Chin Lung Chang

Microfluidic Mixing: A Review

The aim of microfluidic mixing is to achieve a thorough and rapid mixing of multiple samples in microscale devices. In such devices, sample mixing is essentially achieved by enhancing the diffusion effect between the different species flows. Broadly speaking, microfluidic mixing schemes can be categorized as either “active”, where an external energy force is applied to perturb the sample species, or “passive”, where the contact area and contact time of the species samples are increased through specially-designed microchannel configurations. Many mixers have been proposed to facilitate this task over the past 10 years. Accordingly, this paper commences by providing a high level overview of the field of microfluidic mixing devices before describing some of the more significant proposals for active and passive mixers.

Rapid Microfluidic Mixers Utilizing Dispersion Effect and Interactively Time-Pulsed Injection

In this paper, we present a novel active microfluidic mixer utilizing a dispersion effect in an expansion chamber and applying interactively time-pulsed driving voltages to the respective inlet fluid flows to induce electroosmotic flow velocity variations for developing a rapid mixing effect in a microchannel. Without using any additional equipment to induce flow perturbations, only a single high-voltage power source is required for simultaneously driving and mixing sample fluids, which results in a simple and low-cost system for mixing. The effects of the applied main electrical field, interactive frequency, and expansion ratio on the mixing performance are thoroughly examined experimentally and numerically. The mixing ratio can be as high as 95% within a mixing length of 3000 µm downstream from the secondary T-form when a driving electric field strength of 250 V/cm, a periodic switching frequency of 5 Hz, and the expansion ratio M=1:10 are applied. In addition, the optimization of the driving electric field, switching frequency, expansion ratio, expansion entry length, and expansion chamber length for achieving a maximum mixing ratio is also discussed in this study. The novel method proposed in this study can be used for solving the mixing problem in the field of micro-total-analysis systems in a simple manner.

Soldering with solid state and diode lasers: Energy coupling, temperature rise, process window

The increasing number of electrical contacts in automobiles in combination with more complex and miniaturized components leads to higher requirements for the joining technologies. In that context, laser soldering represents an interesting alternative to conventional techniques. So far, solid state lasers [Nd:yttrium–aluminum–garnet (YAG)] and CO2 lasers have been successfully applied in industrial production. Recently, the development of high power diode lasers has offered a new laser source for soldering with technological advantages. Absorptivity of laser radiation on metals, generally increases with shorter wavelength and, consequently, diode lasers may lead to a higher process efficiency compared to Nd:YAG lasers. The absorptivity of copper alloys with different surface conditions has been measured at 808 nm (diode) and 1064 nm (Nd:YAG). When heating up the solder joint, the intensity distribution of the different laser spots becomes important, too. This effect is demonstrated by means of process mode...

A low-leakage sample plug injection scheme for crossform microfluidic capillary electrophoresis devices incorporating a restricted cross-channel intersection

This study develops a crossform CE microfluidic device in which a single‐circular barrier or a double‐circular barrier is introduced at the cross‐channel intersection. Utilizing a conventional crossform injection scheme, it is shown that these barriers reduce sample leakage and deliver a compact sample band into the separation channel, thereby ensuring an enhanced detection performance. A series of numerical and experimental investigations are performed to investigate the effects of the barrier type and the barrier ratio on the flow streamlines within the microchannel and to clarify their respective effects on the sample leakage ratio and sample plug variance during the injection process. The results indicate that a single‐circular barrier injector with a barrier ratio greater than 20% and a double‐circular barrier injector with a barrier ratio greater than 40% minimize the sample leakage ratio and produce a compact sample plug. As a result, both injectors have an excellent potential for use in high‐quality, high‐throughput chemical analysis procedures and in many other applications throughout the micro‐total analysis systems field.

Experimental and Numerical Analysis of High-Resolution Injection Technique for Capillary Electrophoresis Microchip

This study presents an experimental and numerical investigation on the use of high-resolution injection techniques to deliver sample plugs within a capillary electrophoresis (CE) microchip. The CE microfluidic device was integrated into a U-shaped injection system and an expansion chamber located at the inlet of the separation channel, which can miniize the sample leakage effect and deliver a high-quality sample plug into the separation channel so that the detection performance of the device is enhanced. The proposed 45° U-shaped injection system was investigated using a sample of Rhodamine B dye. Meanwhile, the analysis of the current CE microfluidic chip was studied by considering the separation of Hae III digested ϕx-174 DNA samples. The experimental and numerical results indicate that the included 45° U-shaped injector completely eliminates the sample leakage and an expansion separation channel with an expansion ratio of 2.5 delivers a sample plug with a perfect detection shape and highest concentration intensity, hence enabling an optimal injection and separation performance.

Numerical Simulation of Electromagnetic Actuator for Impedance Pumping

This study designs and analyzes an impedance pump utilizing an electromagnetic actuator. The pump is designed to have three major components, namely a lower glass substrate patterned with a copper micro-coil, a microchannel, and an upper glass cover plate attached a magnetic PDMS diaphragm. When a current is passed through the micro-coil, an electromagnetic force is established between the coil and the magnetic diaphragm. The resulting deflection of the PDMS diaphragm creates an acoustic impedance mismatch within the microchannel, which results in a net flow. Overall, the simulated results reveal that a net flow rate of 52.8 μl/min can be obtained using a diaphragm displacement of 31.5 μm induced by a micro-coil input current of 0.5 A. The impedance pump proposed in this study provides a valuable contribution to the ongoing development of Lab-on-Chips (LoCs) systems.

Analysis of Fluid Film Lubrication of an MHD Journal Bearing Subjected to an Axially Applied External Magnetic Field

This work simulates the steady-state flow field in a magnetohydrodynamic journal bearing. A uniform magnetic field is applied in the axial direction across the bearing. Current results indicate that a crescent-shape secondary flow will develop at E = 0.8. The velocity profiles at the location of the minimum and maximum film thickness are almost independent of the strength of the external magnetic field unless the eccentricity ratio is relatively small and magnetic field is strong. There exists an eccentric dependent threshold Ha beyond which the shear stress on the journal increases with Ha while that on the bearing decreases.

A New Simplex Method Based Strategy for Automatic Multiple DOF Laser-Fiber Packaging Alignment

This paper focused on a fine alignment process for automatic multiple degree-of-freedom (DOF) laser-fiber packaging. Among the alignment algorithms used, the Kings modified simplex method (KMSM) has been proven to be very suitable for the fine alignment process because of its fast convergence and ease of implementation. However, direct implementation of this method on a multiple DOF alignment process would lead to local peak trapping problem. In this paper, a KMSM based algorithm is proposed in which the search of the global peak is divided into two stages, namely, 2D alignment and 3D optic axis alignment. The method for the selection of key parameters to prevent the simplex from sticking on the local peaks and to accelerate the search is proposed. The experimental works have proven that the proposed idea is more effective than the conventional KMSM in terms of the rates of success and convergence in all the 100 alignment trials for 10 different LD-fiber pairs, regardless of the initial positions

Smoke Propagation in an Inclined Semi-Circular Long Tunnel

This work used FDS to simulate tunnel fires occur in a semi-circular longitudinally ventilated tunnel. By varying the parameters such as the tunnel gradient, the fire size, and the ventilation velocity, their influence on the backlayering effect and downstream propagation rate can be recognized. Under weak ventilation, the backlayering effect either advances or vanishes depending on the slope of the tunnel. Under stronger ventilation, the backlayering effect would break up. The temperature distributions may become less and less dependent on the tunnel gradient when the ventilation velocity is increased. Although the hot gases and smoke in uphill tunnels propagate faster than those in downhill tunnels, their difference reduces with ventilation velocity.

Rapid Microfluidc Biochips Fabrication by Femtosecond Laser on Glass Substrate

This paper uses a femtosecond laser scriber to perform the direct-writing ablation of glass substrate for the development of microfluidic biochips. The surface quality of the ablated microchannels was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurement techniques. The developed femtosecond laser ablation system provides a versatile and economic approach for the fabrication of glass-base microfluidic chips. In the laser writing process, the desired microfluidic patterns are designed using commercial computer software and are then transferred to the laser scriber to ablate the trenches. The results show that a very smooth channel wall can be achieved through the annealing process at the temperature 650°C and 5 hours. The system provides an economic and powerful means of rapid glass microfluidic biochips development.