Jung-Eun Bae

Microfluidic device for sheathless particle focusing and separation using a viscoelastic fluid

Continuous sheathless particle separation with high efficiency is essential for various applications such as biochemical analyses and clinical diagnosis. Here, a novel microfluidic device for highly efficient, sheathless particle separation using an elasticity-dominant non-Newtonian fluid is proposed. Our device consists of two stages: sheathless three-dimensional focusing (1 st stage) and separation (2nd stage). It is designed based on the principle of a viscoelasticity-induced particle lateral migration, which promises precise separation of particles in a microfluidic device. Particles of 5- and 10-μm diameters were all focused at the centerline of a circular channel at the 1st stage and successfully separated at the 2nd stage with an efficiency of ∼99.9% using size-based lateral migration of particles induced by the viscoelasticity of the medium. We also demonstrated the capability of our device for separation of blood cells into multiple fractions. The tunability of separable particle size could be achieved by changing the viscoelastic property of the medium and flow rate.

Semianalytical methods for the determination of the nonlinear parameter of nonlinear viscoelastic constitutive equations from LAOS data

Various viscoelastic constitutive equations have been developed to describe nonlinear viscoelastic flows. Most equations contain two kinds of parameters: Nonlinear and linear ones. The linear parameters correspond to relaxation time spectrum and can be determined from linear viscoelastic data. Meanwhile, the nonlinear parameters cannot be determined by linear viscoelastic data. The determination of the nonlinear parameters requires both reliable nonlinear data and complex procedures for fitting numerical solution of differential equations to the nonlinear data. If an analytical solution of viscoelastic model is available then dramatic reduction of difficulty is expected in the determination of the nonlinear parameters. Previous studies on analytical solution of large amplitude oscillatory shear (LAOS) are based on series expansion which is effective up to only third harmonic. Since it is practically impossible to obtain the analytical solution of higher order than fifth, we suggest a new method which extr...

Logarithmic method for continuous relaxation spectrum and comparison with previous methods

Various algorithms for determining relaxation time spectrum have been to the fore, because relaxation spectrum plays an important role in interconversion between various viscoelastic functions. In this study, a new algorithm is developed by means of double-logarithmic power series approximation and the Levenberg–Marquardt method. Compared with most previous algorithms, new algorithm is applicable not only to dynamic data but also to the Laplace transform of relaxation modulus, which is available for optical data from the generalized Stokes–Einstein equation or creep data with ringing. It is worth noticing that the new algorithm determines continuous relaxation spectrum without any additional parameters and precedent procedures. This paper suggests such framework of newly developed algorithm and its applications to various kinds of viscoelastic data.

Extraction of viscoelastic functions from creep data with ringing

Sometimes the creep ringing phenomena are observed as results of the coupling between the instrumental inertia and the viscoelasticity of the materials. Especially, biopolymer solutions are apt to be the victim of creep ringing because creep test is usually conducted to enlarge the frequency range of dynamic data. To overcome the creep ringing problems, we developed a general method which extracts pure material response functions from creep data with ringing by the use of Laplace transform and nonlinear regression method. The method is not based on spring-dashpot models and gives continuous relaxation time spectrum.

Comparison of stress-controlled and strain-controlled rheometers for large amplitude oscillatory shear

In linear viscoelastic region, it is well known that dynamic modulus and dynamic compliance can be converted to each other. However, it is questionable whether there exists an interconversion between large amplitude oscillatory shear (LAOS) data measured from different types of rheometers—stress-controlled and strain-controlled rheometers. Hence, we tried to prove the existence by use of polyethylene oxide (PEO) aqueous solutions with well-developed entanglements. From this experiment, we can conclude that a stress-controlled rheometer can simulate LAOS behavior measured from a strain-controlled rheometer under the conditions where inertia effect is not significant. Furthermore, it is investigated whether the LAOS data of the stress-controlled rheometer obey stress–frequency superposition as the strain–frequency superposition found by Cho et al. (J Rheol 54:27–63, 2010) from LAOS data measured by the strain-controlled rheometer. This scaling relation shows that the dimensionless stress amplitude is a function of zeta which is the product of the stress amplitude and linear viscoelastic function J′(ω). The plot shows that all of the data are superposed in a single curve without regard to frequency, molecular weight, and concentration of PEO aqueous solutions.

Andrographolide inhibits HMGB1‐induced inflammatory responses in human umbilical vein endothelial cells and in murine polymicrobial sepsis

Nuclear DNA‐binding protein high‐mobility group box 1 (HMGB1) protein acts as a late mediator of severe vascular inflammatory conditions, such as septic shock, upregulating pro‐inflammatory cytokines. Andrographolide (AG) is isolated from the plant of Andrographis paniculata and used as a folk medicine for treatment of viral infection, diarrhoea, dysentery and fever. However, the effect of AG on HMGB1‐induced inflammatory response has not been studied.

Effect of temporary network structure on linear and nonlinear viscoelasticity of polymer solutions

We investigated the effects of temporary network structures on linear and nonlinear viscoelasticity of polymer solutions by use of oscillatory shear (LAOS) flow. We tested two different types of polymer solutions: entanglement systems and ion complex systems. It was found that the entanglement network is difficult to show shear-thickening while network of ion complex gives rise to shear-thickening. The objectives of this paper are the test of strain-frequency superposition for various polymer solutions and to suggest a new method classifying complex fluids consisting temporary networks using LAOS data.

Analytical studies on the LAOS behaviors of some popularly used viscoelastic constitutive equations with a new insight on stress decomposition of normal stresses

Shear stress of Large Amplitude Oscillatory Shear (LAOS) is known to be decomposed to elastic and viscous stresses. According to the parity of normal stress with respect to shear strain and shear rate, it also can be mathematically decomposed into two parts: NEE (even symmetry part for both strain and strain rate) and NOO (odd symmetry part for both shear strain and shear rate). However, the physical meaning of the decomposed normal stress is questionable. This paper is to prove the conjecture that NEE is elastic and NOO is viscous under the condition of time-strain separability. For the purpose of the proof, we developed mathematical tools for the analytical solutions of LAOS. We applied the mathematical methods to some popularly used constitutive equations such as the convected Maxwell models, the separable Kaye-Bernstein-Kearsley-Zepas (K-BKZ) model, the Giesekus model, and the Phan-Thien and Tanner model.Shear stress of Large Amplitude Oscillatory Shear (LAOS) is known to be decomposed to elastic and viscous stresses. According to the parity of normal stress with respect to shear strain and shear rate, it also can be mathematically decomposed into two parts: NEE (even symmetry part for both strain and strain rate) and NOO (odd symmetry part for both shear strain and shear rate). However, the physical meaning of the decomposed normal stress is questionable. This paper is to prove the conjecture that NEE is elastic and NOO is viscous under the condition of time-strain separability. For the purpose of the proof, we developed mathematical tools for the analytical solutions of LAOS. We applied the mathematical methods to some popularly used constitutive equations such as the convected Maxwell models, the separable Kaye-Bernstein-Kearsley-Zepas (K-BKZ) model, the Giesekus model, and the Phan-Thien and Tanner model.

Determination of continuous relaxation spectrum based on the Fuoss-Kirkwood relation and logarithmic orthogonal power-series approximation

In this study, we suggest a new algorithm for inferring continuous spectrum from dynamic moduli data. The algorithm is based on the Fuoss-Kirkwood relation (Fuoss and Kirkwood, 1941) and logarithmic powerseries approximation. The Fuoss-Kirkwood relation denotes the existence of the uniqueness of continuous spectrum. If we know the exact equation of dynamic moduli, then continuous spectrum can be inferred uniquely. We used the Chebyshev polynomials of the first kind to approximate dynamic moduli data in double-logarithmic scale. After the approximation, a spectrum equation can be derived by use of the complex decomposition method and the Fuoss-Kirkwood relation. We tested our algorithm to both simulated and experimental data of dynamic moduli and compared our result with those obtained from other methods such as the fixed-point iteration (Cho and Park, 2013) and cubic Hermite spline (Stadler and Bailly, 2009).

An Empirical Model for the Viscosity of Reactive Polymeric Fluids

The rheological properties of reactive materials are of importance in industrial or academic purpose. However, it is hard to characterize these properties for overall range of reaction, because curing reaction accompanies enormous and intricate structural changes. Consequently, it is demanded to establish the model which elucidates the rheological changes as a function of degree of reaction. In this study, we observed the curing behavior of novolac epoxy/phenol novolac/triphenylphosphane (TPP) system in stoichiometrically balanced state. We developed a new empirical model describing the change of rheological profile of epoxy system for the wider range of degree of reaction, α. Newly suggested empirical equation as a function of α is applied to analyze the evolution of rheological variables during polymerization of polymethyl methacrylate. It is shown that the new empirical model is suitable to analyze the viscosity profile for systems including reactions such as curing or polymerization.