Dr. R.D. Groot
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Applications of dissipative particle dynamics
Over the last two decades, most simulation studies have concentrated on the motion of individual atoms in systems of a few nm and a few ns. Other simulation methods concentrate exclusively on the macroscopic world of planes, trains and automobiles. However, between these size ranges lie some forty decades in volume and time. The holy grail of theoretical physics is to bridge this gap,[1] because in some cases simulation of this intermediate regime is essential to understand macroscopic phenomena. This is the case when molecules spontaneously order on mesoscopic length and time scales. This category of problems includes life and biological phenomena like membrane structuring, perforation and trafficking. Further this list contains all soft condensed matter including surfactants, polymers and (multi)block copolymers, that show microphase separation, or form gels or glassy systems.
The
question thus arises how these phenomena can be modelled. One approach is the
dissipative particle dynamics method (DPD). Here a number of atoms are taken
together into one simulation bead, that is used as a new simulation
element. The reliability of the result obviously depends on how the underlying
atoms translate into the interaction parameters between the DPD beads. In this
lecture some semi-empirical methods will be discussed, and a number of
published examples based on this will be given. Then we concentrate on two
problems: the mesophase formation of block copolymers,[2] and the interaction
of biological membranes with surfactant.[3]
Block copolymers consist of two blocks, A and B, that are linked together. If these blocks don’t dissolve well into each other, domains of A and B form, that grow together to form a network and finally an equilibrium phase, see picture. The second problem discussed involves the phase structure of a lamellar bilayer of molecules that make up a cell membrane. When surfactant is added, membranes show a transition to a perforated phase, and their ability to stretch is strongly reduced even by small amounts of surfactant.
[1]R.D. Groot and P.B. Warren, Dissipative Particle Dynamics - bridging the gap between atomistic simulation and mesoscopic simulation, J. Chem. Phys., 107, 4423 (1997).
[2]R.D. Groot and T.J. Madden, Dynamic simulation of diblock copolymer microphase separation, J. Chem. Phys., 108, 8713 (1998); R.D. Groot, T.J. Madden and D.J. Tildesley, On the role of hydrodynamic interactions in block copolymer microphase separation, J. Chem. Phys., 110, 9739 (1999).
[3]R.D. Groot and K.L. Rabone, Mesoscopic simulation of cell membrane damage, morphology change and rupture by nonionic surfactants, Biophysical Journal, 81, 725-736 (2001).