Woo Il Lee

Self-Organized Anisotropic Wrinkling of Molecularly Aligned Liquid Crystalline Polymer

Anisotropic wrinkling which utilizes the anisotropic nature of liquid crystalline polymer (LCP) is demonstrated as a means of physical self-assembly to produce periodic microstructures. Through the plasma treatment on the molecularly aligned LCP film surface, one-dimensionally ordered wrinkle pattern was spontaneously formed on glass substrates without employing external thin-film deposition or prestrain control of the system. Experimental results indicate that the directionality of the wrinkle pattern can be tailored by the structural ordering of LCP molecules in the bilayer system of a hard skin layer on a soft substrate. Studies on process variables, such as the plasma treatment time and the film thickness, were conducted to figure out the effect on the wrinkling morphology. Due to its spatial periodicity over a large area and undemanding requirement of the process, this approach can be a candidate for the microfabrication in various applications.

A SOLUTION METHOD FOR A NONLINEAR THREE-DIMENSIONAL INVERSE HEAT CONDUCTION PROBLEM USING THE SEQUENTIAL GRADIENT METHOD COMBINED WITH CUBIC-SPLINE FUNCTION SPECIFICATION

In this article, an inverse method for retrieving time- and space-varying heat flux on the surface of a three-dimensional slab with temperature-dependent thermophysical properties from temperature scanning on the opposite surface is investigated. The sequential gradient method, which implements the gradient method in a sequential manner, is employed to solve the nonlinear inverse heat conduction problem. To stabilize the solution algorithm, a cubic-spline function is specified in space and a constant or linear function is specified in time without the use of the regularization term. The proposed method is verified with presented computational results for several cases.

Finite element analysis of flow and heat transfer with moving free surface using fixed grid system

A numerical study was performed on flow and heat transfer involving moving free surfaces that occurs in mold filling processes such as casting and injection molding. In these problems, the calculation domain changes continuously and the numerical treatment of the moving interface tends to cause artificial diffusion. Among the solution algorithms based on the Eulerian method, the volume-of-fluid (VOF) method was used because the method is simple and efficient in handling the complex flow patterns inside the cavity. To solve the transport equation of free surface without artificial smearing of the interface the baby-cell method was employed in the geometric reconstruction of the free surface. Furthermore, a predictor–corrector method was adopted in the time integration of volume-of-fluid (VOF) transport equation to increase the accuracy. The proposed scheme was verified through several benchmark problems. In order to show the capability of the proposed method, several three-dimensional mold filling processes were solved. The current algorithm was applied to the floating body problem. Three-dimensional floating body problems were tested.

Density variation of nanoscale patterns in thermal nanoimprint lithography

Density variation of nanoscale patterns in thermal nanoimprint lithography was studied both by experiments and molecular dynamics simulations. A simple soft imprinting technique was used to fabricate various nanopatterns (70nm and 600nm lines and 150nm dots) over a large area (2×3cm2). Local density was measured by the relative magnitude of van der Waals interactions between a sharp tip and the patterned surface. In order to investigate the mechanism of density variation, molecular dynamic simulations were performed. Experimental and simulation results demonstrated that the density of the pressed region (valleys) was higher than that of the cavity region (hills) when a simple amorphous polymer is thermally imprinted with a patterned mold.

Evolution of transient meniscus in a wettable microchannel for Newtonian fluid

A simple numerical approach based on the volume of fluid (VOF) method reveals that a W-shaped, transient meniscus is ubiquitous during the formation of a uniformly curved meniscus within a microchannel, which has been believed to be dominant for the transients. The time that is needed to maintain the transient meniscus is correlated with viscosity, surface tension and geometry of the cavity. A generalized correlation is presented to predict the persistent time of the transient meniscus in a wettable microchannel (contact angle, θ < 90°) for Newtonian fluid.

Molecular Dynamic Simulation on the Effect of Polymer Molecular Size in Thermal Nanoimprint Lithographic (T-NIL) Process

Molecular dynamic simulation of thermal nanoimprint lithographic (T-NIL) process has been performed to study the effect of the polymer molecular size on polymer flow for various mold cavity geometries. First, simulations of T-NIL process with several temperature settings were performed to determine the optimal temperature for the process. Simulations were also done to obtain the size of polymer molecule represented by the radius of gyration (Rg). Then, the relation between the Rg of the polymer and the processibility was investigated for various mold cavity sizes. The results showed that there existed a minimum cavity size for the Rg value of polymer for successful processing. Based on the results, it was shown that the polymer cannot be well patterned if the mold cavity size becomes 2Rg of polymer or smaller. Therefore, it could be concluded that the Rg value of a polymer can be a good indicating parameter when choosing the suitable pattern size.

Effects of the process temperature and rolling speed on the thermal roll-to-roll imprint lithography of flexible polycarbonate film

Thermal roll-to-roll imprint lithography (R2RIL) is a simple and low-cost process for the mass production of micro/nanopatterns. However, in that it relies on highly viscous thermoplastic resists, it is limited in its ability to imprint precise patterns at a high speed. Moreover, the concentrated imprint force applied in R2RIL can damage the resist material which is structurally vulnerable at high process temperatures. Therefore, it is important to understand the temperature- and time-dependent characteristics of the resist material as well as the imprinting mechanism when using thermal R2RIL. In this work, the effects of the process temperature and rolling speed on thermal R2RIL of polycarbonate (PC) films were investigated to improve the process efficiency. Micro-scale line patterns were successfully transferred onto PC films from nickel (Ni) mold stamps. Consequently, line patterns with widths in the range of 5–80 µm were achieved at a traveling speed of 28.6 mm s–1 and process temperature of 150 °C, which is just above the glass transition temperature (Tg). In addition, the patterning performance was investigated for different temperatures, rolling speeds and pattern sizes. The imprinted pattern profiles were measured by an alpha-step surface profiler to investigate the patterning performance. The results show that a much better imprint performance was achieved at 150 °C, compared to the result at temperatures below Tg. The physical mechanisms of thermal R2RIL on a PC film were studied by a finite-element analysis and the patterning process was successfully demonstrated by a visco-plastic deformation model.

Measurement of pull-off force on imprinted nanopatterns in an inert liquid

We report on the measurement of the pull-off force on nanoscale patterns that are formed by thermal nanoimprint lithography (t-NIL). Various patterns with feature sizes in the range of 50-900 nm were fabricated on silicon substrates using a rigiflex polymeric mold of ultraviolet curable polyurethane acrylate (PUA, Youngs modulus approximately 1 GPa) or perfluoropolyether (PFPE, Youngs modulus approximately 10.5 MPa) and a resist layer of polystyrene (PS) of three different molecular weights (M(w) = 18,100, 211,600 and 2043,000). The pull-off force was measured in non-polar, non-reactive perfluorodecalin (PFD) solvent between a sharp atomic force microscopy (AFM) tip and an imprinted pattern. Our experimental data demonstrated that the measured pull-off forces were in good agreement with a simple adhesion model based on Lifshitz theory. Also, the force on the pressed region (valley) is higher than that on the cavity region (hill), with the ratio (hill/valley) decreasing with the decrease of pattern size and the increase of molecular weight. The confinement effects were more pronounced for smaller patterns (<300 nm) and higher molecular weights (M(w) = 211,600 and 2043,000) presumably due to sluggish movement of polymer chains into nano-cavities. Finally, the experimental observations were compared with molecular dynamic simulations based on a simplified amorphous polyethylene model.

A Semi-Implicit Method for the Analysis of Two-Dimensional Fluid Flow with Moving Free Surfaces

Flow with moving free surfaces is analyzed with an the Eulerian coordinate system. This study proposes a semi-implicit filling algorithm using VOF in which the PLIC (Piecewise Linear Interface Calculation)-type interface reconstruction method and the donor-acceptor-type front advancing scheme are adopted. Also, a new scheme using extrapolation of the stream function is proposed to find the velocity of the node that newly enters the computational domain. The effect of wall boundary conditions on the flow field and temperature field is examined by numerically solving a two-dimensional casting process.

An Effective Calculation Method for Radiative Exchange in an Enclosure with Specular Surfaces

A new calculation method is proposed to determine the radiative heat transfer in enclosures composed of specularly reflecting surfaces. This method is based on the concept of the transition reflection in the context of the ray-tracing method, by which only two reflections of ray tracing are necessary to consider in the evaluation of net radiative transfer rate. This method can also be used not only for diffuse and specular surfaces but also for directional surfaces such as non-Lambert surface. Because the radiative exchange rate can be obtained by considering only two reflections of ray tracing, the matrix inversion is not required, and thus computational time can be reduced substantially. In order to validate the present method, the radiative exchange between two parallel plates is calculated, and the results are compared with those of previous methods including analytic solutions and stochastic methods. The parameters tested are the wall emissivity and the ratio of the distance between two plates to the length of plates. For all cases considered, the present results are in excellent agreement with exact solutions within 0.5% error, and show better accuracy than other methods. The new calculation method is also tested for nondiffusely emitting and reflecting surfaces of platinum and glass.