soluble polymers with very important applications in medicine and chemical technology. These studies were the first two steps for a more ambitious project, aiming to use the spacing between the lamellae as a template to obtain polyacrylamides with modified properties.

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