A laminar flow perturbation theory for small gas volume fraction in high aspect ratio bubble columns is developed. The model captures the approximate hydrodynamics, mass transfer, microbicidal gas consumption as well as the hydrostatic head appropriate to vessels in the range of 10–50 m heights. The model is validated against a finite element multiphysics two fluid bubbly flow model for limiting cases of low heights. The key parameter in the mixing is found to be the ratio between stripping length scale, and the hydro-static pressure variation. If this is small, the mixing efficiency in the unaerated region increases like the bubble radius to the -3rd power, but for large values it is independent of the bubble radius. The overall mass transfer continues to increase as the bubble radius decreases. We find that decreasing the bubble radius from 1 cm scale to 1 mm results in an order of magnitude increase in our mixing metric.

Spring bead models are commonly used in the constitutive equations for polymer melts. One such model based on kinetic theory—the finitely extensible nonlinear elastic dumbbell model incorporating a Peterlin closure approximation (FENE-P)—has previously been applied to study concentration-dependent anisotropy with the inclusion of a mean-field term to account for intermolecular forces in dilute polymer solutions for background profiles of weak shear and elongation. These investigations involved the solution of the Fokker–Planck equation incorporating a constitutive equation for the second moment. In this paper, we extend this analysis to include the effects of large background shear and elongation beyond the Hookean regime. Further, the constitutive equation is solved for the probability density function which permits the computation of any macroscopic variable, allowing direct comparison of the model predictions with molecular dynamics simulations. It was found that if the concentration effects at equilibrium are taken into account, the FENE-P model gives qualitatively the correct predictions, although the over-shoot in extension in comparison to the infinitely dilute case is significantly underpredicted.

Viscoelastic fluids can be difficult to model due to the wide range of different physical behaviors that polymer melts can exhibit. One such feature is the viscous elastic boundary layer. We address the particular problem of a viscoelastic shear-dependent fluid flowing past a corner and investigate how the properties of the boundary layer change for a White-Metzner fluid. The boundary layer equations are derived and the upstream layer is matched with the far-field flow. It was found that if the fluid is sufficiently shear thinning then the viscoelastic boundary layer formulation fails due to the inertial forces becoming dominant. The depth of the boundary layer is controlled by the shear-thinning parameters. These effects are not a feature of other shear-thinning models, such as the Phan-Thien-Tanner model. This study provides insight in the different effects of some commonly used viscoelastic models in corner flows in the upstream boundary layer, the downstream boundary layer is not addressed.

In this paper a cross-slot geometry for which the height of the channel is small compared to the other channel dimensions is considered. The normal components of the viscoelastic stresses are found analytically for a second order fluid up to numerical inversion. The validity of the theoretical analysis was corroborated by comparison with numerical simulations based on a stabilized Galerkin least squares finite element method using an Oldroyd B fluid. Close agreement was found between numerical predictions and analytical results for Weissenberg numbers up to 0.2. An explicit expression is formulated for viscoelastic parameters in terms of the variation and strength of the first normal stress difference around the stagnation point. The analysis is generalized for the case where the inlet channel width is different from the outlet channel width. For such configurations it was found that uniformity of the elongation rate was reduced.

A cost effective, fast, and accurate technique was needed to measure the vapor composition of a binary system (ethanol-water) and also that of a liquid composition in a ternary system (acetic acid-acetol–water) in a microbubble distillation unit. Cheap TGS-series gas sensors were used for this purpose with both calibrations and measurements carried out in a specially designed chamber. A single parameter polynomial regression was fitted to the binary system, and a two parameter polynomial with an interaction term was fitted to the ternary system. The correlation coefficient, R-squared, was found to be greater than 0.99 for both systems, thus validating the implementation of this novel sensor.

Many fluids of scientific and industrial interest exhibit non-Newtonian behavior under the stress applied. This phenomena often arises from the ability of large polymer molecules to extend under the stress. Non-Newtonian fluids exhibit both shear and extensional properties, which are of great interest for analysis of flow dynamics. Stagnation point flows allow to obtain a broad range of shear rates in the vicinity of the point with zero velocity, providing an information for determination of constitutive parameters of a fluid. Boger fluids are the specific types of viscoelastic liquids with constant shear viscosity and high extensional viscosity. One Newtonian fluid and two Boger fluids with different concentration of polyacrylamide were used in order to investigate relaxation time of viscoelastic fluids using inverse method. Oldroyd-B model was developed to realize inverse procedure.

Taylor's classical paint scraping problem provides a framework for analyzing wall-driven corner flow induced by the movement of an oblique plane with a fixed velocity U. A study of the dynamics of the inertialess limit of a Carreau fluid in such a system is presented. New perturbation results are obtained both close to, and far from, the corner. When the distance from the corner r is much larger than U Gamma, where Gamma is the relaxation time, a loss of uniformity arises in the solution near the region, where the shear rate becomes zero due to the presence of the two walls. We derive a new boundary layer equation and find two regions of widths r(-n) and r(-2), where r is the distance from the corner and n is the power-law index, where a change in behavior occurs. The shear rate is found to be proportional to the perpendicular distance from the line of zero shear. The point of zero shear moves in the layer of size r(-2). We also find that Carreau effects in the far-field are important for corner angles less than 2.2 rad.

In chemical engineering applications, the operation of condensers and evaporators can be made more efficient by exploiting the transport properties of interfacial waves excited on the interface between a hot vapor overlying a colder liquid. Linear theory for the onset of instabilities due to heating a thin layer from above is computed for the Marangoni–Bénard problem. Symbolic computation in the long wave asymptotic limit shows three stationary, non-growing modes. Intersection of two decaying branches occurs at a crossover long wavelength; two other modes co-exist at the crossover point—propagating modes on nascent, shorter wavelength branches. The dispersion relation is then mapped numerically by Newton continuation methods. A neutral stability method is used to map the space of critical stability for a physically meaningful range of capillary, Prandtl, and Galileo numbers. The existence of a cut-off wavenumber for the long wave instability was verified. It was found that the effect of applying a no-slip lower boundary condition was to render all long waves stationary. This has the implication that any propagating modes, if they exist, must occur at finite wavelengths. The computation of 8000 different parameter sets shows that the group velocity always lies within 1 2 to 2 3 of the longwave phase velocity.

The current work investigates two phase flow visualization in a micro channel using micron resolution particle image velocimetry (μPIV). Droplets of both oil-in-water and water-in-oil emulsions were generated in a T-shaped PDMS micro fluidic device and the corresponding flow fields were measured down-stream in the divergent section of the device. The oil-in-water emulsions were formed in a hydrophilically modified PDMS micro channel using plasma polymerization of acrylic acid. To obtain the velocity fields in both types of emulsions, fluorescent particles of 0.86 m size were added into the dispersed water phase in the case of water-in-oil emulsions, and in the continuous water phase in the case of oil-in-water emulsions. The phenomena of flow behavior within the droplets and around the droplets were investigated in detail in a diverging micro channel

The feasibility of separating the azeotropic mixture of ethanol‐water using microbubble‐mediated batch distillation is presented. The effects of the depth of the liquid mixture in the bubble tank and of the inlet air microbubble temperature on the process efficiency were investigated. The enrichment of ethanol in the vapor phase was higher than that achieved at equilibrium conditions for all liquid ethanol mole fractions considered, including the azeotrope. On decreasing the depth of the liquid mixture and increasing the temperature of the air microbubbles, the separation efficiency of ethanol was improved. Ethanol with purity of about 98.2 vol % was obtained using the lowest liquid level (3 mm) in conjunction with the highest air microbubble temperature (90°C). Separation was achieved with a small rise in the temperature of the liquid mixture (4°C) at a depth of 3 mm and evaporation time of 90 min making this system suitable for treating thermally sensitive mixtures. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1192–1199, 2016

A computational model of a single gas microbubble immersed in a liquid of ethanol-water mixture is developed and solved numerically. This complements earlier binary distillation experiments in which the ethanol-water mixture is stripped by hot air microbubbles achieving around 98% vol. ethanol from the azeotropic mixture. The proposed model has been developed using Galerkin finite element methods to predict the temperature and vapor content of the gas microbubble as a function of its, residence time in the liquid phase. This model incorporates a-novel rate law that evolves on a time scale related to the internal mixing of microbubbles of 10(-3)s. The model predictions of a single bubble were shown to be in very good agreement with the existing experimental:data, demonstrating, that the ratio of ethanol:to water in the microbubble regime are higher than the expected ratios that would be consistent with equilibrium theory for all initial bubble temperatures and all liquid ethanol mole fractions considered and within the very short contact times appropriate for thin liquid layers. Our previous experiments showed a decrease in the liquid temperature with decreasing liquid depth in the bubble tank, an increase in the outlet gas temperature with decreasing liquid depth, and an improvement in the stripping efficiency of ethanol upon decreasing the depth of the liquid mixture and increasing the temperature of the air microbubbles, all of which are consistent with the predictions of the computational model.

A novel dual-plane dye laser particle image velocimetry (PIV) technique used to analyze helicity and energy dissipation in an unexcited turbulent swirling jet of pressurized cold air has established that regions within the flow field of the jet exhibiting high helicity are correlated regions of high turbulent kinetic energy dissipation. This PIV configuration provides estimates of all components of the velocity gradient tensor, facilitating calculation of the helicity from the vorticity components. Application of this novel dual-plane PIV technique is extended in this study to investigate helical structures in a turbulent swirling jet where the underlying shear flow is subjected to external acoustic sinusoidal forcing in a plane perpendicular to the central axis of the jet. It was found that acoustic excitation had a significant effect on the mean velocity profile parallel to the direction of the jet. The horizontal forcing resulted in the generation of vorticity that was skewed with a pitch that favored a distribution of angles around 90° with respect to the velocity vector. The distribution of the time-averaged helicity angle indicated organized helical activity, but such activity is not dominated by large-scale coherent structures of maximal helicity.

A surface modification method for bonded polydimethylsiloxane (PDMS) microchannels is presented herein. Polymerization of acrylic acid was performed on the surface of a microchannel using an inline atmospheric pressure dielectric barrier microplasma technique. The surface treatment changes the wettability of the microchannel from hydrophobic to hydrophilic. This is a challenging task due to the fast hydrophobic recovery of the PDMS surface after modification. This modification allows the formation of highly monodisperse oil-in-water (O/W) droplets. The generation of water-in-oil-in-water (W/O/W) double emulsions was successfully achieved by connecting in series a hydrophobic microchip with a modified hydrophilic microchip. An original channel blocking technique to pattern the surface wettability of a specific section of a microchip using a viscous liquid comprising a mixture of honey and glycerol, is also presented for generating W/O/W emulsions on a single chip.

The extent to which the carrier fluid wets the walls of a microchannel is crucial in the droplet formation process for segmented flow microfluidic applications and can be influenced by the use of surfactants. Surfactants dynamically modify the microchannel surface leading to stabilization of the two phase interface, affecting the droplet formation process. An experimental study of the influence of hydrophobic surfactant (Span 80) during the formation of water-inoil droplets in a T-shaped microchannel geometry is presented and the wetting properties of the microchannel walls were characterized. The range of data to be analyzed on the microscale is estimated from the macroscopic interfacial tension and contact angle measurements. The critical micelle concentration (CMC) level at the microscale was estimated by observing the trend of droplet length variation with concentration of surfactant in a microchannel. Microchannels used in this work were fabricated using softlithography methods and bonded using a custom-made plasma bonding setup that does not require an ultra high vacuum chamber and hence saves the fabrication cost.

A non-thermal atmospheric pressure microplasma generated from pure argon (Ar) and a mixture of argon–ethylenediamine vapors (Ar/EDA) has been characterized in this study. A sinusoidal power supply operating at 30 kHz was used to excite microplasma in a rectangular borosilicate glass capillary (4×0.4mm2). The monomer EDA was mixed with Ar in order to perform plasma polymerization inside the microchannel. The analyses were made by measuring spectroscopic and electrical parameters of the discharge. The effects of EDA mixing on plasma parameters such as electron, excitation and rotational temperatures during the process of surface coating of the microchannel were investigated. These parameters play an important role in the deposition process. The plasma temperatures estimated through spectroscopic measurement were found in the sequence Te>Texc>Tvib>Trot, which indicated the non-thermal characteristics of the proposed DBD microplasma. The parameters of the Ar discharge were also numerically computed using plasma simulations. The numerical predictions of electron temperature (2D simulations) and electron density (3D simulations) were found to be in close agreement to those estimated through experiments.

We report a technique of microplasma copolymerization for depositing a stable amino functional film on the inner walls of a glass microcapillary. The monomers ethylenediamine (EDA), 3 aminopropyltriethoxysilane (APTS) and hexamethyldisiloxane (HMDSO) were used as organic precursors for copolymerization. The ratio of the monomer mixture in the plasma was precisely fed using an original technique of liquid injection and helical mixing. The coatings obtained from EDA–HMDSO were found to be more stable than those produced using EDA and APTS. The films were characterized using X-ray photoelectron, Fourier transform infrared and UV–vis spectroscopy, atomic force microscopy and growth rate measurements. The hydrophobic and hydrophilic characteristics of the films were analyzed through water contact angle and surface free energy measurements. The stability of the coatings formed using this method in aqueous media indicated that they could be utilized for biological and microfluidic applications.

This paper presents an overview of the developments that have been made towards the design of an inline rheometer that has the capabilities for monitoring in real time the viscous constitutive parameters of non-Newtonian fluids in a pipe flow. This has potential applications for a wide range of fluids, including hydrocolloid solutions and polymer solutions. This is of relevance to many industries, for example the pharmaceutical, lubrication, food and printing industries. The use of mathematical algorithms for inferring rheological parameters from properties of flow field statistics is explored. Particular focus is given to the development of a flow cell rheometer containing a T-junction geometry with the capacity to induce a range of shear rates in the vicinity of the bend, and a distribution of elongational viscosities along the back-wall. Such features create an information-rich flow field that is beneficial for the development of a rheometer with a fast response time that is suitable for commercial purposes.

The surface coating of sealed microchannels is useful for a variety of applications in microfluidics and lab-on-a-chip devices. This paper demonstrates a novel coating technique for a bonded polydimethylsiloxane (PDMS) microchannel using atmospheric pressure microplasma. Plasma was generated by using two types of electrode arrangements: (i) needle-aluminum foil with dielectric barrier (DB) and (ii) needle-needle configuration without DB. The microplasma configuration with the DB was selected for deposition in this study. The monomer ethylenediamine was used as organic precursor for plasma polymerization. Oil-in-water microemulsions were formed in a T-shaped microchannel in order to test the stability and durability of the plasma polymerized films. The coatings were characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurements.

Atmospheric pressure plasma deposition is a beneficial technology due to its low cost and flexibility in terms of its operation and integration for in-line processing. This paper presents the use of an atmospheric pressure dielectric barrier discharge (DBD) to deposit an amine functional polymer film onto the inner surface of a glass microcapillary. A micro discharge was generated in a DBD chip made from a rectangular borosilicate glass capillary using externally attached parallel plate electrodes. A new microplasma configuration which consists of a perforated high voltage electrode and a ground electrode with large surface area is implemented to sustain a stable glow discharge at atmospheric pressure and a temperature of 35°C. Polymerization was performed using a laboratory made plasma source working at a frequency of 8kHz with 50% duty ratio of the inverter. The monomer precursors allylamine and ethylenediamine were selected to optimize the polymerization conditions at atmospheric pressure. The hydrophobicity and philicity of the deposited surface were controlled as functions of plasma power. The atmospheric pressure plasma polymerized (APPP) films were characterized using Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), plasma emission spectroscopy, contact angle measurements and growth rate analysis. An average growth rate of 1.18μgs−1 for plasma polymerized ethylenediamine (PPEDA) and 1.91μgs−1 for plasma polymerized allylamine (PPAA) was obtained at a discharge power of 15W and 10sccm of monomer flow rate. The film thickness of 0.9μm for PPEDA and 2.1μm for PPAA was determined using AFM for deposition time of 10min. Polymerization results showed that the properties of APPP films can be controlled through optimization of parameters such as discharge power, treatment time and flow rates of the main gas and monomer vapors.

The formation of droplets at a T-junction in a microchannel network is primarily influenced by the pressure difference across the interface in the squeezing regime. Accurate measurements of droplet velocity and pressure profiles are difficult to obtain experimentally, yet these are the basic parameters required for understanding the physics governing the droplet formation process and for shaping the optimum design of microfluidic devices. The current work presents predictions from two dimensional numerical simulations of microdroplet generation at a T-junction. The simulation results are validated with the experimental observations. Detailed profiles of the predicted pressure evolution across the channel upstream of the T-junction indicate that the pressure variation is sensitive to small changes in the wetting properties of the continuous phase. (C) 2013 Elsevier Ltd. All rights reserved.