Green's function method for aerosol penetration in channels under laminar flow conditions

A generalized method based on Green's function technique is developed to calculate the penetration fraction of particles by diffusion, formed from a given source in a gas under laminar flow, in cylindrical and rectangular channels. Both the initial value and the formation-in-flight types of problems are brought together in one formal procedure. The task of having to solve the convective-diffusion equation, for a given distribution of source density in the case of formation-in-flight problems, is altogether eliminated. The effect of a first-order reaction, like radioactive decay, is taken into account. Linear approximation for the shift in the eigenvalues due to the reaction term is given for small values of the dimensionless reaction rate constant. Finally, the solution available for an exponentially decreasing source in a cylindrical channel is found to be incorrect and the correct expression is derived using this method.     

Continuous separation of lipid particles from erythrocytes by means of laminar flow and acoustic standing wave forces

Improved continuous acoustic particle separation (separation efficiency close to 100%) and separation of erythrocytes (red blood cells) from lipid microemboli in whole blood is reported.     

Mixing process of ternary solvents prepared through microchannels in a microchip under laminar flow conditions

The mixing process of ternary solvents (water-hydrophilic/hydrophobic organic mixture) prepared in microchannels in a microchip was examined by fluorescence observation of the dyes dissolved in the solvents under laminar flow conditions. A microchip incorporating microchannels was used. In it, three narrow channels were combined to form one wide channel. Water-acetonitrile (hydrophilic) mixture containing relatively hydrophilic Eosin Y (green) was fed into the narrow center channel and an acetonitrile-ethyl acetate (hydrophobic) mixture containing hydrophobic perylene (blue) was fed into the two narrow side channels in the microchip. The mixtures in the narrow channels combined in the wide channel to prepare the ternary solvents of water-acetonitrile-ethyl acetate, causing the tube radial distribution of the solvents. We observed the mixing process of the ternary solvents in the wide channel through fluorescence of the green and blue dyes, including an aqueous-organic interface. For example, the green dye that was fed into the center channel was distributed near the inner side walls and the blue dye that was fed into the two side channels was distributed around the center area in the wide channel. Such specific mixing behavior was not observed for two-component solvents in the wide channel, such as water-acetonitrile mixture and water-ethyl acetate mixture.     

Deposition of spherical particles onto cylindrical solid surfaces. I. Numerical simulations

The permeability of fractal porous aggregates with realistic three-dimensional structure is investigated theoretically using model aggregates composed of identical spherical primary particles. Synthetic aggregates are generated by several techniques, including a lattice-based method, simulation of aggregation by differential settling and turbulent shear, and the specification of simple cubic structures, resulting in aggregates characterized by the number of primary particles, solid fraction, characteristic radius, and fractal dimension. Stokesian dynamics is used to determine the total hydrodynamic force on and the distribution of velocity within an aggregate exposed to a uniform flow. The aggregate permeability is calculated by comparing these values with the total force and velocity distribution calculated from the Brinkman equation applied locally and to the entire aggregate using permeability expressions from the literature. The relationship between the aggregate permeability and solid fraction is found to be best predicted by permeability expressions based on cylindrical rather than spherical geometrical elements, the latter tending to underestimate the aggregate permeability significantly. The permeability expressions of Jackson and James or Davies provide good estimates of the force on and flow through porous aggregates of known structure. These relationships are used to identify a number of general characteristics of fractal aggregates.     

Deposition of spherical particles onto cylindrical solid surfaces. II. Experimental studies

This paper presents experimental studies of the deposition of silicone oil drops onto two different solid surfaces in an aqueous solution. A series of deposition tests were conducted to measure the dimensionless mass transfer rate (Sherwood number). The effects of three kinds of aqueous solutions and two solid surfaces on the deposition process were studied and compared with the numerical predictions based on the well-known DLVO theory. More specifically, both the experimentally measured and the numerically predicted Sherwood numbers monotonically decrease as the pH value of the aqueous solution increases. It was also found that two ionic surfactant solutions have similar influences while the electrolyte solutions have opposite effects on the deposition rate on different solid surfaces. Finally, comparison of all the experimental results for the bare glass surface with the numerical simulations shows that the deposition process of the silicone oil drops onto the hydrophilic solid surface can be satisfactorily described by the classical DLVO theory. However, the deposition data for the FC725 precoated surface are significantly larger than the numerical predictions. This fact suggests that the so-called non-DLVO attractive interaction is involved in the deposition process with the hydrophobic solid surface. This additional non-DLVO attractive interaction, which is generally called the hydrophobic interaction, still remains to be incorporated into the existing DLVO theory, if this is possible.     

Delamination of Na-montmorillonite particles in aqueous solutions and isopropanol under shear forces

Delamination of montmorillonite (Na-MMT) is an outstanding property of the dispersed MMT particles. Na-MMT particles delaminated in water and isopropanol under shear forces have been studied in this work. The difference in the intercalation and delamination of Na-MMT by water and isopropanol was studied by molecule dynamic simulation and experiment. Molecule dynamic simulation was carried out on Material Studio (MS) 8.0. The experimental study was performed on a Na-MMT through the measurements of Stokes size, optical size, scanning electron microscope, atomic force microscope, and dynamic molecule simulation. The results demonstrated that under the effect of interlayer hydration, the Na⁺ that resides near the siloxane surface was moved to the middle plane of interlayer space, and the interlayer spacing was opened 1.38A. Compared with the interlayer hydration, the interlayer spacing was increased only a little (0.32A) treated by isopropanol; meanwhile, the interlayer sheets were joined together by isopropanol molecule. Because of that the effect of water and isopropanol in the interlayer of Na-MMT was totally different, the Na-MMT particles were indeed delaminated into plate-like super fine particles in water instead of in isopropanol, and delamination was closely correlated with shear force only if hydration was occurred in the interlayer.     

Electroosmotic flow through porous media: cylindrical and annular models

Electroosmotic flow rates through porous media are predicted through the cylindrical and annular geometrical models. Differences in the results obtained from the two models are compared and discussed by making reference to the electrokinetic remediation of contaminated soil. In the annular-model case, when the electrical potential on the outer wall is less than on the inner wall and/or the double-layer thickness is much shorter than the outer radius, a maximum flux is predicted at some outer-to-inner-radii ratio. At a given soil porosity, the cylindrical model predicts a higher flow rate than the annular model, and the difference becomes increasingly more pronounced as the radii ratio approaches unity, which corresponds to a low-porosity soil.     

Hydrodynamic forces and critical stresses in low-density aggregates under shear flow

The distribution of stresses in rigid colloidal aggregates under a shear flow was investigated numerically for particle-cluster and cluster-cluster aggregates with fractal dimensions ranging from 1.7 to 2.3. stokesian dynamics was used to calculate the hydrodynamic force on each monomer, while the internal intermonomer interactions were calculated by applying force and torque balances on each primary particle. Although the hydrodynamic forces act mainly on the periphery of the clusters, their filamentous structure propagates and accumulates internal stresses toward the inner region of the aggregates, where consequently the most loaded intermonomer bonds are located. The spatial stress distribution, when scaled by the proper power of the radius of gyration, is independent of aggregate size and fractal dimension. This feature has made it possible to identify the most probable locations of bond failure in the structure and to estimate the relationship between shear rate and particle size for the occurrence of restructuring and of breakage.     

Chemiluminescence from singlet oxygen under laminar flow condition in a micro-channel

Singlet oxygen was generated by reaction of sodium hypochlorite and hydrogen peroxide in a micro-channel. The two reagent solutions were delivered into the micro-channel by syringe pumps, providing a laminar flow. Such a laminar flow forms a liquid–liquid interface instantly in a micro-channel, and then the interface collapses gradually through molecular diffusion with the residence times. The chemiluminescence from the singlet oxygen was emitted in the course of the collapse of the interface under laminar flow condition. The chemiluminescence intensity was observed continuously and stably in the micro-channel as long as the reagents were fed into the channel. We examined the features of the chemiluminescence emitted in the micro-channel by changing the flow rates of reagents and the detection points in the micro-channel. The data obtained were considered along with the residence times and diffusion lengths. We also examined the effects of antioxidants, such as sodium azide, histidine, nitroblue tetrazolium, and 2-propanol on the chemiluminescence intensity.     

Effect of liquid slip in electrokinetic parallel-plate microchannel flow

Liquid slip at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in parallel-plate microchannels by combining the electrokinetic transport phenomena with Navier's slip condition. Our parametric results suggest that electrokinetic transport phenomena and liquid slip at channel walls are both important and should be considered simultaneously. Their significance depends on channel wall material, electrolyte concentration, and pH. For pressure-driven-flow, liquid slip counteracts the effect by the electrical double layer and induces a larger flow rate. A higher apparent viscosity would be predicted if slip is neglected. For electroosmotic flow, liquid slip alters the flow rate by about 20% for a thick electrical double layer. Our results provide design guidelines to precisely control time-dependent microflow in hydrophobic microfluidic microelectromechanical system devices.     

Luminescence and electrification in a flow of dielectric liquids through narrow channels

Blue-violet luminescence was observed in a mineral oil, which appeared under hydrodynamic cavitation conditions in a channel orifice 1 mm in diameter in a transparent throttling device at inlet pressures higher than 2 MPa. The appearance of electric pulses when a dielectric liquid flew through a thin channel orifice was observed much earlier than luminescence arose. A device for continuously scanning electric potential along a flow without disturbing it was developed. According to the oscillograms obtained, the electric signal was high-frequency, could not be synchronized, and its separate peaks reached 1000 mV. Light emission flux decreased as the temperature of the liquid increased to 30–35°C and inlet pressure grew. The appearance of luminescence and its intensity depended on the sharpness of the entrance edge of the throttle. Studies of hydrodynamic luminescence revealed hysteresis of light emission. A mechanism of localized light emission based on an important role played by electrokinetic phenomena was suggested.     

Multiple flow profiles for two-phase flow in single microfluidic channels through site-selective channel coating

An approach to control two-phase flow systems in a poly(dimethylsiloxane) (PDMS) microfluidic device using spatially selective surface modification is demonstrated. Side-by-side flows of ethanol?:?water solutions containing different polymers are used to selectively modify both sides of a channel by laminar flow patterning. Introduction of air pockets during modification allows for control over the length of the channel section that is modified. This approach makes it possible to achieve slug flow and side-by-side flow of water : 1-octanol simultaneously within the same PDMS channel, without the need of additional structural elements. A key finding is that conditioning of the PDMS channels with 1-octanol before polymer deposition is crucial to achieving stable side-by-side flows.     

Pharmaceutical modulation of diffusion potentials at aqueous-aqueous boundaries under laminar flow conditions

In this work, the modulation of the diffusion potential formed at the microfluidic aqueous-aqueous boundary by a pharmaceutical substance is presented. Co-flowing aqueous streams in a microchannel were used to form the stable boundary between the streams. Measurement of the open circuit potential between two silver/silver chloride electrodes enabled the diffusion potential at the boundary to be determined, which is concentration dependent. Experimental results for protonated propranolol as well as tetrapropylammonium are presented. This concept may be useful as a strategy for the detection of drug substances.     

Diffusiophoresis and electrophoresis of elliptic cylindrical particles

An exact analysis is presented for the diffusiophoresis and electrophoresis of a rigid elliptic cylindrical particle in a uniform applied field oriented arbitrarily with respect to its axis. The range of the interaction between the solute species and the particle surface is assumed to be small relative to the minimum dimension of the particle, but the effect of polarization of the diffuse species in the thin particle-solute interaction layer is incorporated. To solve the conservative equations governing the system, a slip velocity of fluid and normal fluxes of solute species at the outer edge of the thin diffuse layer which balance convection and diffusion of the solute species along the particle surface are used as the boundary conditions for the fluid domain outside the diffuse layer. Expressions for the migration velocity of the particle are obtained in closed forms for the cases of diffusiophoresis in a nonionic solute concentration gradient, diffusiophoresis in a concentration gradient of symmetric electrolyte, and electrophoresis in an external electric field. An interesting feature is found that the diffusiophoretic or electrophoretic velocity of the particle decreases with the reduction of the maximum length of the particle in the direction of migration. Also, the average migration velocity for an ensemble of identical, noninteracting elliptic cylinders with random orientation distribution is obtained for each case considered.     

Analysis of thermo-hydraulic performance and entropy generation characteristics for laminar flow through triangular corrugated channel

Journal of Thermal Analysis and Calorimetry - We analyse the thermo-hydraulic performance and entropy-generation characteristics for laminar flow through triangular corrugated channel. Results are...