Joo Sung Lee

Microfluidic four-roll mill for all flow types

A microfluidic four-roll mill device that can cover the entire spectrum of flow types including purely rotational flow was designed using pseudo-three-dimensional simulations. In experiments using high aspect ratio devices etched in silicon, the authors observed the whole range of flow type by changing only the flow rate ratio. This microfluidic four-roll mill device can be applied to examining microdrop deformation and the dynamics of single molecules in a mixed flow or to enhancing mixing efficiency by sinusoidal changes of the inlet flow rate.

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...

Tumbling dynamics of rod-like and semi-flexible polymers in simple shear and mixed flows

In this work, we focus on the tumbling dynamics of rod-like and semi-flexible polymers in mixed flows, which vary from simple shear to pure rotation. By employing a bead-rod model, the tumbling pathways and periods are examined with a focus on the angular distribution of their orientation. Under the mixed flows, the tumbling dynamics agreed well with earlier studies and confirmed the predicted scaling laws. We found that the angular distribution deviates from that of shear flow as the flow type approaches pure rotation. Finally, we investigated the angular distribution of λ-DNA in a shear flow and found that the present numerical simulations were in quantitative agreement with the previous experimental data.

Helical instability in film blowing process: Analogy to buckling instability

The film blowing process is one of the most important polymer processing operations, widely used for producing bi-axially oriented film products in a single-step process. Among the instabilities observed in this film blowing process, i.e., draw resonance and helical motion occurring on the inflated film bubble, the helical instability is a unique phenomenon portraying the snake-like undulation motion of the bubble, having the period on the order of few seconds. This helical instability in the film blowing process is commonly found at the process conditions of a high blow-up ratio with too low a freezeline position and/or too high extrusion temperature. In this study, employing an analogy to the buckling instability for falling viscous threads, the compressive force caused by the pressure difference between inside and outside of the film bubble is introduced into the simulation model along with the scaling law derived from the force balance between viscous force and centripetal force of the film bubble. Th...