Large scale molecular simulations for flow
The research in my group focuses on algorithms as well as applications for large scale molecular dynamics (MD) simulations. During the few decades that have passed since birth of molecular simulation, the time scales one would like to simulate for many applications have always exceeded what is computationally possible by one or several orders of magnitude. This is still the case, even though the computational power has increased exponentially with Moore's law. The reason for is is that as computers and software get faster, new scientific problems come within reach. Currently the increase in computational power mainly comes from the increase in CPU and GPU core count. This means that scientific codes need to extract more parallelism from scientific problems to be able to make full use of current and future supercomputers. This is the main focus of the work funded by my ERC starting grant entitled ``Million-Core Molecular Simulation'', which will soon end. Recently the emphasis of the applications in my group has shifted to the investigation of molecular aspects of flow. In flow there are both fundamental aspects that are not well understood, especially at surfaces, as well as questions about particular applications where molecular aspects become more important due to the smaller scales in micro- and nanofluidics. Although even in nanofluidics most of the system is still best described by continuum (or meso-) dynamics, details of molecular interactions can play an essential role. One example, studied here, is the three-phase contact line in wetting. Continuum models have a singularity here and a significant amount of the energy dissipation can occur within a nanometer from the contact line. Molecular dynamics simulations are to ony way to study these effects in detail. A second example is specific molecular interactions and interactions between ions in particular. Electrostatic interactions are strong, long-ranged and mediated by the solvent and other molecules in the system. One project studies electrowetting. Here there are both generic as well as ion specific effects of ions transported to and from the contact line and the structure of solvated ions at the contact line. In particular the well studied effect on contact angle saturation is not understood. A second project which will start soon looks at ordering of cellulose fibrils with the end goal of producing stronger materials. Ordering of the charged fibrils can be achieved through flow, which will be studied at the meso scale using rod models. The effective forces between the fibrils will be parametrized using molecular dynamics simulations which can take into account the nature of surface groups and the ionic composition of the solution. These simulations will run faster with an efficient long-range electrostatics algorithm, which I am working on together with Rio Yokota at Tokyo Tech. All this work in done using the open-source GROMACS molecular simulation package and all algorithmic improvements will be made directly available to the community.