Rapid prototyping of PMMA microfluidic chips utilizing a CO<Subscript>2</Subscript> laser

A commercially available CO₂ laser scriber is used to perform the direct-writing ablation of polymethyl-methacrylate (PMMA) substrates for microfluidic applications. The microfluidic designs are created using commercial layout software and are converted into the command signals required to drive the laser scriber in such a way as to reproduce the desired microchannel configuration on the surface of a PMMA substrate. The aspect ratio and surface quality of the ablated microchannels are examined using scanning electron microscopy and atomic force microscopy surface measurement techniques. The results show that a smooth channel wall can be obtained without the need for a post-machining annealing operation by performing the scribing process with the CO₂ laser beam in an unfocused condition. The practicality of the proposed approach is demonstrated by fabricating two microfluidic chips, namely a cytometer, and an integrating microfluidic chip for methanol detection, respectively. The results confirm that the proposed unfocused ablation technique represents a viable solution for the rapid and economic fabrication of a wide variety of PMMA-based microfluidic chips.     

On-chip CO<Subscript>2</Subscript> incubation for pocket-sized microfluidic cell culture

We have developed an on-chip CO₂ incubation system based on mass/heat transfer from aqueous solutions of bicarbonate source to cell culture media through a permeable poly(dimethylsiloxane) (PDMS) wall. Heating a carbonate-buffered bicarbonate solution successfully regulated CO₂ generation without any feedback control. Because a microfluidic cell culture chip with the incubation system does not require an external chamber or gas supply, the entire microfluidic cell culture setup becomes pocket sized. Using 5 ml of 0.8 M sodium bicarbonate with 65 mM sodium carbonate as the water jacket, the chip maintained the temperature, osmolality, and pH of 750 μl cell culture medium within physiological levels when the chip was placed on a 37°C surface. The osmolality shift and pCO₂ of the media reservoir stabilized within <5 mmol/kg and 5.0 ± 1.0% over at least 9 days. The incubation capabilities were demonstrated through microfluidic culture of COS-7 epithelial cells under an inverted microscope for 17 days.     

Fabrication of polystyrene microfluidic devices using a pulsed CO<Subscript>2</Subscript> laser system

In this article, we described a simple and rapid method for fabrication of droplet microfluidic devices on polystyrene substrate using a CO₂ laser system. The effects of the laser power and the cutting speed on the depth, width and aspect ratio of the microchannels fabricated on polystyrene were investigated. The polystyrene microfluidic channels were encapsulated using a hot press bonding technique. The experimental results showed that both discrete droplets and laminar flows could be obtained in the device.     

Surface polishing of quartz-based microfluidic channels using CO<Subscript>2</Subscript> laser

Recently, microgrinding using a polycrystalline diamond tool has been introduced to fabricate microchannels and structures from quartz (fused silica). Compared to wet or dry etching processes, the grinding process is very simple and time-efficient for prototyping. However, the roughness of the machined surface remains an issue, because the surface is covered with many small cracks. Poor surface roughness can affect fluid flow in the microfluidic channels. To reduce the surface roughness of microchannels generated by a grinding process, this study presents the laser polishing of quartz and investigates the effects of the translational speed and pitch of a laser spot on the surface roughness and shape accuracy of microchannels.     

Experimental study of drying effects during supercritical CO<Subscript>2</Subscript> displacement in a pore network

Underground storage in geological aquifers is one of the most important options for large-scale mitigation of CO₂. During the supercritical CO₂ (scCO₂) injection process, water dissolved in scCO₂ may have significant impact on the displacement process. In this study, a series of wet scCO₂ (WscCO₂, 100% water saturation) and dry scCO₂ (DscCO₂, 0% water saturation) displacement experiments were conducted in micromodels for a large range of flow rates. The displacement was visualized using fluorescence microscopy. Results showed that DscCO₂ saturations were up to 3.3 times larger than WscCO₂ saturations when the capillary fingering dominated the displacement. The specific interfacial areas and mobile fractions for the DscCO₂ displacements were also much larger than those for WscCO₂. The capillary forces combined with drying effects are identified as the leading causes for the considerably higher DscCO₂ sweep efficiency. Results from this study showed the important impact of mutual solubility of scCO₂ and water on the displacement process and saturation of scCO₂ (S_scCO2), suggesting that the conventional model describing the relationship between capillary pressure and S_scCO2 needs to be modified for the effect of the mutual dissolution of multiple phases to more adequately describe the scCO₂ displacement process in saline aquifer formation.     

Microfluidic analysis of CO<Subscript>2</Subscript> bubble dynamics using thermal lattice-Boltzmann method

By means of microfluidic analysis with a thermal lattice-Boltzmann method, we investigated the hydrophilic, thermal and geometric effects on the dynamics of CO₂ bubbles at anode microchannels (e.g., porous layers and flow channels) of a micro-direct methanol fuel cell. The simulation results show that a more hydrophilic wall provides an additional attractive force to the aqueous methanol in the flow direction and that moves the CO₂ bubble more easily. The bubble propagates quicker in the microchannel with a positive temperature gradient imposed from the inlet to the exit, mainly due to the Marangoni effect. Regarding the geometric effect of the microchannel, the bubble moves more rapidly in a divergent microchannel than in a straight or convergent channel. On the basis of the quantitative evaluation of hydrophilic, thermal and geometric effects, we are able to design the bubble-removal technique in micro fuel cells.     

Remote measurement of chemical warfare agents by differential absorption CO<Subscript>2</Subscript> lidar

The possibilities of remote sensing of chemical warfare agents by the differential absorption method are analyzed. The CO₂ laser emission lines suitable for sensing chemical warfare agents with accounting for disturbing absorption by water vapor were chosen. The detection range of chemical warfare agents is obtained for a lidar based on CO₂ laser. Factors influencing upon the sensing range have been analyzed.     

Influence of doping upon the phase change characteristics of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript>

The influence of doping upon the phase change characteristics of Ge₂Sb₂Te₅ (GST) has been determined with a variety of techniques including four-point-probe electrical resistance measurements, grazing incidence X-ray diffraction (XRD), X-ray reflectometry (XRR) and a variable incident angle spectroscopic ellipsometer (VASE) and a static tester. Doping with Bi, Sn or In maintains the NaCl-type crystalline structure of GST but expands the lattice due to the larger atomic radii. Sufficient optical contrast is exhibited and can be presumably correlated with the pronounced density change upon crystallization. In the Bi and Sn doped case transition temperatures are reduced with regard to the undoped case. Ultra-fast crystallization within 10 ns is demonstrated, which is correlated with a single NaCl-structure phase and a lower transition temperature arising from the weaker bonds. In the In doped case, however, crystallization is retarded, which can be correlated with the observed phase separation and the increased transition temperature.     

Thermal waveguide and fine-scale periodic relief on the semiconductor’s surface induced by TEA CO<Subscript>2</Subscript> laser radiation

The results of experimental study of linear CO₂ laser radiation interaction with semiconductor monocrystals (silicon, germanium, gallium arsenide,) are presented. It was shown that produced surface relief of micro- and nano-structures with spatial periods d ~ λ/n, d ~ λ/2n and d ~ λ/8n (for germanium) are well explained in the framework of universal polariton model of laser-induced condensed-matter damage.     

Pareto optimal generalized <Emphasis Type="Italic">H</Emphasis><Subscript>2</Subscript>-control and vibroprotection problems

We consider a novel multi-objective control problem where the criteria are generalized H ₂-norms of transfer matrices of individual channels from the disturbance input to various objective outputs. We obtain necessary conditions for Pareto optimality. We show that synthesis of Pareto optimal controls can be done in terms of linear matrix inequalities based on optimizing Germeier’s convolution, which also turns out to be the generalized H ₂-norm of the closed-loop system with output composed of the objective outputs multiplied by scalars. As applications we consider multi-objective problems of vibration isolation and oscillation suppression with new types of criteria.     

H<Subscript>2</Subscript>-optimal tuning algorithms for fixed structure controllers

An H₂-method of optimal tuning is proposed for a fixed order controller. The SISO plant model is considered in state space. The H₂-method of tuning parameter design is based on the minimization of a transient process closeness criterion for appropriate open-loop and closed-loop control systems and their reference models. The controller tuning algorithms use the plant parameter estimations obtained during the plant parameter identification. The analytical expressions are obtained for the square of H₂-norm of a stable dynamic system. The following theorem is proven: the minimum necessary conditions for the functionals of transfer function H₂-norm of open-loop and closed-loop systems are the same as the minimum necessary conditions for the Frobenius norm of the controller parameter tuning polynomial.     

Micromachined SiO<Subscript>2</Subscript> microcantilever for high sensitive moisture sensor

Ultra-sensitive and selective moisture sensors are needed in various industries for processing control or environmental monitoring. As an outstanding sensor platform, surface-stress sensing microcantilevers have potential application in moisture detection. To enlarge the deflection of the microcantilever under surface stress induced by specific reactions, a new SiO₂ microcantilever is developed which features a much lower Young’s modulus than conventional Si or SiN_x microcantilevers. For comparing SiO₂ cantilever with Si cantilevers, a model of the cantilever sensor is given by using both analysis and simulation, resulting in good agreement with the experimental data. The results demonstrate the SiO₂ cantilever can achieve a much higher sensitivity than the Si cantilever. In order to fabricate this device, a new fabrication process using isotropic combined with anisotropic dry etching to release the SiO₂ microcantilever beam by ICP (Inductively Coupled Plasma) was developed and investigated. This new process not only obtains a high etch rate at 9.1 μm/min, but also provides good profile controllability, and a flexibility of device design. Attributed to the high sensitivity, a significant deflection amplitude of the surface modified SiO₂ microcantilever was observed upon exposure to 1% relative humidity. The SiO₂ cantilevers are promising for inexpensive and highly sensitive moisture detection.     

<Emphasis Type="BoldItalic">H</Emphasis><Subscript>2</Subscript>-Control and the Separation Principle for Discrete-Time Markovian Jump Linear Systems

In this paper we consider the H₂-control problem of discrete-time Markovian jump linear systems. We assume that only an output and the jump parameters are available to the controller. It is desired to design a dynamic Markovian jump controller such that the closed-loop system is mean square stable and minimizes the H₂-norm of the system. As in the case with no jumps, we show that an optimal controller can be obtained from two sets of coupled algebraic Riccati equations, one associated with the optimal control problem when the state variable is available, and the other associated with the optimal filtering problem. This is the principle of separation for discrete-time Markovian jump linear systems. When there is only one mode of operation our results coincide with the traditional separation principle for the H₂-control of discrete-time linear systems. Date received: June 1, 2001. Date revised: October 13, 2003.     

CdF<Subscript>2</Subscript>:Ga crystal application for dynamic joint transform correlator

Dynamical holographic joint transform correlator that uses cadmium fluoride crystal with gallium bistable impurity centers to generate real-time matched filters is proposed. Optical scheme features and the correlator performance are presented. Output correlation signals of object discrimination both experimentally obtained and theoretically calculated are demonstrated. Industrial object recognition illustrates possible practical application of the correlator proposed. The article is published in the original.     

Electrical properties and crystallization behavior of Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>100−<Emphasis Type="Italic">x</Emphasis></Subscript> thin films

The structural transformation and transformation kinetics of Sb _x Se_100−x films (60 ≤ x ≤ 70) were studied to investigate the feasibility of applying Sb _x Se_100−x alloys in phase-change nonvolatile memories. The temperature-dependent van der Pauw measurements, Hall measurements, X-ray diffraction and a static tester were used to investigate the electrical properties and crystallization behavior of the Sb _x Se_100−x films. The sheet resistance difference between amorphous and crystalline state was higher than 10⁴ Ω per square According to Hall measurement, Sb _x Se_100−x films have p-type conduction and the Hall mobility and carrier concentration increases with the increase in Sb content. The crystalline structure of the metastable phase of Sb _x Se_100−x alloys, which plays a major roll in fast crystallization, is similar to that of Sb₂Te (rhombohedral structure). The transition temperature, sheet resistance and activation energy for transformation decrease as the amount of Sb increases in the Sb _x Se_100−x film. Applying the Kissinger method, the activation energies for crystallization were in the range from 1.90 ± 0.15 to 4.16 ± 0.28 eV. The desired crystallization speed can be obtained by a systematic change of the composition owing to the variation of the activation barrier with stoichiometry.     

Surface modification of a-Si<Subscript>0.60</Subscript>C<Subscript>0.40</Subscript>:H films by Al-assisted photochemical etching: humidity sensing application

The paper investigates the formation of thin porous amorphous silicon carbide (PASiC) by Al-assisted photochemical etching using HF/AgNO₃ solution under UV illumination at λ = 254 nm. Different etching times varying from 2 to 10 min have been used on thin a-Si_0.60C_0.40:H films, which are elaborated by co-sputtering DC magnetron using a single crystal Si target and who deposited onto 86 of hot pressed polycrystalline 6H-SiC stripes of 12.5 mm³. Because of the high electrical resistivity of the thin a-Si_0.60C_0.40:H film higher than 2 MΩ cm, and in order to facilitate the chemical etching, a thin metallic film of high purity aluminum (Al) has been deposited under vacuum, follow-up of a thin palladium deposited under a grid to reduce attacked surface and reinforced solution etching. The etched surface was characterized by scanning electron microscopy, infrared spectroscopy, spectrophotometer UV, and photoluminescence. Results show that the morphology of etched a-Si_0.60C_0.40:H surface evaluates with etching time and presents a spongy and macroporous layers. Where, the diameter of pore size increases with the increasing etching time. A humidity sensors were fabricated through evaporating coplanar interdigital gold electrodes on PASiC and the humidity sensing properties were tested, it show, that the measured resistance Au-PASiC structure, depends highly on the applied bias voltage. Finally, the sensing performances are attributed to the unique surface structure, morphology of the pore and its size, that provide an effective pathway for vapor transportation and enlarged the sensing area of Au-PASiC.     

An a posteriori error estimate and a comparison theorem for the nonconforming <Emphasis Type="Italic">P</Emphasis><Subscript>1</Subscript> element

A posteriori error estimates for the nonconforming P ₁ element are easily determined by the hypercircle method via Marini’s observation on the relation to the mixed method of Raviart–Thomas. Another tool is Ainsworth’s application of the hypercircle method to mixed methods. The relation on the finite element solutions is also extended to an a priori relation of the errors, and the errors of four different finite element methods can be compared.      

<Emphasis Type="Italic">I</Emphasis><Subscript>DDT</Subscript>: Fundamentals and test generation

It is the time to explore the fundamentals ofI _DDT testing when extensive work has been done forI _DDT testing since it was proposed. This paper precisely defines the concept of average transient current (I _DDT) of CMOS digital ICs, and experimentally analyzes the feasibility ofI _DDT test generation at gate level. Based on the SPICE simulation results, the paper suggests a formula to calculateI _DDT by means of counting only logical up-transitions, which enablesI _DDT test generation at logic level. The Bayesian optimization algorithm is utilized forI _DDT test generation. Experimental results show that about 25% stuck-open faults are withI _DDT test generation. 2.5, and likely to beI _DDT testable. It is also found that mostI _DDT testable faults are located near the primary inputs of a circuit under test.I _DDT test generation does not require fault sensitization procedure compared with stuck-at fault test generation. Furthermore, some redundant stuck-at faults can be detected by usingI _DDT testing.