Shear induced microstructure

The above features of a sheared colloidal crystal appear to be similar in both BCC and FCC structures. However, there are differences in details, and perhaps even within a given symmetry the flow behavior might vary with particle concentration or charge density. For example, Chen et al. (1994) have shown that between the strained crystal and sliding-layer microstructures there can be a polycrystalline structure, the formation of which produces a discontinuous drop in shear stress (see Fig. 6-33). Ackerson and coworkers gave a detailed description of the fascinating shear-induced microstructures of these systems (Ackerson and Clark 1984 Ackerson et al. 1986 Chen et al. 1992, 1994). 

Figure 19 Shear-induced microstructure in a Weeks-Chandler-Andersen fluid of 500 particles under low shear (T = 0.722, p = 0.844, and y = 0.6). The unit of length is the WCA potential a. The direction of flow (x) is out of the page, and the particles are projected on the yz plane.

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). 

Wolf, B., Scirocco, R, Frith, W. J., and Norton, I. T. 2000. Shear-induced anisotropic microstructure in phase-separated biopolymer mixtures. Food Hydrocolloids 14 217-225. 

Shear-induced crystallization had a much greater effect in bulk systems than emulsified systems (Fig. 6) and resulted in an accelerated rate of crystallization. Prior to, and during, the initial stages of crystallization, intradroplet fat is protected from interdroplet crystallization by the spherical shape and pressure of the droplet and is not directly available to the shear field, i.e., no protruding crystals. This observation is consistent with microstructure work where limited destabilization was observed in droplets with no visible crystals. Initially, droplet interfaces in the PSCO system showed that the crystallized fat was not available at the surface, limiting the occurrence of crystal-induced flocculation and coalescence. Droplets remained stable until their interfaces were disturbed by the shear fleld or crystal interaction. 

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. 

However, due to the microstructure of HP-PE fibres, viz. extended polyethylene chains with relatively weak intermolecular Van der Waals interactions, their resistance to static loadings is relatively poor (creep). For similar reasons the compressive strength is not very impressive, due to shear induced failure. Consequently the new generation of HP-PE fibres seem less suitable as reinforcing elements for structural composites. 

Thixotropy comes about first because of the finite time taken for any shear-induced change in microstructure to take place. Microstructure is brought to a new equilibrium by competition between, on the one hand the processes of tearing apart by stress diuing shearing, and on the other hand build-up due to flow- and Brownian-motion-induced collision, over a time that can be minutes. Then, when the flow ceases, the Brownian motion (the only force left) is able to slowly move the elements of the microstructure around to more favourable positions and thus rebuild the structure this can take many hours to complete. The whole process is completely reversible. 

In fact, any shear-induced change in the microstructure of a suspension takes time to occur. This is true for the simplest changes such as the transition from the random-at-rest situation for simple suspensions to the breakdown of floes to primary particles when flocculated suspensions are sheared hard. The former change is often over before the relatively slow mechanics of most viscometers could detect it, but the latter kind of change can take hours or even days to complete ... 

Furthermore, NEMD enables the fluid microstructure to be studied in nonequilibrium steady states and to compare this structure with experiment (Hess Hanley 1982). The nonequilibrium distributions of particle positions and momenta are reflected in the thermophysical properties of viscosity, dilatancy and normal pressure differences. In molecular fluids such as lubricants, nonlinear fluid behavior is brought about in part by shear induced changes in molecular conformation. ... 

Deposition Conditions describes how the relative rates of condensation and evaporation during deposition affect film microstructure and presents evidence for shear-induced restructuring, alignment, or ordering. 

Kromkamp, J., van den Ende, D. T. M., Kandhai, D., van der Sman, R. G. M., and Boom, R. M. 2005. Shear-induced self-diffusion and microstructure in non-Brownian suspensions at non-zero Reynolds numbers. /. Fluid Mech. 529, 253-278. 

---