Vorticity Plane Research Articles

Ordering Kinetics and Alignment of Block Copolymer Lamellae under Shear Flow

The effect of large-amplitude oscillatory shear flow on a concentrated block copolymer solution with lamellar order was studied by in-situ small-angle neutron scattering. Microstructural changes were studied as a function of temperature, frequency of the oscillatory flow field, and thermal history prior to turning on the shear field. We find that the alignment path depends mainly on thermal history prior to turning on the shear field and is independent of frequency and temperature. At long times, the lamellae were aligned parallel to the shearing plates, regardless of frequency, temperature, and thermal history. We refer to this as the parallel orientation. Monotonic changes from the unaligned to the aligned state were found when the shear field was turned on after the sample was completely ordered. The alignment kinetics, in this case, occurs in two stages. The first stage consists of a rapid rotation of the grains so that the lamellar normals lie in the velocity gradient−vorticity plane. This is followed by a slower process wherein the lamellar normals get increasingly localized in the velocity gradient direction. We also studied ordering kinetics under shear, by turning on the shear field before significant ordering had taken place. In this case, the first stage of ordering resulted in the formation of lamellae aligned perpendicular to the shearing plates in addition to the parallel lamellae, regardless of temperature and frequency. Eventually the perpendicular lamellae were transformed to parallel lamellae via an undulation instability.

Flow properties of liquid crystal phases of the Gay–Berne fluid

We have calculated the viscosities of a variant of the Gay–Berne fluid as a function of the temperature by performing molecular dynamics simulations. We have evaluated the Green–Kubo relations for the various viscosity coefficients. The results have been cross-checked by performing shear flow simulations. At high temperatures there is a nematic phase that is transformed to a smectic A phase as the temperature is decreased. The nematic phase is found to be flow stable. Close to the nematic–smectic transition point the liquid crystal model system becomes flow unstable. This is in agreement with the theoretical predictions by Jähnig and Brochard [F. Jähnig and F. Brochard, J. Phys. 35, 301 (1974)]. In a planar Couette flow one can define the three Miesowicz viscosities or effective viscosities η1, η2, and η3. The coefficient η1 is the viscosity when the director is parallel to the streamlines, η2 is the viscosity when the director is perpendicular to the shear plane, and η3 is the viscosity when the director is perpendicular to the vorticity plane. In the smectic phase η1 is undefined because the strain rate field is incommensurate with the smectic layer structure when the director is parallel to the streamlines. The viscosity η3 is found to be fairly independent of the temperature. The coefficient η2 increases with the temperature. This is unusual because the viscosity of most isotropic liquids decreases with the temperature. This anomaly is due to the smectic layer structure that is present at low temperatures. This lowers the friction because the layers can slide past each other fairly easily.

Shear-Induced Structures in Concentrated Surfactant Micellar Phases

Constant and oscillatory Couette shear flow have been used in combination with small-angle neutron scattering to observe the shear-induced ordering in concentrated surfactant micellar phases. For the lamellar phase of hexaethylene glycol monohexadecyl ether, C16 E 6, two distinct lamellae orientations have been identified. At low shear gradients the lamellae are ordered parallel to the flow–vorticity plane, whereas at higher shear gradients the lamellae order parallel to the flow-shear gradient plane, corresponding to a rotation through 90° of the axis of orientation. At intermediate values of constant shear and for oscillatory shear, both lamellae orientations are simultaneously observed for the first time in a surfactant lamellar phase. For the lamellar phase, a dispersion of the binary surfactant mixtures of dioleyl cationic and 2-ethyl hexaglycerol monoether surfactants, a high degree of alignment, in the direction parallel to the flow–vorticity plane, is observed at zero and low shear. With time, during the application of a shear gradient of 25 s−1, the lamellar phase transforms to a highly ordered solution of monodisperse multilamellar vesicles.

Observation of the Structure and the Deformation of A Liquid-Crystalline Polymer Under Shear Flow

We determined the evolution of the main-chain conformation and of the liquid crystalline structure of a liquid crystal polymethacrylate in the smectic phase, versus shear rate. It is shown that the macroscopic shear was transmitted to the smectic layers which ensured an efficient shear of the polymer main-chains. The outcome was a macroscopic orientation of the smectic monodomains with the smectic planes parallel to the shear plane. The polymer main-chains already confined by the mesogenic layers, were in addition, the more elongated along the velocity direction that the stronger, the shear was. These results were obtained in situ by the observations in the shear plane and in the vorticity plane.

From phase orientation to phase destruction opposite shear flow induced effects

We observe the structure of the smectic phase of a side-chain liquid-crystalline polymer when subjected to shear flow. We distinguish two regimes against shear rate. At low velocity, shear induces an alignment of the smectic layers parallel to the shear plane. Whereas for high velocity, instabilities occur which tend to destroy the previous shear induced alignment. Because of the layer orientation, the study has to be carried out in the vorticity plane. A new shear cell has been designed using the specific properties of neutron scattering to give access for the first time to the direct observation of this plane.

Shear-Induced Transformations in the Lamellar Phase of Hexaethylene Glycol Monohexadecyl Ether

The shear-induced ordering of the lamellar phase of hexaethylene glycol monohexadecyl ether, C16E6, in a Couette shear cell has been observed by small angle neutron scattering, SANS. Two distinct lamellae orientations have been identified. At low shear gradients the lamellae are ordered parallel to the flow−vorticity plane, whereas at higher shear gradients the lamellae order parallel to the flow−shear gradient plane corresponding to a rotation through 90° of the axis of orientation. At shear gradients intermediate to those two extremes both orientations are observed simultaneously, this being the first report of such a morphology. A novel extension of shear alignment reported here is the application of oscillatory Couette flow. Here, orientation parallel to the flow−vorticity plane and the intermediate state corresponding to components oriented in the two orthogonal directions are observed. These observations are discussed in the context of recent results of shear-induced effects observed in related syst...

The shear‐distorted microstructure of adhesive hard sphere dispersions: A small‐angle neutron scattering study

In a small‐angle neutron scattering experiment, we measured the shear‐distorted microstructure of a concentrated model dispersion of adhesive hard spheres in the low shear rate regime (Pe≤1) using a couette shear cell. The nonequilibrium structure factor was probed in the velocity–vorticity plane. By changing the temperature, the interaction potential of the particles was varied from a hard sphere repulsion to a strong attraction. The particle distribution which is isotropic at rest, is anisotropically disturbed by the shear flow. At high temperatures, the dispersion is effectively a hard sphere system. In this case the main distortion of the scattering pattern is in the direction of the fluid flow, the shear flow shifts the first maximum in the structure factor toward smaller scattering angles. In the vorticity direction, no significant distortion of the equilibrium structure is observed. Qualitative agreement is found with theoretical predictions for the shear‐distorted structure factor. With decreasing...

Structure factor distortion for hard-sphere dispersions subjected to weak shear flow: Small-angle neutron scattering in the flow–vorticity plane

Small angle neutron scattering (SANS) was used as a probe of the structure factor distortion for concentrated hard-sphere dispersions subjected to steady shear flow. The dispersions consisted of stearically stabilized silica spheres of 40 nm radius suspended in cyclohexane, with volume fractions 0.36<φ<0.52. We subjected the dispersions to weak shear flow (Peclet number Pe<1) in a Couette flow cell, and collected intensity measurements in the flow–vorticity plane. We conducted the experiments using the D11 spectrometer at the Institut Laue-Langevin in Grenoble, France. The data permit a thorough evaluation of the existing theories for the shear-induced distortion of the structure factor. We find that the theories as they stand severely underpredict the distortion at low wave vectors (‖k‖<0.1 nm−1), but the theories can be made to qualitatively agree with the data by adjusting Pe in the equations to a higher value. The theories are based on a perturbation expansion that is linear in small Pe, but such an expansion is inadequate for describing our data. We show that the nonequilibrium contribution to the structure factor is proportional to fractional powers of Pe, with the exponent depending on volume fraction φ. The failure of the theories is probably due to their neglect of hydrodynamic interactions, which are important in concentrated dispersions. The distortion of the structure factor is greatest around its first maximum, with significant distortion being exhibited in both the flow and vorticity directions. The peak in the structure factor broadens, indicating that weak shear flow produces a more disordered structure relative to the equilibrium structure. The data reveal that the shear flow results in a decreased concentration of particles in the flow direction. The concentration of particles in the vorticity direction increases when shear flow is applied, and the formation of temporal clusters is probable.