Cessation Of Shear Research Articles

Structural transformations and viscoelastic response of sheared fingerprint cholesteric textures

Analysis and computer simulations of structural transformations and viscoelastic response functions are presented for the steady rectilinear simple shear flows of a flow-aligning cholesteric liquid crystal oriented with the helix axis along the vorticity direction of the flow; the helix orientation is known as the fingerprint texture. Computation and analysis of orientation and stress relaxation after cessation of shear are also given. The governing parameter that controls the orientation response to the imposed shear is the Ericksen number E (ratio of viscous to elastic effects). For a sufficiently high value of E, a sheared cholesteric liquid crystal with a fingerprint texture originally aligned along the vorticity axis is found to undergo the helix uncoiling transition and adopt the nematic flow-aligned ordering. On the other hand if the Ericksen number is low, the helix rotates driven by vorticity and the orientation state is characterized by traveling twist waves. The transition between the traveling twist wave mode and the stationary nematic mode is found to be mediated by the expansion of the cholesteric pitch, and occurs at a critical value of E, denoted by E c , Close form solutions for the helix pitch as a function of the Ericksen number and for the critical Ericksen number E c for helix uncoiling are obtained and shown to be in good agreement with the numerical solutions. The viscoelastic modes during shear are identified and analyzed. For steady shearing the classical three-region apparent viscosity curve is obtained. Stress relaxation after cessation of shear is computed and explained in terms of the governing scales of the model and in terms of elastic storage modes that operate during shear. Typical oscillatory underdamped stress relaxation oscillations generally present in liquid crystalline materials are found.

A preliminary morphological evidence for the existence of back-flow effect associated with the formation of band texture after cessation of shear for a polymeric liquid crystal

The band texture of the hydroxypropylcellulose (HPC) film is observed after cessation of shear by optical microscopy and scanning electron microscopy (SEM). The sinusoidal supramolecular structure associated with the band texture of HPC is verified by polarized optical microscopy, so the band texture corresponds to the spatial periodicity of local director or molecular orientation. On the other hand, however, the pleated morphology, a depth periodicity, is confirmed by SEM and the pleat period is consistent with the band period. What is more, even without polarizer and analyser, the bands can still be found in an optical microscope with the same periodicity as those under cross polars, and these bands might be attributed to a density periodicity. Both the depth and density periodicities result from the mass flow after cessation of shear and must be coupled to the orientation periodicity resulting from director rotation. As a consequence, we propose that the back-flow effect is striking in the formation of band textures and should be taken into consideration in order to give a plausible and explicit mechanism of band formation.

Anisotropic Small Angle Light and Neutron Scattering from a Lyotropic Lamellar Phase under Shear

The influence of shear on the lamellar phase of a semi-dilute solution (36% (w/w)) of tetra ethylenglycol dodecylether (CH 3 [CH 2 ] 11 [OCH 2 CH 2 ) 4 OH, C 12 E 4 ) in water was investigated by Small Angle Light Scattering (SALS), Small Angle Neutron Scattering (SANS) and rheology. A plateau modulus was determined by low amplitude oscillatory shear experiments. A strong scattering peak perpendicular to the flow direction and a small peak along the flow direction were observed in rheo-SANS. The rheo-SALS patterns were also anisotropic. At low shear rates, a four lobe pattern was found in depolarized light scattering (H v ) and scattering perpendicular to the flow direction was observed in polarized scattering (H H ). A butterfly pattern was found in light scattering at high shear rates. The butterfly pattern was stable after cessation of shear, but it was changed by slow shear flow. The scattering data can be explained by a shear induced formation of deformed vesicles and the butterfly pattern might be correlated with an assembly of vesicles disordered in the direction of shear.

Structure development under transient shear flow in semidilute polystyrene solution

AbstractThe 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 γC 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 γC. 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.

Small-angle neutron scattering from a hexagonal phase under shear

The shear orientation of a micellar hexagonal liquid crystalline phase was investigated by small-angle neutron scattering. The hexagonal phase in the quiescent state showed a symmetrical scattering pattern typical of a polydomain structure. Enhanced scattering along the flow direction was observed during shear and the anisotropy of scattering intensity became stronger with increasing shear rate. The anisotropic scattering pattern corresponds to an orientation perpendicular to the flow direction and can be interpreted as a log-rolling state. The oriented sample did not relax after cessation of shear. The results from small-angle neutron scattering confirm data obtained previously from rheo-small angle light scattering measurements and are discussed in comparison to shear alignment of lyotropic liquid crystalline polymer solutions.

Isotropic and Anisotropic Composition Fluctuations Close to the Order-to-Disorder Transition in an Asymmetric Diblock Copolymer Melt Subjected to Reciprocating Shear Fields

An asymmetric poly(ethylene-propylene)-poly(ethylethylene) (PEP-PEE) diblock copolymer was examined near the order-to-disorder transition temperature, TODT, by small-angle neutron scattering as a function of shear rate, $\dot{\gamma}$. Heating in the absence of shear disorders the material from a hexagonal structure at TODT=155±1 ○C. Application of a reciprocating simple shear field markedly influences the phase behavior near the ODT. Application of shear while heating increases TODT consistent with the theoretical prediction, $T_{\rm ODT}\sim\dot{\gamma}^{2} $. During fast cooling studies we observe an additional characteristic temperature, TS, at which the material spontaneously orders, which we interpret as a stability (i.e., spinodal) limit. this result also agrees with the theoretical prediction, $\tau_{\rm S}\sim\dot{\gamma}^{-1/3}$. Upon cessation of shear at temperatures close to but above the quiescent TODT a transient phase is observed before the system macroscopically disorders. The transient structure has a symmetry compatible with spheres arranged on a body centered cubic (BBC) space lattice, or undulating cylinders of the minority component (PEE). Anisotropic (i.e. lattice pinned) fluctuations can also be observed close to, but below, TODT upon heating the hexagonal structure under shear free conditions.

Relaxation of Concentration Fluctuations in a Shear Field

We have investigated relaxations in a semidilute solution of a high molecular weight polystyrene in the vicinity of ϑ conditions, using three different techniques: dynamic light scattering, relaxation of flow after cessation of shear recorded through the decay of the intensity of scattered light, and flow relaxation after cessation of shear recorded rheologically. We have used a solution at the ϑ temperature of polystyrene with molecular weight Mw = 4.9 × 106 in dioctyl phthalate. All three techniques yield similar values of the viscoelastic relaxation time, at various temperatures between 12 and 35 °C. Two additional modes are observed with dynamic light scattering: the well-established gel diffusion mode of the transient network of the entangled semidilute solution and an additional mode intermediate to the previous two. It is found that the shear field enhances only the longest viscoelastic relaxation mode.

Shear-Induced Alignment of a Hexagonal Lyotropic Liquid Crystal as Studied by Rheo-NMR

The shear-alignment process of the hexagonal lyotropic liquid-crystalline phase formed by a solution of hexa(ethylene glycol) monododecyl ether (C 12 E 6 ) in heavy water is examined. Experiments were performed with a novel cone-and-plate rheo-NMR apparatus, which allows the simultaneous measurement of shear viscosity and deuterium nuclear magnetic resonance. The NMR spectra show that initially disordered samples having polydomain structures become macroscopically aligned during shear. The aligned state shows no relaxation after cessation of shear. With proceeding shear alignment, the apparent shear viscosity decreases. The changes of the deuterium NMR spectra observed during the alignment process reveal that the reorientation of the domains depends not on the shear rate but on the shear strain. NMR experiments using different shear geometries show that at shear rates of ca. 2 s -1 the hexadic axis becomes aligned parallel to the flow. At lower shear rates, we cannot yet distinguish between an orientation of the micelles in the flow direction or perpendicular to the shear plane ; an orientation along the flow direction, however, appears more likely.

The stress jump of a semirigid macromolecule after shear: Comparison of the elastic stress to the birefringence

In this work it is shown that the amount of stress retained at the instant of shear cessation, denoted as the elastic stress by Doi, and not the total shear stress is proportional to the shear equivalent of the optic birefringence even in the nonlinear shear stress–shear rate region. The system studied was xanthan gum in a viscous solvent, fructose dissolved in water, and may be considered as a system with rigid constraints. Coaxiality of the elastic stress and refractive index ellipsoids has been predicted theoretically for these systems; however, previous studies have only compared the total stress to the birefringence which generally leads to invalidation of the stress‐optic relation. The results of this work have three consequences; it reinforces the fact that stress jumps are present, the stress‐optic relation is only true when the elastic and not the total stress is used, and two unique types of stress constitute the total stress as predicted by theory for systems with rigid constraints.

Nonequilibrium molecular dynamics of liquid crystals

During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied to study transport phenomena in fluids that are isotropic at equilibrium. A natural extension is therefore to study liquid crystals, which are anisotropic al equilibrium. The lower symmetry of these systems means that the linear transport coefficients are considerably more complicated than in an isotropic system. Part of the reason for this is that there are crosscouplings between tensors of different rank and parity. Such couplings arc symmetry-forbidden in isotropic phases. In this paper. we review some of fundamental theoretical results we have derived concerning the rheology of liquid crystals. report NEMD simulations of thermal conductivity and shear viscosity of liquid crystals, and present NEMD simulations of shear cessation phenomena. All of the NEMD results are presented for a model liquid crystal fluid which is a modification of the Gay-Borne fluid. The results obtained are in qualitative agreement with experimental measurements on liquid crystal systems.

The viscous stress contribution to lyotropic hydroxypropylcellulose solutions in the biphasic and liquid‐crystalline regions

The rheological properties of polymeric liquid crystals are quite complicated and theories over the last decade and a half have made great progress in explaining this seemingly unique behavior. Most theories have considered only the elastic stress and neglected the viscous stress which can be measured by determining the amount of stress instantaneously lost on cessation of shear. The elastic stress is the stress that is left which subsequently decays with time after cessation. Data is presented that shows the viscous stress contribution determined from shear cessation experiments is present for both the biphasic and liquid‐crystalline mesophase of hydroxypropylcellulose‐water solutions. This system shows the three regions for the shear viscosity at higher concentrations. The viscous stress is the dominant contribution at high shear rates, well within region III, and is caused by viscous drag on the molecules. The viscous stress contribution is still present in the intermediate region II although to a lesser extent (approximately 20% of the total); however, as region I is approached it becomes more prominent. We explain this behavior as due to a phenomenon similar to superplasticity by viscous dissipation between domains or within defects.

Defect density evolution and steady rheological behaviors of liquid‐crystalline polymers

In a polydomain model for textured liquid‐crystalline polymers, domain shrinkage and texture coarsening at low shear rates are considered to be driven by the deviations of defect density L from the initial (L0) and steady (Ls) states, respectively. The densities of L0 and Ls correspond to the average domain sizes of a0 and as at each state. The introduction of L0 into the Larson–Doi defect density evolution equation provides a limit of texture length scale, eliminating the indefinite coarsening of texture. Then, the defect density evolution is refreshed in each case of the steady state, shear inception, and shear cessation. With the inclusion of L0, the shear thinning behavior of a steady‐state viscosity curve can be developed in Onogi–Asada’s region I as well as the Newtonian plateau behavior in region II. Moreover, another Newtonian plateau region over very low shear rates, called ‘‘region 0,’’ can be predicted when smaller domains are considered to be isotropically distributed at the initial state.

Dynamic and Transient Flow Behavior of Thermotropic Homopolyester with Nonamethylene Flexible Unit

Rheological properties of a semiflexible polyester having nonamethylene flexible spacer were investigated by small amplitude oscillatory shear flow and intermittent forward flow both in liquid crystalline state and in isotropic state. In dynamic measurements, it was found that the slope of the modulus versus, frequency in terminal region was uniquely determined by the intrinsic nature of each phase and the slope was higher in isotropic state than in liquid crystalline state. The viscoelastic data measured in the isotropic state at different temperatures can be drawn on a master curve, which reflects the homogeneous nature of the phase. The complex viscosity showed Arrhenius type decrease with increasing temperature in isotropic phase. In transient experiments, the large stress overshoot and negative N1 undershoot were observed upon start-up of shear flow in the liquid crystalline state. The negative N1 undershoot was significantly affected by the shear history and did not reappear under subsequent flow even after long rest. The stress relaxed almost instantly after cessation of shear for all shear rates, implying that high structural distortion was introduced by shear flow. The large stress overshoot was not observed in isotropic state due to homogeneous structure.

Phase separation in liquids due to quench by cessation of shear

Shear flow can induce a shift in the coexistence curve of partially miscible liquids and easily mix domains of two coexisting phases below T c. These properties are used to (i) homogenize a liquid mixture at temperatures below the critical one, (ii) induce a phase separation by stopping the shear. Experimental studies at different temperatures, shear rates and volume fractions by light scattering show that the process of phase separation is generally quite identical to the process induced by a thermal quench.

Novel Kinetic Effects in Vkcoelasttc Surfactant Solutions Under Shear

ABSTRACTUsing small-angle neutron scattering (SANS), we have investigated the transient alignment and relaxation under Couette shear of viscoelastic aqueous micellar solutions of cetyltrimethylarmmonium 3,5-dichlorobenzoate (CTA3,5C1) and CTA3,5C1/CTAB mixtures at concentrations well above ф* (but below 1.0 wt. %). Time constants of the order of ten's of minutes are reported for alignment and relaxation, orders of magnitude slower than any previously observed in similar micellar systems. The collective properties of the network of entangled, threadlike micelles, rather than the individual micellar segments, dominate the alignment and relaxation behavior. At low micellar surface charge density (σ) (e.g., in pure CTA3,5C1), the first observation of alignment proceeding in two stages has been made. Increasing a decreases by an order of magnitude the shear rate required to reach full alignment and provides a comparable decrease in the rate constant for relaxation after cessation of shear.

Strain-thickening transition in ferric-oxide suspensions under oscillatory shear

Shear-strain-thickening transition under oscillatory flow was observed in flocculated ferric-oxide suspensions in mineral oil. The value of the dynamic modulus of the suspensions that was measured at small strain amplitude after cessation of shear also became higher when the strain amplitude of the applied shear had been within or above the transition region.

The relation between CDM instability and Deborah number in differential type rheological equations

Previous studies have argued that rheological equations of the differential type, such as second-order fluid models, are inadequate because they result in unstable solution after cessation of steady shear. If the sign of the viscoelastic coefficient is selected so that the storage modulus is positive, the fluid velocity increases indefinitely and the flow does not decay by viscous dissipation, in contradiction to thermodynamic laws. This study mitigates this problem by demonstrating that the solution of such equations is actually stable at low values of Deborah number De, where these equations are only valid for other reasons. In fact, second order and higher order differential type equations are applicable only if the relaxation time of the fluid is low relative to a characteristic time of the flow. The study shows how to determine the characteristic time and thus clarifies, in practical terms, the limits of the region where differential type equations can be applied.

Mode selection in the dynamics of sheared polymer solutions.

We report transient light scattering measurements upon the cessation of shear for a semidilute polymer solution under theta conditions. The polymer solution demonstrates two modes of relaxation for concentration fluctuations in equilibrium. The transient mensurements show that the mechanism that couples the concentration fluctuafions to the shear flow, and thereby enhances concentration fluctuations, selectively enhances only the slower of the two equilibrium modes

Image analysis of shear-induced textures in liquid-crystalline polymers

Abstract Textures of several liquid-crystalline solutions of poly(benzylglutamate) and a solution of hydroxypropylcellulose were videorecorded during and after shear and the images were Fourier transformed. We find for all the solutions that bands perpendicular to the flow direction form after cessation of prolonged shearing if the Deborah number De exceeds a critical value Dec≈0·1. Here De = τ , where is the shear rate and τ is the characteristic molecular relaxation time. The bands are characterized by an initially broad spectrum of wavelengths; for the poly(benzylglutamate) solutions this spectrum narrows and shifts towards longer wavelengths as time progresses after cessation of shear, leading to an increase in the characteristic band spacing b(t) with time t. The dependences of the band spacing on the shear rate and on the solution viscosity n in poly(benzylglutamate) are very weak, but the time to form bands after shearing ceases is roughly inversely proportional to 0. Our results suggest that both ...

Anisotropic phase separation kinetics in a polymer blend solution following cessation of shear studied by light scattering

A light scattering instrument capable of monitoring the scattering from samples under the influence of a simple shear field has been constructed. The apparatus consists of transparent cone-and-plate fixtures and a two-dimensional charge coupled device array detector. The detector unit is also capable of measuring the scattering patterns as a function of time. The phase separation kinetics of an 8% solution of a polystyrene/polybutadiene (50:50) blend in dioctyl phthalate following cessation of a steady shear is monitored with this instrument. The sample is two-phase in the quiescent state and the applied shear is sufficient to suppress all scattering observed from the quiescent sample (shear-induced mixing). The evolution of the scattering profiles following cessation of shear is quite different parallel and perpendicular to the original flow direction. In the normal direction, a spinodal growth and coarsening mechanism similar to that observed in temperature quench experiments is observed. In the parallel direction, a different mechanism is followed. We believe this is the first report of this type of anisotropic behaviour in the phase separation kinetics of a polymer blend.