Transient flow patterns and bubble slug lengths in parallel microchannels with oxygen gas bubbles produced by catalytic chemical reactions

Transient flow patterns and bubble slug lengths were investigated with oxygen gas (O-2) bubbles produced by catalytic chemical reactions using a high speed camera bonded with a microscope. The microreactor consists of an inlet liquid plenum, nine parallel rectangular microchannels followed by a micronozzle, using the MEMS fabrication technique. The etched surface was deposited by the thin platinum film, which is acted as the catalyst. Experiments were performed with the inlet mass concentration of the hydrogen peroxide from 50% to 90% and the pressure drop across the silicon chip from 2.5 to 20.0 kPa. The silicon chip is directly exposed in the environment thus the heat released via the catalytic chemical reactions is dissipated into the environment and the experiment was performed at the room temperature level. It is found that the two-phase flow with the catalytic chemical reactions display the cyclic behavior. A full cycle consists of a short fresh liquid refilling stage, a liquid decomposition stage followed by the bubble slug flow stage. At the beginning of the bubble slug flow stage, the liquid slug number reaches maximum, while at the end of the bubble slug flow stage the liquid slugs are quickly flushed out of the microchannels. Two or three large bubbles are observed in the inlet liquid plenum, affecting the two-phase distributions in microchannels. The bubble slug lengths, cycle periods as well as the mass flow rates are analyzed with different mass concentrations of hydrogen peroxide and pressure drops. The bubble slug length is helpful for the selection of the future microreactor length ensuring the complete hydrogen peroxide decomposition. Future studies on the temperature effect on the transient two-phase flow with chemical reactions are recommended.

Study on the kinetics of hydrate formation in a bubble column

Gas hydrate formation experiments were performed using methane in the presence of tetrahydrofuran (THF) in aqueous solution in a transparent bubble column in which a single pipe or a sintered plate was used to produce bubbles. The mole fraction of THF in aqueous solution was fixed at 6%. The hydrate formation kinetic behaviors on the surface of the rising bubble, the mechanical stability of hydrate shell formed on the surface of the bubble, the interactions among the bubbles with hydrate shell were observed and investigated morphologically. The rise velocities of individual bubbles with hydrate shells of different thickness and the consumption rates of methane gas were measured. A kinetic model was developed to correlate the experimentally measured gas consumption rate data. It was found that the hydrate formation rate on the surface of the moving bubble was high, but the formed hydrate shell was not very easy to be broken up. The bubbles with hydrate shells tended to agglomerate rather than merge into bigger bubble. This kind of characteristic of hydrate shell hindered the further formation of hydrate and led to the lower consumption rate of methane. The consumption rate of methane was found to increase with the decrease of temperature or increase of pressure. The increase of gas flux led to a linear increase in consumption rate of methane. It was demonstrated that the developed kinetic model could be used to correlate the consumption rate satisfyingly.

Bleaching kinetics of indoly-benzylfulgimide in PMMA

Indoly-benzlfulgimide belongs to the photochromic fulgide family and follows photochemical first order kinetics. Its bleaching kinetics is investigated at 633 nm and 640 nm by spectroscopy, by the time dependence of transmission and of diffraction from holographically induced gratings. The non-exponential decay law resulting for diffraction experiments with a Gaussian beam profile is calculated and verified experimentally. For a quasi-homogeneous beam profile the time constant determined from diffraction decay is half the one for absorbance decay. The photochemical reaction rate of indoly-benzylfulgimide in PMMA is (3.9 +/- 0.3) cm(2)/J at 650 nm. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Absorbance kinetics of dye-doped systems with photochemical first order kinetics

Often it is assumed that absorbance decays in photochromic materials with the time dependence of the photochemical kinetics, i.e. exponentially for first order kinetics. Although this may hold in the limiting case of vanishing absorbance, deviations are to be expected for realistic samples, because the local photochemical kinetics slows down with increasing initial absorption and penetration depth of the radiation. We discuss the theory of the kinetics of initially homogeneous photochromic samples and derive analytical solutions. In extension of Tomlinson's theory we find an analytical solution that holds with good approximation even for samples that exhibit a small residual absorption in the saturation limit. The theoretical time dependence of the absorbance originating from photochemical first order kinetics of dye-doped systems is compared with experimental data published by Lafond et al. for fulgides doped in different polymer matrices. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kinetics of picosecond laser pulse induced charge separation and proton transfer in bacteriorhodopsin

Bacteriorhodopsin (BR) films oriented by an electrophoretic method are deposited on a transparent conductive ITO glass. A counterelectrode of copper and gelose gel is used to compose a sandwich-type photodetector with the structure of ITO/BR film/gelose gel/Cu. A single 30-ps laser pulse and a mode-locked pulse train are respectively used to excite the BR photodetector. The ultrafast failing edge and the bipolar response signal are measured by the digital oscilloscope under seven different time ranges. Marquardt nonlinear least squares fitting is used to fit all the experimental data and a good fitting equation is found to describe the kinetic process of the photoelectric signal. Data fitting resolves six exponential components that can be assigned to a seven-step BR photocycle model: BR-->K-->KL-->L-->M-->N-->O-->BR. Comparing tests of the BR photodetector with a 100-ps Si PIN photodiode demonstrates that this type of BIR photocletector has at least 100-ps response time and can also serve as a fast photoelectric switch. (C) 2003 Society of Photo-Optical Instrumentation Engineers.

Transient and nonlinear analysis of slow-light pulse propagation in an optical fiber via stimulated Brillouin scattering

We investigate slow-light pulse propagation in an optical fiber via transient stimulated Brillouin scattering. Space-time evolution of a generating slow-light pulse is numerically calculated by solving three-wave coupled-mode equations between a pump beam, an acoustic wave, and a counterpropagating signal pulse. Our mathematical treatments are applicable to both narrowband and broadband pump cases. We show that the time delay of 85% pulse width can be obtained for a signal pulse of the order of subnanosecond pulse width by using a broadband pump, while the signal pulse is broadened only by 40% of the input signal pulse. The physical origin of the pulse broadening and distortion is explained in terms of the temporal decay of the induced acoustic field. (C) 2009 Optical Society of America

Kinetics of photoinduced anisotropy in bacteriorhodopsin film under two pumping beams

Photoinduced anisotropy in bacteriorhodopsin (BR) film arises from the selective bleaching of BR molecules to linearly polarized light. The kinetics of photoinduced anisotropy excited by single and two pumping beams are investigated theoretically and experimentally. Compared with a single pumping beam (650 nm), which produces comparatively small photoinduced anisotropy, dual-wavelength linearly polarized pumping beams (650 and 405 nm) can obviously change the photoinduced anisotropy. When the polarization orientation of the 405 ran pumping beam is perpendicular to that of the 650 nm pumping beam, the peak and steady values of the photoinduced anisotropy kinetic curves are remarkably enhanced. But when the two pumping beams have parallel polarization orientation, the peak and steady values are restrained. At a fixed intensity of the 650 nm pumping beam, there exists an optimal intensity for the 405 nm pumping beam to maximize the value of the photoinduced anisotropy. The photoinduced transmittance of the polarizer-BR-analyzer system is modulated by the polarization angle of the 405 nm pumping beam in an approximate-cosine form. (C) 2008 Optical Society of America.

Effect of reconstruction beam polarization on the kinetics of anisotropic gratings in bacteriorhodopsin

Anisotropic gratings are recorded on bacteriorhodopsin films by two parallelly polarized beams, and the effect of the polarization orientation of the reconstructing beam on the diffraction efficiency kinetics is studied. A theoretical model for the diffraction efficiency kinetics of the anisotropic grating is developed by combining Jones-matrix and photochromic two-state theory. It is found that the polarization azimuth of the reconstructing beam produces a cosine modulation on the kinetics of the diffraction efficiency, being positive at the peak efficiency and negative for steady state. By adding auxiliary violet light during grating formation, the saturation of the grating can be restrained. As a result, the negative cosine modulation for the steady-state diffraction efficiency changes to a positive one. In addition, the steady-state diffraction efficiency is increased appreciably for all reconstructing polarization orientations. (c) 2008 Optical Society of America.

SBS-based slow light in optical fibers: optimum design considerations for undistorted slow-light signal propagation in steady-state and transient regimes

SPIE

DETERMINATION OF THE SUBBAND ENERGY IN THE V-SHAPED POTENTIAL WELL OF DELTA-DOPED GAAS BY DEEP-LEVEL TRANSIENT SPECTROSCOPY

Using deep level transient spectroscopy (DLTS) the conduction-subband energy levels in a V-shaped potential well induced by Si-delta doping in GaAs were determined. Self-consistent calculation gives four subbands in the well below the Fermi level. Experimentally, two DLTS peaks due to electron emission from these subbands were observed. Another two subbands with low electron concentration are believed to be merged into the adjacent DLTS peak. A good agreement between self-consistent calculation and experiment was obtained. (C) 1994 American Institute of Physics.