Consultant: Robert C. Lance lance lance armstrong.

Department: Massachusetts Institute of Technology. Dept. of Chemical Engineering.

Author: Massachusetts Institute of Technology

Date Issued: 2008

Dissipative particle dynamics (DPD) could be a mesoscale simulation technique which utilizes soft potentials between large particles to breed liquid behavior. Healthy, DPD resembles molecular dynamics, as all matter is symbolized by point particles which speak with one another via, pairwise forces. The procedure was introduced noisy . 1990’s, and possesses since experienced numerous refinements that have put it to use a effective thermodynamic footing. DPD is notable for the versatility it really is the modeler for building complex fluid systems. DPD was utilized to look at simple molecular fluids, polymer and colloid solutions, and phase behavior of block copolymer melts. Lately, numerous workers used DPD to look at the flow of polymer solutions in many geometries for example microchannels, pores, and sudden contractions. While these kinds of flows are extremely-suitable for DPD’s relative strengths, an essential step remains skipped. Prior to the link between these complex flows may be recognized, you have to show the rheological predictions produced by DPD are frequently reliable. The key factor reason behind this thesis should be to show the rheology of polymer solutions may be simulated effectively with DPD. The rheology in the solution of DPD dumbbells having a FENE spring pressure law is studied partially one of the thesis via simulation of steady shear flow and steady planar elongational flow.

The rheological solutions are in comparison to dilute Brownian dynamics simulations from the FENE dumbbell model. The amount of coarse-graining within the DPD fluid is varied by altering how large the DPD dumbbell in compliance while using particle size, and a ongoing extensibility parameter. Generally, the viscosity, first normal stress coefficient, and dumbbell extension in shear flow calculated with DPD come in agreement while using the BD results. The 2 methods aren’t perfectly alike however, and two systematic variations relating to the DPD and BD solutions are observed. An excluded volume effect which occurs naturally in DPD and isn’t inside the BD simulations leads to elevated viscosity and dumbbell extension within the zero-shear-rate regime.(cont.) It seems sensible much more proficient at DPD dumbbells for coarse-grained. At high shear rates within the power-law regime, DPD systematically overpredicts the speed of shear-thinning, while using the finest deviation occurring within the most coarse-grained dumbbells. This is often hypothesized to get outcomes of hydrodynamic interaction which will come naturally from DPD’s explicit control of the solvent. The HI effect is examined when using the Giesekus anisotropic drag tensor. Immediately after its introduction, the complaint is produced that DPD’s dynamic solutions are suspect because it possesses a small, gas-like Schmidt number, and so momentum and mass are transported while using DPD medium at similar rates.

This is often compared to physical fluids, that have large Schmidt figures. While using the Lowe-Anderson formulation of DPD enables the Schmidt amount method to be varied for the same polymer model. Shear flow simulations of identical dumbbells under different Schmidt number conditions give leads to excellent agreement with one another, indicating the Schmidt number isn’t an important consider exercising polymer rheology with DPD. Steady planar elongational flow is simulated the very first time in DPD when using the Kraynik and Reinelt boundary conditions, that are periodic wide and time, permitting simulations of planar elongational flow by having an limitless time period. The planar elongational flow link between FENE dumbbells can also be convinced with BD, butshow exactly the same systematic deviations noticed in shear flow. The 2nd factor relating to this thesis examines a much more complex polymer solution using DPD, with simulations of semidilute solutions of longer N = 20 bead-spring chain polymers undergoing shear and planar elongational flow. In addition to concentration effects, the need for the solvent quality can also be examined with simulations of polymer solutions in good and theta solvents. To be able to capture concentration dependency, a spring -spring repulsion pressure is determined to the DPD model to avoid polymer springs from passing though one another. A effective concentration addiction to a extended relaxation time is noted.(cont.) In planar elongational flow, each solution encounters a coil-stretch transition inside the theoretically predicted strain rate De = .5. In shear flow, the rheological solutions will be in qualitative agreement with theory, showing a plateau at low De, along with a transition in a shear-thinning regime beginning at De = 1. Since the planar elongational flow results show apparent addiction to the answer relaxation time, the shear results show an assorted addiction to the general solution relaxation time, which reflects the concentration dependence, along with the relaxation rate in the isolated chain, suggesting that merely some regions of the shear rheology are afflicted by the concentration. To conclude in the thesis is the fact DPD is able to faithfully reproduce reliable rheological behavior with bead-spring polymer models. We uncover however, the computational costs connected while using the explicit simulation within the solvent put DPD in the disadvantage for systematic rheology studies compared to Brownian dynamics. The very best costs early in the year-spring repulsion pressure implementation are particularly restricting. In complex systems where DPD’s natural versatility in molecular architecture and chemistry make sure it is the best option, rheological results has become recognized with elevated confidence.

Thesis (Ph. D.)–Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2008.Includes bibliographical references (p. 191-200).

Keywords: Chemical Engineering.