Tai Hun Kwon

A barrier embedded chaotic micromixer

Mixing enhancement has drawn a great attention to designing of micromixers, since the flow in a microchannel is usually characterized by a low Reynolds number (Re) which makes mixing quite a difficult task to complete. In this regard, we present a new chaotic passive micromixer, called a barrier embedded micromixer (BEM). In the BEM, chaotic flow is induced by periodic perturbation of the velocity field due to periodically inserted barriers along the top surface of the channel while a helical type of flow is obtained by slanted grooves on the bottom surface in the pressure driven flow. A T-channel and a microchannel with only slanted grooves were fabricated for the purpose of experimental comparison. Mixing performance has been experimentally characterized in two ways: (i) change of average mixing intensity by means of phenolphthalein and (ii) mixing patterns via a confocal microscope. Experimental results showed that BEM has better mixing performance than the other two. A characteristic required mixing length, defined in view of intensity change, increases logarithmically with Re in BEM. The confocal microscope images indicated that BEM could achieve almost complete mixing. The chaotic mixing mechanism, proposed in this study can be easily applied to integrated microfluidic systems, such as micro-total-analysis-systems, lab-on-a-chip and so on.

Effects of intrinsic hydrophobicity on wettability of polymer replicas of a superhydrophobic lotus leaf

The present work suggests a mass-producible and large-scale fabrication method of superhydrophobic polymeric surfaces by means of material processing equipments which can maximize productivity and cost effectiveness. We fabricated two types of polymeric lotus leaf replicas using a nickel mold, i.e. R1 from intrinsically hydrophobic polydimethylsiloxane by means of polymer casting (PC) and R2 from an intrinsically hydrophilic UV-curable photopolymer by means of UV-nanoimprint lithography (UV-NIL). In the case of R1 from PC, although the nano-scaled structures were not well reproduced, the contact angle (CA) was remarkably high and the sliding angle (SA) was also close to that of the original lotus leaf, resulting in a superhydrophobic surface. In contrast to R1, in the case of R2 from UV-NIL, the nano-scaled structures as well as micro-scaled structures were also relatively well reproduced and the CA was increased noticeably by around 99° in comparison to a flat photopolymer. However, unexpectedly, the SA of R2 was much higher than that of R1. This work provides useful tips of polymeric material selection for the industrial mass production of the superhydrophobic polymer surface.

A split and recombination micromixer fabricated in a PDMS three-dimensional structure

In this paper we propose a new split and recombination (SAR) micromixer that is compatible with the microfabrication process of polydimethylsiloxane (PDMS). We evaluate the mixing efficiency of the fabricated SAR micromixer and find that it increases interfaces exponentially. Simulation using CFD-ACE+ shows a cross-sectional view of the flow and estimates the mixing efficiency of the SAR micromixer and the pressure drop for a unit of the SAR micromixer. A mixing experiment involving phenolphthalein and NaOH solutions shows that interfaces, represented as red lines, are increased by SAR mixing. The result of our mixing experiment involving blue dye and water is evaluated to determine the mixing efficiency by calculating the standard deviation (stdev) of the pixel intensity of the observed image. After the seventh unit of the SAR micromixer, solutions are mixed to 90% at Re 0.6. The number of units needed to reach a mixed state in which the stdev is lower than 0.05, a 90% mixed state, increases from 5 to 10 for a flow rate ranging from 0.1 µl min−1 (Re 0.012) to 1000 µl min−1 (Re 120) including numerical analysis results. The pressure drop increases proportionally from 2.8 Pa to 35 000 Pa when the flow rate increases from 0.1 µl min−1 (Re 0.012) to 1000 µl min−1 (Re 120) in the numerical analysis results.

A barrier embedded Kenics micromixer

It is of great interest to enhance the mixing performance in a microchannel in which the flow is usually characterized by a low Reynolds number (Re) so that good mixing is quite difficult to achieve. In this regard, we present a new chaotic passive micromixer, named the barrier embedded Kenics micromixer (BEKM). In the BEKM, a higher level of chaotic mixing can be achieved by combining two general chaotic mixing mechanisms: splitting/reorientation and stretching/folding. The splitting/reorientation mechanism is obtained by the alternating arrangement of helical elements in the original Kenics mixer design. The stretching/folding mechanism is induced by periodic perturbation of the velocity field due to periodically inserted barriers along the cylinder wall while a relative helical flow is maintained by a helical element inside the pipe. In this study, the fully three-dimensional geometry of the BEKM was realized by micro-stereolithography, along with the Kenics micromixer and a circular T-pipe. Mixing performance was experimentally characterized in terms of an average mixing intensity via colour change of phenolphthalein. Experimental results show that the BEKM has better mixing performance than two other micromixers. The chaotic mixing mechanism proposed in this study could be applied to integrated microfluidic systems, such as the micro-total-analysis system, lab-on-a-chip and so on, as a mixing component.

A chaotic micromixer using obstruction-pairs

A micromixer is one of the most important components for a chemical and/or diagnostic analysis in microfluidic devices such as a micro-total-analysis-system and a lab-on-a-chip. In this paper, a novel chaotic micromixer is developed in a simple design by introducing obstruction-pairs on the bottom of a microchannel. An obstruction-pair, which is composed of two hexahedron blocks arranged in an asymmetric manner, can induce a rotational flow along the down-channel direction due to the anisotropy of flow resistance. By utilizing this characteristic of the obstruction-pair, four mixing units are designed in such a way that three obstruction-pairs induce three rotational flows which result in a down-welling and a hyperbolic point in the channel cross-section. There can be a variety of micromixer geometries by arranging the mixing units in various sequences along the microchannel, and their mixing performances will differ from each other due to different flow characteristics. In this regard, numerical investigations are carried out to predict and characterize the mixing performances of various micromixers. Also experimental verifications are carried out by a flow visualization technique using phenolphthalein and sodium hydroxide solutions in a polydimethylsiloxane-based micromixer.

Replication Quality of Flow-Through Microfilters in Microfluidic Lab-on-a-Chip for Blood Typing by Microinjection Molding

In the present study, replication of flow-through microfilters in the newly developed microfluidic lab-on-a-chip for blood typing by microinjection molding process was experimentally investigated. As a precise replication of the microfilters was required in order to effectively filter out agglutinated red blood cells, the effects of important processing conditions on the replication of the flow-through microfilters were investigated. By using a mold insert fabricated by a nickel electroplating process and a newly designed mold base, microinjection molding experiments were carried out. A three-dimensional solid model reconstruction method was proposed with the help of specific features characterizing the geometry of microfilters, and accordingly, the feature values of the replicated microfilters were measured by a noncontact optical measurement system. So reconstructed solid modeling result was then used to investigate the effects of various processing conditions, such as a flow rate, a mold temperature, and a packing pressure. Amongst the processing conditions investigated in the present study, the flow rate was found to be the most important one.

Barrier embedded chaotic micromixer

In this paper, we present a new chaotic passive micromixer named barrier embedded micromixer (BEM), with mixing visualization experimental results. Chaotic mixing in the microchannel has been successfully achieved by introducing periodic barriers on one microchannel wall which cause periodic perturbation in the velocity field of the helical flow. The BEM was made by PDMS (polydimethylsiloxane) from SU-8 masters which were fabricated by conventional photolithography. Two fluid flows containing different dyes were driven by a syringe pump in the constant flow rate with Re/spl ap/25. Experimental results showed an exponential growth of the interfacial area between two fluid flows, which confirms the chaotic mixing in the BEM.

A novel low temperature bonding technique for plastic substrates using X-ray irradiation

The present study proposes a new low temperature bonding technique for two plastic substrates using deep X-ray irradiation. The deep X-ray irradiation causes the decrease of molecular weight of PMMA, which in turn decreases the glass transition temperature (T/sub g/). Lowered T/sub g/ on the surface of two PMMA sheets makes bonding possible at low temperature. The present study also offers a tool to predict the decrease of T/sub g/ based on the relationship between X-ray dose and molecular weight. Bonding experiments are conducted at various bonding temperatures near the desired T/sub g/. Then, the bonding strength is measured by means of a tensile test machine (MTS system). The bonding is found to be successfully achieved when the bonding temperature is 8/spl sim/15/spl deg/C higher than the desired T/sub g/. The bonding strength is about 0.96 MPa.

Fabrication of a Hydrophilic Poly(dimethylsiloxane) Microporous Structure and Its Application to Portable Microfluidic Pump

In this paper, we present a simple and low-cost fabrication technique of a novel hydrophilic poly(dimethylsiloxane) (PDMS) microporous structure (or microsponge) based on a modified sugar leaching technique. A surfactant, Silwet L-77 enabled us to achieve the wettability conversion of PDMS from hydrophobic to hydrophilic. The wettability changes of PDMS surfaces and microsponges were characterized at various weight percentages of Silwet L-77 and it was found that a hydrophilic PDMS microsponge containing 0.6% of Silwet L-77 rapidly absorbed water droplets. The average porosity of the fabricated PDMS microsponges was measured as 0.55. We have successfully demonstrated the hydrophilic PDMS microsponge as a portable pump for a microfluidic device. The hydrophilic PDMS microsponge was firmly bonded at the inlet of a PDMS microchannel via oxygen plasma treatment. A water-phase sample was easily loaded into the hydrophilic PDMS microsponge and it was released and injected into the microchannel by simply pushing the microsponge.

Physical modeling and analysis of microlens formation fabricated by a modified LIGA process

In this paper, we present the physical modeling and analysis method of microlens formation using deep x-ray lithography followed by the thermal treatment of a polymethylmethacrylate (PMMA) sheet. According to the modeling, x-ray irradiation causes a decrease of the molecular weight of PMMA, which in turn decreases the glass transition temperature and consequently causes a net volume increase during the thermal cycle, resulting in a swollen microlens. In this modeling, the free volume theory including volume relaxation phenomena was considered during the thermodynamically non-equilibrium cooling process. Based on this modeling, an analysis method has been proposed to predict the shape of the microlens with the surface tension effect taken into account. The analysis results are favorably compared with experimental data. The present analysis method enables us to predict the fabricated microlens shapes and the variation pattern of the maximum heights of the microlens, which depend on the conditions of the thermal treatment. The prediction model could eventually be used in determining the detailed thermal treatment conditions for a desired microlens shape fabricated by a modified LIGA process.