Hyun Wook Jung

Continuous separation of microparticles in a microfluidic channel via the elasto-inertial effect of non-Newtonian fluid

Pure separation and sorting of microparticles from complex fluids are essential for biochemical analyses and clinical diagnostics. However, conventional techniques require highly complex and expensive labeling processes for high purity separation. In this study, we present a simple and label-free method for separating microparticles with high purity using the elasto-inertial characteristic of a non-Newtonian fluid in microchannel flow. At the inlet, particle-containing sample flow was pushed toward the side walls by introducing sheath fluid from the center inlet. Particles of 1 μm and 5 μm in diameter, which were suspended in viscoelastic fluid, were successfully separated in the outlet channels: larger particles were notably focused on the centerline of the channel at the outlet, while smaller particles continued flowing along the side walls with minimal lateral migration towards the centerline. The same technique was further applied to separate platelets from diluted whole blood. Through cytometric analysis, we obtained a purity of collected platelets of close to 99.9%. Conclusively, our microparticle separation technique using elasto-inertial forces in non-Newtonian fluid is an effective method for separating and collecting microparticles on the basis of size differences with high purity.

Analysis of the stabilizing effect of spinline cooling in melt spinning

Abstract Despite the fact that understanding of draw resonance in spinning process has steadily advanced with its onset readily predictable by the linear stability analysis method, as [C.J.S. Petrie, Progress Trends Rheol. II (1988) 9] eloquently elaborated, there are still many issues to be answered. For one, the stabilizing effect of spinline cooling has been proven by both experiments and the linear stability analysis but the question of why the cooling performs such a stabilizing role is not yet explained. The same can be said of other process conditions and material properties like elasticity over their roles in spinning stability. The governing physics and the hyperbolic nature of the spinning equations tell us that spinline tension represents the key link in relaying disturbances from the take-up to the spinneret to perpetuate draw resonance. In this simulation study the spinline tension sensitivity to disturbances has been found decreasing as the spinline cooling increases, i.e., stability enhanced by the cooling. This finding explains the success of an ingenious device called draw resonance eliminator of [P.J. Lucchesi, E.H. Roberts, S.J. Kurtz, Plast. Eng. 41 (1985) 87] which renders the spinline tension very insensitive to disturbances using maximum cooling air blown onto the spinline (the film in their case). It also explains why spinning with constant force boundary conditions is always stable by providing the reason that the transmission links between disturbances and the tension are completely disconnected in this case. Newtonian and upper convected Maxwell fluids have been tested to reveal that spinline cooling reduces the tension sensitivity to disturbances, resulting in increased stability.

Effect of fluid viscoelasticity on the draw resonance dynamics of melt spinning

The effect of fluid viscoelasticity on the draw resonance dynamics of melt spinning has been examined using White-Metzner and Phan Thien-Tanner fluid models into the governing equations of the process, in a continued effort to study the effects of process conditions and material properties on draw resonance, following up the earlier study [J. Non-Newtonian Fluid Mech. 87 (1999) 165] dealing with the effect of spinline cooling on the same draw resonance. Whether or not the fluid viscoelasticity stabilizes melt spinning has turned out to coincide with whether or not the spinline tension sensitivity decreases with the increasing fluid viscoelasticity. This is because the spinning stability is always enhanced by a decrease in tension sensitivity to process disturbances and this tension sensitivity was then found in the said earlier study to be moving opposite to the level of the spinline tension: the higher spinline tension, the smaller tension sensitivity. It has been found in the present study that the effect of fluid viscoelasticity on spinning stability can be classified into two diametrically different kinds: for extension-thickening fluids an increase in viscoelasticity increases tension, decreases tension sensitivity and, thus, stabilizes the spinning, whereas it decreases tension, increases tension sensitivity and, thus, destabilizes the spinning of extension-thinning fluids.

Kinematic waves and draw resonance in film casting process

Draw resonance, one of major instabilities frequently occurring in fiber spinning, film casting and film blowing processes, arises as the drawdown ratio is increased beyond its critical value and is manifested by sustained periodic variations in spinline variables such as cross-sectional area and tension. The approach which was introduced by Hyun and coworkers [1–6] based on kinematic waves traveling on the spinline to explain the physics behind this draw resonance in spinning and to derive its criterion, has been applied to film casting in the present simulation study. It has then been revealed that the same mechanism and criterion govern the draw resonance in film casting as in spinning, but at the same time, some differences also have emerged. Particularly, the nonlinear dynamics of the film width whose counterpart does not exist in spinning has been found quite complex and also very sensitive to process parameters like fluid viscoelasticity and the aspect ratio of the casting equipment. This contrasts dramatically with that of the film thickness which shows rather simple dynamic patterns, almost insensitive to the changing parameters values. The good control of film width is of as much importance as that of film thickness in many industrial processes including paper coating. An ingenious coating method [7] exemplifies how critical the film width control is for the successful operation of extrusion coating of polymer films on paper substrates.

Transient and steady-state solutions of 2D viscoelastic nonisothermal simulation model of film casting process via finite element method

The various aspects of the nonlinear dynamics and stability of nonisothermal film casting process have been investigated solving a two-dimensional (2D) viscoelastic simulation model equipped with the Phan-Thien-Tanner (PTT) constitutive equation by employing a finite element method. This study represents an extension of the earlier report [Kim, Lee, Shin, Jung, and Hyun, J. Non-Newtonian Fluid Mech. 132, 53–60 (2005)] in that two important points are additionally addressed here on the subject: the nonisothermal nature of the film casting, and the differentiation of extension-thickening (strain hardening) and extension-thinning (strain softening) fluids in their different behavior in the film casting process. The PTT model, known for its robustness in portraying dynamics in the extensional deformation processes which include the film casting of this study along with film blowing and fiber spinning as well, renders the transient and steady state solutions of the dynamics in the 2D, viscoelastic, nonisothermal, film casting capable of explaining the effects of various process and material parameters of the system on the film dynamics of the process. Especially, the different behavior displayed by two polymer groups, i.e., the extension-thickening low density polyethylene (LDPE) type and the extension-thinning high density polyethylene (HDPE) type, in the film casting can be readily explained by the PTT equation-included simulation model. The three nonlinear phenomena commonly observed in film casting, i.e., draw resonance oscillation, edge bead, and neck-in, have been successfully delineated in this study using the simulation and experimental results.The various aspects of the nonlinear dynamics and stability of nonisothermal film casting process have been investigated solving a two-dimensional (2D) viscoelastic simulation model equipped with the Phan-Thien-Tanner (PTT) constitutive equation by employing a finite element method. This study represents an extension of the earlier report [Kim, Lee, Shin, Jung, and Hyun, J. Non-Newtonian Fluid Mech. 132, 53–60 (2005)] in that two important points are additionally addressed here on the subject: the nonisothermal nature of the film casting, and the differentiation of extension-thickening (strain hardening) and extension-thinning (strain softening) fluids in their different behavior in the film casting process. The PTT model, known for its robustness in portraying dynamics in the extensional deformation processes which include the film casting of this study along with film blowing and fiber spinning as well, renders the transient and steady state solutions of the dynamics in the 2D, viscoelastic, nonisotherm...

Stability of Isothermal Spinning of Viscoelastic Fluids

The stability of isothermal spinning of viscoelastic fluids which have strain-rate dependent relaxation time has been investigated using the linear stability analysis method. The instability known as draw resonance of the system was found to be dependent upon the material functions of the fluids like fluid relaxation time and the strain-rate dependency of the relaxation time as well as upon the draw-down ratio of the process. Utilizing the fundamental physics of the system characterized by the traveling kinematic waves, we also have developed a simple, approximate method for determining this draw resonance instability; it requires only the steady state velocity solutions of the system, in contrast to the exact stability analysis method which requires solving the transient equations. The stability curves produced by this simple, fast method agree well with those by the exact stability method, proving the utility of the method. The stability of other extensional deformation processes such as film casting and film blowing can also be analyzed using the method developed in this study.

The sensitivity and stability of spinning process using frequency response method

The sensitivity and stability by frequency response of the final filament to several sinusoidal disturbances have been investigated in viscoelastic spinning by using various novel numerical algorithms. Amplitudes, or gains of the spinline cross-sectional area at the take-up, show resonant peaks, which are frequently encountered in hyperbolic systems. To effectively solve the complex system of the frequency response equation, alternative ways have been performed and compared. Interestingly, in the one-dimensional systems considered, integrating the linearized equations over the spinline length to shoot at the take-up boundary condition using two initial guesses (“two-shot” method) proved far more efficient than modal analysis using eigenfunction data or solving the matrix problem from the entire length by a direct method or an iterative one (GMRES). Also, the methodology to determine the stability of the system by using frequency response data, as suggested in Kase and Araki [1982], has been revamped to viscoelastic spinning system.

Multiplicity, bifurcation, stability and hysteresis in dynamic solutions of film blowing process

The complicated nonlinear dynamics in film blowing process has been investigated focusing on the multiplicity, bifurcation, and stability in the dynamic solutions of the system using both the theoretical model simulation and experiments. A number of interesting findings have been revealed about the dynamics of the process including a fundamentally different behavior of the system with a maximum of three steady states for the nonisothermal operations in contrast to the isothermal approximation where only two steady states were predicted. These differences have been identified as stemming from the fact that multiple values of a bubble radius at the freezeline height can give the same value of the air pressure inside the bubble depending upon the process conditions and the values of the bifurcation parameters of the system. The stability of the three steady states also displays many different patterns dictated by the process conditions, including the hysteresis in the bifurcation diagrams. These stability re...

Effect of shim configuration on flow dynamics and operability windows in stripe slot coating process

The slot coating method is a strong candidate for the manufacture of secondary batteries and electrodes for electronics. For the production of such devices, one may need to coat multiple lanes simultaneously which is usually done by implementing specially designed stripe shim inside the die manifold. The effects of shim configurations on the stripe-patterned coated film were analyzed in this study. We employed computational and experimental analyses to estimate flow patterns and corresponding desirable operating condition changes for uniform, converging and diverging slit channels. It was found that the slit channel shape, which is determined by the shim design, can manipulate the die-exit velocity distribution, capable of controlling edge shape of the crossflow film thickness profile. However, other than producing a uniform channel, this might decrease the scope of operability windows due to the aggravation of bead break-up near the edge of the slit channel.

Fast dynamics and relaxation of colloidal drops during the drying process using multispeckle diffusing wave spectroscopy.

The fast dynamics generated by the Brownian motion of particles in colloidal drops, and the related relaxation during drying, which play key roles in suspension systems, were investigated incorporating multispeckle diffusing wave spectroscopy (MSDWS). MSDWS equipment was implemented to analyze the relaxation properties of suspensions under a nonergodic and nonstationary drying process, which cannot be elucidated by conventional light scattering methods, such as dynamic light scattering and diffusing wave spectroscopy. Rapid particle movement can be identified by the characteristic relaxation time, which is closely related to the Brownian motion due to thermal fluctuations of the particles. In the compacting stage of the drying process, the characteristic relaxation time increased gradually with the drying time because the particles in the colloidal drop were constrained by themselves. Moreover, variations of the initial concentration and particle size considerably affected the complete drying time and characteristic relaxation time, producing a shorter relaxation time for a low concentrated suspension with small particles.