Kinetics of the shear-induced isotropic-to-lamellar transition of an amphiphilic model system: a nonequilibrium molecular dynamics simulation study

The shear-induced isotropic-to-lamellar phase transition in the amphiphilic systems in the vicinity of the quiescent order-to-disorder transition point is investigated by the large-scale parallel nonequilibrium molecular dynamics simulations of simple amphiphilic model systems. There is a shear-induced upward shift of the ordering temperature. The initial isotropic phase orders into a lamellar phase perpendicular to the shear vorticity. The phase diagram as a function of temperature and shear rate is established. The dependency of the ordering transition on interaction strength and shear rate is rationalized by the competition between shear rate and chain relaxation. The time evolution of morphology reveals that the shear-induced ordering proceeds via nucleation and growth, a signature of a first-order phase transition. At low shear rate, a single ordered domain grows after an incubation period. With increasing shear rate ordering speeds up, but eventually develops in a lamellar system with disordered shear bands. The time dependence of the order parameter follows that of the mean-squared end-to-end distance, shear viscosity, and bulk pressure, and follows an Avrami scheme with an Avrami exponent between 2 and 4.     

Structure of AOT reverse micelles under shear

Reverse micelles in the AOT/water/isooctane system, at various water contents (W(0)), were studied using rheometry and small angle X-ray scattering (SAXS) experiments under static conditions and under shear. The SAXS analysis confirmed the spherical shape of the micelles at low water content and revealed a transition into elongated micelles at higher water content. A population of spherical micelles was found to coexist with the cylindrical ones, even above the percolation threshold. The shape transformation was correlated with a viscosity leap observed in the rheometry measurements. Reverse micelles at low water content under shear act as a Newtonian fluid, without any detectable shape changes. In contrast, reverse micelles at high water content behave as a shear thinning fluid. SAXS measurements at high water content under shear force have shown that the shear forces induced alignment of the cylindrical micelles in the flow direction, without any other changes in the micelle dimensions. The anisotropy parameter, a measure of the degree of the spatial order, was found to increase with increasing water content and shear rate.     

Rheology of branched wormlike micelles

The topology of self-assembled surfactant solutions includes varying degrees of micellar branching, ranging from linear wormlike micelles to a micellar network. Micellar branching acts as an effective attraction between micelles such that network condensation can lead to phase separation. Unlike chemical branching in polymers, micellar branches are labile. Movement of branches along a micelle contour has therefore been proposed as a mechanism of stress relaxation that leads to a reduction in the structural relaxation time and thus, the zero-shear viscosity. Branching is also thought to suppress flow alignment, and for lower levels of branching, may also suppress instabilities such as shear banding. The suppression of shear banding can lead to a lesser degree of shear-thinning in the apparent viscosity at higher shear rates, as well as a reduction in extensional thickening. However, for higher levels of branching, shear can induce branching for samples in proximity to such a phase transition, which can result in shear banding due to shear-induced phase separation. Recent modeling and simulations of the energetics of branching, as well as experiments on model systems, show that the reduction in zero-shear viscosity is due to micelle branching. Current research includes efforts to develop a more mechanistic, quantitative understanding of micellar branching and more generally, its effects on micellar solution rheology.     

Formation and shear-induced processing of quantum dot colloidal assemblies in a multiphase microfluidic chip

The controlled self-assembly of polymer-stabilized quantum dots (QDs) into mesoscale aqueous spherical assemblies termed quantum dot compound micelles (QDCMs) using a two-phase gas-segmented microfluidic reactor is described. Self-assembly is initiated by the fast mixing of water (approximately 1 s) with a blend solution of polystyrene-coated QDs and amphiphilic polystyrene-block-poly(acrylic acid) stabilizing chains via chaotic advection within liquid plugs moving through a sinusoidal channel. Subsequent recirculating flow within a post-formation channel subjects the dynamic QDCMs to shear-induced processing, controlled via the flow rate and channel length, before a final quench into pure water. During processing, larger QDCMs within the initial population undergo breakup into smaller particles, resulting in smaller mean particle sizes, smaller relative standard deviations, and more skewed distribution shapes, as the overall shear exposure is increased. For these cases, shear-induced size reduction is sufficient to dominate surface tension-driven growth.     

Giant micellar worms under shear: a rheological study using SANS

Flow-SANS experiments were performed on viscoelastic aqueous solutions of erucyl bis(hydroxyethyl) methylammonium chloride in the presence of potassium chloride. This cationic surfactant has the ability to form very long and flexible wormlike micelles upon addition of salt. The effects of the key-parameters-shear rate, temperature, surfactant and salt concentration-on the ability of the micelles to align in the flow-field were investigated. The scattering data were analyzed in terms of an anisotropy factor (Af). It was found that the wormlike micelles aligned in the direction of the applied shear rate and that the anisotropy factor increased with shear rate. In addition, an increase in temperature caused a decrease of the anisotropy factor (Af) due to the formation of shorter worms. Furthermore, the branching of the micelles at high ionic strength caused the anisotropy factor to decrease in comparison with the values obtained from linear wormlike micelles, hence revealing that the formation of 3-way junctions restricts the alignment of the micelles in the shear-flow. Furthermore, the total surfactant concentration was found to affect the shear-induced patterns significantly, and different behaviors were observed depending on the ionic strength.     

Shear-induced phase separation in solutions of wormlike micelles

Polymer solutions in the vicinity of the theta-point are known to undergo shear-induced turbidity or phase separation. The present study shows that a similar phenomenon also occurs for certain wormlike micellar solutions. Wormlike micelles are the self-assembled counterparts of polymers and are characterized by their ability to reversibly break and recombine. In the system of interest, the micelles are formed by the cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC), in conjunction with a salt such as sodium chloride (NaCl) or sodium salicylate (NaSal). Micellar samples that become turbid under shear show evidence of critical concentration fluctuations and may contain predominantly branched micelles. The shear-induced turbidity in these samples correlates with the appearance of flow-dichroism in rheooptic experiments and with an increase in low-q scattering in small-angle light scattering under flow (flow-SALS) experiments. The characteristic "butterfly" pattern, with enhanced scattering in the flow direction and a dark streak perpendicular to the flow direction, is typically observed in flow-SALS. The results suggest that the turbidity is due to a shear-induced growth of concentration fluctuations, which in turn manifests as large anisotropic domains, typically oriented along the vorticity axis.     

Non-Newtonian behavior in simple fluids

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheology of a microscopic sample of simple fluid. The calculations were performed using a configurational thermostat which unlike previous nonequilibrium molecular dynamics or nonequilibrium Brownian dynamics methods does not exert any additional constraint on the flow profile. Our findings are in agreement with experimental results on concentrated "hard sphere"-like colloidal suspensions. We observe: (i) a shear thickening regime under steady shear; (ii) a strain thickening regime under oscillatory shear at low frequencies; and (iii) shear-induced ordering under oscillatory shear at higher frequencies. These results significantly differ from previous simulation results which showed systematically a strong ordering for all frequencies. They also indicate that shear thickening can occur even in the absence of a solvent.     

In situ small-angle neutron scattering and rheological measurements of shear-induced gelation

The microscopic structure of shear-induced gels for a mixed solution of 2-hydroxyethyl cellulose and nanometer-size spherical droplets has been investigated by in situ small-angle neutron scattering (SANS) with a Couette geometry as a function of shear rate gamma. With increasing gamma, the viscosity increased rapidly at gamma approximately 4.0 s(-1), followed by a shear thinning. After cessation of shear, the system exhibited an extraordinarily large steady viscosity. This phenomenon was observed as a shear-induced sol-gel transition. Real-time SANS measurements showed an increase in the scattering intensity exclusively at low scattering angle region. However, neither orientation of polymer chains nor droplet deformation was detected and the SANS patterns remained isotropic irrespective of gamma. It took about a few days for the gel to recover its original sol state. A possible mechanism of gelation is proposed from the viewpoint of shear-induced percolation transition.     

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.     

Hydrogen bonding in ethanol under shear

We study the dependence of viscosity of ethanol on shear rate using constant volume and constant pressure nonequilibrium molecular dynamics simulations, with the emphasis of the interrelationship between breaking, stability, and alignment of hydrogen bonds and shear thinning at high shear rates. We find that although the majority of hydrogen bond breakings occur at low shear rates, we do not observe shear thinning until there is some shear-induced alignment of the hydrogen bonds with the direction of shear.     

Shear-induced conformational ordering, relaxation, and crystallization of isotactic polypropylene

The shear-induced coil-helix transition of isotactic polypropylene (iPP) has been studied with time-resolved Fourier transform infrared spectroscopy at various temperatures. The effects of temperature, shear rate, and strain on the coil-helix transition were studied systematically. The induced conformational order increases with the shear rate and strain. A threshold of shear strain is required to induce conformational ordering. High temperature reduces the effect of shear on the conformational order, though a simple correlation was not found. Following the shear-induced conformational ordering, relaxation of helices occurs, which follows the first-order exponential decay at temperatures well above the normal melting point of iPP. The relaxation time versus temperature is fitted with an Arrhenius law, which generates an activation energy of 135 kJ/mol for the helix-coil transition of iPP. At temperatures around the normal melting point, two exponential decays are needed to fit well on the relaxation kinetic of helices. This suggests that two different states of helices are induced by shear: (i) isolated single helices far away from each other without interactions, which have a fast relaxation kinetic; (ii) aggregations of helices or helical bundles with strong interactions among each other, which have a much slower relaxation process. The helical bundles are assumed to be the precursors of nuclei for crystallization. The different helix concentrations and distributions are the origin of the three different processes of crystallization after shear. The correlation between the shear-induced conformational order and crystallization is discussed.     

Effect of ionic strength on the rheological behavior of aqueous cetyltrimethylammonium p-toluene sulfonate solutions

The influence of ionic environment on the rheological properties of aqueous cetyltrimethylammonium p-toluene sulfonate (CTAT) solutions has been studied under three different flow fields: simple shear, opposed-jets flow and porous media flow. Emphasis was placed in the experiments on a range of CTAT concentration in which wormlike micelles were formed. It is known that these solutions exhibit shear thickening in the semi-dilute regime, which has been explained in terms of the formation of shear-induced, cooperative structures involving wormlike micelles. In simple shear flow, the zero shear viscosity exhibits first an increase with salt addition followed by a decrease, while the critical shear rate for shear thickening increases sharply at low salt contents and tends to saturate at relatively high ionic strengths. The results are explained in terms of a competition between micellar growth induced by salt addition and changes in micellar flexibility caused by ionic screening effects. Dynamic light scattering results indicate that micelles grow rapidly upon salt addition but eventually achieve a constant size under static conditions. These observations suggest that the wormlike micelles continuously grow with salt addition, but, as they become more flexible due to electrostatic screening, the wormlike coils tend to adopt a more compact conformation. The trends observed in the apparent viscosities measured in porous media flows seem to confirm these hypotheses-but viscosity increases in the shear thickening region-and are magnified by micelle deformation induced by the elongational nature of the local flow in the pores. In opposed-jets flow, the solutions have a behavior that is close to Newtonian, which suggests that the range of strain rates employed makes the flow strong enough to destroy or prevent the formation of cooperative micellar structures.     

Small-angle X-ray scattering study of a poly(oxyphenylethylene)–poly(oxyethylene) diblock copolymer gel under shear flow

 The structure and flow behaviour of a micellar “cubic” phase is studied, using small-angle X-ray scattering (SAXS) and constant stress rheometry on a poly(oxyphenylethylene)–poly(oxyethylene) diblock copolymer in water. The predominant structure is a face-centred cubic (fcc) array of spherical micelles, which under shear undergoes layer sliding to give a scattering pattern from stacked hexagonal close-packed layers. A detailed analysis of the SAXS data indicates the presence of a fraction of grains with a structure distorted from a fcc phase. The additional reflections that characterize this structure can be indexed to a rhombohedral unit cell, space group R3ˉm, with the same volume as the fcc unit cell. The rhombohedral unit cell corresponds to a cubic cell that has been “stretched” along a [111] direction, and it is suggested that such a structure results from the gradient in shear velocity in the Couette cell employed. Shearing at high shear rates leads to a “smearing out” of the reflections, but upon cessation of shear under these conditions a highly oriented SAXS pattern is obtained, which confirms the persistence of rhombohedral ordering. The shear-induced changes in orientation are correlated to a plateau observed in the stress plotted against shear rate, such a plateau being a sign of inhomogeneous flow. Received: 8 September 2000 Accepted: 29 November 2000     

Diagram of State of Stiff Amphiphilic Macromolecules

Summary: We studied coil-globule transitions in stiff-chain amphiphilic macromolecules via computer modeling and constructed phase diagrams for such molecules in terms of solvent quality and persistence length. We showed that the shape of the phase diagram essentially depends on the macromolecule degree of polymerization. Relatively short amphiphilic molecules always form a spherical globule in a poor solvent, and the coil-globule transition includes one or two intermediate conformations, depending on the chain's stiffness. These are a disk-like globule in case of high enough Kuhn segment length, and a pearl necklace-like structure of spherical micelles and a disk-like globule in case of relatively flexible chains. The phase diagram of a long stiff amphiphilic chain was found to be more complex still. Thus three specific regions can be distinguished in the poor solvent region, depending on the chain stiffness. These correspond to a cylindrical globule without any specific backbone ordering, a cylindrical globule containing blobs with collagen-like ordering of the chain, and co-existence of collagen-like and toroidal globules. In the intermediate transition region in this case, apart from the pearl necklace-like conformations with spherical micelles, necklace conformations can be also observed where the polymeric chain has collagen-like ordering within each bead.     

Molecular dynamics simulation of a reverse micelle self assembly in supercritical CO2

In this communication we report on molecular dynamics computer simulations of self-assembly of reverse micelles in supercritical carbon dioxide. The reverse micelles contain perfluoropolyether ammonium carboxylate surfactants and an aqueous core. We observed a quick self-assembly of these micelles over time periods of approximately 5 ns, irrespective of initial conditions. In most cases, the self-assembled perfluorinated reverse micelles have a nice spherical shape and properties consistent with experiments. When the fluorinated surfactant is replaced by its hydrogenated analogue, the assembled aggregate contains a region of direct contact between water and carbon dioxide, indicating that hydrogenated surfactant is not a good agent for creation of microemulsions in water/carbon dioxide mixtures.     

Model shape transitions of micelles: spheres to cylinders and disks

We present a microscopic analysis of shape transitions of micelles of model linear nonionic surfactants. In particular, symmetric H(4)T(4) and asymmetric H(3)T(6) surfactants have been chosen for the study. In a previous work, it has been observed that symmetric surfactants have a strong tendency to prefer spherical micelles over a wide range of chemical potentials, while asymmetric surfactants undergo shape transitions between a spherical micelle at low concentration to other forms, mainly finite cylindrical micelles. This study combines the application of a two-dimensional single-chain mean-field theory (SCMFT) with Monte Carlo (MC) simulations of exactly the same systems. On the one hand, the characteristics of the SCMFT make this method suitable for free energy calculations, especially for small surfactants, due to the incorporation of relevant microscopic details in the model. On the other hand, MC simulations permit us to obtain a complete picture of the statistical mechanical problem, for the purpose of validation of the mean-field calculations. Our results reveal that the spherical shape for the symmetric surfactant is stable over a large range of surfactant concentrations. However, the asymmetric surfactant undergoes a complex shape transition that we have followed by calculating the standard chemical potential as a function of the aggregation number. The results indicate that the system forms prolate spheroids prior to developing short capped cylinders that gradually grow in length, with some oscillations in the energy of formation. The most important result of our work is the evidence of a bifurcation where, together with the elongated objects, the system can develop oblate aggregates and finally a torus shape similar to a red blood cell.     

Structure and transport properties of polymer grafted nanoparticles

We perform molecular dynamics simulations on a bead-spring model of pure polymer grafted nanoparticles (PGNs) and of a blend of PGNs with a polymer melt to investigate the correlation between PGN design parameters (such as particle core concentration, polymer grafting density, and polymer length) and properties, such as microstructure, particle mobility, and viscous response. Constant strain-rate simulations were carried out to calculate viscosities and a constant-stress ensemble was used to calculate yield stresses. The PGN systems are found to have less structural order, lower viscosity, and faster diffusivity with increasing length of the grafted chains for a given core concentration or grafting density. Decreasing grafting density causes depletion effects associated with the chains leading to close contacts between some particle cores. All systems were found to shear thin, with the pure PGN systems shear thinning more than the blend; also, the pure systems exhibited a clear yielding behavior that was absent in the blend. Regarding the mechanism of shear thinning at the high shear rates examined, it was found that the shear-induced decrease of Brownian stresses and increase in chain alignment, both correlate with the reduction of viscosity in the system with the latter being more dominant. A coupling between Brownian stresses and chain alignment was also observed wherein the non-equilibrium particle distribution itself promotes chain alignment in the direction of shear.     

Comparison of aggregation behaviors between branched and linear block polyethers: MesoDyn simulation study

The comparison of aggregation behaviors between the branched block polyether T1107 (polyether A) and linear polyether (EO)₆₀(PO)₄₀(EO)₆₀ (polyether B) in aqueous solution are investigated by the MesoDyn simulation. Polyether A forms micelles at lower concentration and has a smaller aggregation number than B. Both the polyethers show the time-dependent micellar growth behaviors. The spherical micelles appear and then change to rod-like micelles with time evolution in the 10 vol% solution of polyether A. The micellar cluster appears and changes to pseudo-spherical micelles with time evolution in the 20 vol% solution of polyether A. However, the spherical micelles appear and change to micellar cluster with time evolution in the 20 vol% polyether B solution. The shear can induce the micellar transition of both block polyethers. When the shear rate is 1?×?10⁵ s^?1, the shear can induce the sphere-to-rod transition of both polyethers at the concentration of 10 and 20 vol%. When the shear rate is lower than 1?×?10⁵ s^?1, the huge micelles and micellar clusters can be formed in the 10 and 20 vol% polyether A systems under the shear, while the huge micelles are formed and then disaggregated with the time evolution in the 20 vol% polyether B system.     

Analysis of shear-induced and extensional-induced associating polymer assemblies: Brownian dynamics simulation

In order to examine the difference between shear-induced and extensional-induced associating polymer assemblies at the molecular level, Brownian dynamics simulations with the bead-spring model were carried out for model DNA molecules with sticky spots. The radial distribution of molecules overestimates from that in the absence of flow and increases with increasing Weissenberg number in extensional flow, but slightly underestimates without regard to shear rate in shear flow. The fractional extension progresses more rapidly in extensional flow than in shear flow and the distribution of fractional extension at the formation time has a relatively sharper peak and narrower spectrum in extensional flow than in shear flow. In shear flow, the inducement of the assembly mainly results from the progress of the probability distribution of fractional extension. However, in extensional flow, the assembly is induced by both the progress of the probability distribution and increasing the values of the radial distribution.     

Elongational flow induced ordering in surfactant micelles and mesophases

We have used small angle neutron scattering, SANS, to investigate the elongational flow induced ordering in surfactant micelles and mesophases. Spatially resolved SANS measurements have been used to determine the distribution of orientational ordering over the flow velocity pattern in an elongational flow cell, and comparison with the effects of shear flow are made. Two different surfactant systems have been studied, the charged wormlike mixed micelles of hexaethylene monododecyl ether, C16E6/hexadecyl trimethylammonium bromide, C16TAB (3% C16E(6)/5 mol% C16TAB), and the Lalpha lamellar phase of C16E6 (50.6 wt% C16E6 at 55 degrees C), and a substantially different response is observed. The orientational distribution of the Lalpha lamellar phase of C16E6 reflects the flow velocity pattern distribution within the cell, whereas for the wormlike mixed micelles of C16E6/C16TAB this is not the case, and this is associated with the shear thinning behavior of that system.