Chih-Yung Wen

Rapid magnetic microfluidic mixer utilizing AC electromagnetic field.

This paper presents a novel simple micromixer based on stable water suspensions of magnetic nanoparticles (i.e. ferrofluids). The micromixer chip is built using standard microfabrication and simple soft lithography, and the design can be incorporated as a subsystem into any chemical microreactor or a miniaturized biological sensor. An electromagnet driven by an AC power source is used to induce transient interactive flows between a ferrofluid and Rhodamine B. The alternative magnetic field causes the ferrofluid to expand significantly and uniformly toward Rhodamine B, associated with a great number of extremely fine fingering structures on the interface in the upstream and downstream regions of the microchannel. These pronounced fingering patterns, which have not been observed by other active mixing methods utilizing only magnetic force, increase the mixing interfacial length dramatically. Along with the dominant diffusion effects occurring around the circumferential regions of the fine finger structures, the mixing efficiency increases significantly. The miscible fingering instabilities are observed and applied in the microfluidics for the first time. This work is carried with a view to developing functionalized ferrofluids that can be used as sensitive pathogen detectors and the present experimental results demonstrate that the proposed micromixer has excellent mixing capabilities. The mixing efficiency can be as high as 95% within 2.0 s and a distance of 3.0 mm from the inlet of the mixing channel, when the applied peak magnetic field is higher than 29.2 Oe and frequency ranges from 45 to 300 Hz.

Non-equilibrium dissociating flow over spheres

Previous work on the correlation of dissociative non-equilibrium effects on the flow field in front of blunt bodies considered the dependence of the dimensionless shock stand-off distance on the dimensionless dissociation rate immediately after the normal shock in the simple case of a diatomic gas with only one reaction. In this paper, the correlation is corrected to take into account the additional parameter of the dimensionless free-stream kinetic energy, and extended to the case of complex gas mixtures with many species and many reactions, by introducing a new reaction rate parameter that has a clear physical meaning, and leads to an approximate theory for the stand-off distance. Extensive new experimental results and numerical computations of air, nitrogen and carbon dioxide flow over spheres were obtained over a large range of total enthalpy. The results comprise surface heat flux measurements and differential interferograms. Both experimental results and numerical computations substantiate the approximate theory.

Quantification of the pulse wave velocity of the descending aorta using axial velocity profiles from phase-contrast magnetic resonance imaging

The pulse wave velocity (PWV) of aortic blood flow is considered a surrogate for aortic compliance. A new method using phase‐contrast (PC)‐MRI is presented whereby the spatial and temporal profiles of axial velocity along the descending aorta can be analyzed. Seventeen young healthy volunteers (the YH group), six older healthy volunteers (the OH group), and six patients with coronary artery disease (the CAD group) were studied. PC‐MRI covering the whole descending aorta was acquired, with velocity gradients encoding the in‐plane velocity. From the corrected axial flow velocity profiles, PWV was determined from the slope of an intersecting line between the presystolic and early systolic phases. Furthermore, the aortic elastic modulus (Ep) was derived from the ratio of the brachial pulse pressure to the strain of the aortic diameter. The PWV increased from YH to OH to CAD (541 ± 94, 808 ± 184, 1121 ± 218 cm/s, respectively; P = 0.015 between YH and OH; P = 0.023 between OH and CAD). There was a high correlation between PWV and Ep (r = 0.861, P < 0.001). Multivariate analysis showed that age and CAD were independent risk factors for an increase in the PWV. Compared to existing methods, our method requires fewer assumptions and provides a more intuitive and objective way to estimate the PWV. Magn Reson Med, 2006.

Investigation of Pulsatile Flowfield in Healthy Thoracic Aorta Models

Cardiovascular disease is the primary cause of morbidity and mortality in the western world. Complex hemodynamics plays a critical role in the development of aortic dissection and atherosclerosis, as well as many other diseases. Since fundamental fluid mechanics are important for the understanding of the blood flow in the cardiovascular circulatory system of the human body aspects, a joint experimental and numerical study was conducted in this study to determine the distributions of wall shear stress and pressure and oscillatory WSS index, and to examine their correlation with the aortic disorders, especially dissection. Experimentally, the Phase-Contrast Magnetic Resonance Imaging (PC-MRI) method was used to acquire the true geometry of a normal human thoracic aorta, which was readily converted into a transparent thoracic aorta model by the rapid prototyping (RP) technique. The thoracic aorta model was then used in the in vitro experiments and computations. Simulations were performed using the computational fluid dynamic (CFD) code ACE+® to determine flow characteristics of the three-dimensional, pulsatile, incompressible, and Newtonian fluid in the thoracic aorta model. The unsteady boundary conditions at the inlet and the outlet of the aortic flow were specified from the measured flowrate and pressure results during in vitro experiments. For the code validation, the predicted axial velocity reasonably agrees with the PC-MRI experimental data in the oblique sagittal plane of the thoracic aorta model. The thorough analyses of the thoracic aorta flow, WSSs, WSS index (OSI), and wall pressures are presented. The predicted locations of the maxima of WSS and the wall pressure can be then correlated with that of the thoracic aorta dissection, and thereby may lead to a useful biological significance. The numerical results also suggest that the effects of low WSS and high OSI tend to cause wall thickening occurred along the inferior wall of the aortic arch and the anterior wall of the brachiocephalic artery, similar implication reported in a number of previous studies.

A MEMS-based valveless impedance pump utilizing electromagnetic actuation

This study presents a planar valveless impedance-based micro pump for biomedical applications. The micro pump comprises four major components, namely a lower glass substrate containing a copper micro coil, a microchannel, an upper glass cover plate and a PDMS diaphragm with a magnet mounted on its upper surface. When a current is passed through the micro coil, an electromagnetic force is established between the coil and the magnet. The resulting deflection of the PDMS diaphragm creates an acoustic impedance mismatch within the microchannel, which results in a net flow. The performance of the micro pump is characterized both experimentally and numerically using Ansoft/Maxwell3D FEA software. The results show that the mechanical integrity of the micro pump is assured provided that the diaphragm deflection does not exceed 110 µm. This deflection is obtained by supplying the micro coil with an input current of 0.6 A, and results in a flow rate of 7.2 ml min−1 when the PDMS membrane is driven by an actuating frequency of 200 Hz.

Numerical analysis of a rapid magnetic microfluidic mixer

This paper presents a detailed numerical investigation of the novel active microfluidic mixer proposed by Wen et al. (Electrophoresis 2009, 30, 4179–4186). This mixer uses an electromagnet driven by DC or AC power to induce transient interactive flows between a water‐based ferrofluid and DI water. Experimental results clearly demonstrate the mixing mechanism. In the presence of the electromagnets magnetic field, the magnetic nanoparticles create a body force vector that acts on the mixed fluid. Numerical simulations show that this magnetic body force causes the ferrofluid to expand significantly and uniformly toward miscible water. The magnetic force also produces many extremely fine finger structures along the direction of local magnetic field lines at the interface in both upstream and downstream regions of the microchannel when the external steady magnetic strength (DC power actuation) exceeds 30 Oe (critical magnetic Peclet number Pem,cr = 2870). This study is the first to analyze these pronounced finger patterns numerically, and the results are in good agreement with the experimental visualization of Wen et al. (Electrophoresis 2009, 30, 4179–4186).The large interfacial area that accompanies these fine finger structures and the dominant diffusion effects occurring around the circumferential regions of fingers significantly enhance the mixing performance. The mixing ratio can be as high as 95% within 2.0 s. at a distance of 3.0 mm from the mixing channel inlet when the applied peak magnetic field supplied by the DC power source exceeds 60 Oe. This study also presents a sample implementation of AC power actuation in a numerical simulation, an experimental benchmark, and a simulation of DC power actuation with the same peak magnetic strength. The simulated flow structures of the AC power actuation agree well with the experimental visualization, and are similar to those produced by DC power. The AC and DC power actuated flow fields exhibited no significant differences. This numerical study suggests approaches to maximize the performance of the proposed rapid magnetic microfluidic mixer, and confirms its exciting potential for use in lab‐on‐a‐chip systems.

Two-dimensional vortex shedding of a circular cylinder

The Strouhal–Reynolds number relationship for the two-dimensional (2-D) vortex shedding of a circular cylinder at low to medium Reynolds numbers (Re, ranging from 45 to 560) is investigated experimentally. Both horizontal and vertical soap film tunnels are used to set up a truly 2-D experiment. It is found that two separate 3-D instabilities of the natural wake at Re≅180 and 260 disappear. The Strouhal–Reynolds number curve is in good agreement with the 2-D computations of Barkley and Henderson [J. Fluid Mech. 322, 215 (1996)]. The 2-D asymptote of 0.2417 of Strouhal number is also approached.

An experimental investigation of thermal contact conductance across bolted joints

Abstract An experimental study of thermal contact conductance was conducted with pairs of aluminum alloy (6061-T6) specimens jointed by bolts. The individual aluminum samples have a square cross-section ( 63.5 mm × 63.5 mm ) and a height of 50 mm. Three different bolt patterns were adopted in this study, including single-bolt, 4-bolt, and 8-bolt configurations. The bolt–shaft diameters were 3, 5, and 8 mm, and the torque applied on each bolt was between 1 and 10 N m. The heat flux through the test specimens ranged from 4 to 20 kW/m 2 . The interfacial contact pressure of bolt-jointed specimens was determined by a pressure-measuring film inserted between samples. Results show that the interfacial contact pressure increases with an increase of either the applied torque or the number of bolts. The interfacial temperature difference across the junction was substantially reduced for bolt-jointed specimens, when compared with two superimposed samples without bolts. With the same bolt number, the variation of bolt–shaft diameter from 5 to 8 mm yields nearly no influence on the thermal contact conductance. However, when the size of bolt was kept constant the thermal contact conductance of samples jointed by 8 bolts was greatly larger than that of 4-bolt samples. The increase of contact surface roughness of test specimens leads to a decrease of the thermal contact conductance. When an RTV silicon layer was used as the interstitial material, the total joint conductance of Al/RTV/Al was much lower than the contact conductance of bare aluminum contact. The total joint conductance of Al/RTV/Al was increased with a decrease of the thickness of RTV silicon layers.

Flow visualization of natural convection of magnetic fluid in a rectangular Hele-Shaw cell

Abstract The nature convection of a magnetic fluid in a Hele-Shaw cell with aspect ratio of one is studied experimentally. Results obtained from heat transfer measurements and shadowgraphs revealed that the vertically imposed magnetic field has a destabilizing influence. The flow instability mode becomes different from that without the magnetic field.

Design, analysis and optimization of an electromagnetic actuator for a micro impedance pump

This study designs and optimizes an electromagnetic actuator for use in a valveless micro impedance pump. The actuator is modeled to have an electroplated permanent magnet mounted on a flexible PDMS diaphragm and a planar copper micro-coil patterned on a bottom glass substrate. The constituent parts of the actuator, namely the diaphragm, the micro-coil and the magnet, are modeled, analyzed and optimized in such a way as to maximize the actuating force while simultaneously ensuring the mechanical integrity of the device. In performing the analyses, theoretical and mathematical models of the stroke volume and diaphragm deflection are developed based on thin plate theory. The design models are verified theoretically and numerically, and the relationships between the electromagnetic force, the diaphragm displacement and the diaphragm strength are systematically explored. Overall, the results reveal that in the optimized device, the target diaphragm deflection of 20 μm can be obtained using a compression force of 12 μN developed by a micro-coil input current of 0.8 A. The electromagnetic actuator proposed in this study provides an ideal solution for the pumping requirements of a variety of biomedical chips and microfluidic applications and therefore represents a valuable contribution to the ongoing development of lab-on-a-chip systems. (Some figures in this article are in colour only in the electronic version)