Cessation Of Shear Flow Research Articles

Rheology of liquid crystalline oligomers for 3-D printing of liquid crystalline elastomers.

Liquid crystalline monomers can be oligomerized and subsequently 3-D printed to prepare liquid crystalline elastomers (LCEs) with spatial variation of the nematic director to create soft materials that undergo complex shape change when subject to stimulus. Here, we detail the correlation of alignment in 3-D printed LCE on the shear history of the oligomeric ink. This coupling is evident both in the polymerization of sheared LCE samples as well as steady-state rheological experiments that quantify the time-dependent flow behaviors of these complex fluids. Under a steady shear flow, oligomeric LC inks transition from a nematic state with unaligned (polydomain) orientation to a uniaxially aligned (monodomain) nematic phase over a large range of applied strain. After cessation of shear flow, the oligomeric LC inks return the polydomain orientation over approximately 30 minutes. The alignment of liquid crystalline segments in the LCE (and the associated stimuli-response of the materials) is ultimately correlated to the degree of strain applied to the ink.

Nonmonotonic Stress Relaxation after Cessation of Steady Shear Flow in Supramolecular Assemblies.

Stress relaxation upon cessation of shear flow is known to be described by single-mode or multimode monotonic exponential decays. This is considered to be ubiquitous in nature. However, we found that, in some cases, the relaxation becomes anomalous in that an increase in the relaxing stress is observed. Those observations were made for physicochemically very different systems, having in common, however, the presence of self-associating units generating structures at large length scales. The nonmonotonic stress relaxation can be described phenomenologically by a generic model based on a redistribution of energy after the flow has stopped. When broken bonds are reestablished after flow cessation, the released energy is partly used to locally increase the elastic energy by the formation of deformed domains. If shear has induced order such that these elastic domains are partly aligned, the reestablishing of bonds gives rise to an increase of the overall stress.

A singular perturbation study of the Rolie-Poly model

A singular perturbation analysis for the Rolie-Poly model is presented for the limit of large relaxation time. The model describes entangled linear polymer melts and time scales for a Rouse time and reptation time. First, the case of the Rouse time approaching zero is named the nonstretching model, and the addition of a solvent model gives the small parameter to be the ratio of the retardation time to the slow reptation time. This analysis is instructive in formulating the perturbation analysis for the full Rolie-Poly model without the solvent, where the small parameter is the ratio of Rouse time to reptation time. The resulting formulae elucidate the characteristic features of thixotropic yield stress fluids, including yield stress hysteresis, delayed yielding and long term persistence of a decreased viscosity after cessation of flow. Two fundamental initial value problems are solved: startup of shear flow and cessation of shear flow. The scalings lead naturally to simplified dynamical systems that describe regimes of fast, slow and yielded dynamics, and show how the combination of these regimes can be used to describe the entire flow evolution.

Viscoelasticity of a colloidal gel during dynamical arrest: Evolution through the critical gel and comparison with a soft colloidal glass

We consider the gelation of colloidal particles in suspension after cessation of shear flow. Particle aggregation is driven by a temperature-tunable attractive potential which controls the growth of clusters under isothermal conditions. A series of frequency resolved time sweeps is used to systematically reconstruct the frequency dependent dynamic moduli as a function of time and temperature or attraction strength. The data display typical hallmarks of gelation with an abrupt transition from a fluid state into a dynamically arrested gel state after a characteristic gelation time tg that varies exponentially with temperature and serves to collapse the evolution of the system onto a universal curve. We observe the viscoelastic properties of the critical gel where we find that G′(ω)≊G″(ω)∼ωnc, where nc = 0.5 in a narrow time window across all attraction strengths. We measure a dynamic critical exponent of κ = 0.25 which is similar to that observed in cross-linked polymer gels. The approach to the critical gel is therefore governed by zero-shear viscosity η0∼−ϵ−s and plateau modulus Ge∼ϵz with s = z = 2, where ϵ = p/pc − 1 is the distance to the gel point in appropriate reaction coordinates. Remarkably, the relaxation moduli of the near critical gels are identical across the temperatures considered, with G(t) ≈ 0.33 t−0.5. This suggests an underlying strong similarity in gel structure in the regime of attraction strengths considered, despite the differences in aggregation kinetics. We contrast these findings with the behavior of a colloidal glass undergoing dynamical arrest where no critical state is observed and where the arrest time of the system displays a marked frequency dependence. These findings highlight the underlying structural differences between colloidal gels and glasses which are manifest in their dynamic properties in the vicinity of the liquid-to-solid transition.

Mode-coupling analysis of residual stresses in colloidal glasses.

We present results from computer simulation and mode-coupling theory of the glass transition for the nonequilibrium relaxation of stresses in a colloidal glass former after the cessation of shear flow. In the ideal glass, persistent residual stresses are found that depend on the flow history. The partial decay of stresses from the steady state to this residual stress is governed by the previous shear rate. We rationalize this observation in a schematic model of mode-coupling theory. The results from Brownian-dynamics simulations of a glassy two-dimensional hard-disk system are in qualitative agreement with the predictions of the theory.

Effects of particles on stability of flow-induced precursors

The effect of two colorant particles with different surface geometries on the stability of shear-induced precursors in isotactic polypropylene was studied after the cessation of shear flow at 140 °C. In the absence of particles, the shear-induced precursors survived for at least 100 s after the shear flow ended. The presence of particles was found to stabilize lower molecular weight chains assisting in the formation of additional shear-induced precursors. The precursors thus formed in the samples containing particles contained two oriented clusters with different molecular weights. Incorporation of lower molecular weight chains in the precursors led to increased dissolution rates of the shear-induced precursors. Particle surface geometry was found to influence precursor dissolution, with planar particles stabilizing the shear-induced precursors to a much greater extent than curved particles. The particles investigated thus act like structural probes to follow quantitatively the dissolution process of precursors after shear and importantly to infer the formation of precursors during shear.

Interaction of toroidal focal conic defects with shear flow

Toroidal focal conic defects form in smectic liquid crystals due to antagonistic surface anchoring conditions. The resulting layer structure, defect size, and the formation of ordered patterns of defects have been studied in detail. Here, we investigate the effect of shear flow on toroidal focal conic defects. The defects are formed and subjected to a steady linear Couette flow in a microscale shear cell. In situ visualization of the evolution of the polarization texture of individual defects reveals the impact of three distinct flow regimes on the layer structure. At low Ericksen numbers, individual defects exhibit an early time elastic regime in which the optical intensity of the layer structure does not change in the presence of the applied shear flow. At increasing applied strains, the defect layer structure deforms, resulting in a monotonically increasing optical intensity within its original footprint. At even larger applied strains, satellite defects are emitted from the anchored defect. Cessation of shear flow at moderate strains, prior to the formation of satellite defects, leads to an exponential decay of the polarization intensity of the defect, suggesting that relaxation of the layer structure occurs with a time constant that is consistent with that predicted by a permeation flow mechanism. This study establishes the relevant length and time scales involved in the interaction of shear stresses and elastic stresses in smectic liquid crystalline samples whose structure deviates significantly from an idealized aligned lamellar configuration.

Supersaturated dispersions of rodlike viruses with added attraction

The kinetics of isotropic-nematic (I-N) and nematic-isotropic (N-I) phase transitions in dispersions of rodlike fd viruses are studied. Concentration quenches were applied using pressure jumps in combination with polarization microscopy, birefringence, and turbidity measurements. The full biphasic region could be accessed, resulting in the construction of an experimental analog of the bifurcation diagram. The N-I spinodal points for dispersions of rods with varying concentrations of depletion agent (dextran) were obtained from orientation quenches using cessation of shear flow in combination with small-angle light scattering. We found that the location of the N-I spinodal point is independent of the attraction, which was confirmed by theory. Surprisingly, the experiments showed that also the absolute induction time, the critical nucleus, and the growth rate are insensitive of the attraction if the concentration is scaled to the distance to the phase boundaries.

Shear Flow and Large Amplitude Oscillation Shear Study of Solutions of Aggregating Micellar Casein Particles

Abstract Small micellar casein particles were formed in aqueous solutions of native casein after addition of polyphosphate. These so-called submicelles aggregated and gelled with a rate that increased with increasing temperature. The evolution of the viscosity during this process was determined at constant shear rate or shear stress. When applying a small shear stress the viscosity increased strongly until the shear rate became immeasurably slow, but when the applied shear stress exceeded a critical value (σc) the aggregates broke up and the viscosity reached a maximum. At longer times the viscosity decreased rapidly at first, followed by a very slow decrease. σc was independent of the shear rate and heating temperature, but increased strongly with increasing casein concentration. At constant shear rate the stress remained close to σc, but fluctuated irregularly. After cessation of shear flow, gels were formed rapidly. Oscillation shear measurements for σ > σc showed a strongly non-linear response at the time of maximum viscosity.

Rheology and microstructures formation of immiscible model polymer blends under steady state and transient flows

AbstractViscoelastic properties of model immiscible blend were studied here under steady state condition at different initial conditions and transient flow conditions. The flow‐induced microstructure has been studied on these model blends. For this system, the elastic properties of the blend are mainly governed by the interface. Measurement of the dynamic modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to prove the blend morphology. The dynamic moduli after cessation of shear flow, the mean diameter of the disperse phase as generated by the shear flow, have been calculated using the model of Palierne. A procedure based on a direct fitting of the dynamic moduli with the model is compared with the one that uses a weight relaxation spectrum. On the other hand, the steady state normal stress data have been related to the morphology of the blend by means of Doi and Ohta model. The specific interfacial area is found to be inversely proportional to the ratio of interfacial tension over shear stress for the blend. The flow behavior during transient shear flow was also discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3519–3533, 2005

Phase diagram, rheology and interfacial tension of aqueous mixtures of Na-caseinate and Na-alginate

In this work, the system Na-alginate/Na-caseinate/water has been characterised in terms of phase diagram, rheological properties of the equilibrium phases, and interfacial tension as a function of tie-line position. In water, the two biopolymers are poorly miscible and the system separates in two phases, each highly enriched of only one of the two biopolymers. The rheological properties of the equilibrium phases are essentially determined by the biopolymer present to a larger extent. Thus, as for the aqueous solutions of each biopolymer, the Na-caseinate rich phases are Newtonian, while the Na-alginate rich phases show shear-thinning behaviour at shear rates larger than 1 s −1. In the region close to the critical point, the Na-alginate rich phases are more viscous than the Na-caseinate rich ones. The opposite trend is found as the total amount of biopolymers increases. The interfacial tension between the equilibrium phases was measured by analysing drop retraction upon cessation of shear flow. The interfacial tension decreases as the tie-line approaches the critical point. A linear relation was found to hold between interfacial tension and tie-line length.

Structural Evolution During and After Cessation of the Shear Flow for a Side-Chain-Type Liquid Crystalline Polysiloxane Above the Isotropic-Liquid Crystalline Phase Transition Temperature

Evolution of the anisotropic texture during and after cessation of shear flow for a side-chain-type liquid crystalline polysiloxane (LCS) was studied by shearing microscopy at temperatures above the isotropic to liquid crystalline phase transition temperature (Tc=309.2 K if LCS is cooled from the isotropic phase in the quiescent state). The anisotropic texture of LCS during the shear flow was found at a temperature of 313 K and a shear rate of 50 s−1. After cessation of the shear flow, the anisotropic texture disappeared. At a temperature of 311 K and a shear rate of 50 s−1, LCS under shear flow also showed the anisotropic texture. It is noteworthy that the anisotropic texture was stable at this temperature, even after cessation of the shear flow. Furthermore, the evolution of the anisotropic texture was described quantitatively from the behavior of the birefringence as a function of time after cessation of the shear flow.

Measurement of average drop size in aqueous mixtures of Na-alginate and Na-caseinate by linear oscillatory tests

The linear viscoelastic behaviour of two-phase immiscible systems is characterised by an increased elasticity at low frequencies, which can be quantitatively related to the average size of the dispersed phase droplets and to the interfacial tension between the two phases (Rheol. Acta 29 (1990) 204). In the case of aqueous biopolymer mixtures, however, the application of classical emulsion models, such as the one of Palierne, is not trivial, due to sedimentation of the dispersed phase and to the extremely low values of interfacial tension which are typical of these systems. In this work, we investigated the application of Palierne model to aqueous mixtures of Na-alginate and Na-caseinate. The volume average drop radius was calculated by fitting the Palierne model to experimental data of elastic modulus vs frequency right after cessation of shear flow. A comparison with direct drop size measurements was performed by shearing the mixtures in a parallel-plate apparatus coupled with an optical microscope. In this apparatus, the mixtures were subjected to the same flow history as in the rheometer and drop size was measured by image analysis. Good agreement was found in all the four mixtures investigated, covering two volume fractions of dispersed phase, i.e. 5 and 10%, and two values of viscosity ratio (drop viscosity/continuous phase viscosity), i.e. 0.34 and 3.0.

A thermodynamic approach to the rheology of highly interactive filler-polymer mixtures: Part I - Theory

The rheological behavior of highly interactive filler-polymer mixtures is simulated utilizing a double network created by the entangled polymer matrix and the adsorbed polymer. Both networks are represented by a nonlinear viscoelastic constitutive equation. The dependence of rheological properties on filler concentration is taken into account through the bridging density resulting from polymer-filler interactions and a hydrodynamic reinforcement. The relative contribution of both networks is computed through the energy balance consistent with the thermodynamics of the polymer-filler chemical interactions and fluid mechanics. This self-consistent approach allows one to calculate the strain dependence of dynamic properties under oscillatory flow and shear rate dependence of stresses under steady simple shear flow and upon start up and cessation of shear flow.

Phase Behavior, Rheology, and Morphology of Binary Blends of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers

The phase behavior, rheology, and morphology of binary blends of semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs) were investigated. Specifically, binary blends consisting of poly[(phenylsulfonyl)-p-phenylene alkylene-bis(4-oxybenzoate)]s (PSHQn) having five methylene groups (PSHQ5) and 11 methylene groups (PSHQ11) were prepared by solvent casting. It was found from differential scanning calorimetry (DSC) that PSHQ5, PSHQ11, and their blends are glassy thermotropic polymers, exhibiting only glass-to-nematic and nematic-to-isotropic (N−I) transitions. Approximate phase diagrams were constructed for PSHQ5/PSHQ11 blends based on DSC data. Using a cone-and-plate rheometer, transient shear flow experiments were conducted for the PSHQ5/PSHQ11 blends (i) at 160 °C in the biphasic region where PSHQ11 forms an isotropic phase and PSHQ5 forms a nematic phase and (ii) at 130 °C in the nematic region where both PSHQ5 and PSHQ11 formed the nematic phase. It was found for such PSHQ5/PSHQ11 blends that the steady-state shear viscosity at 130 °C (in the nematic region) is lower than that at 160 °C (in the biphasic region). However, the first normal stress difference at 130 °C exhibits a very large overshoot followed by an oscillatory decay until reaching a steady state, while it is virtually zero at 160 °C. The time evolution of morphology for the PSHQ5/PSHQ11 blends, upon shear startup and also upon cessation of shear flow, was investigated using a specially designed optical microrheometer equipped with a polarizing optical microscope. Contrasting observations are reported for the case of nematic PSHQ5 in isotropic PSHQ11 when compared to the nematic PSHQ5/nematic PSHQ11 blend. Shearing of a nematic/nematic blend induces a much larger birefringence change than does shearing a nematic/isotropic blend, and a shear-induced isotropic-to-nematic transition is observed from a mixture of isotropic phases containing two TLCPs.

Monte Carlo simulation of self-avoiding lattice chains subject to simple shear flow Part II. Three-dimensional results and comparison with experiments

This paper has extended a lattice Monte Carlo (MC) method to simulate the simple shear flow of multiple self-avoiding chains in three dimensions following our research work in two dimensions. Comparisons of simulation outputs with experimental observations, theoretical predictions and other simulation results are made. The steady-state scaling analysis to scattering functions of deformed chains confirms the existence of anisotropic scaling laws at fixed reduced shear rates found in molecular dynamics (MD) simulation. The exponent of chain deformation shown in the MC simulation falls into a normal regime measured from neutron scattering and light scattering experiments. The relation between orientation angles and shear rates is consistent with some scattering experiments. Both Newtonian and non-Newtonian regimes are reproduced in our lattice MC simulation. Non-zero first and second normal stress differences and their dependence of the shear rate are found, as well as the shear thinning effect. The stress growth at inception and stress decay after cessation of shear flow is also examined. The validity of our novel simulation approach is thus confirmed. Since both chain conformations and rheological properties under shear flow can be studied, our MC approach can be used to reveal non-linear viscoelasticity of polymer fluids and polymer-flow interaction.

Rheo-Optical Evidence of a Flow-Induced Isotropic−Nematic Transition in a Thermotropic Liquid-Crystalline Polymer

A direct rheo-optical characterization of the flow-induced isotropic-nematic (I-N) transition in a semiflexible thermotropic liquid-crystalline polymer (TLCP) was investigated, using a specially constructed apparatus enabling in-situ optical microscopic observations at elevated temperatures, along with cone-and-plate rheometry. For the investigation, an aromatic polyester, poly((phenylenesulfonyl)- p-phenylene 1,10-decamethylenebis(4-oxybenzoate)) (PSHQ10), was synthesized via solution polymeri- zation. Above the equilibrium isotropic-nematic transition temperature for this polymer, T ) 170.5 °C, application of steady-state shear flow above a certain critical value of shear rate, A o c, produces a first- order I-N transition, with A o c increasing with temperature. This transition is evidenced by the formation of elongated nematic (birefringent) domains in the isotropic matrix, accompanied by a drastic decrease in shear viscosity ( ). Remarkably, the nematic domains that form for A o> A o care optically uniform under cross-polarized optical microscopy; i.e., they are apparently free of disclinations (defects), typical of textured TLCPs. The flow-induced I-N transition in PSHQ10 is found to be reversible; i.e., upon cessation of shear flow, the domains melt to the original isotropic phase and the dynamic moduli rise toward the pretransition values. The observed flow-induced I-N transition may find important applications, such as envisaging new routes for processing TLCPs with better mechanical properties and helping to understand bioprocesses such as silk thread spinning.

WAXS studies of global molecular orientation induced in nematic liquid crystals by simple shear flow

Global molecular orientation function coefficients for the nematic liquid crystal 4-cyano 4'-nn -pentylbiphenyl (5CB) in shear flow are presented, being extracted from 2-dimensional Wide-Angle X-ray Scattering data. A linear increase in orientation parameter P2 is observed with a logarithmic increase in shear rate. It is proposed that this arises from an increased number of LC directors aligning to the shear axis. Upon cessation of shear flow, the anisotropy is seen to relax away completely, over a time scale which is inversely proportional to the previously applied shear rate.

Transient rheological behavior of a thermotropic liquid‐crystalline polymer. II. Intermittent shear flow and evolution of dynamic moduli after cessation of shear flow

The intermittent shear flow behavior in the nematic state of a well‐characterized thermotropic liquid‐crystalline polymer, poly[(phenylsulfonyl)‐p‐phenylene 1,10‐decamethylene‐bis (4‐oxy‐ benzoate)] (PSHQ10), was investigated using a cone‐and‐plate rheometer. For the study, PSHQ10 was synthesized in our laboratory. In order to erase the thermal history associated with polymerization and sample preparation and control initial morphology, an as‐cast specimen was first heated to the isotropic region (190 °C), sheared there at a rate of 0.085 s−1 for 10 min, and then cooled slowly down to a predetermined nematic state (130, 140, 150, or 160 °C). A fresh specimen was used for each intermittent shear flow experiment, thus circumventing the effect of previous shear history on subsequent transient rheological responses. Upon startup of shear flow, the growths of shear stress σ+(t,γ̇) and first normal stress difference N+1(t,γ̇) were recorded as functions of applied shear rate, temperature, and rest period. We found that the peak values of σ+(t,γ̇) and N+1(t,γ̇), respectively, increased with increasing rest period and applied shear rate, and with decreasing temperature. In order to facilitate an analysis of the data obtained under different experimental conditions, the transient stresses were rescaled using the equilibrium value as a reference. The time evolution of dynamic storage and loss moduli (G’ and G‘) after cessation of shear flow was monitored, by applying small amplitude oscillatory deformations to the specimens. We found that the value of G‘ increased with time, the rate of which becoming greater with increasing applied shear rate, while the value of G’ remained more or less constant, for a rest period of 150 min.

SANS Study of Chirality and Order in Liquid Crystalline Cellulose Suspensions

ABSTRACTSmall angle neutron scattering, SANS, was used to describe the magnetic alignment and in situ shear ordering of polyelectrolytic, liquid crystalline cellulose microfibrils in aqueous (D2O) suspension. In a 2.4 Tesla magnetic field, microfibril suspensions exhibit anisotropic chiral nematic (cholesteric) ordering in which the distance between nematic planes along the cholesteric axis is shorter than between rods within a nematic plane. This is consistent with the hypothesis that cellulose microfibrils are helically twisted rods. During shear, the SANS interference peaks perpendicular to the flow direction sharpen with increasing shear rate. Yet, the highest degree of alignment (for microfibrils with axial ratios of ~45) was observed a short period after the cessation of shear flow.