Maj Thijs Michels

Thermodynamic consistency in dissipative particle dynamics simulations of strongly nonideal liquids and liquid mixtures

Dissipative particle dynamics (DPD) is a mesoscopic simulation method for studying hydrodynamic behavior of complex fluids. Ideally, a mesoscopic model should correctly represent the thermo- and hydrodynamic properties of a real system beyond certain length and time scales. Traditionally defined DPD quite successfully mimics hydrodynamics, but is not flexible enough to accurately describe the thermodynamics of a real system. The so-called “multibody” DPD (MDPD) is a pragmatic extension of the classical DPD that allows one to prescribe the thermodynamic behavior of a system with only a small performance impact. Here we present a practical improvement to the “multibody” DPD model and test it on a number of single-component examples. We also generalize MDPD to multicomponent systems, which are an important target of DPD studies. The improved model provides a correction for particle correlations in strongly nonideal systems that were neglected in the original MDPD model. The implications of the coarse-grainin...

Calculation of excitonic properties of conjugated polymers using the Bethe-Salpeter equation

The excitonic spectra of a number of conjugated polymers, polythiophene (PT), polyphenylenevinylene (PPV), ladder-type poly-para-phenylene (LPPP), trans-polyacetylene (PA), and polydiacetylene (PDA), are calculated by solving the Bethe–Salpeter equation for the two-body Green function of the electron–hole pair. The only input to the calculation is the bulk dielectric constant. We find intermediate binding energies of 0.4–0.6 eV for the lowest optical 1 1Bu exciton, obtaining good agreement with recent experiments for each polymer individually. The agreement for the singlet–triplet splittings and the 1 1Bu polarizabilities is satisfactory. This also holds for the 1 1Bu–2 1Ag splittings in PT, LPPP, and PPV. For PA and PDA we find the 2 1Ag exciton above the 1 1Bu exciton, in contrast with experiment; a possible explanation for this discrepancy is the absence of lattice relaxation effects and double excitations in our calculations.

How ions distribute in a drying porous medium: A simple model

Salt crystallization at surfaces is an important problem for buildings and monuments. We do not consider the formation of salt crystals as such, but focus on transport properties of ions in a drying porous medium. We deal with the first phase of the drying process, where the water is still uniformly distributed throughout the medium. An approximate model is presented, which accounts for both convection and diffusion. It is shown that the key parameter is the Peclet number at the evaporating surface, Pe≡hL/eD, where h, L, e, and D are the drying rate, sample size, porosity, and diffusion constant, respectively. When Pe≪1 (diffusion dominates over convection) the ions remain uniformly distributed throughout the system. Strong accumulation at the evaporating surface occurs for Pe≫1 (convection dominates over diffusion). Crossover behavior is found for Pe≈1. Therefore, it is likely that the first crystals will be formed both in the bulk and at the interfaces of the material when Pe≪1. For high values of Pe th...

Mesoscale simulations of surfactant dissolution and mesophase formation

The evolution of the contact zone between pure surfactant and solvent has been studied by mesoscale simulation. It is found that mesophase formation becomes diffusion controlled and follows the equilibrium phase diagram adiabatically almost as soon as individual mesophases can be identified, corresponding to times in real systems of order 10 micros.

DOPANT-INDUCED CROSSOVER FROM 1D TO 3D CHARGE TRANSPORT IN CONJUGATED POLYMERS

The interplay between inter- and intrachain charge transport in bulk polythiophene in the hopping regime has been clarified by studying the conductivity

Deforming glassy polystyrene : Influence of pressure, thermal history, and deformation mode on yielding and hardening

\ensuremath{\sigma}

Systematic study of percolative network formation and effective electric response in low-concentration-carbon-black/polymer composites

as a function of frequency

Ab initio prediction of the electronic and optical excitations in polythiophene: Isolated chains versus bulk polymer

\ensuremath{\omega}/2\ensuremath{\pi}

The microstructure of petroleum vacuum residue films for bituminous concrete: a microscopy approach

(up to 3 THz), temperature

Volume fraction dependence and reorganization in cluster-cluster aggregation processes

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