Seung S. Lee

Theoretical and experimental study of MHD (magnetohydrodynamic) micropump

This paper presents a novel micropump of which pumping mechanism is based upon magnetohydrodynamic (MHD) principles. MHD is the study of flow of electrically conducting liquids in electric and magnetic fields. Lorentz force is the pumping source of conductive, aqueous solutions in the MHD micropump. Conducting fluid in the microchannel of the MHD micropump is driven by Lorentz force in the direction perpendicular to both magnetic and electric fields. The performance of the micropump is obtained by measuring the pressure head difference and flow rate as the applied voltage changes from 10 to 60 VDC at 0.19 and 0.44 Tesla (T). The pressure head difference is 18 mm at 38 mA and the flow rate is 63 μl/min at 1.8 mA when the inside diameter of inlet/outlet tube is 2 mm and the magnetic flux density is 0.44 T. Bubble generation by the electrolysis of the conducting liquid can be observed. The performance of the MHD micropump obtained theoretically in single phase is compared with the experimental results.

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.

A simple method for microlens fabrication by the modified LIGA process

Microlenses and microlens arrays were fabricated using a novel fabrication technology based on the exposure of a resist (usually PMMA) to deep x-rays and subsequent thermal treatment. The fabrication technology is very simple and produces microlenses and microlens arrays with good surface roughness (less than 1 nm). The molecular weight and glass transition temperature of PMMA is reduced when it is irradiated with deep x-rays. The microlenses were produced through the effects of volume change, surface tension, and reflow during thermal treatment of irradiated PMMA. The geometry of the microlens was determined by parameters such as the x-ray dose applied to the PMMA, the diameter of the microlens, along with the heating temperature, heating time and cooling rate in the thermal treatment. Microlenses were produced with diameters ranging from 30 to 1500 μm. The modified LIGA process was used to construct not only hemispherical microlenses, but also structures that were rectangular-shaped, star-shaped, etc.

Carbon nanotube film piezoresistors embedded in polymer membranes

We present carbon nanotube film (CNF) piezoresistors embedded in polymer membranes. CNFs by vacuum filtration are patterned on an Au-deposited Si-wafer and transferred onto the poly-dimethylsiloxane (PDMS) using the weak adhesion property between Au-layer and Si-wafer. Transmittance and I-V characteristic are measured to confirm transferred CNFs as transparent electrodes. The pressure sensor consists of CNF piezoresistors embedded in 130 μm thick circular PDMS membranes. The gauge factor of CNFs at different thickness is obtained around 10–20 when the resistance increases from 2.7 to 5.6 kΩ with applied pressure, which shows that CNFs can be used as transparent piezoresistors in polymer-based microelectromechanical systems.

Piezoelectric cantilever acoustic transducer

We present a piezoelectric acoustic transducer fabricated on a bulk-micromachined cantilever diaphragm. Use of the cantilever as a supporting diaphragm produces a highly sensitive microphone. In addition, when the device is driven electrically as an output transducer, a microspeaker, the relatively large deflections produce significant acoustic output. A voltage-to-frequency converter has also been demonstrated with piezoelectric cantilever transducers. The micromachined transducer has a zinc oxide (ZnO) piezoelectric thin film on a 1.5 m thick cantilever diaphragm, made of LPCVD low-stress silicon nitride. The measured cantilever microphone sensitivity is fairly constant around 3 mV in the low-frequency range below the first resonant frequency, which occurs at 1.8 kHz. The microspeaker output is approximately 100 dB SPL at 4.8 kHz and 12 (peak-peak) input drive. The voltage-to-frequency conversion is accomplished by the addition of a conducting plate and an aluminum (Al) sputtered layer on the underside of the cantilever. The resonant frequency of the microspeaker is changed by the potential applied between the top conducting plate and the lower Al layer. As the potential is changed from 0 to 40 , the resonant frequency shifts down from 14.5 kHz to 11.5 kHz while the amplitude of the output pressure is increased by 12.5 dB SPL. In the potential range of 15 to 25 , the frequency shift is fairly linear with the potential change and the sensitivity (frequency shift per unit applied potential change) is 200 Hz around 13 kHz.

Output-feedback H/sub /spl infin// control of discrete-time switching fuzzy systems

This paper suggests a new H/sub /spl infin// output-feedback controller for discrete-time switching fuzzy systems, which have high- and low-level weighting functions, namely, crisp switching-region weighting functions and local fuzzy weighting functions. Based on a new piecewise fuzzy weighting-dependent Lyapunov function (PFWLF) consisting of current-time states and a set of one-step-past local fuzzy weighting-dependent Lyapunov matrices, the new controller directly uses the current-time information on the high-level weighting functions as well as the current-time and the one-step-past information on the low-level weighting functions. This resulting controller is formulated in terms of parametric linear matrix inequalities (PLMIs), which are local fuzzy weighting-dependent conditions.

Piezoelectric membrane acoustic devices

This paper reports the 3/spl times/3/spl times/0.003 mm/sup 3/ piezoelectric membrane acoustic device, which works as a microphone and a microspeaker. It has a 0.5 /spl mu/m thick zinc oxide (ZnO) piezoelectric thin film on a 1.5 /spl mu/m thick low stress silicon nitride membrane, made of LPCVD. The maximum deflection in the center of membrane, using laser Doppler vibrometer, is 1 /spl mu/m at 7.3 kHz with input drive 15 V/sub P-K/ (zero-peak). The output sound pressure level (SPL) of microspeaker is 76.3 dB SPL at 7.3 kHz, and 83.1 dB SPL at 13.3 kHz with input drive 15 V. The distance between reference microphone and piezoelectric microspeaker is 1 cm. The sensitivity of microphone is 0.51 mV/Pa at 7.3 kHz with noise level of 18 dB SPL.

Piezoelectric cantilever voltage-to-frequency converter

Abstract A micromachined piezoelectric cantilever acoustic device that functions as a microspeaker and a voltage-to-frequency converter has been designed, fabricated and tested. The 2 × 2 × 0.0047 mm 3 cantilever has a 1.3-μm-thick zinc oxide (ZnO) piezoelectric thin film on a supporting layer of LPCVD low-stress silicon nitride. When measured with 8 V P-P (peak-peak) input drive, the sound pressure level (SPL) of the cantilever microspeaker output is higher than 70 dB at 5 kHz at a distance of 0.5 cm. The microspeaker also can be used as a voltage-to-frequency converter by the addition of a silicon top plate and an aluminum (Al) sputtered layer on the backside of the cantilever. The resonant frequency of the microspeaker is changed by the potential applied between the top plate and the backside Al layer. As the potential is changed from 0 to 40 V P_P , the resonant frequency shifts down from 14.5 to 11.5 kHz. In the potential range of 15 to 25 V P-P , the frequency shift is fairly linear with the potential change and the sensitivity (frequency shift/applied potential change) is 200 Hz/V around 13 kHz.

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.

Microfabrication of 3D Oblique Structures by Inclined UV Lithography

This paper presents a novel microfabrication technology of 3D oblique microstructures by inclined UV lithography with negative photoresist, SU-8. Various 3D oblique microstructures were simply fabricated such as embedded channels, bridges, V-grooves, and so on.