Dissipative Particle Dynamics Simulation of Suspensions Rheology, and Electroosmotic Flow in Nanochannels

The dissipative particle dynamics (DPD) method is developed using innovative numerical techniques and extensively examined in the contexts of rheology and electroosmosis. In Chapters 3-5, it is attempted to classify practical ranges of DPD parameters under a variety of simulation settings, thermostating schemes and shearing methods. Through a calibration process, useful windows of parameters are categorised so that DPD users can model a wide range of rheological systems conveniently with proper temperature control and equilibrium statistics. DPD was found to perform poorly under certain dissipation rates and shear rates when sheared via original Lees-Edwards boundary condition. Hence, a modified version of this shearing method is shown to be an effective remedy to improve the hydrodynamics and thermal stability of sheared DPD systems. These achievements shed light on unclear correlations between input parameters and simulation outputs, and relatively rectifies the lack of predictability embedded in DPD method. In Chapter 6, it is shown that plain DPD is inherently a flexible numerical tool to reproduce experimental behaviour of dilute to dense suspensions. This is achieved via a simple calibration of parameters without unnecessary and computationally intensive modifications to DPD underlying formulas. In Chapter 7, contrary to existing DPD modellings of electroosmotic flow (EOF), soft-core electrostatic interactions are treated fully explicitly by inclusion of charge clouds around DPD soft beads and adopting the corrected Ewald sum method (EW3DC). The developed DPD platform is then calibrated to match the results of molecular dynamics, and reproduce experimental trends. A new system of unit conversion between DPD reduced units and SI units is introduced, which is also useful in other electrokinetic applications. The coarse-graining degree of beads is set to unity to challenge DPD performance in the smallest possible length scale, i.e. in a nanochannel sized at 3.8 nm

Thermostatic and rheological responses of DPD fluid to extreme shear under modified Lees-Edwards boundary condition

Thermodynamic, hydrodynamic and rheological interactions between velocity-dependent thermostats of Lowe-Andersen (LA) and Nosé-Hoover-Lowe-Andersen (NHLA), and modified Lees-Edwards (M-LEC) boundary condition were studied in the context of Dissipative Particle Dynamics method. Comparisons were made with original Lees-Edwards method to characterise the improvements that M-LEC offers in conserving the induced shear momentum. Different imposed shear velocities, heat bath collision/exchange frequencies and thermostating probabilities were considered. The presented analyses addressed an unusual discontinuity in momentum transfer that appeared in form of nonphysical jumps in velocity and temperature profiles. The usefulness of M-LEC was then quantified by evaluating the enhancements in obtained effective shear velocity, effective shear rate, Péclet number, and dynamic viscosity. System exchange frequency ($\Gamma$) with Maxwellian heat bath was found to play an important role, in that its larger values facilitated achieving higher shear rates with proper temperature control at the cost of deviation from an ideal momentum transfer. Similar dynamic viscosities were obtained under both shearing modes between LA and NHLA thermostats up to $\Gamma = 10$, whilst about twice the range of viscosity ($1 10$%). The main benefits of this modification were to facilitate momentum flow from shear boundaries to the system bulk. In addition, it was found that there exist upper thresholds for imposing shear on the system beyond which temperature cannot be controlled properly and nonphysical jumps reappear

The relationship between coronary artery distensibility and fractional flow reserve.

Discordance between angiography-based anatomical assessment of coronary stenosis severity and fractional flow reserve (FFR) has been attributed to several factors including lesion length and irregularity, and the myocardial territory supplied by the target vessel. We sought to examine if coronary arterial distensibility is an independent contributor to this discordance. There were two parts to this study. The first consisted of "in silico" models of 26 human coronary arteries. Computational fluid dynamics-derived FFR was calculated for fully rigid, partially distensible and fully distensible models of the 26 arteries. The second part of the study consisted of 104 patients who underwent coronary angiography and FFR measurement. Distensibility at the lesion site (DistensibilityMLA) and for the reference vessel (DistensibilityRef) was determined by analysing three-dimensional angiography images during end-systole and end-diastole. Computational fluid dynamics-derived FFR was 0.67±0.19, 0.70±0.18 and 0.75±0.17 (P<0.001) in the fully rigid, partially distensible and fully distensible models respectively. FFR correlated with both DistensibilityMLA (r = 0.36, P<0.001) and DistensibilityRef (r = 0.44, P<0.001). Two-way ANCOVA analysis revealed that DistensibilityMLA (F (1, 100) = 4.17, p = 0.031) and percentage diameter stenosis (F (1, 100) = 60.30, p < 0.01) were both independent predictors of FFR. Coronary arterial distensibility is a novel, independent determinant of FFR, and an important factor contributing to the discordance between anatomical and functional assessment of stenosis severity