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PubMed2005

Mixing in microchannels based on hydrodynamic focusing and time-interleaved segmentation: modelling and experiment.

This paper theoretically and experimentally investigates a micromixer based on combined hydrodynamic focusing and time-interleaved segmentation. Both hydrodynamic focusing and time-interleaved segmentation are used in the present study to reduce mixing path, to shorten mixing time, and to enhance mixing quality. While hydrodynamic focusing reduces the transversal mixing path, time-interleaved sequential segmentation shortens the axial mixing path. With the same viscosity in the different streams, the focused width can be adjusted by the flow rate ratio. The axial mixing path or the segment length can be controlled by the switching frequency and the mean velocity of the flow. Mixing ratio can be controlled by both flow rate ratio and pulse width modulation of the switching signal. This paper first presents a time-dependent two-dimensional analytical model for the mixing concept. The model considers an arbitrary mixing ratio between solute and solvent as well as the axial Taylor-Aris dispersion. A micromixer was designed and fabricated based on lamination of four polymer layers. The layers were machined using a CO2 laser. Time-interleaved segmentation was realized by two piezoelectric valves. The sheath streams for hydrodynamic focusing are introduced through the other two inlets. A special measurement set-up was designed with synchronization of the mixer's switching signal and the camera's trigger signal. The set-up allows a relatively slow and low-resolution CCD camera to freeze and to capture a large transient concentration field. The concentration profile along the mixing channel agrees qualitatively well with the analytical model. The analytical model and the device promise to be suitable tools for studying Taylor-Aris dispersion near the entrance of a flat microchannel.

Microfluidic Analytical Techniques↗
PubMed2000

Cholinergic regulation of biological hydrodynamics.

The structures of biological life are formed in water. Their function depends on changes in the entropy of water. It is regulated by the cholinergic system. The initiating event is the ChE-splitting of water with liberation of free protons. They will draw electrons from the fairly inert dioxygen. The induced oxygen reactivity will give liberation and transfer of electrons and hydrodynamic pH-dependent changes in protein configurations. A multitude of sub-systems will be activated. The sequence of events normally ends with the formation of water, thus preventing uncontrolled radical chain reactions. Cholinergic receptors appear as restricting units of the general disordering entropy tendency. ChE-induced hydrodynamics is propagated to the inner of cells by the water soluble protons and the electrolytes. Especially Ca appear to have a strong influence on the hydrodynamic dipole moment of water. Because water is an integral structure of DNA genetics also will be influenced. Conditions caused by deprivation of oxygen or of reactive oxygen and disorders by hyperactivity and inactivity are briefly discussed. The CNS takes the shape of a large-scale quantum computer with a function far beyond our ability of immediate perception. The atomic nuclear proportions of quantum bits (qubits) will admit the functional one-cell unit of immune memory cells. Cholinergic hydrodynamics appear to substantiate the much discussed chaos theory.

Cholinesterases↗
PubMed2001

Multiparticle adhesive dynamics: hydrodynamic recruitment of rolling leukocytes.

The slow rolling motion of leukocytes along the walls of blood vessels mediated by specific receptor-ligand adhesion is important in inflammation and occurs in postcapillary venules over a wide range of wall shear stresses and vessel diameters. The ability of hydrodynamic collisions between cells to induce capture of free-stream leukocytes to a selectin-bearing surface under shear flow was studied experimentally by using a cell-free assay. It was found that carbohydrate-coated spherical beads, representing model leukocytes, tend to attach to the adhesive wall 4-5 cell diameters up- or downstream of a slowly rolling or stationary adhesive bead. A key feature of such "hydrodynamic recruitment" is that only glancing, indirect collisions occurring close to the plane will result in downstream attachment. A direct numerical simulation of cell capture and rolling that includes multiparticle hydrodynamic interactions is shown to reproduce the observed behavior accurately. The theory predicts that hydrodynamic recruitment will occur in the absence of buoyancy effects and over a range of shear rates, suggesting that the mechanism may be important in vivo. This theory is supported by measurements of leukocyte capture in vivo using the hamster cheek pouch model.

Cell Adhesion↗
PubMed1994

Hydrodynamic steering effects in protein association.

Protein-ligand reaction rates are often limited by the rate of diffusional encounter of the protein and ligand in solution. Reaction rates, however, can be much greater than expected, given the necessity for correct orientation before reaction. A number of forces can affect the orientation of the protein and ligand in solution, and thus increase the reaction rate. We have considered hydrodynamic forces, produced when water molecules between protein and ligand must be pushed out of the way to allow their encounter. We have used the cleft enzymes as a model system, as they could be expected to show strong hydrodynamic effects. One particular type of hydrodynamic interaction stands out: a steering torque which occurs when the enzyme and substrate move toward each other in solution. The magnitude of this steering torque is compared to the mutual torque experienced by interacting "protein-sized" dipoles in solution. A simple model is used to demonstrate that the hydrodynamic steering torque can be 2 orders of magnitude greater than the electrostatic torque.

Biophysical Phenomena↗
PubMed2004

Roughness-dependent removal of settled spores of the green alga Ulva (syn. Enteromorpha) exposed to hydrodynamic forces from a water jet.

Topographic features change the hydrodynamic regime over surfaces subjected to flow. Hydrodynamic microenvironments around topographic structures may have consequences for recruitment and removal of propagules of marine benthic organisms. The settlement and adhesion of zoospores from the green alga Ulva linza (syn. Enteromorpha linza) to defined topographies was investigated. A range of topographic size scales (Rz: 25-100 microm) was manufactured from plankton nets, creating patterns with ridges and depressions. The topographic scales span a roughness similar to that of natural substrata and antifouling coatings. Spores were removed from the surfaces by a calibrated water jet. Fewer spores were removed from the smallest topographic structure tested (Rz: 25 microm) compared to both the smooth (Rz: 1) and the roughest (Rz: 100 microm) structures. Zoospores that settled in depressions were less likely to be removed compared to spores on the ridges. The results in terms of the interaction between surface topography and hydrodynamic forces have implications for both natural substrata exposed to wave action and antifouling surfaces on ships' hulls. The possible effects of topography on increasing zoospore adhesion and offering a refuge from hydrodynamic forces are discussed.

Adhesiveness↗
PubMed1999

Determination of the quench velocity and rewetting temperature of hot surfaces: Formulation of a nonisothermal microscale hydrodynamic model.

A nonisothermal microscale model of the three-phase, solid-liquid-gas, contact zone is formulated in the context of rewetting phenomena. The model incorporates hydrodynamics, heat transfer, interfacial phenomena, and intermolecular long range forces, in a two-dimensional proximal region of the order of 1000 A in width and 100 A in thickness. The model comprises scaled mass, momentum, and energy balances, and their corresponding scaled boundary conditions. The small contact angles which are characteristic of rewetting situations facilitate the use of the lubrication approximation, and the dynamics of the liquid and gas phases is decoupled by applying the one-sided simplification. The microscale hydrodynamic model reflects the strong effect of the solid-liquid interactions on the film profile, and the attendant flow and thermal fields. Thinner films having smaller contact angles involve stronger solid-liquid attraction forces, and consequently they exhibit higher rewetting temperatures and lower evaporation and vapor recoil effects. Thermocapillary and evaporation and conduction effects are expressed by appropriate dimensionless numbers. A set of such numbers is defined in the context of the differential equations of the microscale model. This model covers the hydrodynamic aspect of rewetting phenomena, which are also controlled by thermodynamic and macroscale constraints. This calls for interfacing and appropriate combination between the microscale hydrodynamic model, thermodynamics, and other macroscale rewetting models, for the determination of rewetting temperatures and quench velocities of liquids on hot solid surfaces. This is addressed elsewhere, in subsequent papers that follow this work.

Journal Article↗
PubMed2001

Origin of the hydrodynamic Lyapunov modes.

Recent studies of the Lyapunov spectrum of the hard sphere fluid reveal that there are "hydrodynamic" Lyapunov exponents corresponding to collective perturbations in phase space. We show that these collective perturbations are due to the conservation of certain quantities during collisions. These new conservation laws generate new hydrodynamic fields, just as the conservation of mass, momentum, and energy generate the density, velocity, and temperature fields. We then construct a detailed theory of the new hydrodynamic fields using a kinetic theory approach. This theory predicts several properties of the modes, but not all of them. This suggests that the underlying idea is correct, but a detailed theory must be elaborated in another way. The hydrodynamic exponents are not related in a simple way to the transport coefficients.

Journal Article↗
PubMed2002

Two-component fluid membranes near repulsive walls: Linearized hydrodynamics of equilibrium and nonequilibrium states.

We study the linearized hydrodynamics of a two-component fluid membrane near a repulsive wall, using a model that incorporates curvature-concentration coupling as well as hydrodynamic interactions. This model is a simplified version of a recently proposed one [J.-B. Manneville et al., Phys. Rev. E 64, 021908 (2001)] for nonequilibrium force centers embedded in fluid membranes, such as light-activated bacteriorhodopsin pumps incorporated in phospholipid egg phosphatidyl choline (EPC) bilayers. The pump-membrane system is modeled as an impermeable, two-component bilayer fluid membrane in the presence of an ambient solvent, in which one component, representing active pumps, is described in terms of force dipoles displaced with respect to the bilayer midpoint. We first discuss the case in which such pumps are rendered inactive, computing the mode structure in the bulk as well as the modification of hydrodynamic properties by the presence of a nearby wall. These results should apply, more generally, to equilibrium fluid membranes comprised of two components, in which the effects of curvature-concentration coupling are significant, above the threshold for phase separation. We then discuss the fluctuations and mode structure in the steady state of active two-component membranes near a repulsive wall. We find that proximity to the wall smoothens membrane height fluctuations in the stable regime, resulting in a logarithmic scaling of the roughness even for initially tensionless membranes. This explicitly nonequilibrium result is a consequence of the incorporation of curvature-concentration coupling in our hydrodynamic treatment. This result also indicates that earlier scaling arguments which obtained an increase in the roughness of active membranes near repulsive walls upon neglecting the role played by such couplings may need to be reevaluated.

Journal Article↗
PubMed2003

Hydrodynamic modes for granular gases.

The eigenfunctions and eigenvalues of the linearized Boltzmann equation for inelastic hard spheres (d=3) or disks (d=2) corresponding to d+2 hydrodynamic modes are calculated in the long wavelength limit for a granular gas. The transport coefficients are identified and found to agree with those from the Chapman-Enskog solution. The dominance of hydrodynamic modes at long times and long wavelengths is studied via an exactly solvable kinetic model. A collisional continuum is bounded away from the hydrodynamic spectrum, assuring a hydrodynamic description at long times. The bound is closely related to the power law decay of the velocity distribution in the reference homogeneous cooling state.

Journal Article↗
PubMed2004

Kinetic derivation of the hydrodynamic equations for capillary fluids.

Based on the generalized kinetic equation for the one-particle distribution function with a small source, the transition from the kinetic to the hydrodynamic description of many-particle systems is performed. The basic feature of this interesting technique to obtain the hydrodynamic limit is that the latter has been partially incorporated into the kinetic equation itself. The hydrodynamic equations for capillary fluids are derived from the characteristic function for the local moments of the distribution function. Fick's law appears as a consequence of the transformation law for the hydrodynamic quantities under time inversion.

Journal Article↗
PubMed2005

Kinetics of the polymer collapse transition: the role of hydrodynamics.

We investigate numerically the dynamical behavior of a polymer chain collapsing in a dilute solution. The rate of collapse is measured with and without the presence of hydrodynamic interactions. We find that hydrodynamic interactions accelerate polymer collapse. We present a scaling theory describing the physical process responsible for the collapse kinetics. Predicted collapse times in a hydrodynamic (tauH approximately N(4/3)) and a Brownian heat bath (tauB approximately N2) agree well with the numerical results (tauH approximately N(1.40+/-0.08) and tauB approximately N(1.89+/-0.09)) where N denotes chain length. The folding kinetics of Go models of proteins is also examined. We show that for these systems, where many free energy minima compete, hydrodynamics has little effect on the kinetics.

Journal Article↗
PubMed2005

Effect of hydrodynamic flow on low-field spin-lattice relaxation in liquids in the nanoscopic vicinity of solid surfaces: theory and Monte Carlo simulations of model pore spaces.

It is shown that slow hydrodynamic flow with velocities of a few millimeters per second reduces the spin-lattice relaxation rate of fluids confined to pores of a diamagnetic, polar, solid material. The effect is predicted by an analytical theory and Monte Carlo simulations of model pore spaces. Adsorbate molecules diffusing in the vicinity of pore surfaces can perform adsorption, desorption, and readsorption cycles, effectively leading to displacements along the surface (also termed "bulk mediated surface diffusion" or BMSD). Since the surface determines the orientation of the adsorbed molecule relative to the external magnetic field, desorption at one site and readsorption at another site of a nonplanar surface will cause molecular reorientation. This is the basis of the "reorientation mediated by translational displacements" (RMTD) relaxation mechanism. If hydrodynamic flow is superimposed on diffusion, the RMTD process will be accelerated in a sort of rotational analog to translational hydrodynamic (or Taylor-Aris) dispersion. This reveals itself by a prolongation of spin-lattice relaxation times at low frequencies. The flow-relaxation effect takes place in the vicinity of the pore surfaces on the order of nanometers. The conclusions are (i) the BMSD and RMTD relaxation mechanism of fluids in porous materials is corroborated, (ii) hydrodynamic dispersion affects molecular displacements at surfaces, and (iii) interfacial slip in the sense of a molecular hopping, i.e., a desorption-readsorption process takes place.

Journal Article↗
PubMed2000

Effective screening of hydrodynamic interactions in charged colloidal suspensions.

We investigate the hydrodynamic interaction in suspensions of charged colloidal silica spheres. The volume fraction as well as the range of the electrostatic repulsion between the spheres is varied. Using a combination of dynamic x-ray scattering, cross-correlated dynamic light scattering, and small angle x-ray scattering, the hydrodynamic function H(q) is determined experimentally. The effective hydrodynamic interactions are found to be screened, if the range of the direct interaction is relatively long and the static density correlations are strong. This observation of effective hydrodynamic screening is in marked contrast to hard-sphere-like systems.

Journal Article↗
PubMed2002

Hydrodynamic contributions to multimodal guidance of prey capture behavior in fish.

Water movements, of both abiotic and biotic origin, provide a wealth of information of direct relevance to the guidance of prey capture behavior. To gather hydrodynamic information, fish have sensors of two basic types: those scattered over the surface of the body known as superficial neuromasts and similar sensors embedded in subdermal lateral line canals. Recently, the anatomical dichotomy between superficial and canal neuromasts has been matched by demonstrations of a corresponding functional dichotomy. Prey detection and localization are evidently mediated by canal neuromasts, whereas superficial neuromasts are more sensitive to water flows over the surface of the fish and participate in the orientation to water currents, a behavior known as rheotaxis. However, rheotaxis in combination with chemosensory inputs can also guide fish to their prey. Thus there is evidence that both lateral line sub-modalities either alone or in concert with other senses play a role in prey capture. Are there circumstances where prey capture requires integration of information from both lateral line sub- modalities? Recent evidence shows that fish are capable of tracking other fish on the basis of the hydrodynamic trails left behind by their swimming motion. Pharmacological and physical ablation of lateral line end organs shows that indeed integration of information from both sub-modalities is required for the complex hydrodynamic task of natural prey capture in the dark. Furthermore, these experiments provide an excellent demonstration of the integration of hydrodynamic, chemosensory, tactile and visual information for the multimodal guidance of prey capture behavior.

Animals↗
PubMed1991

Decreased hydrodynamic resistance in the two-phase flow of blood through small vertical tubes at low flow rates.

The aggregation of red blood cells in blood flowing through small tubes at very low shear rates leads to the two-phase flow of an inner core of rouleaux surrounded by a cell-depleted peripheral layer. The formation of this layer is known to be accompanied by a decrease in hydrodynamic resistance to flow. To quantitate this effect, we measured the pressure gradient, flow rate, and the radius of the red blood cell core in suspensions flowing through tubes of 172-microns radius at mean linear flow rates (U) from 50 to 0.15 tube diameters.sec-1. Washed red blood cells were suspended in 1.5% buffered dextran 110 at hematocrits of 34-52%. Using syringe pumps, blood flowed from a stirred reservoir through a vertical 12-cm length of tube in either the upward or downward direction. The pressure drop was measured with transducers. Mean values in distributions in the core radius were obtained by analyzing cine films of flow taken through a microscope with flow in the upward direction, measuring the core radius at five equally spaced axial positions of the tube in each of 100 frames. At 34% and 46% hematocrit, the hydrodynamic resistance increased as U decreased from 50 sec-1, reaching a maximum at U-2 sec-1. It then decreased to a minimum at U less than 0.5 sec-1 as the red blood cell core formed in the tube, and the mean core radius/tube radius ratio decreased from 0.98 to 0.74 with marked axial fluctuations at the lower U. At higher hematocrits, both the increase and decrease in hydrodynamic resistance were greater. In a red blood cell albumin-saline suspension, where there is no aggregation of red blood cells and no two-phase flow, hydrodynamic resistance increases linearly with decreasing U. The experimental results were compared with the predictions of a two-phase steady-flow model, assuming axisymmetric flow of a core surrounded by cell-free suspending medium. Two models were considered, one in which the core is solid, the other in which the rheological properties of the suspension in the core are given by the Quemada equation. The effects of sedimentation of the core resulting in a zero net flow pressure gradient were taken into account. Provided that an experimentally extrapolated value for the zero pressure gradient was used, the Quemada-fluid model gave good agreement with the experimentally observed core radius as a function of U and hematocrit.

Blood Flow Velocity↗
PubMed2000

Hydrodynamics, shell shape, behavior and survivorship in the owl limpet Lottia gigantea.

On wave-swept rocky shores, limpets are subjected to water velocities in excess of 20 m s(-1), which may impose large hydrodynamic forces. Despite the extreme severity of this flow environment, predictions from conical models suggest that limpets' shells are typically far from the optimal shape that would minimize the risk of dislodgment, a deviation that is allowed by the high tenacity of the limpets' adhesive system. In this study, we test this conclusion using an actual limpet. The shell of Lottia gigantea differs substantially from the hydrodynamic optimum in that its apex is displaced anteriorly to form a plough, which is used to defend the limpet's territory. The hydrodynamic effects of this shape are similar to those observed in conical models: the animal experiences an increased lift when facing into the flow and a decreased lift when the flow is at its back. However, neither effect has a substantial impact on the risk of dislodgment. When the animal is stationary, its adhesion to the substratum is very strong, and its risk of being dislodged is small regardless of its orientation to the flow and despite its sub-optimal shape. In contrast, when the animal is crawling rapidly, its adhesion is substantially decreased, and it would probably be dislodged by rapid flow even if the shell were shaped optimally. The risk of dislodgment by waves is therefore functionally independent of shell shape. In essence, despite the extremely high water velocities to which this species is subjected, its shell has had the 'permission' of the flow environment to respond to other selective factors, in particular those associated with its aggressive, territorial behavior. The result is a shell that is both a potent territorial weapon and a functional (albeit less than optimal) hydrodynamic shape.

Adaptation, Physiological↗
PubMed2001

Running in the surf: hydrodynamics of the shore crab Grapsus tenuicrustatus.

When locomoting in water, animals experience hydrodynamic forces due to ambient water motion and their own motion through the water. Because an aquatic pedestrian must maintain contact with the substratum to locomote, hydrodynamic forces which can dislodge an animal have the capacity to constrain the postures, gaits and speeds an animal can use. This study measured hydrodynamic forces on the amphibious shore crab Grapsus tenuicrustatus in aquatic and terrestrial postures. The crabs' locomotory speeds and ambient water velocities in their habitat were considered in predicting the conditions under which a crab is likely to overturn or wash away. A non-moving crab can withstand 200% faster flow in the aquatic posture than in the terrestrial posture. A crab using the terrestrial posture while locomoting through still water experiences 132% greater drag and 17% greater acceleration reaction forces than it does in the aquatic posture. Due to the lower hydrodynamic forces in the aquatic posture, a crab could locomote up to 50% more quickly or through a faster water flow environment than it could in the terrestrial posture. In faster flow environments like wave-swept rocky shores, a crab in either posture would have to actively grasp the substratum to keep from being dislodged, preventing it from using a punting gait. In slower flow environments, animals can locomote faster and take advantage of different gaits that are not available to them in faster flow environments.

Animals↗
PubMed2005

Estimation of bioreactor efficiency through structured hydrodynamic modeling case study of a Pichia pastoris fed-batch process.

In this article, two theories are unified to investigate the effect of hydrodynamics on a specific bioprocess: the network-of-zones (NOZ) hydrodynamic structured modeling approach (developed by several researchers but applied to only a few bioprocesses) and the effectiveness factor eta approach. Two process scales were investigated (20 and 500 L), and for each, hydrodynamics were quantified using an NOZ validated by homogeneity time measurements. Several impeller combinations inducing quite different hydrodynamics were tested at the 20-L scale. After this step, effectiveness factors were determined for each fermentation run. To achieve this, a perfectly mixed microbial kinetic model was evaluated by using simple Monod kinetics with a fed-batch mass balance. This methodology permitted determination of the effectiveness factor with more accuracy because of the relation with the perfect case deduced from the Monod kinetics. It appeared that for the small scale, eta decreased until reaching a value of approx 0.7 (30% from the ideal case) for the three impeller systems investigated. However, stirring systems that include hydrofoils seemed to maintain higher effectiveness factors during the course of the fermentation. This effect can be attributed to oxygen transfer performance or to homogenization efficiency exhibited by the hydrofoils. To distinguish the oxygen transfer from the homogenization component of the effectiveness factor, these phenomena were analyzed separately. After determining the evolution of etaO2 linked to oxygen transfer for each of the fermentation runs, the NOZ model was employed to quantify substrate gradient appearance. After this step, another effectiveness factor, etamix, related to mixing was defined. Consequently, it is possible to distinguish the relative importance of the mixing effect and oxygen transfer on a given bioprocess. The results have highlighted an important scale effect on the bioprocess that can be analyzed using the NOZ model.

Bioreactors↗