Kyung Hyun Ahn

Large amplitude oscillatory shear as a way to classify the complex fluids

Abstract Rheological properties of some polymer solutions in both the linear and non-linear regions have been investigated. The solutions include poly(vinyl alcohol) solution (with and without sodium borate), hyaluronic acid solution, and xanthan gum solution; some of which form microstructures depending on their deformation history, and thus can be classed as complex fluids. Among the rheological properties, the large amplitude oscillatory shear (LAOS) behavior was found to be very sensitive to the interactions or the shear-induced formation of microstructures. Depending on the interactions between the microstructures, it was found that there exist at least four types of LAOS behavior: type I, strain thinning ( G ′, G ″ decreasing); type II, strain hardening ( G ′, G ″ increasing); type III, weak strain overshoot ( G ′ decreasing, G ″ increasing followed by decreasing); type IV, strong strain overshoot ( G ′, G ″ increasing followed by decreasing). It is suggested that the LAOS behavior can be effectively used as a tool for classifying the complex fluids.

A geometrical interpretation of large amplitude oscillatory shear response

Although the stress of oscillatory shear flow can be decomposed into elastic and viscous parts in the linear regime, it is not yet known how to decompose the stress of a large amplitude oscillatory shear (LAOS) flow. We developed a method of analyzing LAOS data, which decomposes the stress into elastic and viscous components on the basis of a sound mathematical and physical foundation. This method is based on the symmetry of the stress and is a generalization of linear viscoelasticity from the viewpoint of geometry. The proposed method is more powerful than previous methods such as Fourier transform analysis and the Lissajous plot, in that it is more sensitive to the presence of nonlinearities and it is easier to determine nonlinear parameters.

Large amplitude oscillatory shear behavior of complex fluids investigated by a network model: a guideline for classification

Large amplitude oscillatory shear (LAOS) behavior of complex fluids, which form microstructures depending on their deformation history, has been investigated by using a network model. According to recent experimental observations, the LAOS behavior of complex fluids could be classified by at least four types: type I, strain thinning (G � , G �� decreasing); type II, strain hardening ( G � , G �� increasing); type III, weak strain overshoot ( Gdecreasing, G �� increasing followed by decreasing); type IV, strong strain overshoot ( G � , G �� increasing followed by decreasing). To understand such complex behavior, we have applied a general network model. As there is little information available on the form of creation and loss rates of network junctions, we have modeled the creation and loss rates as exponential functions of shear stress. By adjusting the model parameters that define the creation and loss rates, the types of LAOS behavior observed in the experiments could be reproduced. Despite highly simplistic modeling, the model reproduced the types of LAOS behavior observed in the experiments, which means that the behavior can be explained in terms of the model parameters, that is, the creation and loss rates of network junctions. It is also suggested that the LAOS behavior can be effectively used as a tool for classifying complex fluids.

Fourier-transform rheology under medium amplitude oscillatory shear for linear and branched polymer melts

Nonlinear response of linear and branched polymers has been investigated under medium strain amplitude oscillatory shear (strain amplitude range from 10% to 100%) with Fourier-transform rheology. A power law relationship was found between the relative third intensity (I3∕I1), which is an indicator of nonlinearity, and the strain amplitude at low and medium strain amplitudes. On a log-log plot, the intercept and slope of I3∕I1 were investigated at different excitation frequencies and temperatures. Simulation results with three different constitutive equations [Giesekus, exponential Phan-Thien Tanner (E-PTT), pom-pom model] were also compared. Experimental results show that the intercept was affected by the excitation frequency and temperature, and the slope of I3∕I1 for linear polymer remained constant regardless of molecular weight, molecular weight distribution, and excitation frequency in accordance with the predictions of the constitutive equations (Giesekus and E-PTT). It should be noted that the slop...

Nonlinear viscoelasticity of polymer nanocomposites under large amplitude oscillatory shear flow

In this study, the nonlinear response of polymer nanocomposites under large amplitude oscillatory shear (LAOS) flow was investigated. We first investigated polycaprolactone (PCL)/multiwall nanotube (MWNT) composites under LAOS flow using different analyzing methods including Lissajous plot analysis, stress decomposition, and Fourier transform rheology (FT-rheology). The nonlinear parameter Q (≡I3/1/γ02) was obtained from the FT-rheology as a function of strain amplitude, and the zero-strain nonlinearity Q0 (≡limγ0→0Q) was also calculated. We compared the linear and nonlinear viscoelastic properties as we increase MWNT concentration (ϕ). It was found that the zero-strain nonlinearity (Q0) was more sensitive to detect the effect of MWNT concentration than the linear viscoelastic properties. We also investigated the effect of particle shape on nonlinear viscoelastic properties of the polymer composites containing particles of different shape, e.g., PCL/MWNT (one-dimensional thread shape), PCL/organomodified ...

Three-dimensional dynamics simulation of electrorheological fluids under large amplitude oscillatory shear flow

Large amplitude oscillatory shear (LAOS) behavior of electrorheological (ER) fluids has been investigated using three-dimensional particle-level dynamics simulation. As an ER device usually operates in a dynamic mode with large deformation, it is important to understand the LAOS behavior as well as its underlying mechanism for the development of an effective ER fluid. Simulation predicted most of the experimental observations including strain overshoot, distorted stress signal, dogbone-type Lissajous curve, and stripe pattern formation, to list a few. By careful investigation of cluster statistics as well as microstructures, we found that the strain overshoot phenomenon, which is often observed in complex fluid systems with little explanation, arises from the cluster reformation process in addition to a slight rearrangement within a cluster. Fourier transformation analysis was also performed, and the scaling behavior of the intensities of higher harmonics was investigated. The intensities of higher harmon...

Liquid Crystal Order in Colloidal Suspensions of Spheroidal Particles by Direct Current Electric Field Assembly

DC electric fields are used to produce colloidal assemblies with orientational and layered positional order from a dilute suspension of spheroidal particles. These 3D assemblies, which can be visualized in situ by confocal microscopy, are achieved in short time spans (t < 1 h) by the application of a constant voltage across the capacitor-like device. This method yields denser and more ordered assemblies than had been previously reported with other assembly methods. Structures with a high degree of orientational order as well as layered positional order normal to the electrode surface are observed. These colloidal structures are explained as a consequence of electrophoretic deposition and field-assisted assembly. The interplay between the deposition rate and the rotational Brownian motion is found to be critical for the optimal ordering, which occurs when these rates, as quantified by the Peclet number, are of order one. The results suggest that the mechanism leading to ordering is equilibrium self-assembly but with kinetics dramatically accelerated by the application of the DC electric field. Finally, the crystalline symmetry of the densest structure formed is determined and compared with previously studied spheroidal assemblies.

Latex Migration in Battery Slurries during Drying

We used real-time fluorescence microscopy to investigate the migration of latex particles in drying battery slurries. The time evolution of the fluorescence signals revealed that the migration of the latex particles was suppressed above the entanglement concentration of carboxymethyl cellulose (CMC), while it was significantly enhanced when CMC fully covered the surfaces of the graphite particles. In particular, a two-step migration was observed when the graphite particles flocculated by depletion attraction at high CMC/graphite mass ratios. The transient states of the nonadsorbing CMC and graphite particles in a medium were discussed, and the uses of this novel measurement technique to monitor the complex drying processes of films were demonstrated.

Drying of the Silica/PVA Suspension: Effect of Suspension Microstructure

The particle/polymer/solvent suspension system shows complicated microstructure. When the suspension system experiences an industrial process such as coating and drying, the system experiences microstructural change. In this study, we investigated the microstructural change during the drying of a silica/polyvinyl alcohol (PVA) suspension, with an emphasis on suspension stability. We controlled the amount of PVA adsorption on the silica surface by adjusting the pH (1.5, 3.6, and 9) of the silica/PVA suspension. The amount of adsorption was measured to increase with decreasing pH, and the degree of flocculation in the silica/PVA suspension became stronger with decreasing pH. However, through the measurement of stress development during drying and the observation of film microstructure after drying, we found that the more strongly flocculated suspension became a more disperse, close-packed film after drying. By evaluating the potential energy, we could suggest the role of adsorbed polymers in structural change during the drying of the silica/PVA suspension. As pH decreases, the adsorbed polymers could bridge the particles and lead to a flocculated suspension before drying. As the solvent evaporates during drying, the adsorbed polymers introduce steric repulsion between approaching particles, leading to a change from flocculated to dispersed microstructure. This implies that the required silica/PVA film performance and the microstructure of the silica/PVA suspension can be tailored through controlling the polymer adsorption in suspension.

Numerical simulation of three-dimensional viscoelastic flow using the open boundary condition method in coextrusion process

Abstract Three-dimensional numerical simulation of coextrusion process of two immiscible polymers through a rectangular channel has been performed using the finite element method. The upper convected Maxwell (UCM) model and the Phan-Thien and Tanner (PTT) model were considered as viscoelastic constitutive equations. The elastic viscous stress splitting (EVSS) method was adopted to treat the viscoelastic stresses, and the streamline upwinding (SU) method was applied to avoid the failure of convergence at high elasticity. The problem arising from the ambiguous outlet boundary condition that has previously been used in the three-dimensional simulation of a viscoelastic coextrusion process could be avoided by introducing the open boundary condition (OBC) method. The abrupt change or deviation of contact line position near the outlet that was observed when the fully developed outlet boundary condition was applied could be clearly removed by using the OBC method. The effects of viscoelastic properties, such as the shear viscosity ratio, the elasticity, the second normal stress difference, and the extensional viscosity on the interface distortion, the interface curvature, and the degree of encapsulation along the downstream direction have been investigated. The shear viscosity ratio between the polymer melts was the controlling factor of the interface position and the encapsulation phenomena. The interface distortion seems to increase as the elasticity ratio increases under constant shear viscosity, even though it is not so large. The degree of encapsulation seems to increase with increasing the ratio of the second normal stress differences. The extensional viscosity had minor effect on the encapsulation phenomena. The second normal stress difference was found to have a great influence on the increasing of the degree of encapsulation along the downstream direction as compared to the effect of the first normal stress difference.