Unstable Mixtures Research Articles

Computational study of gaseous cellular detonation diffraction and re-initiation by small obstacle induced perturbations

A gaseous detonation wave that emerges from a channel into an unconfined space is known as detonation diffraction. If the dimension of the channel exit is below some critical value, the incident detonation fails to re-initiate (i.e., transmit into a self-sustained detonation propagating) in the unconfined area. In a previous study, Xu et al. [“The role of cellular instability on the critical tube diameter problem for unstable gaseous detonations,” Proc. Combust. Inst. 37(3), 3545–3533 (2019)] experimentally demonstrated that, for an unstable detonable mixture (i.e., stoichiometric acetylene–oxygen), a small obstacle near the channel exit promotes the re-initiation capability for cases with a sub-critical channel size. In the current study, two-dimensional numerical simulations were performed to reveal this obstacle-triggered re-initiation process in greater detail. Parametric studies were carried out to examine the influence of obstacle position on the re-initiation capability. The results show that a collision between a triple-point wave complex at the diffracting shock front and the obstacle is required for a successful re-initiation. If an obstacle is placed too close or too far away from the channel exit, the diffracting detonation cannot be re-initiated. Since shot-to-shot variation in the cellular wave structure of the incident detonation results in different triple-point trajectories, for an obstacle at a fixed position, the occurrence of re-initiation is of a stochastic nature. The findings of this study highlight that flow instability generated by a local perturbation is effective in enhancing the re-initiation capability of a diffracting cellular detonation wave in an unstable mixture.

A comprehensive study on laminar burning velocity and flame stability of oxy-producer gas mixtures. Part-1: Gas mixture composition and flame stability analysis based on Lewis number

The objective was to understand the effect of change in the composition of combustible (CC)/ non-combustible components (NCC) on the unstretched laminar burning velocity (LBV), burned gas Markstein length (Lb) and Lewis number (Le) of premixed producer gas (PG)-oxygen mixtures. Design of composition and flame stability aspects based on Le are presented in part-1. PG comprised of CC(CH4:CO:H2) + NCC(CO2:N2) and the extent of variation was identified from 800 compositions reported in 123 research works. In this work, forty-five PG mixtures were studied where CC:NCC varied from 25:75–45:55, CO2:N2 from 0:100–100:0 and CH4:CO:H2 from 25:25:50–50:25:25. Spherical flame method was used to measure the LBV at 1 bar, 300 K and ϕ = 0.8 in a 4.5L spherical chamber. Validation experiments were performed with (50%H2 + 50%CO)-air mixtures and it compared well with the literature and computational data. Unstable mixtures identified by effective (volume-based model) and critical Lewis numbers corroborated well with the experimental burned gas Markstein length.

Propagation mechanism of gaseous detonations in annular channels with spiral for acetylene-oxygen mixtures

The propagation mechanism of detonations in the annular channel with a spiral is experimentally studied for an acetylene and oxygen mixture with different levels of argon dilution. Detonation velocity is measured through the placement of regularly spaced photodiodes along the test tube. A high-speed camera is also used to supplement the photodiodes when the luminosity decreases. Smoked foils of long length are inserted into the annular channel to register the cellular structures. The results indicate that the effect of the spiral on detonation propagation highly depends on the intrinsic instability of the detonation. With a decrease in the initial pressure, the propagation changes from spiral-induced multi-head mode to spiral-induced single-head mode and then spiral-induced shock-flame complex mode; then, the propagation fails as the limit is approached. Discontinuities in normalized velocity are found for mixtures with high argon dilution, which indicates the transition from single-head spin to shock-flame complex, as observed from the smoked foils. Such a phenomenon is not observed in cases with an unstable mixture of C2H2 + 2.5O2 or in cases with large roughness. This is because of the different mechanisms stable and unstable detonations and the corresponding response to the boundary change. The critical width of the annular channel normalized by cell size for the discontinuity is found to slightly larger than 1/π, which corresponds to the lowest quenching limit in smooth tubes, particularly for stable detonations. The critical diameter normalized by cell size for the discontinuity in the circular tube is almost twice the critical width in the annular channel, equal to the geometrical scaling based on front curvature theory.

Pickering emulsions stabilized by sphere-spheroid mixtures

Stable oil-in-water Pickering emulsions (PEs) are prepared by using hydrophilic and oppositely charged spherical (silica) and nonspherical (hematite) nanoparticle mixture. By varying aqueous dispersion pH, the surface potentials of the particles are tuned. For fixed sizes of silica and hematite, stable emulsions are formed at low concentrations of silica when the surface potential mismatch between the particles is high. Conversely, when the latter is small, stable emulsions are found up to high concentration of silica nanoparticles. These findings are explained based on the formation of silica-hematite heteroaggregates and on their surface-active characteristics. It is found that for a given pH and particle concentration (expressed as number ratio NS-H, which is the ratio of the total number of spheres to the total number of spheroids), highly stable oil-in-water emulsions are formed only when the unstable aqueous binary mixture of colloids and the oil phase are emulsified. The result is presented as a state diagram, which shows the particle composition corresponding to the formation of stable emulsions, the composition range where the emulsions that result are unstable and the region where the emulsions do not form at all. As the formation of PE is kinetically controlled, we demonstrate that at conditions where emulsions formed via one-step emulsification for a given NS-H are unstable, following a two-step emulsification procedure yields highly stable emulsions. The work highlights the role of (i) NS-H, (ii) aqueous dispersion pH, (iii) surface potential mismatch and (iv) emulsification protocol on the formation and stability of solid particle stabilized emulsions.

Proposition of non-standard method useful for viscosity measurements of unstable two-phase systems coupled with examples of its application

Abstract A tubular rheometer was proposed for testing rheological properties of heterogeneous two-phase systems flowing through a pipe. The paper describes the idea of its design and assembly in a flow system. The research methodology and guidelines for interpretation of measurement data are given. Based on the study performed for the water-oil system, it was demonstrated that the use of standard rotational rheometers could not provide similar measuring possibilities. It was possible to determine the rheological characteristics of unstable two-phase mixtures and variability in viscosity in the conditions of actual flow. The interpretation of the measurement results also provided unusual means to identify the phase inversion phenomenon. The areas of application of the proposed method extend beyond research of water-oil systems and scope of experiments reported in this paper. Despite the focus on a specific mixture and its exceptional characteristics, the study also demonstrated other areas of application of this method.

The re-initiation mechanism of detonation diffraction in a weakly unstable gaseous mixture

Numerical simulations were performed to investigate the re-initiation mechanism of a diffracted detonation wave near the critical channel width for a weakly unstable gas. Two scenarios were examined: diffraction of a planar detonation wave and of a cellular detonation wave inside the inlet channel. The results revealed that the critical channel width predicted using a cellular detonation wave is smaller than that predicted using a planar detonation wave. The re-initiation mechanisms are described in detail by tracing massless particles along both the plane of symmetry and the re-initiation path. For planar detonation diffractions, a compression wave is formed in the far field behind the diffracted shock. Re-initiation is closely related to the amplification of this compression wave and its coalescence with the diffracted shock. Depending on the inlet channel width, the strength of the reflected rarefaction wave is responsible for weakening the strength of the compression wave and its coalescence with the diffracted shock, consequently hindering the reaction of particles behind the diffracted shock wave. In cellular cases, the continuous collisions of transverse waves, which generate local explosion sites, sustain detonation wave propagation.

Effects of stress state and fine fraction on stress transmission in internally unstable granular mixtures investigated via discrete element method

Low effective stress in the fine fraction of internally unstable materials is the key mechanism leading to the internal instability. In the present study, the effects of stress state and fine fraction on the stress transmission in gap-graded internally unstable granular mixtures are investigated at microscale by discrete element method. It is found that the fine fraction plays a more dominant role than the stress state of soil in the stress transmission. The sensitive range of fine fraction with respect to number of contact (25–35%) is relatively lower than that of contact force (30–40%). The influence of stress state is only significant within a specific range of fine fraction, and the contributions of isotropic pressure and stress ratio to stress state influence are different. The internally unstable materials with an interactive-overfilled load-bearing fabric could switch to internally stable by increasing the isotropic pressure. The influence of stress ratio is associated with the magnitude of isotropic pressure.

Emulsifying ability of exudate gums obtained from three plant species in Ghana

Food emulsions are thermodynamically unstable mixtures which can be stabilized with the application of an emulsifier. In Ghana emulsifiers are imported, and this increases the final cost of food emulsions. In this study, gums obtained from three tree plant species in Ghana, i.e. Albizia zygia (Albizia), Khaya senegalensis (Khaya), and Anarcardium occidentale (Cashew), were used to stabilize oil-in-water emulsion, and the stability of the emulsions were measured after centrifugation at 1300 x g for 5 min, and upon pH adjustment (from 2 to 3.5). Quantity of gum (mass), solubility of gum in the continuous phase, viscosity, oil volume fraction, and pH were investigated to determine how they affect emulsion stability.The results indicated with the exception of viscosity, emulsion stability is influenced by all the other factors studied. In addition there was no significant difference (P<0.05) between the emulsion stabilities of food emulsions stabilized by gums of Cashew (0.77-0.86) and Acacia (0.78-0.87). The Pearson’s co-efficient of correlation indicated that the emulsion stability values of the emulsions positively correlated with the solubility of the gums (R2 =0.983 at P<0.05, and0.997 at P<0.01). Although there were no significant differences in the emulsion stability values of emulsions stabilized with Albizia and Khaya gums, both recorded significantly lower (P<0.05) emulsion stability values (0.76-0.85 and 0.75-0.81 respectively) than the Acacia gum (control). The Cashew gum has the potential to be utilized as an emulsifier in the food industry.
 Keywords: Emulsion, Emulsifier, Acacia gum, Oil-in-water Emulsion, Emulsion stability

Near-limit detonations of methane–oxygen mixtures in long narrow tubes

The near-limit gaseous detonation behavior of three different methane–oxygen mixtures was investigated. Experiments were performed in transparent tubes of three different inner diameters (d). Due to the relatively large tube length l ( $$\frac{l}{d} > 2500$$ except $$\frac{l}{d} > 1000$$ for the largest diameter), the tube was arranged in a spiral configuration for the convenience of testing. Photodiodes were spaced at uniform intervals along the tube to provide a high-resolution velocity measurement, such that up to eight cycles of the galloping mode were registered. From the velocity histogram and the probability distribution function, a bimodal behavior was observed in all galloping regimes for different unstable mixtures, with dominant modes near 70% of the Chapman–Jouguet detonation velocity ( $$0.7D_{\mathrm{CJ}}$$ ) and $$D_{\mathrm{CJ}}$$ . With decreasing pressure, the lower-velocity mode became more prevalent until the failure of detonation. The range of initial pressures, within which galloping detonations were observed, decreased rapidly with increasing tube diameter and with increasing mixture stability. These results suggest that both the instability and the boundary effect influence the existence of galloping detonations. The normalized wavelength of the galloping cycle ( $$\frac{L}{d}$$ ) was in the range of 200–450 for the three different mixture compositions and exhibited a general trend that the wavelength increased with decreasing initial pressure.

Partially hydrolyzed polyvinyl alcohol for fusion-based pharmaceutical formulation processes: Evaluation of suitable plasticizers.

The present study evaluates the effect of several pharmaceutical plasticizers on the thermo-physical and physicochemical properties of partially hydrolyzed poly(vinyl alcohol) (PVA) used in fusion-based pharmaceutical formulation processes. Specifically, the effect of mannitol (MAN), sorbitol (SOR), sucrose (SUC), anhydrous citric acid (CA), triethyl citrate (TEC) and low-molecular weight polyethylene glycol (PEG400) on PVA's melting properties, physical state and thermal degradation was evaluated via differential scanning calorimetry (DSC), powder X-ray diffractometry (pXRD) and thermo-gravimetric analysis (TGA). Results showed that the use of MAN, SOR, SUC and PEG400 led to the reduction of PVA's melting onset temperature, while MAN, SUC, CA and SOR were amorphously dispersed within PVA's matrix, and the addition of SUC and CA resulted in significant reduction of PVA's crystallinity. TGA results showed the formation of thermally highly unstable PVA mixtures in the cases of CA and TEC (degradation started from ~150°C and ~125°C, respectively), while significant molecular interactions were identified by FTIR in the cases of PVA-MAN, PVA-SOR and PVA-SUC. Hot-stage polarized microscopy (HSM) revealed PVA's melt miscibility only with MAN and SOR, while melt flow index (MFI) measurements showed that the use of MAN, SOR and PEG400 resulted in a significant improvement of PVA's melt flow properties. Finally, MD simulations were in close agreement with the experimental observations, indicating that they can be considered as a promising tool for the theoretical modelling of such systems.

Insights into the Approach, Fabrication, Application, and Lacunae of Nanoemulsions in Drug Delivery Systems.

Many of the recently approved drug molecules that are therapeutically successful are found to be incompatible for the development of a novel delivery system and to take part in various health care management. Regardless of having better therapeutic properties, these molecules are barred from their effective clinical uses. The main reason attributed to it is poor solubility and/or poor permeability of drugs which finally emerges the drug to be low bioavailable. Nanoemulsions are one of the most acceptable nanolipoidal drug delivery system and appears to be a hope for the delivery of many of the Biopharmaceutical Classification System (BCS) class II and IV drugs. A nanoemulsion is a thermodynamically unstable isotropic mixture of oil, surfactant, and co-surfactants and is biphasic in nature. It can be either water in oil or oil in water and droplets are found in the range of 5 to 500 nm. The manufacturing and fabrication of nanoemulsions involve various natural, synthetic and semi synthetic materials using either low or high-energy methods. Application of nanoemulsions as a novel drug delivery system through several routes, especially oral, transdermal, ophthalmic, and intranasal, have been increased for various pharmacological aspects such as cardiovascular, anticancer, antimicrobial, and ophthalmic due to their stability, high solubilization capacity, and ease of preparation. The objective of this review is to focus on the aspects of manufacturing, fabrication, application, and some toxicological concerns related to nanoemulsions.

Variations in surface quality of self-consolidation and highly workable concretes with formwork material

The effect of the forming materials on surface quality of the self-consolidating and highly workable concretes was experimentally evaluated. Several mixtures having wide ranges of workability and rheology were investigated. The mixtures were placed in a Z-shaped mold. No mechanical consolidation was applied at all. The sides of the mold were built up using plywood, PVC, steel, and permeable formwork liner using a polyester filter. The concrete surface quality was evaluated in terms of the voids, bleeding, segregation, and insufficient self-consolidation, using an image processing technique. The surface diameter and area of the voids found on the formed concrete surfaces were statistically correlated to the concrete rheology. The permeable lined formwork was shown to produce concrete surfaces with better quality compared to impermeable formwork materials. The same outcome was also noticed for the concrete mixtures having insufficient self-consolidation and even the unstable mixtures that manifested signs of bleeding. The permeable lined formwork was shown to provide an effective way for the entrapped air/water bubbles and the bleed water to escape. The maximum surface dimension of the voids found on the concrete surfaces was limited to 3, 6, 7, and 10 mm for the mixtures cast using the permeable liner, steel, PVC, and plywood formwork materials, respectively.

The effects of differential diffusion in counter-flow premixed flames with dilution and hydrogen enrichment

The effects of differential diffusion on local flamelet velocities, turbulent burning rates, and structure of lean turbulent premixed flames in the thin reaction zone regime are investigated using aerodynamically stabilized flames in a counter-flow apparatus. Various fuel–oxidizer–inert mixtures with different transport properties, representative of distinct effective Lewis numbers, are studied. In order to minimize the effects of mixture reactivity in these experiments, unstretched laminar flame speed is kept constant during mixture dilution, and hydrogen enrichment of hydrocarbon flames, through changing the mixture equivalence ratio. Furthermore, bulk-flow properties and stagnation surface temperature are kept constant; hence, the study focuses on the effects of differential diffusion, which is the change in transport properties of the mixture, i.e., fuel and heat diffusivities, in the context of fuel flexibility. Highly strained laminar flame measurements are also reported as a reference of comparison. Local instantaneous statistics of various flame parameters within the imaged plane, such as flame location, flame velocity, and flame-front topology, are quantified using high-speed particle image velocimetry (PIV) and Mie scattering flame tomography at two levels of turbulence intensity. These parameters are presented as probability density functions using sufficiently large data sets to ensure statistical accuracy. The results for various flame-parameter statistics, which are measured over a wide range of Lewis numbers, show that the effects of differential diffusion are important in turbulent flames in the thin reaction zone regime. At constant turbulence intensity, differential diffusion increases the burning rates of turbulent flames in thermo-diffusively unstable mixtures through two main mechanisms: (1) increasing local flamelet displacement velocity, and (2) increasing flame-surface area. The relative contribution of these two parameters in increasing turbulent burning rates is approximately 76% and 24%, respectively, which is not dependent on the fuel, oxidizing-gas mixture, or turbulence intensity, and the results overlap over a wide range of Lewis numbers.

Rheological characteristics of unstable heavy crude oil-water dispersed mixtures

The rheological characteristics of unstable heavy crude oil-water dispersed mixtures were investigated to improve pipeline transportation. The viscosity of heavy crude oil is 5439 mPa s, and density is 955 kg/m3 at 30 °C. A series of factors, including oil volume fraction, shear rate, and temperature, was considered systematically to investigate the phenomenon of phase inversion and stability of oil-water mixtures. The results show that the apparent viscosity of a heavy crude oil-water mixture reaches its maximum value at the phase inversion point, and that the non-Newtonian fluid characteristics of unstable heavy crude oil-water mixtures are different than those of fine emulsions or light oil-water mixtures. The addition of emulsifier influences the apparent viscosity and phase inversion point by changing the droplet size distribution. Furthermore, the apparent viscosity of a heavy crude oil-water mixture continues to increase as the median diameter of the dispersed phase decreases. All these are beneficial to the analysis of the rheological characteristics of heavy crude oil-water dispersed mixtures.

The effect of instability of detonation on the propagation modes near the limits in typical combustible mixtures

At the near-limit conditions, the behavior of detonation propagation manifests unstable modes; those modes are classified into three types, i.e., steady mode, fluctuation mode and fast failure mode. This work aims to elucidate essential physical relation between those unstable propagation modes and the instability of detonation. The velocity of the combustion wave in five mixtures (i.e., C2H4-6N2O, CH4-2O2, 2H2-O2, 70% and 85% Ar diluted C2H2-2.5O2) are experimentally measured to analyze velocity behavior near the limits. The stability parameter χ is computed from CHEMKIN package and chemical kinetics. According the detonation cellular pattern and the calculation of χ, C2H4-6N2O and CH4-2O2 are considered as unstable mixtures (with highly irregular cells), 2H2-O2 is characterized as stable mixture (with regular structure), 70% and 85% Ar diluted C2H2-2.5O2 are determined to be highly stable mixtures (with highly regular pattern). In the above unstable/stable/highly stable mixtures, the detonation propagation behavior at the near-limit condition presents Slow Decay/ Fluctuation/ Fast Failure modes. Evidences confirm that instability (or cell regularity) dominates the propagation modes as detonation approaches the limits. The competition mechanism between detonation instability and boundary effect resulting in the slow decay of propagation velocity as the condition is outside the limits. However, for stable mixtures, due to the instability has very limited effect on the propagation of detonation, this competition mechanism is lacking, and resulting in detonation rapidly attenuates to a fast flame. In addition, the neutral stability curve (Ng et al., Combust Theor Model 2005;9:385-401) is an appropriate separatrix between Fast Failure/Fluctuation modes, a new second stability curve is proposed in this work, which is the boundary of Fluctuation/Slow Decay modes.

Development and use of a mixed-reactant fuel cell

Mixed-reactant fuel cells are unconventional electrochemical power sources which use a thermodynamically unstable mixture of fuel and oxidant. In the work described here a 2-phase mixture of alkaline potassium formate (liquid)/oxygen(gas) is the fuel/oxidant combination used in the “flow-by” mode with mixed-reactant fuel cells, developed in the laboratory from a 10E-4 m2 single cell to a bipolar stack of nineteen 35E-4 m2 cells. With Pd/Ag anode/cathode catalysts, operating at 500 kPa(abs), 80 °C the single cell with a Pd load of 0.054 kg m−2 reaches a superficial power density of 4,000 W m−2 at 10,000 A m−2. Tests of the 19 cell stack with a Pd load of 0.036 kg m−2 at 300 kPa(abs), 60 °C give power output up to 120 W, with a corresponding volumetric power density about 400 kW m−3. To demonstrate the fuel cell performance three nominally 100 W 19 cell stacks are wired in parallel to a 250 W electric motor to drive a modified electric scooter, with on-board fuel, oxidant and an adult passenger, for about 15 minutes at 10 km h−1. To our knowledge this is the first time mixed-reactant fuel cells have been used alone to power a vehicle.

Velocity behavior downstream of perforated plates with large blockage ratio for unstable and stable detonations

Due to the excellent thermal propulsion performances of detonation, it has been applied for the purpose of aerospace propulsion devices, e.g., pulse detonation engines (PDEs), rotating detonation engines (RDEs), and oblique detonation wave engines (ODWEs). However, it remains challenging for developing those new-concept propulsion devices, mainly because it is a formidable task to establish a steady and self-sustained detonation in the hypersonic flow and combustible mixture. One of the fundamental problems is to understand the interaction of diffractions and the detonation waves within the engines. In this study, perforated plates with various blockage ratios are seated at the beginning of the detonation propagation to explore the diffractions that generated from the plates on the detonation propagation mechanism. Two explosive mixtures of C2H2 + 5N2O and C2H2 + 2.5O2 + 70% Ar are studied to illustrate the difference in propagation velocity behavior of detonation after it suffers from large-scale diffractions. The results show that diffractions have less effect on the propagation of highly unstable mixture, and the effect becomes obvious as BR increases to 0.962; this phenomenon occurs because detonation has an irregular cellular pattern and sub-structures are characterized by highly unstable detonation, in which the detonation instabilities are amplified by the large perturbations of obstacles, leading to an augment in forming more cellular cells of detonation in its front that cover the weakening effects from diffractions. In contrast, the diffractions significantly affect the stable mixture, manifested by a remarkable increase of the critical pressure for a self-sustained detonation downstream of the obstacle with the augment of blockage ratio; this phenomenon can be attributed to the diffractions being distributed along the curvature over the detonation surface, thereby causing more excessive curvature of the entire detonation front, which in turn exacerbates the failure of detonation.

Apparent viscosity characteristics and prediction model of an unstable oil-in-water or water-in-oil dispersion system

A series of theoretical analysis and experimental measurements were done to analyze the characteristic of the apparent viscosity of oil-water unstable mixtures which display the similar distributions as the mixtures in pipeline flow. Based on the experimental results, it can be concluded that the unstable oil-water dispersed mixtures are always show the characteristic of non-Newtonian fluid. The shear rate has a great influence on the apparent viscosity, particular for the water-in-oil mixtures. The effect of dispersed phase volume fraction on the apparent viscosity of the unstable mixtures displays the similar rules as the stable emulsions. Also, the influences of the dispersed phase distribution and viscosity on the apparent viscosity were studied through the corresponded experiments systematically. Furthermore, a new kind of prediction model was obtained in the present study. The unstable and non-Newtonian fluid characteristics of oil-water two-phase dispersed mixtures were considered in this model to improve their range of application and the prediction accuracy. Moreover, this kind of apparent viscosity prediction model was validated by using the experimental data from some other literatures including different kinds of oils such as white oil and crude oil, and the average deviation was always less than 9% which was a great accuracy of apparent viscosity prediction.

Effect of acoustically absorbing wall tubes on the near-limit detonation propagation behaviors in a methane–oxygen mixture

Methane (CH4) is the main component of natural gas and produces less carbon dioxide for each unit of heat released and more heat per unit mass than other hydrocarbon fuels, and it is thus considered to be an environmental-friendly fuel. The explosion and detonation hazards associated with methane mixtures deserve special attention because of their potential safety hazards. Acoustically absorbing materials are effective in damping out the transverse wave of a detonation structure. A detonation could be attenuated or prohibited after passing over this material, but the failure mechanism of the detonation still needs further exploration. In this study, tubes made of an acoustically absorbing material (hole diameters of the wall are from 30 μm to 300 μm) are inserted in the smooth rigid wall tube to investigate the effect of the porous-walled material on the detonation propagation at the near-limit conditions. Porous-walled tubes with three different scales (L/D = 3.85, 7.69, and 15.38) are studied. Photodiodes and smoked foils are employed to simultaneously measure the local velocity of the combustion waves and record the cellular detonation structures, respectively. The results show that, for shorter porous-walled tubes, the prohibition effect of the absorbing material on the detonation propagation is only prominent at the critical and sub-critical conditions, but the material has a minor effect on the detonation propagation at the super-critical condition. In addition, the prohibition effect of the porous material on the detonation propagation becomes more evident with the increased length of the acoustically absorbing material. This outcome occurs because a transverse wave plays an important role in the propagation of a self-sustained detonation, as it is partially damped during the transmission of the detonation through the porous-walled tube; thus, extending the length of the porous-walled section results in increasing the losses of the incident and reflected shock waves due to a greater expansion and mass divergence into porous material. Therefore, the velocity of the combustion wave decreases faster in the downstream of the porous material with an increased length. On the other hand, the methane-oxygen mixture has a highly irregular cellular pattern and is characterized as an unstable mixture with a high degree of instability. The strong instability leads to an enhanced ability to generate new transverse waves in the far downstream, and therefore, at the super-critical condition (a relatively higher initial pressure), the instability partly compensates for the negative effect of the acoustically absorbing material on the detonation propagation.

Redox Reactions in Chromium-Containing Fixatives for Biological Materials

Abstract—A study of redox reaction kinetics in chromium-containing fixatives (mixtures of chromic acid, bichromate, formaldehyde, and acetic acid) showed that these fixatives were unstable chemical mixtures characterized by rapid kinetics of redox processes. Therefore, biological material fixation occurs in a non-stationary solution of a variable composition, including chromic acid, bichromates (chromium(VI)), formaldehyde, acetic acid, formic acid, and chromium(III) acetate. We propose the division of chemical fixation in these fixatives into two successive stages (“two-step fixation protocol”): the tissue is fixed in an acetic acid–formaldehyde mixture (a rapidly penetrating fixing mixture) at the first stage and washed thoroughly, and treated by the mordant for the staining at the second stage. Such a protocol will at least preserve the original composition of chromium-containing fixatives.