Shear-induced parallel-to-perpendicular orientation transition in the amphiphilic lamellar phase: A nonequilibrium molecular-dynamics simulation study

The relationship between structure and rheology in a three-dimensional sheared lamellar mesophase

The evolution of a lamellar mesophase from an initially disordered state under shear is examined using simulations of a mesoscale model based on a concentration field ψ that distinguishes the hydrophilic and hydrophobic components. The Landau-Ginzburg free-energy functional is augmented by a term that is minimised for sinusoidal modulations in the concentration field with wavelength λ = (2π/k), and the dynamical equations are the model H equations. The structure and rheology are determined by the relative magnitudes of the diffusion time for coarsening, (λ²/D) and the inverse of the strain rate ^-1, and the Ericksen number, which is the ratio of the shear stress and the layer stiffness. When the diffusion time is small compared with the inverse of the strain rate, there is a local formation of misaligned layers, which are deformed by the imposed flow. There is near-perfect ordering with isolated defects at low values of the Ericksen number, but the defects result in a significant increase in viscosity due to the high layer stiffness. At high values of the Ericksen number, the concentration field is deformed by the mean shear before layers form via diffusion. Cylindrical structures aligned along the flow direction form after about 8-10 strain units, and these evolve into layers with disorder through diffusion perpendicular to the flow. The layers are not perfectly ordered, even after hundreds of strain units, due to the creation and destruction of defects via shear. The excess viscosity is low because the layer stiffness is small compared with the applied shear at a high Ericksen number. This study provides guidance on how the material parameters and imposed flow can be tailored to achieve the desired rheological behaviour.

Microscopic shear flow simulations of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid

We have studied the behaviour of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid in shear flow by molecular dynamics simulation using the SLLOD equation of motion. This is facilitated by the fact that the biaxial symmetry allows linear relations between the pressure and the velocity gradient. This means that linear irreversible thermodynamics can be applied independently of the simulations to obtain the torques determining the orientations of the system and that the predictions of this theory can be cross-checked by the simulations. It turns out that there is a torque turning the smectic layers to the orientation parallel to the vorticity plane if the simulation is started in another orientation. In the orientation parallel to the vorticity plane where the director formed by the long axes of the molecules, nw, is perpendicular to the vorticity plane there is another torque keeping the director formed by the normals of the broadsides of the molecules, nu, parallel to this plane at a constant alignment angle, ψ relative to the streamlines independently of the strain rate. Moreover, this alignment angle seems to be the one where the irreversible energy dissipation rate, , is minimal. This is in agreement with a recently proven theorem according to which is minimal in the linear regime of a nonequilibrium steady state. Finally, we studied the orientation of nu when the smectic layers are parallel to the shear plane. In a simulation this orientation is stabilised by the periodic boundary conditions. Then we found that there was a nonlinear torque turning nu to the orientation perpendicular to the streamlines thus minimising the value of even though this value is larger than the value of in the orientation parallel to the vorticity plane. This means that is minimized given the external boundary conditions.

Shear flow simulations of smectic liquid crystals based on the Gay–Berne fluid and the soft sphere string-fluid

Smectic liquid crystal undergoing shear flow.

Shear-induced parallel and transverse alignments of cylinders in thin films of diblock copolymers

Coarse-grained Langevin dynamics simulations were performed to investigate the alignment behavior of monolayer films of cylinder-forming diblock copolymers under steady shear, a structure of significant importance for many technical applications such as nanopatterning. The influences of shear conditions, the interactions involved in the films, and the initial morphology of the cylinder-forming phase were examined. Our results showed that above a critical shear rate, the cylinders can align either along the shearing direction or transverse (log-rolling) to the shearing direction depending on the relative strength between the interchain attraction in the cylinders (εAA) and the surface attraction of the confining walls with the film (εBW). To understand the underlying mechanism, the microscopic properties of the films under shear were systematically investigated. It was found that at low εAA/εBW, the majority blocks of the diblock polymer that are adsorbed on the confining walls prefer to move synchronously with the walls, inducing the cylinder-forming blocks to align along the flow direction. When εAA/εBW is above a threshold value, a strong attraction between the cylinder-forming blocks restrains their movement during shear, leading to the log-rolling motions of the cylinders. To predict the threshold εAA/εBW, we developed an approach based on equilibrium thermodynamics data and found good agreement with our shear simulations.

Diblock-copolymer thin films under shear

The behavior of lamellae forming diblock-copolymer melts confined by two non-selective substrates under shear is studied by means of molecular dynamics simulations. Since the substrate/copolymer preferential interaction is absent, the vertically oriented lamellae (L_⊥) are formed. The response of L_⊥ phase under transverse and perpendicular modes of shear is studied for a wide range of shear rates, γ̇. In particular, shear deformation and reorientation transition, flow behavior, and difference in the macroscopic response under the two modes of shear are discussed. We show that an inclined lamellae state observed for transverse shear below a critical shear rate γ̇^* is stabilized by a cyclic motion of chains close to the substrates. The value of γ̇^*, at which lamellae dissolve and reorient along the flow field during transverse shear, coincides with the onset of shear-thinning. For γ̇<γ̇^*, the shear viscosity for transverse shear is much larger compared to that observed in perpendicular shear, while there is no difference for γ̇>γ̇^*.

Dynamic orientation transition of the lyotropic lamellar phase at high shear rates

The dynamic orientation behavior of the lamellar phase of a triblock copolymer is studied in a wide range of shear rates as a function of solvent composition. We find that various phases can be induced by increasing the shear rate. At low shear rates, the onion phase forms from planar lamellae with many defects. A further increase of the shear rate caused the onion structure to break down, and the lamellar phase recovers with fewer defects. Finally, the transition of the orientation from parallel to perpendicular is observed at high shear rates. In the orientation transition at high shear rates, a stable intermediate structure, to our knowledge, is found for the first time. We also find that the critical shear stress of the rupture of the onion phase coincides with the orientation transition. The consistency of the critical shear stress suggests that all orientation transitions at a high shear rate are dominated by a mechanical balance between the applied viscous stress and the internal relaxation mode of the lamellae.

Morphology and self-arraying of SDS and DTAB dried on mica surface

Dewetting phenomena produce interesting patterns that may impart new properties to solid surfaces. Sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) aqueous solutions, dried on mica surfaces under different drying conditions, undergo dewetting events forming structured deposits that were imaged by scanning electron microscopy (SEM), atomic force (AFM) and Kelvin force microscopy (KFM). Dry SDS, in most situations, displays long branched stripes formed due to fingering instability, while DTAB undergoes stick-slip motion forming patterns of parallel continuous or split stripes. In both systems, independently of drying conditions, surfactants pack forming lamellar structures, but with different orientations: SDS lamellae are aligned parallel to the substrate whereas DTAB lamellae are normal to the mica plane. Electric potential maps of SDS obtained by KFM show well-defined electrostatic patterns: surfactant layers deposited on mica are overall negative with a larger excess of negative charge in the interlamellar space than in the lamellar faces.

Rheodielectric study on shear-induced structural change in the smectic-A phase of 4-n-octyl-4'-cyanobiphenyl (8CB)

Simultaneous measurements of rheological and dielectric properties are made to investigate shear-induced structural change in the smectic-A phase of 8CB (4-n-octyl-4'-cyanobiphenyl). With increasing the shear rate, the fluidity changes from non-Newtonian to Newtonian flow via an unstable flow region, accompanied by a characteristic change in the dielectric permittivity. In the non-Newtonian flow region, a dielectric dispersion, which can be ascribed to an undulation motion of smectic layer, is recognized. On the basis of these results, it is suggested that with increasing the shear rate the undulation changes to a chaotic structure, which is followed by a more simple structure with the layer normal along the neutral axis, and that these structural changes are responsible for the fluidity change.

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.

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.