Aging of the Nonlinear Optical Susceptibility of colloidal solutions

Using Z-scan and dynamic light scattering measurements we investigate the nonlinear optics response of a colloidal solution undergoing dynamics slowing down with age. We study the high optical nonlinearity of an organic dye (Rhodamine B) dispersed in a water-clay (Laponite) solution, at different clay concentrations (2.0 wt% - 2.6 wt%), experiencing the gelation process. We determine the clay platelets self diffusion coefficient and, by its comparison with the structural relaxation time, we conclude that the gelation process proceeds through the structuring of interconnecting clay platelets network rather than through clusters growth and aggregation.Comment: 4 figures, 4 page

Effect of viscosity on homogeneous–heterogeneous flow regime transition in bubble columns

Experiments were performed in a cylindrical 0.14m diameter bubble column with a metal perforated plate. Air and aqueous solutions of glycerol with viscosity 1–22mPa s were the phases. Gas holdup was measured and plotted against the gas flow rate. The critical point where the homogeneous–heterogeneous regime transition begins was determined by the drift-flux plot of the primary data. The homogeneous regime stability was expressed by the critical values of the gas holdup and gas flow rate. The results show that moderate viscosity (3–22 mPa s) destabilizes the homogeneous regime and advance the transition. The results indicate that low viscosity (1–3 mPa s) could stabilize the homogeneous regime. The destabilizing effect of the column height proved previously for air–water system applies also to viscous batches.Comissão Europeia (CE) - Marie Curie Training Site Fellowship Contract Number HPMT-CT-2000-00074.Grantová agentura České republiky (GA CR) - Grant No. 104/01/0547

Meniscus dynamics in bubble formation : a parametric study

This study brings new experimental data on the bubble formation process with a particular focus on the behaviour of the gas–liquid interface (meniscus) inside a transparent orifice on a perforated plate. The meniscus plays an important role by coupling the gas and liquid sides. The goal was to find the effect of five important control parameters on the bubble production and the meniscus behaviour (orifice diameter, plate thickness, gas chamber size, liquid height, and liquid viscosity). The gas input was also varied. Three physical signals were taken in the measurements to extract the information needed (gas pressure in plenum, acoustic pressure in liquid, and video record of meniscus motion). Several relevant quantities were measured to characterize the bubbling process (bubbling period, active orifice time, and periods of meniscus oscillations). The data obtained show how these quantities depend on the control parameters. The recognition of the role of the meniscus dynamics in the gas dispersing process with perforated plates offers a deeper insight into this important and complicated problem

Accretion of Warm Chondrules in Weakly Metamorphosed Ordinary Chondrites and Their Subsequent Reprocessing

To better understand chondrite accretion and subsequent processes, the textures, crystallography, deformation, and compositions of some chondrite constituents in ten lithologies of different cluster texture strength were studied in seven weakly metamorphosed (Type 3) and variably shocked ordinary chondrites (Ragland—LL3 S1, Tieschitz—H/L3 S1, NWA 5421—LL3 S2, NWA 5205—LL3 S2, NWA 11905—LL3-5 S3, NWA 5781—LL3 S3, NWA 11351—LL3-6 S4) using optical and electron microscopy and microtomography techniques. Results support a four-stage model for chondrite formation. This includes 1) limited annealing following collisions during chondrule crystallization and rapid cooling in space prior to accretion, as evidenced by olivine microstructures consistent with dislocation recovery and diffusion; 2) initial accretion of still-warm chondrules into aggregates at an effective chondrite accretion temperature of ∼900-950 °C with nearly in situ impingement deformation between adjacent chondrules in strongly clustered lithologies (NWA 5781, Tieschitz, NWA 5421, NWA 5205 Lithology A), as evidenced by intragranular lattice distortions in olivine consistent with high-temperature slip systems, and by evidence that some olivine-rich objects in Tieschitz accreted while partly molten; 3) syn- or post-accretion bleaching of chondrule mesostases, which transferred feldspathic chondrule mesostasis to an interchondrule glass deposit found in strongly clustered lithologies, as evidenced by chemical data and textures; and 4) post-bleaching weak or strong shocks that resulted in destruction of interchondrule glass and some combination of brecciation, foliation of metal and sulfide, and melting and shock-overprinting effects, as evidenced by poor cluster textures and presence of clastic texture, alignment of metal and sulfide grains caused by shock compression, presence of impact-generated glass, and changes in olivine slip systems. The data support the model of Metzler (2012), who suggested that chondrules in ordinary chondrites accreted while still warm to form cluster chondrite textures as a “primary accretionary rock” (our Stage 2), and that subsequent brecciation destroyed this texture to create chondrites with weak cluster texture (our Stage 4)

CFD simulation and experimental measurement of gas holdup and liquid interstitial velocity in internal loop airlift reactor

This paper documents experiments and CFD simulations of the hydrodynamics of our two-phase (water, air) laboratory internal loop airlift reactor (40 l). The experiments and simulations were aimed at obtaining global flow characteristics (gas holdup and liquid interstitial velocity in the riser and in the downcomer) in our particular airlift configurations. The experiments and simulations were done for three different riser tubes with variable length and diameter. Gas (air) superficial velocities in riser were in range from 1 to 7.5 cm/s. Up to three circulation regimes were experimentally observed (no bubbles in downcomer, bubbles in downcomer but not circulating, and finally the circulating regime). The primary goal was to test our CFD simulation setup using only standard closures for interphase forces and turbulence, and assuming constant bubble size is able to capture global characteristics of the flow for our experimental airlift configurations for the three circulation regimes, and if the simulation setup could be later used for obtaining the global characteristic for modified geometries of our original airlift design or for different fluids. The CFD simulations were done in commercial code Fluent 6.3 using algebraic slip mixture multiphase model. The secondary goal was to test the sensitivity of the simulation results to different closures for the drag coefficient and the resulting bubble slip velocity and also for the turbulence. In addition to the simulations done in Fluent, simulation results using different code (CFX 12.1) and different model (full Euler–Euler) are also presented in this paper. The experimental measurements of liquid interstitial velocity in the riser and in the downcomer were done by evaluating the response to the injection of a sulphuric acid solution measured with pH probes. The gas holdup in the riser and downcomer was measured with the U-tube manometer. The results showed that the simulation setup works quite well when there are no bubbles present in the downcomer, and that the sensitivity to the drag closure is rather low in this case. The agreement was getting worse with the increase of gas holdup in the downcomer. The use of different multiphase model in the different code (CFX) gave almost the same results as the Fluent simulations

Effect of solid particles on flow regimes in bubble columns

Grantová agentura České republiky (GA ČR) - Grant No. 104/04/0827.Economic Community (EC) - BEMUSAC Project No. G1MA-CT-2002-04019.Institute of Chemical Process Fundamentals - Marie Curie Training Site Fellowship - Contract Number HPMT-CT-2000-00074

Effect of solids on flow regime transition in three-phase bubble columns

In bubble column reactors there are two principal flow regimes [1,2,3], the homogeneous (HoR) and the heterogeneous (HeR). These reactors have different behaviour in HoR and HeR, thus the dependences of the rates of mass, heat and momentum transfer on the design and operating parameters (such as reactor geometry, gas and liquid flow rates and properties of the contacting phases) are also very different. Therefore, for rational reactor design and operation it is of crucial importance to know the range of parameters over which a certain regime prevails and the regime transition conditions [4]. The effect of the presence of solids on gas holdup in bubble columns has been extensively investigated. Most of the published work reports that the gas holdup decreases with increasing solid concentration [5-14]. However, a favourable effect of solids on gas holdup has been also observed by Douek et al. [15]. In spite of all these efforts, information on the effect of solids on homogeneous-heterogeneous regime transition is very scarce. Krishna et al. [16] found that the transition gas holdup was significantly reduced due to the presence of silica particles. Xie et al. [17] reported that as pulp consistency is increased, the regime transition is delayed. The goal of this work was to examine the influence of solid particles on homogenous regime stability and regime transition in a three phase bubble column. For that, two studies were done, one focused on the regime transition and the other on visualization of bubbleparticle interaction. The regime transition experiments were performed in a cylindrical bubble column of 0.14 m diameter. Air, distilled water and calcium alginate beads (deq=2.1mm) at concentrations up to 30%(v/v) were the phases. The dependence of the gas holdup (e) on the gas flow rate (q) was measured. At low solid loading, the experimental data e(q) show a slight increase of gas holdup with solid concentration, which is not normally observed. At higher solid loading, a significant reduction of gas holdup with solid concentration is observed. This suggests that solid concentration could play a dual role in the homogeneous regime stability: low concentrations stabilize, while high concentrations destabilize. The critical point, where the HoR loses stability and the transition begins, was evaluated using the drift flux plot. The critical values of gas flow rate and voidage were the measures of the homogeneous regime stability. For low solid volume fraction, a stabilizing effect of the presence of solids is shown by an increase in the critical values, while for higher solids volume fraction ( φ s>3%) the critical values decrease, witnessing a destabilizing effect of the solids. The visualization study was performed in a 0.07 m diameter cylindrical bubble column, using a standard and high-speed cameras. This work shows important phenomena that result from the bubble-particle interactions, which partly explains the regime transition results. At low solid loading, bubble-particle collisions lead to reduction of bubble rise velocity that results in higher gas holdup and a delay of the transition. At higher solid loading, the bubble coalescence rate increases with the solid content that decreases the gas holdup and advances the transition.Commission of the European Communities for the Marie Curie Training Site - contract No. HPMT-CT-2000-0074

Effect of solids on homogeneous–heterogeneous flowregime transition in bubble columns

Experiments were conducted to study the effect of the presence of the solid phase on the homogeneous–heterogeneous flow regime transition in a bubble column 0.14m diameter. Air, distilled water and calcium alginate beads (2.1 mm, 1023 kg/m3) at concentrations c=0–30% (vol.) were the phases. The basic data were the voidage–gas flow rate dependences. The critical point, where the homogeneous regime loses stability and the transition begins, was evaluated by the drift flux model. The critical values of voidage and gas flow rate were the quantitative measures of the homogeneous regime stability. These were plotted against the solid phase concentration. It was found, that both the voidage and the critical values increased with the solid content at low solid loading, approx. c=0–3%, and decreased at higher loading, c>3%. The homogeneous regime was thus first stabilized and then destabilized. To explain this dual effect, possible physicalmechanisms of the solid phase influence on the uniform bubble bed were discussed

Effect of surfactants on homogeneous regime stability in bubble columns

Grant Agency of the Czech Republic (GA CR) - Grant No. 104/01/0547.European Commission - Maria Curie Training Sites Project (MCTS)- Contract No. HPMT-CT-2000-00074