Applied Shear Flow Research Articles

Shear-induced structural transition in the lamellar phase of the C16E7/D2O system. Time evolution of small-angle neutron scattering at a constant shear rate

The time evolution of small-angle neutron scattering is measured for the lamellar phase of a nonionic surfactant C16H33(OC2H4)7OH (C16E7) in D2O at 48 wt% at 343 K under shear flow. At the shear rates of 0.3, 1 and 3 s−1, a new diffraction peak appears at higher q [where q = (4π/λ)sin θ, and λ and 2θ are the wavelength of the neutron beam and the scattering angle, respectively] about 1–2 h after applying shear flow and coexists with the initial diffraction peak. The coexistence of two peaks continues even after 5 h at 0.3 s−1 whereas at 1 and 3 s−1 the peak at lower q disappears after about 3 h. These results indicate that the repeat distance decreases discontinuously and so suggest some sort of transition. A plot of the repeat distance after 5 h versus shear rate shows a minimum at 1 s−1, which is in good agreement with our previous results obtained by increasing the shear rate stepwise.

Nonequilibrium molecular dynamics simulation study on the orientation transition in the amphiphilic lamellar phase under shear flow

By the extensive large-scale nonequilibrium molecular dynamics simulation on an effective generic model-A2B2 tetramer for amphiphiles, we investigate the shear-induced parallel to perpendicular orientation transition in the lamellar phase as a function of segregation degree and shear rate. Under low rate shear flow the evolution of parallel lamellar configurations at different segregation strengths shows a similar kinetic pathway independent of the segregation degree. While under high rate shear flow in which the lifetime of undulation instability exceeds the characteristic time of the applied shear flow, the kinetic pathway of the shear-induced parallel-to-perpendicular orientation transition in lamellar systems is the segregation degree dependent. Comparing the temporal mesoscopic domain morphology, the microscopic chain conformation, and macroscopic observable-viscosity changes with the experimentally proposed mechanisms, we find that the undulation instability, partial breakup of monodomain, grain rotation, and recombination combined with defect migration and annihilation are the kinetic pathway for the parallel-to-perpendicular orientation transition in the lamellar phase in or near the intermediate segregation limit, and that the undulation instability, domain dissolution, and reformation along the preferred direction combined with defect migration and annihilation are the kinetic pathway for the parallel-to-perpendicular orientation transition in the lamellar phase close to the order-to-disorder phase transition point. A detailed underlying microscopic picture of the alignment process illustrates that the orientation transition is driven by the alignment of molecules with shear flow. The orientation diagram that characterizes the steady-state orientations as a function of shear rate and attractive potential depth is built, in which the attractive potential depth takes the role of an inverse temperature, somewhat like the Flory-Huggins interaction parameter. The microscopic mechanism of the critical orientation transition condition is discussed.

Vorticity banding in rodlike virus suspensions

Vorticity banding under steady shear flow is observed in a suspension of semiflexible colloidal rods (fd virus particles) within a part of the paranematic-nematic biphasic region. Banding occurs uniformly throughout the cell gap within a shear-rate interval (.gamma-, .gamma+) , which depends on the fd concentration. For shear rates below the lower-border shear rate .gamma- only shear elongation of inhomogeneities, which are formed due to paranematic-nematic phase separation, is observed. Within a small region just above the upper-border shear rate .gamma+ , banding occurs heterogeneously. An essential difference in the kinetics of vorticity banding is observed, depending on the morphology of inhomogeneities formed during the initial stages of the paranematic-nematic phase separation. Particle tracking and polarization experiments indicate that the vorticity bands are in a weak rolling flow, superimposed on the applied shear flow. We propose a mechanism for the origin of the banding instability and the transient stability of the banded states. This mechanism is related to the normal stresses generated by inhomogeneities formed due to the underlying paranematic-nematic phase transition.

Observations of immiscible polymer blend electrorheological fluids with a confocal scanning laser microscope

By using a new system that combines a confocal scanning laser microscope and a rheometer, we observed structural changes and measured shear stress simultaneously in immiscible polymer blends, while they were being subjected to shear flow and an electric field. In three different blends various structural changes were observed, which were all related to the electrorheological effect. These blends were composed of liquid crystalline polymers (LCPs) and oils. In one blend we observed a change from a LCP droplet-dispersed structure to an oil-dispersed one, i.e., a phase inversion, while in another blend there was a change from a LCP droplet-dispersed structure to a network structure. After applying shear flow an oil layer was three-dimensionally observed in the third blend, which had an oil-dispersed structure. In addition, it was found that the layer disappeared under no shear and the annihilation process was accelerated by the application of an electric field.

Collisional Rayleigh-Taylor instability and shear-flow in equatorial Spread-F plasma

Abstract. Collisional Rayleigh-Taylor (RT) instability is considered in the bottom side of the equatorial F-region. By a novel nonmodal calculation it is shown that for an applied shear flow in equilibrium, the growth of the instability is considerably reduced. Finite but small amounts of diffusion enhances the stabilization process. The results may be relevant to the observations of long-lived irregularities at the bottom-side of the F-layer.Key words. Ionosphere (ionospheric irregularities, equatorial ionosphere, plasma waves and instabilities)

Phase-separating binary fluids under oscillatory shear.

We apply the lattice Boltzmann methods to study the segregation of binary fluid mixtures under oscillatory shear flow in two dimensions. The algorithm allows to simulate systems whose dynamics is described by the Navier-Stokes and the convection-diffusion equations. The interplay between several time scales produces a rich and complex phenomenology. We investigate the effects of different oscillation frequencies and viscosities on the morphology of the phase separating domains. We find that at high frequencies the evolution is almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low viscosity) and diffusive (high viscosity) regimes, respectively. When the period of the applied shear flow becomes of the same order of the relaxation time T(R) of the shear velocity profile, anisotropic effects are clearly observable. In correspondence with nonlinear patterns for the velocity profiles, we find configurations where lamellar order close to the walls coexists with isotropic domains in the middle of the system. For particular values of frequency and viscosity it can also happen that the convective effects induced by the oscillations cause an interruption or a slowing of the segregation process, as found in some experiments. Finally, at very low frequencies, the morphology of domains is characterized by lamellar order everywhere in the system resembling what happens in the case with steady shear.

Shear-induced structural transformation of pentaethylene glycol n-dodecyl ether and lithium perfluorooctane sulfonate mixed-surfactant lamellar solution

The viscoelastic behavior of the shear-induced structural transformation from the lamellar phase to multilamellar vesicles (MLVs) of a mixed-surfactant system was investigated. The transformation was divided into two processes on the basis of the strain dependence of the apparent viscosity. The first stage is a lamellar-to-intermediate structure transformation. It was found that a strain, not an applied shear rate, governed this process. The second stage is an intermediate-to-MLV phase transformation, which was not controlled by the strain. These structure developments were found in the shear-thickening viscosity regime. The MLV phase formed by applying shear flow exhibited shear-thinning viscosity behavior and reversible response to shear flow. The viscoelastic properties of the MLV phase were investigated by dynamic viscoelastic measurements. Under oscillating shear deformation, the amplitude dependence of the dynamic modulus indicated that the viscoelasticity of the MLV depended on the initial structure, such as the number of vesicle shells and the size of the MLV, which is governed by the preshear rate.

Shear flow effects on polymer melts crystallization: kinetics features

In this work the kinetics of the isothermal crystallization from the melt of isotactic polyolefins in quiescent conditions as well as after the application of a step-shear flow is investigated by means of rheological measurements. It is shown that the kinetics of the crystallization, as measured by the increase of the storage modulus, is not affected by the strain amplitude and the frequency of the oscillation, once they are properly chosen. A strong enhancement of the crystallization kinetics has been obtained when the step-shear flow was applied at the crystallization temperature TC=92°C for two different molecular weight poly(1-butene) samples (i-PB400 and i-PB200) and at TC=137°C for a polypropylene (i-PP). In particular, the overall-crystallization rate constants of the i-PB400 increased with increasing the applied shear rate at a constant total strain of 60. At higher shear flow temperatures slower kinetics occurred in all the cases until the effect of the applied shear flow was lost. Moreover, the effect of the molecular weight on the flow induced crystallization phenomenon is investigated on the two i-PB samples and the results have clearly shown that the higher molecular weight i-PB200 polymer is much more sensitive than the i-PB400 to the flow history.

The segregation of sheared binary fluids in the Bray-Humayun model

The phase separation process which follows a sudden quench inside the coexistence region is considered for a binary fluid subjected to an applied shear flow. This issue is studied in the framework of the convection-diffusion equation based on a Ginzburg-Landau free energy functional in the approximation scheme introduced by Bray and Humayun [{\it Phys.Rev.Lett.} {\bf 68}, 1559, (1992)]. After an early stage where domains form and shear effects become effective the system enters a scaling regime where the typical domains sizes $L_\parallel $, $L_\perp$ along the flow and perpendicular to it grow as $t^{5/4}$ and $t^{1/4}$. The structure factor is characterized by the existence of four peaks, similarly to previous theoretical and experimental observations, and by exponential tails at large wavevectors.

Shear effects on the morphology transition in a nonionic surfactant system

We report the small angle neutron scattering studies on the lamellar-to-gyroid morphology transition in a nonionic surfactant system under shear flow condition. The morphology transition is suppressed by the shear flow when the shear rate exceeds a critical value. This means that the shear field suppresses the undulation fluctuation of lamellar the structure above the critical shear rate. On the contrary when we applied shear flow in the gyroid phase, the shear-induced gyroid-to-lamellar transition was observed above the critical shear rate. The critical shear rate for the suppression of the undulation fluctuation agrees with the critical shear rate for shear induced gyroid-to-lamellar transition.

Flow-induced structure in a thermotropic liquid crystalline polymer as studied by SANS

Small-angle neutron scattering is utilized to determine the flow induced alignment of a model thermotropic liquid crystalline polymer (LCP) as a function of shear rate and temperature. The results demonstrate that the flow-induced structures in thermotropic liquid crystalline polymers have similarities and differences to those in lyotropic liquid crystalline polymer solutions. The shear rate dependence of the alignment shows that the flow-induced alignment correlates very well to the viscosity behavior of the LCP in the shear thinning regime, while temperature variation results in a change in the extent of alignment within the nematic phase. Relaxation results also demonstrate that the flow-induced alignment remains essentially unchanged for up to an hour after the shear field has been removed. Last, there exists a regime at low shear rate and low temperature where alignment of the LCP molecule perpendicular to the applied shear flow is stable. These results provide important experimental evidence of the molecular level changes that occur in a thermotropic liquid crystalline polymer during flow, which can be utilized to develop theoretical models and more efficiently process thermotropic polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3017–3023, 1998

Effects of shear stress on metabolic function of the co-culture system of hepatocyte/nonparenchymal cells for a bioartificial liver.

To improve the culture conditions of hepatocytes for use as a bioartificial liver, the effects of shear flow on the co-culture system of hepatocytes/nonparenchymal cells (NPC) were investigated. A flow chamber with a collagen coated rectangular glass plate, where hepatocytes (5 x 10(4) cell/cm2) and NPC (2 x 10(5) cell/cm2) were seeded, was used to attain a shear stress of 4.7 dyne/cm2. Concentrations of ammonia and urea in the medium were measured daily during the 2 week experiment. The metabolic activity of hepatocytes in the homotypic culture were lower than those of the co-culture, especially when the cultivation time exceeded 1 week. In addition, the applied shear flow promoted activity of the co-culture system. An enhancement in the rates of ammonium removal and urea synthesis was obtained in the perfusion systems. Morphologic observation revealed that aggregates of hepatocytes formed abundantly in the perfusion system and hepatocytes developed a cuboid shape. This suggested that perfusion affected the function and morphology of hepatocytes in the co-culture system. Shear flow could induce cell-cell interactions and secretion of extracellular matrix through the activation of NPC.

Flow instabilities in polymer blends under shear

The effect of shear flow on the miscibility behaviour of the model polymer blend polystyrene/poly(vinyl methyl ether) has been investigated in real time by two-dimensional light scattering and optical microscopy. Under certain shear and dynamic temperature conditions large ‘waves’ have been observed which are aligned either in the direction parallel to the applied shear flow or just off it. Two-dimensional light scattering patterns from samples under the same conditions show sharp streaks aligned in the direction perpendicular to the flow. The evidence presented in this work indicates that these ‘waves’ are unlikely to be elongated phase-separated domains; instead, they could be caused by fluid instabilities.

Oscillatory shear alignment of a liquid crystalline polymer

The alignment of a semiflexible main-chain liquid crystalline polymer by oscillatory and steady shear is investigated in the nematic and smectic states by rheometry and X-ray scattering. Steady shear of the nematic produces alignment in the flow direction, whereas dynamic shear produces planar alignment parallel to the flow and neutral directions. In the smectic state, alignment of the layer normal along the neutral direction was predominant-the result of various frequencies and strain amplitudes. The predominance of this alignment arises from the high degree of interlayer connectivity characteristic of main-chain liquid crystals. More surprisingly, for certain alignment procedures (those with strain amplitude less than 5% and moderate to low frequency), alignment of the layer normal is approximately parallel to the flow direction. Alignment parallel to the velocity-gradient direction was found only when another smectic form, having less interlayer connectivity, was induced by the deformation process. This study demonstrates a rich relationship between applied shear flow, phase behavior, and the molecular orientation.

Shear-induced structure distortion in nonaqueous dispersions of charged colloidal spheres via light scattering

The effect of steady-shear flow on the microstructure of nonaqueous dispersions of colloidal silica spheres having medium-range repulsive potentials is studied, with a newly developed two-dimensional light-scattering set-up, as a function of shear rate ⋗g and particle volume fraction φ. When dispersed in a close refractive index matching solvent mixture of ethanol and toluene, the dispersions exhibit an equilibrium disorder-order transition at φ = 12.6%. It is found that the shear deformation of the colloidal liquid structure at high shear rates is a smooth process with no long-range string or layer ordering in the flow-vorticity plane. The structure distortion is in good qualitative agreement with an approximate solution of the two-particle Smoluchowski equation. In particular, the theoretically recognized boundary-layer effect, which has been implicitly known for its relation to the onset of the non-Newtonian behaviour of effective viscosity, is clearly shown in our experiment for the first time. In the crystal phase an applied shear flow leads to the gradual melting of the equilibrium solid-like structure into sliding layers, strings, and finally into a distorted liquid-like structure as ⋗g is raised. There is also an indication that the layer structure contains a significant number of free strings. No shear-thickening or discontinuous behaviour in dispersion viscosity in relation to these translational order transitions is detected. We conclude that the nonequilibrium order of our system is correlated to the corresponding microstructure in equilibrium.