Sangmo Kang

A Review on Mixing in Microfluidics

Small-scale mixing is of uttermost importance in bio- and chemical analyses using micro TAS (total analysis systems) or lab-on-chips. Many microfluidic applications involve chemical reactions where, most often, the fluid diffusivity is very low so that without the help of chaotic advection the reaction time can be extremely long. In this article, we will review various kinds of mixers developed for use in microfluidic devices. Our review starts by defining the terminology necessary to understand the fundamental concept of mixing and by introducing quantities for evaluating the mixing performance, such as mixing index and residence time. In particular, we will review the concept of chaotic advection and the mathematical terms, Poincare section and Lyapunov exponent. Since these concepts are developed from nonlinear dynamical systems, they should play important roles in devising microfluidic devices with enhanced mixing performance. Following, we review the various designs of mixers that are employed in applications. We will classify the designs in terms of the driving forces, including mechanical, electrical and magnetic forces, used to control fluid flow upon mixing. The advantages and disadvantages of each design will also be addressed. Finally, we will briefly touch on the expected future development regarding mixer design and related issues for the further enhancement of mixing performance.

A Review on the Analysis and Experiment of Fluid Flow and Mixing in Micro-Channels

The studies with respect to micro-channels and micro-mixers are expanding in many dimensions. Most significant area of micro-mixer study is the flow analysis in various micro-channel configurations. The flow phenomena in microchannel devices are quite different from that of the macro-scale devices. An attempt is made here to review the important recent literature available in the area of micro-channel flow analysis and mixing. The topics covered include the physics of flow in micro-channels and integrated simulation of the micro-channel flow. Also, the flow control models and electro-kinetically driven micro-channel flows are dealt in detail. A survey of important numerical methods, which are currently popular for micro-channel flow analysis, is carried out. Different options for mixing in microchannels are provided, in sufficient detail.

Numerical and Experimental Study on a Channel Mixer with a Periodic Array of Cross Baffles

In this study we show an enhanced mixing effect with a simple channel having a periodic array of cross baffles. We performed numerical compulation to obtain the steady flow field within the channel at low Reynolds numbers by using a commercial code, ANSYS CFX 10.0. A visualization experiment was also conducted to validate our numerical results qualitatively. In evaluating the mixing performance, we employed the Lyapunov exponent. It was shown that the visualized mixing pattern was in a good agreement with that numerically given. Our Liapunov exponent distribution in the space also demonstrates that the proposed channel design indeed exhibits a chaotic stirring at low Reynolds numbers. Our design is thus assumed to be applicable to designing a microchannel mixer with enhanced mixing effect

Unsteady electroosmotic channel flows with the nonoverlapped and overlapped electric double layers

In micro- and nanoflows, the Boltzmann distribution is valid only when the electric double layers (EDL’s) are not overlapped and the ionic distributions establish an equilibrium state. The present study has numerically investigated unsteady two-dimensional fully-developed electroosmotic flows between two parallel flat plates in the nonoverlapped and overlapped EDL cases, without any assumption of the Boltzmann distribution. For the study, two kinds of unsteady flows are considered: one is the impulsive application of a constant electric field and the other is the application of a sinusoidally oscillating electric field. For the numerical simulations, the ionic-species and electric-field equations as well as the continuity and momentum ones are solved. Numerical simulations are successful in accurately predicting unsteady electroosmotic flows and ionic distributions. Results show that the nonoverlapped and overlapped cases are totally different in their basic characteristics. This study would contribute to further understanding unsteady electroosmotic flows in micro- and nanofluidic devices.

Molecular Dynamics Simulation of Aggregates in the Dodecane/span80 System and Their Behaviour in an Electric Field

Molecular dynamics simulation of self-assembly of surfactant span80 molecules to form reverse micelles in nonpolar liquid dodecane is carried out. Simulations are performed using a united atom model for dodecane and a hybrid model for span80 molecules. Various physical characteristics of reverse micelle are measured, and the same are compared with available experimental results. Presence of charge carriers in the form of solvated ions in the core of reverse micelles is confirmed by the simulation. Movement of reverse micelles under the effect of uniform external electric field is also discussed.

Enhancement of Mixing in a Microchannel by Using AC-Electroosmotic Effect

This study has focused on optimizing the AC-electroosmotic micro-mixer, which is composed of a microchannel with an array of rectangular electrodes attached on the bottom wall. The electrode array is spatial-periodically arranged in pairs symmetric with respect to the longitudinal central line. An AC electrode field is applied to the electrodes, which drives the secondary transverse flow in a circulating cell mode near the electrodes. The main flow along the channel longitudinal direction plus this secondary transverse flow contribute to the stretching and folding of the fluid flow, that is the chaotic behavior, and thus to the enhancement of the fluid mixing. To design the better micro-mixer, numerical simulations have been performed by using a commercial code (CFX 10). In the simulations, the concept of mixing index is employed to evaluate the mixing performance as well as to optimize the size and spacing of each electrode in one pair. It is found that the optimum design of one electrode pair, which leads to the best mixing performance, is not simply harmonic one. When the length ratio of the two electrodes in a pair closes to 2:1, the best mixing effect can be attained. The flow pattern was visualized. Furthermore, the velocity field will be measured with a PTV technique to validate the numerical simulations.Copyright

Numerical Model of Propulsive Behavior of a Rotating Spring in Viscous Fluid

, , . 석하였다. Abstract: In this paper, we study the propulsive behavior related to the flagellar motion of bacteria using a spring model. A commercial program was used to conduct simulations, and we verified the numerical technique by setting an additional rotating domain and conducting a parametric study. The numerical results are in good agreement with slender-body theory, although overall, they are not in agreement with resistive-force theory. We confirm the effect of the rotational velocity, pitch, helical radius, fluid viscosity, and, in particular, the distance from the wall on the propulsion of the spring.

An experimental study on the AC electroosmotic flow around a pair of electrodes in a microchannel

This paper presents an experimental study on the AC electroosmotic flow in a microchannel having a pair of rectangular electrodes on the bottom wall with narrow gap. The microchannel was made of PDMS (Polydimethylsiloxane) and the electrodes of ITO (Indium Tin Oxide). The electrodes were arranged such that the electric field is mainly perpendicular to the channel’s longitudinal direction, thus creating a transversal secondary flow. The primary flow was driven by a pressure force through the fluid-level difference on both reservoirs of the channel. To measure the velocity distributions around the electrodes, we used a micro-PTV (particle tracking velocimetry) technique. We find that on the surface of the electrodes the flow velocity caused by the AC electroosmosis is directed from the electrode edge toward the side wall of the channel, and the maximum crosswise velocity occurs at the frequency 120Hz. A smooth profile of the crosswise velocity component along a vertical line was successfully obtained from the present experimental technique, and it shows a flow reversal due to the mass conservation principle.

Numerical Study on the Dynamics of Organism Motion under Background Flow

We propose a two-dimensional numerical model to investigate the dynamic behaviour of an organism swimming in a background flow in a channel. In this work, the organism is modeled as a neutrally buoyant one-dimensional elastic filament based on an immersed boundary finite volume method. Further, the organism is modeled using discrete number of immersed boundary points and the Navier-Stokes equations governing the flow are solved on a staggered Cartesian grid system. A driving function is applied which results in a wave travelling along the length of the organism from left to right. It is found that under no background flow, the organism swim in the forward direction (right to left) when the wave travel over the organism is in the opposite direction. It is observed that, under a uniform background flow, a non-motile organism is simply dragged by the flow whereas a motile organism swims backward along the direction of flow. Further, it is seen that a propulsion enhancement is found in the case of organism swimming along the flow direction when the wave travel is in the opposite direction as that of the flow.

Finite element analysis and optimization of cylindrical resonant photoacoustic cell

One of the unique properties of the sp2-carbon allotropes, such as fullerenes, carbon nanotubes and graphenes, is that their electronic structures differ significantly among them according to characteristic electron confinement based on their dimensionality and geometric structures, which can be influenced not only by charge injection and chemical bonding but also structural modification. In this talk, I will discuss the electronic structures of various sp2-carbon allotropes on metal substrates investigated by scanning tunneling microscopy and spectroscopy. In particular, it is focused on the one dimensional (1D) electronic structure in a graphene nano wrinkle (GNW) of an epitaxially grown graphene (EG) sheet on Ni(111), the width of which was small enough (less than 5 nm) to cause 1D electron confinement. Use of spatially resolved, scanning tunneling spectroscopy revealed band-gap opening and a 1D van Hove singularity in the GNW, as well as the chemical potential distribution across the GNW. Our demonstration of 1D electron confinement in an EG is the novel possibility of controlling its electronic properties not by chemical modification but by mechanical structuring in a controlled manner. Graphene-based carbon materials such as fullerenes, carbon nanotubes, and graphenes have distinct and unique electronic properties that depend on their dimensionality and geometric structures. Graphene wrinkles with pseudo one-dimensional structures have been observed in a graphene sheet. However, their one-dimensional electronic properties have never been observed because of their large widths. Here we report the unique electronic structure of graphene nanowrinkles in a graphene sheet grown on Ni, the width of which was small enough to cause one-dimensional electron confinement. Use of spatially resolved, scanning tunnelling spectroscopy revealed bandgap opening and a one-dimensional van Hove singularity in the graphene nanowrinkles, as well as the chemical potential distribution across the graphene nanowrinkles. This observation allows us to realize a metallic-semiconducting-metallic junction in a single graphene sheet. Our demonstration of one-dimensional electron confinement in graphene provides the novel possibility of controlling its electronic properties not by chemical modification but by ‘mechanical structuring’. Graphene wrinkles, which are one-dimensional (1D) folded graphene structures, have generally been observed in graphene produced by chemical vapour deposition. These structures have been thought to be the result of the difference in the thermal expansion coefficient between graphene and its substrate. A graphene wrinkle is chemically bonded with surrounding planar epitaxial graphene. Therefore, its unique geometric structure is distinct from those of carbon nanotubes and graphene nanoribbons which are indisputably 1D structures. Hence, we define a graphene wrinkle as a ‘pseudo 1D structure’ to indicate that it has a 1D shape, but is still a part of a two-dimensional structure. In the following, we demonstrate the 1D electron confinement in graphene nanowrinkle (GNW) by scanning tunnelling microscopy/spectroscopy (STM/STS), whose width is <5 nm. Moreover, spatially resolved electronic structures have been investigated, and the manipulation of graphene geometry by STM tip has been demonstrated. Our results imply that a semiconducting property can be realized by the mechanical deformation of the graphene geometry not by chemical modification, which is analogous to the case of a strain-induced pseudo magnetic field that was discovered in deformed ‘graphene nanobubbles’. The lack of surface functionalization in our approach can prevent the mobility decline due to chemical defects. Moreover, the covalent bonding at the metallic pEG-semiconducting GNW junction can reduce the contact resistance. Our results demonstrate that the interfacial interaction between graphene and the metal substrate provides a novel way to realize a metallic-semiconducting-metallic junction within a single graphene sheet. Results: Structural characterization of GNWs Epitaxial graphene with GNWs was synthesized by dissociating acetylene on a clean Ni(111) surface. A rapid cooling process is necessary, which is the most critical step to synthesize GNWs. Most of the GNWs were observed in the region where the terrace width of the underlying Ni surface was as small as several tens of nanometres. These GNWs have been recoloured with orange, and a line profile along the white arrow, which shows that the GNWs on the terrace have larger widths and lower heights than the GNWs at the step edges. We should note that all GNWs were formed at the step edges (red triangles or propagated from kinks at the step edges of the Ni surface, the implication being that the geometrical structure of the underlying Ni must play a crucial role in the formation of GNWs To analyse the structure of the GNWs in detail, we obtained atomically resolved STM images from an isolated GNW on the terrace under different scanning conditions. The top and bottom regions of the GNW in were scanned at a sample bias (Vs) of 1 V and a feedback current (If) of 1 nA, whereas the centre region was scanned with a smaller tip–sample distance.The objective of the paper was to demonstrate feasibility of an ammonia sensor using polymer –inorganic nano-composite thin film upconversion light emitters made by the new double-beam pulsed laser deposition method. The existing pulsed laser deposition vacuum chamber was modified to accommodate two laser beams of different wavelengths for the in-situ ablation of two targets: a polymer host poly(methyl methacrylate) mixed with indicator dye Phenol Red and the brilliant rare earth doped upconversion phosphor NaYF4:Yb3+, Er3+. Nano-composite films were deposited on silicon substrates by the proposed method with near-infra-red laser radiation (1064-nm wavelength) ablating the polymer target dissolved in Gamma-butyrolactone together with the indicator dye, and frozen in circulating liquid nitrogen (matrix assisted pulsed laser evaporation – MAPLE), and visible radiation (532 nm) ablating the inorganic target. The deposited nano-composite films retained bright green upconversion fluorescence with a spectral peak at 540 nm attributed to the inorganic phosphor nano-particles pumped with the 980-nm infrared laser diode. The spectrum of the green emission matched the absorption band of the indicator dye exposed to ammonia. When the films were exposed to ammonia, they demonstrated an optical response in the form of the drop of the intensity of green radiation monitored with a silicon photodiode. The sensitivity of the opto-electronic sensor of ammonia based on the nano-composite films was measured to be close 0.4% ammonia in air, and the response time was 5 minutes.Superconductors possess unique properties such as zero electrical resistance and expulsion of magnetic fields below a critical temperature Tc. They can carry electric current without any energy loss and have many applications. However, understanding superconductivity is a great challenge. Especially, anomalously small isotope effect in some high and low Tc superconductors such as YBa2Cu3O7 (YBCO), Nb3Sn, Zr, created a great challenge for understanding. To solve the puzzle, a new methodology is implemented by integrating first-principles calculations of electronic structures of the materials into the theory of many-body physics for superconductivity. The aim is to seek a unified methodology to study the electronic and superconducting properties of the materials. It is demonstrated from first-principles that the extended saddle point singularities in the electronic structures of the materials such as YBCO, Nb3Sn, Zr, strongly correlate with the anomalous isotope effect in these superconductors. Some guidance for finding new high Tc superconductors will also be discussed.