Multiscale Simulation of Microgel Structure

 

The project aims to study the structural properties of microgels on various length scales by means of atomistic simulations. The problem of concern is the great mismatch between length and time scales accessible in simulations and resolvable by experiments. The computations required for accurate modeling and simulation of such large-scale systems as microgels with atomistic resolution involve a hierarchy of levels of theory: quantum mechanics (QM) to determine the interaction potentials; force fields (FF) to fit effective charges and to obtain atom based forces, molecular dynamics (MD) based on such a FF; mesoscale or coarse grained descriptions that average or homogenize atomic motions; and finally continuum level description.

The prominent challenge for theory and computation involves efficiently bridging, from QM first-principles, into larger length scales with predominantly heterogeneous spatial distributions, and longer timescales of simulation, while retaining physicochemical accuracy and certainty. In an atomistic simulation the definition of the particles is straightforward: each particle is an atom of the element of interest. As a consequence, interaction potentials can be calibrated upon comparison with results obtained from ab initio QM.

In contrast, in mesoscale simulations, the system is still too small to be regarded as a continuum, yet too large to be simulated effectively using atomistic methods. The upscaling is performed by a process of integrating out redundant degrees of freedom, known as ‘coarse graining’, which can be achieved by grouping atoms into larger particles (often known as ‘mapping’). Consequently, one must assume a mean “chemical” structure to account for different interactions between “bonded” and “non-bonded” particles. By a mesoscale bond, we actually refer to a very large number of atomic chemical bonds. The definition of these mesoscale bonds between particles is of utmost importance in the context of the modeling of the thermodynamical and structural properties of microgels.

The ultimate goal is to develop a reversible bottom-up, top-down approach, based on atomistic MD, to characterize properties of thermosensitive microgels at a hierarchy of length and timescales. This will improve the ability to design, analyze, and interpret experimental results, perform model-based prediction of phenomena, and to control precisely the multi-scale nature of microgel systems for multiple applications.