Viscoelastic Memory Functions in Glassy and Polymer Liquids

Scientific context. Viscoelastic fluids are ubiquitous. Examples include glassy liquids or polymer melts. A key property of these fluids are long-lasting dynamic memory effects correlating the flow at a given time to the history of prior motion. Theoretically, these memory effects are described by viscoelastic memory functions (VMFs) which determine the response of the fluid to deformation on various length and time scales. It is a fundamental unsolved problem of condensed matter physics to develop accurate approximation for the VMFs. Progress in developing such approximations has stagnated in the past, since theory shows that the time evolution of the VMFs is not generated by the classical Newtonian dynamics, but by an intricate "projected" dynamics. The projected dynamics cannot be reproduced by standard molecular dynamics (MD) simulations, thereby disabling direct calculation of the exact VMFs to guide the development of approximations.

Here we suggest to close this gap by a new, constrained MD (CMD) method that mimics the projected dynamics directly in the simulation. Through two parallel doctoral theses, we seek to create the CMD method, to make it publicly available by implementation in the opensource LAMMPS code for high-performance computing, and to apply CMD to explore the spatio-temporal dependence of the VMFs for models of glassy [1] and polymer liquids [2].

1. L. Klochko, J. Baschnagel, J. P. Wittmer, H. Meyer, O. Benzerara, A. N. Semenov, Theory of length-scale dependent relaxation moduli and stress fluctuations in glass-forming and viscoelastic liquids, J. Chem. Phys. 156, 164505 (2022).
2. A. N. Semenov, J. Farago, H. Meyer, Length-scale dependent relaxation shear modulus and viscoelastic hydrodynamic interactions in polymer liquids, J. Chem. Phys. 136, 244905 (2012).

Practical context. Two doctoral theses, funded by the French National Research Agency (ANR), are available in the "Theory and Simulation of Polymers (TSP)" group at the Institut Charles Sadron (ICS) in Strasbourg, France. The ICS is a research institute of the CNRS (Centre National de la Recherche Scientifique) and associated with the University of Strasbourg (Unistra). The ICS hosts seven research groups, one of which is the TSP group. All permanent members of the TSP group are involved, at various levels, in the research associated with the two PhD theses proposed. Each PhD student will have a doctoral contract and be enrolled in the Doctoral College of Physics and Chemistry-Physics (ED 182) of the Unistra, providing a structured doctoral training parallel to the doctoral thesis. The doctoral contract is for three years with a monthly net salary of (currently) 1739 euros. 

The Institut Charles Sadron (ICS) was created in 1947 to develop the fundamental science of polymers at the interface between chemistry and physics, and to guide the explosive birth of the polymer industry. Since then, the ICS has been able to establish itself as a global player in various fields related to the science polymers and self-assembled systems.

Located in Cronenbourg, the ICS is a “Unité propre” (UPR 22) of the CNRS, associated with the Institute of Chemistry (INC) of the CNRS, in close interaction with the Institute of Physics (INP) and the Institute of Sciences of engineering and systems (INSIS). It is also associated with the University of Strasbourg and INSA Strasbourg, with strong links to the Faculty of Physics and Engineering, the ECPM and the Faculty of Chemistry. The ICS is a key player in the Carnot Mica Institute and in the Federation of the Materials and Nanosciences Research of the Grand Est, and is involved in numerous national and international networks.

Structured in seven research teams and four platforms, the ICS brings together 53 researchers and teacher-researchers, 38 engineers, technicians and administrative staff and around 100 doctoral students, post-docs, associate researchers and internship students.

Following a multidisciplinary approach, the scientific mission of the Institute covers the science of polymers and self-assembled systems, from the mechanisms of formation of macromolecules to the fundamental understanding of their properties, both in dilute and condensed phases.

The current research efforts may be grouped into four main topics, which are treated in a collaborative manner by different teams at the interface between chemistry, physico-chemistry, experimental and theoretical physics.

We are looking for physicists or theoretical chemists having a good knowledge of statistical physics, experience in programming (e.g., python, C++, etc.), and familiarity with LINUX. Successful candidates must have a strong interest in both theory and numerical work. Numerical work involves code development (e.g., implementing CMD in the C++ LAMMPS code, programming of data analysis tools, etc.), carrying out large-scale MD simulations, and data curation. Development of CMD and analysis of the simulation data critically hinge on the synergy provided by applying the underlying theory that also requires further extension. The positions can be filled at any time, but no later than Oct 1, 2025.