Restructuring and break-up of two-dimensional aggregates in shear flow

We consider single two-dimensional aggregates, containing glass particles, placed at a water/air interface. We have investigated the critical shear rate for break-up of aggregates with different sizes in a simple shear flow. All aggregates break-up nearly at the same shear rate (1.8 +/- 0.2 s(-)(1)) independent of their size. The evolution of the aggregate structure before break-up was also investigated. With increasing shear rate, the aggregates adopt a more circular shape, and the particles order in a more dense, hexagonal structure. A simple theoretical model was developed to explain the experimentally observed break-up. In the model, the aggregate is considered as a solid circular disk that will break near its diameter. The capillary and drag force on the two parts of the aggregate were calculated, and from this force balance, the critical shear rate was found. The model shows a weak size dependence of the critical shear rate for the considered aggregates. This is consistent with the experimental observations.     

Fragmentation and erosion of two-dimensional aggregates in shear flow

We consider single two-dimensional aggregates containing glass particles trapped at a water/oil or water/air interface. Two modes for aggregate break-up are observed: break-up by fragmentation into a few parts and break-up by erosion of single particles. We have studied the critical shear rate for these modes as a function of the aggregate size. Two different particle sizes were used. The smaller particles, with a radius of 65 microm, form aggregates that break up predominantly by erosion at a shear rate between 0.5 and 0.7 s(-1). This value hardly depends on the size of the aggregates. The larger particles, with a radius of 115 microm, form aggregates that break by erosion or by fragmentation. In both modes, the critical shear rate again depends only weakly on the size of the aggregates and ranges between 1.6 and 2.2 s(-1). Also the structural changes inside the aggregate before break-up were studied. The aggregate behavior at the water/air and water/oil interfaces is quite similar. The critical shear rate for break up was also modeled. The model shows in both modes a weak dependence of the critical shear rate on the aggregate size, which is consistent with the experimental observations. The kinetics of the erosion process was also modeled and compared with the experimentally obtained time dependence of the aggregate size. The differences in the large and small particle systems can be attributed to the occurrence of friction forces between the particles, which one expects to be much larger for the large particle system, due to the stronger two-particle interaction.     

Hydrodynamic lubrication in nanoscale bearings under high shear velocity

The setting up process in a nanoscale bearing has been modeled by molecular dynamics simulation. Contrary to the prediction from the classical Reynolds' theory, simulation results show that the load capacity of the nanoscale bearing does not increase monotonically with the operation speed. This is attributed to the change of the local shear rate, which will decrease with the shear velocity of the bearing as the shear velocity exceeds a critical value, i.e., the local shear rate has an upper limit. A simple nonlinear dynamic model indicates that the momentum exchange between the liquid and the solid wall is reduced with the shear velocity when the shear velocity is above a critical value. The weak momentum exchange results in a decrease of the local shear rate, which in turn causes a sharp increase of the slip length.     

Breakage rate of colloidal aggregates in shear flow through stokesian dynamics

We study the first breakage event of colloidal aggregates exposed to shear flow by detailed numerical analysis of the process. We have formulated a model, which uses stokesian dynamics to estimate the hydrodynamic interactions among the particles in a cluster, van der Waals interactions and Born repulsion to describe the normal interparticle interactions, and the tangential interactions through discrete element method to account for contact forces. Fractal clusters composed of monodisperse spherical particles were generated using different Monte Carlo methods, covering a wide range of cluster masses (N(sphere) = 30-215) and fractal dimensions (d(f) = 1.8-3.0). The breakup process of these clusters was quantified for various flow magnitudes (γ), under both simple shear and extensional flow conditions, in terms of breakage rate constant (K(B)), mass distribution of the produced fragments (FMD, f(m,k)), and critical stable aggregate mass (N(c)), defined as the largest cluster mass that does not break under defined flow conditions. The breakage rate K(B) showed a power law dependence on the product of the aggregate size and the applied stress, with values of the corresponding exponents depending only on the aggregate fractal dimension and the type of flow field, whereas the prefactor of the power law relation also depends on the size of the primary particles comprising a cluster. The FMD was fitted by Schultz-Zimm distribution, and the parameter values showed an analogous dependence on the product of the aggregate size and the applied stress similar to the rate constant. Finally, a power law relation between the applied stress and corresponding largest stable aggregate mass was found, with an exponent value depending on the aggregate fractal dimension. This unique and detailed analysis of the breakage process can be directly utilized to formulate a breakage kernel used in solving population balance equations.     

The distribution of stresses in rigid fractal-like aggregates in a uniform flow field

The distribution of stresses in rigid fractal-like aggregates moving in a uniform flow field was investigated for particle-cluster and cluster-cluster aggregates with fractal dimensions ranging from 1.7 to 2.3. The method of reflections was used to calculate the drag force on each monomer, while the internal inter-monomer interactions were calculated by applying force and torque balances on each primary particle. The stress distribution was found to be very dissimilar from that of the applied external forces. Although the highest external forces act on the monomers located at the periphery of the aggregate where the drag is more intense, the most stressed inter-monomer bonds are always located in the internal part of the aggregate. This phenomenon is a consequence of the structure of the studied fractal aggregates, which are made mainly of filaments of monomers: the stress generated by the external forces is propagated and progressively accumulated by such filaments up to their roots, which are situated in the inner part of the cluster. Such a behaviour is different from that exhibited by highly connected structures, in which the loads are absorbed locally by the structure and the largest stresses are normally found in the proximity of the highest applied external forces.     

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.     

Normal stress and shear stress in a viscoelastic liquid under oscillatory shear flow

Normal stress and shear stress of concentrated polystyrene solutions in a chlorinated diphenyl were measured under steady flow and oscillatory shear flow in a Weissenberg rheogoniometer. The normal stress difference was observed to oscillate at double the frequency of the applied shear strain with amplitude proportional to the square of the applied amplitude, while the shear stress was found to oscillate at the same frequency with amplitude proportional to the applied amplitude. A theoretical relation between the displacement of the oscillatory normal stress difference from zero level and the dynamic modulus derived by Lodge and other investigators was confirmed experimentally, and the theoretical predictions of Coleman and Markovitz concerning the relation among steady-flow normal stress difference and dynamic modulus were also confirmed. However, the theoretical predictions of Lodge, of Spriggs, Huppler and Bird, and of Williams on the relation between the amplitude and phase of oscillatory normal stress and those of oscillatory shear stress did not agree with experimental results.     

Hydrodynamic flow and electroosmotic flow in zirconia-packed capillaries

Fused-silica capillaries were packed with Zirchrom-PBD stationary phase for application in CEC, nanoLC and pseudoelectrochromatography (PEC). Acido-basic properties of zirconia can be used to control the EOF even if the zirconia particles were coated by polybutadiene. As for native zirconia, the EOF is pH-dependent and the pI is close to pH 5. The mixed-mode pressure-voltage technique induced a modulation of the mobile-phase velocity as well as an electrophoretic migration of the solutes in order to improve the resolution of the separation. A significant increase of the flow appeared when both hydrodynamic and EOFs were in the same direction. But an important reduction of the electroosmotic velocity was observed when the hydrodynamic flow and EOF were opposed in Zirchrom-PBD columns. This behaviour has been observed at high or low pH on several columns. Separations of neutral and charged compounds have been performed with these columns in PEC mode.     

Association under shear flow in aqueous solutions of pectin

Effects of oscillatory and steady shear flows on intermolecular associations in dilute and semidilute aqueous solutions of pectin in the absence and presence of the hydrogen bond breaking agent urea are reported. A weak oscillatory shear perturbation builds up, depending on polymer concentration, multichain aggregates or networks in the course of time and these association structures are mainly stabilized through hydrogen bonds. The association effect is more pronounced at higher concentrations, and the growth of intermolecular interactions is inhibited by the addition of urea. Steady shear measurements on the pectin-water solutions reveal shear thickening at low shear rates for all the concentrations, except the lowest one, and disruption of intermolecular junctions at high shear rates. In the presence of urea, no shear thickening is detected. The polymer concentration dependence of the viscosity at a low shear rate can be described by a power law η ∼ c^x, with x = 1.9 and 1.4 without and with urea, respectively. When a low constant shear rate is applied to pectin solutions and this monitoring shear rate is interrupted periodically by transitory high shear rates perturbations during a short time, prominent association structures evolve upon return to the monitoring shear rate. This effect is more evident at a lower polymer concentration, and in the presence of urea, the growth of the association complexes is damped. The shear-induced alignment and stretching of polymer chains and the formation of hydrogen-bonded structures are analyzed in the framework of a model, where cooperative zipping of stretched chains play an important role. Viscosity enhancement is found for a semidilute pectin-water solution in the presence of moderate levels of salt addition (NaCl), suggesting that partial screening of electrostatic interactions promotes growth of energetic cross-links.     

Asymptotic relations between shear stresses and normal stresses in general incompressible fluids

It is shown that in the general theory of incompressible simple fluids with fading memory there are, for several types of nonsteady shearing motions, simple universal asymptotic relations between the shear stress S₁₂ and the first normal stress difference N₁ = S₂₂ – S₁₁. The kinematical situations considered include initiation of steady shearing, rest after steady shearing, and sinusoidal oscillation. In, for example, relaxation following cessation of a steady shearing flow with rate of shear κ, there holds, to within an error O(κ⁴): This and the other derived universal relations between N₁ and S₁₂ are either consequences of, or are closely related to a general asymptotic formula [B. D. Coleman and W. Noll, Revs. Mod. Phys., 33 , 239 (1961), eq. (6.15)] expressing N₁ as an integral of the product of the shear relaxation modulus and the square of the history of the relative shear strain.     

Bead–spring model for adsorbed polymer chains in shear flow: Hydrodynamic interaction

We consider the hydrodynamic interaction between two absorbed polymer chains in a simple shear flow, each modeled by a bead connected to the wall by a linear spring. It is concluded that hydrodynamic interaction between the beads or between the beads and the wall cannot be responsible for the experimentally observed increase in hydrodynamic thickness.     

Computational modelling of declination loops under shear flow

Optical observations of sheared lyotropic and thermotropic liquid crystals have shown them to have a rich microstructure dominated by disclination loops and networks. In thermotropic liquid crystalline polymers it has been possible to freeze in the microstructures, which are then sectioned and analysed by optical and electron microscopy, to enable the identification of the predominant types of disclinations. In this work a computational model is presented which simulates the development of texture in liquid crystalline materials. The model has been designed so that it is possible to study large localised distortions which are subjected to a flow field. In the aspect of the work reported here, simulations have been used to study the influence of simple shear on individual disclination loops placed in an otherwise undeformed sample. The subsequent deformation of the loop is shown to be dependent on the angle that the rotation vector of the loop makes with the vorticity direction.     

Dielectric spectroscopy on W/O emulsions under influence of shear forces

The dielectric properties of concentrated w/o-emulsions have been investigated, both at rest and during shear. The volume fraction water ranged from 0.50 to 0.95. The time domain dielectric spectroscopy techniques (TDS) was used to record the dielectric spectra, which covered the frequency region from 25 MHz to 2 GHz. In order to simultaneously record rheological and dielectric data a modified viscometer of the coaxial cylinder type was applied.A close connection between the viscosity and the dielectric properties of w/o emulsions is demonstrated. The very large effects of shear both on the static permittivity and the dielectric relaxation time for the emulsion can partially be ascribed to the degree of flocculation in the system. At high shear rates, at which the emulsions are expected to have a low degree of flocculation, the observed dielectric properties differ from those expected from a theoretical model for spherical emulsion droplets.     

Structure development under transient shear flow in semidilute polystyrene solution

The shear-induced concentration fluctuations or phase separation of a semidilute solution comprised of polystyrene (PS) as a solute and dioctylphthalate (DOP) as a solvent (PS/DOP) was investigated by using real-time and in-situ shear-small-angle light scattering and shear-phase-contrast optical microscopy. When a transient shear flow with a fixed shear rate γ greater than a critical value γ_C was imposed on the solution, a unique anisotropic scattering pattern was observed some time after onset of shear. This pattern was found to be identical to the “butterfly pattern” previously reported for the same solutions under steady shear flow with γ_C. When the shear flow was ceased before the scattered intensity reached a steady state, the scattered intensity rapidly increased toward a maximum intensity, and then decreased toward the intensity of the quiescent solution with time. From the phase-contrast microscopy, this immediate increment of the scattered intensity after the shear cessation was found to arise from the increment in amplitude of the concentration fluctuations along flow direction. The characteristic length scale of the fluctuations was about 2.5 μm in this experiment, almost independent of the shear rate imposed on the solution.     

Production of reactive oxygen species in endothelial cells under different pulsatile shear stresses and glucose concentrations

A hemodynamic Lab-on-a-chip system was developed in this study. This system has two unique features: (1) it consists of a microfluidic network with an array of endothelial cell seeding sites for testing them under multiple conditions, and (2) the flow rate and the frequency of the culture medium in the microchannel are controlled by a pulsation free pump to mimic the flow profile of the blood in the blood vessel under different physiological conditions. The investigated physiological conditions were: (1) the resting condition in a normal shear stress of 15 dyne cm(-2) with a normal heart rate of 70 bpm, (2) an exhaustive exercise condition with a high shear stress of 30 dyne cm(-2) and a fast heart rate of 140 bpm, and (3) a constant high shear stress of 30 dyne cm(-2). Two chemical conditions were investigated (10 mM and 20 mM glucose) to mimic hyperglycemic conditions in diabetes patients. The effects of various shear stresses either alone or in combination with different glucose concentrations on endothelial cells were examined using the developed hemodynamic Lab-on-a-chip system by assessing two parameters. One is the intracellular level of reactive oxygen species (ROS) determined by a fluorescent probe, H(2)DCFDA. Another is the mitochondrial morphology revealed with a fluorescent dye, MitoTracker Green FM. The results showed that ROS level was elevated nearly 4-fold after 60 min of exhaustive exercise. We found that the pulsatile nature of the fluid was the determination factor for causing ROS generation in the cells as almost no increase of ROS was detected in the constant shear stress condition. Similarly, much higher level of ROS was detected when 10 mM glucose was applied to the cells under normal or high pulsatile shear stresses compared with under a static condition. These results suggest that it is necessary to use pulsatile shear stress to represent the physiological conditions of the blood flow, and demonstrate the advantage of utilizing this newly developed hemodynamic Lab-on-a-chip system over the conventional non-pulsatile system in the future shear stress related studies.     

Rheo-optical characterization of dilute polymer mixtures under shear flow

Optical properties can estimate morphological changes of polymer chains under flow. This work proposes a rheo-optical procedure to determine turbidity and both flow and form birefringence of diluted polymer mixtures of polystyrene (PS) and polypropylene (PP) during a controlled shear flow, by measuring the transmitted light intensity with and without crossed polarizers via an own built optical sensor. The turbidity in these dilute mixtures decreased with the increase of the shear rate due to deformation of the dispersed phase droplets, which reduces their cross-sections. The presence of PP as the dispersed phase in the PS matrix caused a decrease in the total birefringence measured, whereas PS as the dispersed phase in the PP matrix caused an increase in it. Both effects are associated to the positive contribution of the form birefringence, produced by the shear-induced elongated morphology of the dispersed phase.     

Crystallization kinetics of colloidal spheres under stationary shear flow

A systematic experimental study of dispersions of charged colloidal spheres is presented on the effect of steady shear flow on nucleation and crystal growth rates. In addition, the nonequilibrium phase diagram as it relates to the melting line is measured. Shear flow is found to strongly affect induction times, crystal growth rates, and the location of the melting line. The main findings are that (1) the crystal growth rate for a given concentration exhibits a maximum as a function of the shear rate; (2) contrary to the monotonic increase in the growth rate with increasing concentration in the absence of flow, a maximum of the crystal growth rate as a function of concentration is observed for sheared systems; and (3) the induction time for a given concentration exhibits a maximum as a function of the shear rate. These findings are partly explained on a qualitative level.     

Oil coating on hydrophilic surfaces from emulsions and under shear flow

We study the formation of silicone oil coating on negatively charged hydrophilic surfaces via emulsion deposition. Cationic surfactants usually adsorb and form bilayers onto negative surfaces. As a result, direct emulsions stabilized with cationic surfactants are paradoxically poorly efficient at coating negative substrates. We show in this work an alternative and new method, still based on electrostatic attractions, to coat negative substrates. Our method consists of using emulsions stabilized with nonionic surfactants and of adding to the oil cationic additives that are non-water-soluble and of high molecular weight to minimize their solubilization in the micelles formed by the neutral surfactant. The positively charged droplets stick efficiently onto the substrates. They form monolayer and uniform coatings. We study the kinetics and the density of the resulting coating using a flow cell experiment. This technique allows us to finely analyze the influence of several physicochemical parameters.