Fast Fluid Flow Research Articles

Fluid mixing from below in unconformity-related hydrothermal ore deposits

Unconformity-related hydrothermal ore deposits typically form by mixing of hot, deep, rock-buffered basement brines and cooler fluids derived from the surface or overlying sediments. Current models invoking simultaneous downward and upward flow of the mixing fluids are inconsistent with fluid overpressure indicated by fracturing and brecciation, fast fluid flow suggested by thermal disequilibrium, and small-scale fluid composition variations indicated by fluid inclusion analyses. We propose a new model where fluids first descend, then evolve while residing in pores and later ascend. We use the hydrothermal ore deposits of the Schwarzwald district in southwest Germany as an example. Oldest fluids reach the greatest depths, where long residence times and elevated temperatures allow them to equilibrate with their host rock, to reach high salinity, and to scavenge metals. Youngest fluids can only penetrate to shallower depths and can (partially) retain their original signatures. When fluids are released from different levels of the crustal column, these fluids mix during rapid ascent in hydrofractures to form hydrothermal ore deposits. Mixing from below during ascent provides a viable hydromechanical mechanism to explain the common phenomenon of mixed shallow and deep fluids in the formation of hydrothermal ore deposits.

Fouling Characteristics and Electrochemical Recovery of Carbon Nanotube Membranes

AbstractThe fouling behavior of carbon nanotube (CNT) membranes is investigated for large protein biomolecules and a wide variety of small molecules. The CNT membranes are largely fouling resistant, even to untreated river water, due to size exclusion and an inert graphitic core that supports fast fluid flow. However, it is found that bovine serum albumin (BSA) and naphthalene significantly foul membranes due to solution coagulation and π–π stacking, respectively. Small single‐walled (SW) CNTs (<1.5 nm i.d.) are difficult to foul with BSA when precipitation is prevented, showing that size exclusion at SWCNT tips can prevent fouling. Electrochemical oxidation, bubble generation and ionic pumping are shown to recover membrane performance. Electrochemical oxidation at greater than +1.4 V is seen to oxidize CNTs as well as biofoulants, but H2 bubble generation at –2 V lifts foulants without damage to the membrane allowing for repeated cycles. Ionic pumping using large cations is seen to remove small molecule foulants adsorbed to the CNT core. The relatively narrow class of foulants and three complementary methods of membrane defouling make the CNT membrane platform a potentially robust system for a wide variety of chemical separations and environmental water treatments.

Microfluidic sorting with a moving array of optical traps

Optical sorting was demonstrated by selective trapping of a set of microspheres (having specific size or composition) from a flowing mixture and guiding these in the desired direction by a moving array of optical traps. The approach exploits the fact that whereas the fluid drag force varies linearly with particle size, the optical gradient force has a more complex dependence on the particle size and also on its optical properties. Therefore, the ratio of these two forces is unique for different types of flowing particles. Selective trapping of a particular type of particles can thus be achieved by ensuring that the ratio between fluid drag and optical gradient force on these is below unity whereas for others it exceeds unity. Thereafter, the trapped particles can be sorted using a motion of the trapping sites towards the output. Because in this method the trapping force seen by the selected fraction of particles can be suitably higher than the fluid drag force, the particles can be captured and sorted from a fast fluid flow (about 150 μm/s). Therefore, even when using a dilute particle suspension, where the colloidal trafficking issues are naturally minimized, due to high flow rate a good throughput (about 30 particles/s) can be obtained. Experiments were performed to demonstrate sorting between silica spheres of different sizes (2, 3, and 5 μm) and between 3 μm size silica and polystyrene spheres.

Two-phase AC electrothermal fluidic pumping in a coplanar asymmetric electrode array

The ability to achieve fast fluid flow yet maintain a relatively low temperature rise is important for AC electrothermal (ACET) micropumping, especially in applications such as bioMEMS and lab-on-a-chip systems. In this paper, we propose a two-phase ACET fluidic micropump using a coplanar asymmetric electrode array. The proposed structure applies a two-phase AC voltage, i.e., voltage of phase 0°/180°, to the narrow electrodes while the wide electrodes are at ground potential. Numerical simulation demonstrates that this simple coplanar electrode configuration can achieve at least 25% faster fluid flow rates than using a single AC signal. By selecting certain design parameters, a two-phase ACET structure can achieve up to 50% faster fluid flow rates than a corresponding single-phase structure. The simple two-phase AC signal sources are easily produced by using inverter buffers, which is a considerable improvement compared to the multi-phase AC signals required by other electrokinetic micropumping methods, such as traveling wave structures.

Maximum Velocity for a Single Water Molecule Entering a Carbon Nanotube

Carbon nanotubes, despite their hydrophobic nature, rapidly fill with water and allow super fast fluid flow through their interior due to the almost frictionless nanotube surface. The question arises as to whether it is possible to maximize the uptake (suction energy) of water and thus generate the highest possible fluid flow. In this paper, we outline the concepts of an acceptance condition and the suction energy and subsequently examine the suction characteristics of a single water molecule entering a carbon nanotube. In particular, we find that for the hydrogen atoms oriented towards the tube end, the radius of the carbon nanotube must be at least 3.464 A (or 0.3464 nm) for acceptance of a water molecule, and that a radius of 3.95 A provides the maximum uptake or suction energy.

Enhanced ac electrothermal fluidic pumping in microgrooved channels

It is important to generate fast fluid flow yet maintain low temperature rise for ac electrothermal (ac ET) pumping in microsystems with conductive fluids. This has been generally the limitation of ac ET driven micropump applications. We present an enhanced ac ET pumping mechanism using low voltage ac signals that can result in a small amount of temperature rise. Different from the published traveling wave and asymmetric electrode structures positioned on insulated flat surfaces, channels with a microgrooved surface are utilized in this study. The effects of the microgroove existence on the modification of the ET body force and recession of the vortex backflows are demonstrated. Forward and backward pumping modes are identified and analyzed. This mechanism utilizes a thin film of asymmetric electrode structure on the microgrooved channel floor that can be fabricated with common planar lithography technologies. This study demonstrates that using the microgrooved structure can increase pumping capacity by five to sixfold as compared to a planar electrode arrangement with the same effective dimensions.

Leuko-polymersomes.

Polymersomes are vesicles whose membranes are comprised of self-assembled amphiphilic block co-polymers. Synthetic control of block co-polymer chemistry provides an advantageous diversity of polymersome functions, ranging from tunable materials strength, superior encaspulation of hydrophobic and hydrophilic drugs and optical dyes, and facile functionalization. We have exploited polymersome tunability to make leuko-polymersomes: polymersomes with the adhesive properties of leukocytes. By functionalizing the terminal groups on the outer shell of the vesicle with biotin, we have used modular avidin-biotin chemistry to attach adhesion ligands that mimic the two critical adhesion pathways that leukocytes utilize to achieve adhesion in the fast fluid flow of blood vessels--selectins and integrins. We demonstrate that adhesion is specific and is supported at hydrodynamic flow rates at which leukocytes adhere. We envision the use of such particles for monitoring or treating inflammation, cancer and cardiovascular disease.

Authigenic seep‐carbonates cementing coarse‐grained deposits in a fan‐delta depositional system (middle Miocene, Marnoso‐arenacea Formation, central Italy)

AbstractSeep‐carbonates (13C‐depleted) are present at different levels within the Miocene terrigenous succession of Deruta (Marnoso‐arenacea Formation, central Italy); they are associated with pebbly sandstones and conglomerates in a tectonically active fan‐delta slope depositional system. Most of these seep‐carbonates are included in slide/slump horizons as scattered blocks. The occurrence of seep‐carbonates is clear evidence of the flow of methane‐rich fluids pervading the sediments. Fluids, probably of biogenic origin, may have reached the sea‐bottom through thrust faults and selectively infiltrated the more permeable coarse‐grained horizons deposited along the slope. Different stages of fluid emissions are documented: slow flux stage, corresponding to the development of large carbonate bodies and dense chemosynthetic communities; and fast fluid flow associated with intense carbonate brecciation, pipes and veins. Large amounts of authigenic carbonates are reworked by slope failures triggered by tectonics and fluids reducing sediment strength; in situ cementation of slide blocks may also have occurred due to remobilization of methane‐rich fluids by mass‐wasting processes.

Residence time distribution measurements and flow modeling in a supercritical water oxidation reactor: Application of transfer function concept

Residence time distribution (RTD) measurements are carried out in a flow-through tubular supercritical water oxidation (SCWO) reactor using ex situ pulse response experiments. The experiments are performed from ambient to supercritical water conditions with a volumetric flow rate fixed at 1 ml/min. The transfer function concept is used to interpret tracer information and to elucidate hydrodynamic flow patterns inside the “hot zone” of the reactor vessel. The experimental RTD curves are modeled by an axially dispersed plug flow. At supercritical conditions, the studied tubular SCWO reactor can be characterized as a mixed flow system (with Péclet numbers of 1.4–2). The results also indicate the presence of fast preferential fluid flow in the reactor below 573 K and 10 MPa.

Voltage Gated Carbon Nanotube Membranes

Membranes composed of an array of aligned carbon nanotubes, functionalized with charged molecular tethers, show voltage gated control of ionic transport through the cores of carbon nanotubes. The functional density of tethered charge molecules is substantially increased by the use of electrochemical grafting of diazonium salts. Functionality can be forced to occur at the CNT tip entrances by fast fluid flow of an inert solvent through the core during electrochemical functionalization. The selectivity between Ru(bi-pyridine)(3)2+ and methyl viologen2+ flux is found to be as high as 23 with -130 mV bias applied to the membrane as the working electrode. Changes in the flux and selectivity support a model where charged tethered molecules at the tips are drawn into the CNT core at positive bias. For molecules grafted along the CNT core, negative bias extends the tethered molecules into the core. Electrostatically actuated tethers induce steric hindrance in the CNT core to mimic voltage gated ion channels in a robust large area platform.

Investigation of a measurement technique to estimate concentration and size of inclusions in droplets

Inhomogeneous droplets including small spherical inclusions are characterized by the estimation of droplet diameter, inclusion concentration and inclusion diameter and a measure for the polydispersity of the inclusions. In most cases, it is reasonable to assume that the material parameters are known and therefore the index of refraction and the shape of inclusions. In this paper, the results from a measurement technique are investigated. The method will be evaluated on the basis of light scattering measurements for a range of scattering angles. These measurements have been taken with a fast CCD line scan camera and appropriate optics. An attempt is made to derive information from these measurements only. The continuous wavelet transform, speckle image analysis and turbidity measurement methods are used to estimate the concentration and the diameter of the monodisperse polystyrene particles within a droplet. The droplets, generated by a drop-on-demand droplet generator, are nearly monodisperse. The volume concentration of the inclusions within the suspensions varies between 0.01% and 9%. The inclusions are monodisperse. However, it seems to be possible that they coagulate due to the fast fluid flows at the droplet generation. As a result, the technique may be used only for the estimation of average values of size and concentration of inclusions from the measurements.

Elasticity Imaging of Polymeric Media

Viscoelastic properties of soft tissues and hydropolymers depend on the strength of molecular bonding forces connecting the polymer matrix and surrounding fluids. The basis for diagnostic imaging is that disease processes alter molecular-scale bonding in ways that vary the measurable stiffness and viscosity of the tissues. This paper reviews linear viscoelastic theory as applied to gelatin hydrogels for the purpose of formulating approaches to molecular-scale interpretation of elasticity imaging in soft biological tissues. Comparing measurements acquired under different geometries, we investigate the limitations of viscoelastic parameters acquired under various imaging conditions. Quasi-static (step-and-hold and low-frequency harmonic) stimuli applied to gels during creep and stress relaxation experiments in confined and unconfined geometries reveal continuous, bimodal distributions of respondance times. Within the linear range of responses, gelatin will behave more like a solid or fluid depending on the stimulus magnitude. Gelatin can be described statistically from a few parameters of low-order rheological models that form the basis of viscoelastic imaging. Unbiased estimates of imaging parameters are obtained only if creep data are acquired for greater than twice the highest retardance time constant and any steady-state viscous response has been eliminated. Elastic strain and retardance time images are found to provide the best combination of contrast and signal strength in gelatin. Retardance times indicate average behavior of fast (1-10 s) fluid flows and slow (50-400 s) matrix restructuring in response to the mechanical stimulus. Insofar as gelatin mimics other polymers, such as soft biological tissues, elasticity imaging can provide unique insights into complex structural and biochemical features of connectives tissues affected by disease.

Sensitivities of flow and transport parameters in fractured porous media using automatic differentiation

AbstractThe hydraulic complexity of hardrock aquifers with, for example, fast fluid flow in fractures, storage effects in the rock matrix, high contrasts in permeability, and a high spatial variability of structures requires a high standard of experimental and numerical techniques. This paper focuses on the interpretation of tracer tests. The sensitivity of tracer breakthrough curves with respect to flow and transport parameters is analysed for different model types in order to identify the relevant flow and transport processes. This leads to a better understanding of the system behaviour and facilitates the interpretation of experimental results. The technique of automatic differentiation is used to evaluate the sensitivities. The main advantage of this technique is that the sensitivities are calculated without truncation error. Copyright © 2005 John Wiley & Sons, Ltd.

Self-Consistent-Field Theory of Viscoelastic Behavior of Inhomogeneous Dense Polymer Systems

By combining the self-consistent-field (SCF) model and a reptation model, we propose a unified molecular model to explicitly account for dynamics of chain conformation in studying fast polymeric fluid flows in which the polymer chains can be severely stretched and the fluid can even become inhomogeneous. In the governing equations of the unified model, we directly couple the probability distribution function P(u;s,r) of the tangent vector u ≡ dR(s)/ds at the sth segment at position r, with the statistical weight, Q(s,r;s‘,r‘), of the subchain between sth and s‘th segments at the positions of r and r‘, respectively. The polymeric stress responses under flow condition can be readily calculated from the second moment of P(u;s,r), A(s,r) ≡ 〈uu〉 ≡ ∫uuP(u;s,r) du. The model was first validated in homogeneous simple shear flow. The quantitative agreements between the polymeric stresses evaluated by this novel method and the results of the reptation theory have been found. The model was then tested in a case stud...

Double‐adiabatic MHD theory for motion of a thin magnetic filament and possible implications for bursty bulk flows

During fast fluid flows in Earth's magnetotail, the plasma distribution function often takes the form of one beam flowing through another, which raises the question of whether bursty bulk flows can reasonably be represented in terms of single‐fluid magnetohydrodynamics, either in global MHD codes or in thin filament theory. An exact kinetic solution is compared with exact fluid solutions for a simplified case of cold, collisionless particles in a pipe, under conditions where there are counterstreaming beams similar to the ones that often occur in Earth's magnetotail. The results from kinetic theory differ from standard fluid theory but are exactly consistent with Chew‐Goldberger‐Low double‐adiabatic fluid theory. Double‐adiabatic MHD equations are derived for the motion of a thin filament through a medium. An initial simulation is presented of a double‐adiabatic filament that starts out with lower gas pressure than nearby flux tubes. As in earlier calculations for the isotropic case, the near‐equatorial part of the filament moves rapidly earthward. A compressional shock wave forms in the filament near the equatorial plane and propagates earthward. The near‐equatorial region of the filament exhibits characteristics similar to a flow burst, while the behavior far from the equatorial plane resembles that of earthward‐streaming plasma sheet boundary layer. The double‐adiabatic filament becomes firehose unstable after the shock wave reflects from the earthward boundary of the simulation and propagates back into the tail.

Simplified Numerical Simulation of Transients in Gas Networks

A set of equations governing an isothermal compressible fluid flow is resolved numerically for two practical cases. The first case concerns the fast fluid flow in short gas pipelines where the equations, written in conservative form, are resolved by a predictor-corrector scheme for the interior mesh points: an improved Lax-Friedricks scheme as a predictor and a leapfrog scheme as a corrector. Characteristics and upwind methods are used for the boundary conditions. The second case is concerned with massic slow fluid flow in relatively long gas pipelines. The equations, written in non conservative form, are resolved by a simple explicit finite difference scheme. The boundary conditions are considered by using the characteristic form of the equations including an inertial multiplier (Yow model) and resolved by a Newton-Raphson method. The obtained results agree with those of other methods. These numerical experiments permit the user to gain more computational time and simplicity in comparison with methods.

MATHEMATICAL MODELING OF THROUGH-HOLE-PLATING IN ELECTROCHEMICAL MICROFABRICATION II. AXISYMMETRIC TWO-DIMENSIONAL CONVECTIVE MASS TRANSFER

Abstract Axisymmetrical, two dimensional mass transfer in a tube and a through-hole was studied. The elliptic convective-diffusion equation was solved numerically by taking into account both the axial and radial diffusion in the entrance region of an infinite tube. The whole mass-transfer region in a tube of finite length was also studied by dividing the domain into several regions according to different mass-transfer mechanisms. A similar analysis is then conducted for a through-hole geometry by assuming a modified Hagen-Poiseuille fluid pattern. Results show that mass-transfer rate in a through-hole is larger than that in a tube by approximately 10%. This is due primarily to faster fluid flow within the concentration boundary layer in a through-hole.