Vorticity banding in rodlike virus suspensions

Harvard University, Cambridge, Massachusetts, United States
Physical Review E (Impact Factor: 2.29). 09/2006; 74(2 Pt 2):026307. DOI: 10.1103/PhysRevE.74.026307
Source: PubMed


Vorticity banding under steady shear flow is observed in a suspension of semiflexible colloidal rods (fd virus particles) within a part of the paranematic-nematic biphasic region. Banding occurs uniformly throughout the cell gap within a shear-rate interval (.gamma-, .gamma+) , which depends on the fd concentration. For shear rates below the lower-border shear rate .gamma- only shear elongation of inhomogeneities, which are formed due to paranematic-nematic phase separation, is observed. Within a small region just above the upper-border shear rate .gamma+ , banding occurs heterogeneously. An essential difference in the kinetics of vorticity banding is observed, depending on the morphology of inhomogeneities formed during the initial stages of the paranematic-nematic phase separation. Particle tracking and polarization experiments indicate that the vorticity bands are in a weak rolling flow, superimposed on the applied shear flow. We propose a mechanism for the origin of the banding instability and the transient stability of the banded states. This mechanism is related to the normal stresses generated by inhomogeneities formed due to the underlying paranematic-nematic phase transition.

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Available from: Kyongok Kang, Aug 22, 2014
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    • "This phase separation (i.e., formation of a chiral nematic phase) in CNC aqueous suspensions is generally governed by basic properties of the nanoparticles , namely, physical dimension, size polydispersity, surface charge, and the ionic strength of the system (Onsager 1949; Odijk 1986; Dong et al. 1998; Hamad and Hu 2010). Due to formation of liquid crystalline ordered domains in these systems, CNC suspensions have a complicated microstructure which controls the macroscopic flow behavior of the material and is sensitive to applied flow fields similar to other systems such as rod-like virus suspensions (Lettinga et al. 2005; Kang et al. 2006; Ripoll et al. 2008; Boluk et al. 2011). "
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    • "Using small-angle neutron-scattering under flow (Rheo-SANS) and video imaging, Herle et al. found anisotropic patterns in both bands, indicating strong alignment of the structures; notwithstanding, the micellar alignment does not correspond to the lower viscosity band. Vorticity banding has also been observed in colloids [29] [30] [31], wherein experiments suggest the association of vorticity banding to an elastic instability similar to the Weissenberg effect [31], implying a relation between the band apparition and normal stresses along the gradient axis. The vorticity structuring has been also associated to velocity rolls stacked along the vorticity direction [32]. "
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    • "The structure comprises micron-sized bands of different director orientations stacked in the vorticity direction, which consist of similar morphologies of droplets in a matrix (either strongly nematic drops in a less nematic matrix or vice versa). Recent experiments suggest that this structure is roll-like, rather than the " canonical " vorticity banding suggested here (Kang et al. 2006). "
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