MMSD 2015 - Non-equilibrium processes in soft matter
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Non-equilibrium thermodynamics embodies a vibrant field of active research dedicated to a better description of equilibrium and dynamical properties of systems evolving out of equilibrium. Examples include crystal growth, organic electronics, and active proton conduction in protein channels. The ever-growing synergy between molecular simulations and experiments in soft-matter systems holds the prospect of an in silico understanding, prediction, and design of non-equilibrium processes to help guide experimental breakthroughs.
The characterization of a non-equilibrium transformation between two equilibrium states was pioneered by the works of Jarzynski and Crooks, who expressed the free-energy difference between equilibrium states with the work carried out on the system [1, 2] and the relationship between forward and backward processes [3, 4], respectively. For microscopic systems, fluctuations play a significant role and generate an ensemble of transformations, and thus work values, between the two states. More generally, notions such as work, heat, and entropy production  of individual trajectories have acquired a theoretical understanding in stochastic thermodynamics . This framework has enabled the analysis of equilibrium properties of single-molecule pulling experiments , where technological limitations impose a finite pulling rate--and thus non-equilibrium effects --but also computer simulations of (bio)molecular systems [9-11].
non-equilibrium simulations offer a promising route to probe intrinsically dynamic material properties. non-equilibrium molecular dynamics (NEMD)  and reverse NEMD  impose a temperature gradient and a flux, respectively, to measure the system’s thermal conductivity. Other methods include the relaxation of different parts in contact that are prepared at different temperatures . These non-equilibrium methods have reached unprecedented levels of accuracy to reproduce experimental data .
Dynamical behavior can occur over time scales that are prohibitive for atomistic simulations (e.g., rheological properties). In this context, coarse-grained (CG) models , which provide a particle-based description of the system while averaging over certain degrees of freedom, offer a promising route to reach long time scales. Carefully parametrized CG models probed under shear flow using non-equilibrium molecular dynamics have shown to be able to faithfully reproduce structural and conformational properties of polymer melt systems, though dynamical quantities prove more challenging .
Ultimately, non-equilibrium processes can lead to complex structures and morphologies. Emerging entanglements of seed material gives rise to the formation of functional structures, such as crystal growth, (bio)mineralization, and organic electronics. For these systems, a proper understanding of the non-equilibrium process that leads to the functionalized state is both scientifically fascinating and paramount to ongoing technological advances. As a promising step in this direction, a recent simulation study introduced new methodologies to probe steady-state evaporation .
This workshop gathers scientists of different backgrounds and expertise, from statistical physicists to biomolecular modelers, who can provide an original view over several different aspects of non-equilibrium processes in soft matter systems. The workshop will be linked to the biennial meeting, Mainz Materials Simulations Days (MMSD 2015), which is by now an established series of meetings organized by the Max Planck Institute for Polymer Research and Mainz University.
How can we establish theoretically-sound methodologies for non-equilibrium computer simulations?
The molecular modeling community has shown much interest in extracting quantitative material properties from non-equilibrium processes. However, finite-size effects and certain linear extrapolations can cause significant difficulties in, e.g., the extraction of thermal conductivity from NEMD , mainly due to the need to describe transport phenomena occurring over excessively long length scales (e.g., micron). The preparation or upkeep of a system in a non-equilibrium state introduces inherent challenges. NEMD and reverse NEMD techniques highlight difficulties pertaining to thermostats, e.g., fast removal of energy and obscure statistical-mechanical ensembles. To what extent can stochastic thermodynamics help the development of simulation protocols with a firmer theoretical basis? By means of interdisciplinary presentations and extensive discussions, interactions between simulators and theoreticians will aim at extending the range of applicability and reliability of computer simulations.
Can coarse-grained models reproduce non-equilibrium phenomena?
The workshop will aim at fostering discussions geared toward carefully crafted and controlled non-equilibrium CG simulations, from the theoretical basis to force-field parametrization and methodological aspects of the simulations. To what extend can CG models that are parametrized on (static) equilibrium properties only be used to study non-equilibrium processes? Can one remove the fast degrees of freedom by coarse-graining without disrupting significantly the accuracy of the model in a non-equilibrium setting? These questions naturally connect to today’s grand challenge of coarse-grained simulations: the faithful description of kinetics and time scales . The workshop will push the theoretical underpinning of CG models that are able to reproduce the hierarchy of time scales in soft matter to accurately model dynamic properties .
What are the prospects for in silico manipulation of structure formation?
The formation of structures and their inherent properties can, to some extent, be manipulated from control parameters and external factors (e.g., temperature, pH, chemical composition) surrounding the physical system, thereby affecting the ensemble of transformations along the non-equilibrium process. Reaching a reliable description of external factors from computer simulations will require new methodologies that embody the diversity of both systems of interest and control parameters . Could an appropriate simulation protocol help guide crystal growth in a preferential direction? Can simulations predict how the type, concentration, and removal of additives impact the morphology of organic-electronic materials? The combined efforts between the statistical physics of non-equilibrium systems, improved simulation methodologies, force-field parametrization endeavors, and increasingly accurate thermostatic and dynamical measurements will push the field toward a predictive tool to describe and guide non-equilibrium processes.
Bjoern Baumeier ( Max Planck Institute for Polymer Research ) - Organiser
Kurt KREMER ( Max Planck Institute for Polymer Research ) - Organiser
Friederike Schmid ( Johannes Gutenberg University Mainz ) - Organiser
Thomas Speck ( Johannes Gutenberg-University Mainz ) - Organiser
Raffaello Potestio ( University of Trento - Physics Dept. ) - Organiser
Tristan Bereau ( University of Amsterdam ) - Organiser