Dissipative particle dynamics simulation of on-chip hydrodynamic chromatography.

Separations of flexible polymers using on-chip hydrodynamic chromatography (HDC) have been studied with dissipative particle dynamic simulations, a novel computational technique that fully accounts for hydrodynamic interactions among polymer segments and with walls. The current study focuses on comparisons of simulation results on elution times with that predicted by earlier theoretical models for HDC separation. The elution times obtained in simulation are found to compare reasonably well with the theoretical prediction when chain stretching is not significant. Deviation from the theoretical prediction occurs when the chain stretching becomes significant. We observe a reversal in elution order and the reversal occurs at a Deborah number about 8, slightly larger than the Deborah number at the onset of chain stretching. The simulations also confirm the applicability of the Aris-Taylor equation for the convective dispersion of small solutes and for solutes like polymer chains, except the latter requires the use of a modified Peclet number.

Hydrodynamic interaction in polymer solutions simulated with dissipative particle dynamics.

The authors analyzed extensively the dynamics of polymer chains in solutions simulated with dissipative particle dynamics (DPD), with a special focus on the potential influence of a low Schmidt number of a typical DPD fluid on the simulated polymer dynamics. It has been argued that a low Schmidt number in a DPD fluid can lead to underdevelopment of the hydrodynamic interaction in polymer solutions. The authors' analyses reveal that equilibrium polymer dynamics in dilute solution, under typical DPD simulation conditions, obey the Zimm [J. Chem. Phys. 24, 269 (1956)] model very well. With a further reduction in the Schmidt number, a deviation from the Zimm model to the Rouse model is observed. This implies that the hydrodynamic interaction between monomers is reasonably developed under typical conditions of a DPD simulation. Only when the Schmidt number is further reduced, the hydrodynamic interaction within the chains becomes underdeveloped. The screening of the hydrodynamic interaction and the excluded volume interaction as the polymer volume fraction is increased are well reproduced by the DPD simulations. The use of soft interaction between polymer beads and a low Schmidt number do not produce noticeable problems for the simulated dynamics at high concentrations, except for the entanglement effect which is not captured in the simulations.

Pressure driven flow of polymer solutions in nanoscale slit pores.

Polymer solutions subject to pressure driven flow and in nanoscale slit pores are systematically investigated using the dissipative particle dynamics approach. The authors investigated the effect of molecular weight, polymer concentration, and flow rate on the profiles across the channel of the fluid and polymer velocities, polymer density, and the three components of the polymers radius of gyration. They found that the mean streaming fluid velocity decreases as the polymer molecular weight and/or polymer concentration is increased, and that the deviation of the velocity profile from the parabolic profile is accentuated with increase in polymer molecular weight or concentration. They also found that the distribution of polymers conformation is highly anisotropic and nonuniform across the channel. The polymer density profile is also found to be nonuniform, exhibiting a local minimum in the center plane followed by two symmetric peaks. They found a migration of the polymer chains either from or toward the walls. For relatively long chains, as compared to the thickness of the slit, a migration toward the walls is observed. However, for relatively short chains, a migration away from the walls is observed.