Shear-induced ordering

X — A/T + B (where A and B are constants) however, the parameters from the fit cannot straightforwardly be interpreted on a molecular basis (Almdal et al. 1996). A transition from shear-induced order to shear-induced disorder on increasing the shear rate has recently been reported in an asymmetric PS-PI diblock in concentrated solution (Balsara and Dai 1996). The low-shear rate ordering was consistent with the suppression of fluctuations, and the high-shear rate disordering was interpreted as arising from fluctuations of the ordered (cylindrical) microstructure (Balsara and Dai 1996). 

Substantial interest has been raised in the problem of the structure and dynamics of suspensions in shear hydrodynamic fields. ° ° The experiments showed that both shear-induced melting and shear-induced ordering can be observed at different particle volume fractions and shear rates. The nonequilibrium microstructure of the suspension under shear can be investigated in these experiments and compared with the predictions from analytical theories and computer simulations. 

Bruce 1. Ackerson Shear induced order and shear processing of model hard sphere suspensions, 1. Rheol., 34() (1990) 553-590... 

J. F. Lutsko, Molecular Chaos, Pair Correlations and Shear-Induced Ordering of Hard Spheres, Phys. Rev. Lett. 77, 2225-2228 (1996). 

Low-molecular weight chains do not experience enough shear force to induce scission. Watson et al." demonstrated (by the intrinsic viscosity characterization of masticated NR) that the limiting molecular weight for the shear-induced degradation is in the order of 0.7-1.0 X 10 Frenkel independently speculated that shear-induced cleavage occurs near the midpoint of the polymer chain. 

The velocity, viscosity, density, and channel-height values are all similar to UF, but the diffusivity of large particles (MF) is orders-of-magnitude lower than the diffusivity of macromolecules (UF). It is thus quite surprising to find the fluxes of cross-flow MF processes to be similar to, and often higher than, UF fluxes. Two primary theories for the enhanced diffusion of particles in a shear field, the inertial-lift theory and the shear-induced theory, are explained by Davis [in Ho and Sirkar (eds.), op. cit., pp. 480-505], and Belfort, Davis, and Zydney [/. Membrane. Sci., 96, 1-58 (1994)]. While not clear-cut, shear-induced diffusion is quite large compared to Brownian diffusion except for those cases with very small particles or very low cross-flow velocity. The enhancement of mass transfer in turbulent-flow microfiltration, a major effect, remains completely empirical. 

In order to obtain solutions with the desired flow properties, shear-induced degradation should be avoided. From mechanical degradation experiments it has been shown that chain scission occurs when all coupling points are loose and the discrete chains are subjected to the velocity field. Simple considerations lead to the assumption that this is obtained when y) is equal to T sp(c-[r ]) (Fig. 18). The critical shear rate can then easily be evaluated [22]. 

Imposing the shear on self-assembling systems may induce ordered morphologies that are not observable in equilibrium [102]. For example, the lamellar... 

As discussed below, the quality of the alignment (and even its direction in the case of lamellar morphology), is influenced by temperature, as well as the frequency and strain amplitude of the aligning shear field. No general theory for the alignment of block-copolymer phases has yet been developed. However, studies of a number of different block-copolymer systems show that in ordered states with cylindrical domains, shear orients the cylinders parallel to the flow, while for lamallar microdomains, two different shear-induced orientations are commonly found, depending on alignment conditions in both of these orientations, the flow direction lies in the plane of the lamellae. 

Within the fluctuation-dominated region of the disordered phase of a diblock near Todt shearing can apparently induce a transition to the ordered state (Koppi et al. 1993). Cates and Milner (1989) predicted such a phenomenon, based on a shear-induced suppression of... 

In order to calculate the thin film viscosity with Eq. (15), the flow-activation entropy near the surface is also needed. An experimental flow-activation entropy is calculated from the spin-off data with Eqs. (15) and (19) as follows. The experimental vj vs. h is determined from the dh/dt during air shear induced flow on a rotating disk. Eq. (19) is then solved for AS vis versus h using Eq. (19) for AE -. Below 2.3 nm, — 1.9kJ/mol, which corresponds to the critical configurational entropy change for flow (-/ ln2 - 5.76J/mol-K). 

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