Myung Suk Chun

Conformational dynamics of sub-micron sized wormlike polyelectrolyte polymer in flow fields

AbstractConformational dynamics of a single chain of wormlike polyelectrolyte xanthan polymer has been investigated in the external flow fields by employing a well-suited coarse-grained Brownian dynamics simulation. This goes beyond other simulations, which do not consider the hydrodynamic interaction between pairs of beads in polyelectrolyte polysaccharide and the long-range electrostatic screening effect. Conformational properties, such as the radius of gyration and the static structure factor, were unchanged with the flow strength parameter (i.e., Weissenberg number) in the uniform flow. However, influences by flow strength as well as flow type were evident in both simple shear and extensional-like flows with non-zero velocity gradients in flow regimes, commonly exhibiting a sigmoidal transition in the radius of gyration. Transition to a higher plateau, and independence of long-range electrostatic screening on chain conformation, can be encountered earlier with increasing flow strength, as a special feature of a polyelectrolyte in extensional-like flow. The translational self-diffusion coefficient increases when increasing either the flow strength or the electrostatic screening effect in uniform and simple shear flows. Scaling behavior of the static structure factor is quite well-correlated with respect to each flow field, where the Flory-Edwards exponent (ν) decreases with higher values of flow strength and flow type parameters, but for lower screening effect. Present results on the mesoscopic scale devoted to the bulk space can readily serve as the basis for further scrutiny of the behavior of wormlike polyelectrolytes within various flow fields in confined spaces.n

A Novel Simulation Architecture of Configurational-Bias Gibbs Ensemble Monte Carlo for the Conformation of Polyelectrolytes Partitioned in Confined Spaces

By applying a configurational-bias Gibbs ensemble Monte Carlo algorithm, priority simulation results regarding the conformation of non-dilute polyelectrolytes in solvents are obtained. Solutions of freely-jointed chains are considered, and a new method termed strandwise configurational-bias sampling is developed so as to effectively overcome a difficulty on the transfer of polymer chains. The structure factors of polyelectrolytes in the bulk as well as in the confined space are estimated with variations of the polymer charge density.

The electrokinetic microfluidic flow in multi-channels with emergent applicability toward micro power generation

In order to elaborate the possible applicability of microfluidic power generation from conceptualization to system validation, we adopt a theoretical model of the electrokinetic streaming potential previously developed for the single channel problem. The ion transport in the microchannel is described on the basis of the Nernst-Planck equation, and a monovalent symmetric electrolyte of LiClO4 is considered. Simulation results provide that the flow-induced streaming potential increases with increasing the surface potential of the microchannel wall as well as decreasing the surface conductivity. The streaming potential is also changed with variations of the electric double layer thickness normalized by the channel radius. From the electric circuit model with an array of microchannels, it is of interest to evaluate that a higher surface potential leads to increasing the power density as well as the energy density. Both the power density and the conversion efficiency tend to enhance with increasing either external resistance or number of channels. If a single microchannel is assembled in parallel with the order of 103, the power density of the system employing large external resistance is estimated to be above 1 W/m3 even at low pressure difference less than 1 bar.

Particle migration in planar Couette–Poiseuille flows of concentrated suspensions

Particle migration phenomena in parallel slit channel flows of concentrated suspensions with asymmetric velocity profiles, which are governed by planar Couette–Poiseuille (C-P) flow conditions, are numerically investigated employing the diffusive flux model (DFM) via the finite volume method. The particle distributions predicted by DFM are confirmed by comparing quantitatively with those by the reported experimental results and the lattice Boltzmann method. The main factors governing the migration dynamics in the DFM, such as particle size, concentration, and flow length from an inlet of the channel, are effectively unified into a nondimensional length element. The effects of the asymmetric C-P flow fields on particle dynamics are clarified by the evolution of the concentration distribution along the nondimensional length element under a different asymmetric velocity and initial concentration conditions. From scale analysis, this asymmetric distribution is analytically interpreted by adopting a concept of...

Secondary flow behavior of electrolytic viscous fluids with Bird-Carreau model in curved microchannels

A curved channel is indispensable for the lab-on-chips system because it provides a convenient scheme for increasing the channel length per unit chip area in the direction of net flow. A secondary Dean flow in curved rectangular microchannels is examined by applying a finite volume scheme with a semi-implicit method for pressure-linked equations revised (SIMPLER) algorithm for the steady flow of non-Newtonian fluids with dilute concentrations in electrolyte solution. The proposed framework is based on a theoretical model coupled with the Cauchy momentum equation of Bird-Carreau fluid, electrostatic interaction, and net charge conservation. The shear thinning effect on the secondary flow at the turn is analyzed with respect to the parameters of interest including zero-shear viscosity and power-law index. The simulation results for the realistic microchannel indicate that the streamwise axial velocity tends to shift toward the inner wall and this is caused by a stronger spanwise pressure gradient, resulting from a sufficiently low Dean number. In the case of the non-Newtonian fluid, the axial velocity becomes a blunt profile by spreading to each side of the wall, and its flow skewness directed asymmetrically to the inner wall is higher than that in the case of the Newtonian fluid. The attaining trend of higher flow skewness is changed by the competing effect of shear thinning and is well matched with the behavior of the secondary flow.

Sensitivity analysis of two-dimensional viscoelastic film casting processes using transfer functions by transient frequency response method

The sensitivity of two-dimensional (2-D) film casting processes for viscoelastic fluids is investigated using a transfer function approach based on the transient frequency response method. The transient responses of state variables to step-changed film tension have been conveniently changed into corresponding transfer functions via the Laplace transform in a tension-controlled system. The transfer function between take-up velocity and film tension plays a key role in elucidating both sensitivity and the stability of film drawing systems. Various amplitude ratios of state variables are effectively evaluated over a wide range of frequencies when an ongoing disturbance is imposed at take-up velocity. The aspect ratio (up to 1 in this study) tends to make the system less sensitive to disturbances. The effect of viscoelasticity on the sensitivity is related to stability patterns, which depend on the aspect ratio and Deborah number.