Large-amplitude oscillatory shear

This shear flow is of interest, because it involves the exponential separation of material points, and thus seems likely to generate chain stretch, but is much easier to generate in the laboratory than extensional flows. However, whereas in extensional deformations the distance between material points on the same streamline increases exponentially with time, in exponential shear it is material points on different streamlines that separate in this way. In other words, uniform extension is an irrotational flow, while shear is an equal mixture of stretching and rotating flows. 

Exponential shear is therefore not a flow with constant stress history. The stress in this flow tends to grow without limit, even in an inelastic or linearly elastic fluid, and this makes the presentation of data an important issue. Doshi and Dealy [57] and Dealy [58] have argued that the results of an exponential shear experiment should be reported in terms of a time-dependent exponential viscosity rf that is defined in terms of the instantaneous shear rate  

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Wiesner U (1997) Lamellar diblock copolymers under large amplitude oscillatory shear flow order and dynamics. Macromol Chem Phys 198 3319-3352... 

The linear viscoselastic properties of the pristine nanocomposites, the effect of large amplitude oscillatory shear on the orientation of the nanocomposites and the viscoelastic properties of these oriented nanocomposites were examined. 

Fig. 25. Preliminary SANS measurements on post large amplitude oscillatory shear confirm the presence of global alignment of the silicate layers.

A.J.Giacomin, R.S.Jeyaseelan, T.Samurkas, J.M.Dealy, Validity of separable BKZ model for large amplitude oscillatory shear, J. Rheol. 37 (1993), 811-826. 

Problem 10.10 You would like to reduce the density of defects present in a smectic A liquid. Which method would be more likely to succeed steady shearing or large-amplitude oscillatory shearing ... 

Unusual stress waveforms are observed during oscillatory deformation on such materials when the strain amplitude is much greater than the yield strain. Watanabe and Kotaka (1984) plotted large-amplitude oscillatory shear data in the form of Lissajous figures— that is, plots of periodic stress versus periodic strain. In the nonlinear regime, above the yield strain, unusual rhombic Lissajous figures were obtained at low frequencies, and bent ellipses were obtained at higher frequencies. Qualitatively, the waveforms are similar to those depicted in Fig. 13-23. 

The correlation between rheology and thermodynamics is likely to prove a fruitful area for investigation in the future. Very little is as yet known about the detailed mechanisms of non-linear viscoelastic flows, such as those involved in large-amplitude oscillatory shear. Mesoscopic modelling will no doubt throw light on the role of defects in such flows. This is likely to involve both analytical models, and mesoscopic simulation techniques such as Lattice... 

MSI) that uses the same time-dependent Ginzburg Landau kinetic equation as CDS, but starts from (arbitrary) bead models for polymer chains. The methods have been summarized elsewhere. Examples of recent applications include LB simulations of viscoelastic effects in complex fluids under oscillatory shear,DPD simulations of microphase separation in block copoly-mers ° and mesophase formation in amphiphiles, and cell dynamics simulations applied to block copolymers under shear. - DPD is able to reproduce many features of analytical mean field theory but in addition it is possible to study effects such as hydrodynamic interactions. The use of cell dynamics simulations to model non-linear rheology (especially the effect of large amplitude oscillatory shear) in block copolymer miscrostructures is currently being investigated. ... 

While in the linear response regime, modulus and density correlator are measurable quantities outside the linear regime, both quantities serve as tools in the ITT approach only. The transient correlator and shear modulus provide a route to the stationary averages because they describe the decay of equilibrium fluctuations under external shear and their time integral provides an approximation for the stationary distribution function. Determination of tlie frequency dependent moduli under large amplitude oscillatory shear has only recently become possible [58], and requires an extension of the present approach to time dependent shear rates in (3) [59],... 

Jeyaseelan and Giacomin (1995) examine the use of large-amplitude oscillatory-shear (LAOS) rheology of filled polymer melts (HDPE filled with carbon black) and use transient-network theory (which separates filler and polymer entanglement effects) to describe the non-linear flow behaviour. 

Debbaut, B., and Burhin, H., Large amplitude oscillatory shear and Fourier-transform rheology for a high-density polyethylene experiments and numerical simulation, J. Rheol., 46,1155-1176 (2002). 

Shear flow fields are a remarkably effective means of orienting copol5mier microdomains. Large amplitude oscillatory shear (LAOS) and extrusion are common methods used to align... 

The effect of the lA approximation has also been examined for large amplitude oscillatory shear deformation. In this case the result of the constitutive equation without using the lA approximation is shown to be in better agreement with experimental results.""... 

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