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Vorticity banding


Vorticity banding is the shear-induced phenomenon where a banded pattern is formed that extends along the vorticity direction. We studied vorticity banding in suspensions of very long and thin colloidal rods with hard-core like interactions. Banding occurs in a limited part within the two-phase isotropic-nematic coexistence region. Particularly the kinetics of vorticity-band formation is studied. These experiments together with particle tracking lead us to propose a mechanism for the vorticity-banding instability that is similar to the Weissenberg effect in polymer systems, where now the role of the polymers is played by the inhomogeneities that are formed during the initial stages of phase separation. The vorticity-banding instability is thus an elastic instability.

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( K. Kang, P. Lettinga )

Phase behaviour and kinetics of rod-like viruses under shear

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Dispersions of colloidal rods are very susceptible to external fields like, for example, shear flow and a magnetic field. It is therefore interesting to investigate the phase behaviour of a dispersion of rods affected by a shear flow. Here we investigate the non-equilibrium phase behaviour for the isotropic to nematic transition as a function of shear rate and rod concentration, using an improved time resolved Small Angle Light Scattering set-up combined with a couette shear cell and confocal microscopy in combination with a counter-rotating cone-plate shear cell MOVIE We access the full phase diagram including the binodal line and the spinodal line, which gives the shear rate and concentration where the dispersions become unstable. We perform these experiments at varying rod concentration and rod-attraction and show that the phase diagrams for all attractions investigated can be collapsed on a single master curve with a simple scaling.

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( P. Lettinga, P. Holmqvist)

Competition between wall slip and shear banding in wormlike micelles

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The interplay between shear band (SB) formation and boundary conditions is investigated in wormlike micellar systems using ultrasonic velocimetry coupled to standard rheology in Couette geometry. Transient strain-controlled experiments are performed on 6 and 10 wt. % CPyCl/NaSal wormlike micelle solutions. Time-resolved velocity profiles measured in smooth and sand-blasted geometries show (i) that boundary conditions strongly influence both the dynamics of SB formation and the SB fluctuations in the steady state and (ii) evidence for metastability close to the onset of shear banding.

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( P. Lettinga )

Dynamic response of block copolymer wormlike micelles to shear flow

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The flow behavior of giant wormlike micelles consisting of Pb-Peo block copolymers in the vicinity of the isotropic-nematic phase transition concentration is studied. We explain the appearance of shear banding close to this transition by critical slowing down of the rotational diffusion. This is evidenced by a combination of Fourier transform rheology, time-resolved Small Angle Neutron Scattering (SANS), using a microscopic theory for stiff rods to interpret the dynamic response of the system.

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( P. Lettinga )

Flow behavior of rod-like viruses in the nematic phase

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The behavior of a colloidal suspension of rodlike fd viruses in the nematic phase, subjected to steady state and transient shear flows, is studied. The monodisperse nature of these rods combined with relatively small textural contribution to the overall stress make this a suitable model system to investigate the effects of flow on the nonequilibrium phase diagram. Transient rheological experiments are used to determine the critical shear rates at which director tumbling, wagging, and flow-aligning occurs. The present model system enables us to study the effect of rod concentration on these transitions. The results are in quantitatively agreement with the Doi-Edwards-Hess model. Moreover, we observe that there is a strong connection between the dynamic transitions and structure formation, which is not incorporated in theory.

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( P. Lettinga, J. Dhont )

Kritische Phänomene unter dem Einfluss von Scherung


In der Nähe des kritischen Punktes zwischen einer gasförmigen und einer flüssigen Phase werden die effektiven Wechselwirkungen sehr langreichweitig und die Dynamik der Konzentrationsfluktuationen wird stark verlangsamt (üblicherweise bezeichnet als "critical slowing down"). Die langreichweitigen Wechselwirkungen sind durch einen sogenannten kritischen Strukturfaktor ausgezeichnet, der mittels Lichtstreuung gemessen werden kann. Beide, die langreichweitigen Wechselwirkungen und das kritische Verlangsamen der Dynamik machen die Mikrostruktur eines kritischen Systems sehr empfindlich auf Schereinflüsse: Strukturänderungen verhalten sich bereits nicht linear bei kleinen Scherraten und eine "out of phase" Komponente des Strukturfaktors entwickelt sich als Antwort auf eine oszillatorische Scherung schon bei kleinen Frequenzen.

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( P. Lettinga )