Shear-banding and microstructure of colloids in shear flow

Abstract

We shall discuss the following phenomena found in various colloidal systems in shear flow. We recently observed shear-banding in suspensions of fd-virus in a cylindrical shear cell. Small angle light scattering experiments revealed that the shear-banding transition is preceded by a relatively fast process (minutes) of nematic-to-paranematic phase separation during which inhomogeneities on the micrometer length scale are formed. After the formation of these inhomogeneities, a slow (hours) appearence of shear-bands is observed, which have a height of a few mm. In the stationary state it is found, by means of polarization microscopy, that inhomogeneities exist within the bands. Small angle, time resolved light scattering experiments on near-critical microstructural order in a mixture of colloidal spheres and free polymer under stationary shear flow are discussed. The unexpected distortion of microstructure in directions perpendicular to the flow direction is quantitatively explained by extending an already existing theory, to include shear-induced short-ranged microstructural distortion. In colloidal systems consisting of "hairy colloids", where a spherical core is decorated with relatively long polymers, shear-induced polymer brush deformation might be important for its structural and rheological behavior. Preliminary neutron scattering and rheology experiments are performed to study polymer brush deformation in (semi-) dilute suspensions.

Osmotic shrinkage in star/linear polymer mixtures

“…Moreover, the reduction of R g /R g0 with c/c lin * reveals the osmotic effect of the added linear homopolymers in reducing the star's radius of gyration. This effect is unique for soft and deformable objects [7,[12][13][14][15][16]. The data of Fig.2a, which are plotted against the weight concentration (or monomer number density), are independent of the linear chain molar mass (Fig.…”

“…Last but not least, whereas the depletion effect was unambiguously confirmed with SANS (and rheological) studies, which conformed to the reduced repulsion of the effective pair interaction potential 3 [4,6,13,14,15], no theoretical rationalization for the single particle shrinkage from first principles is yet available. Yet, this behavior appears to be generic in soft deformable particles [2,[12][13][14][15][16], and a fundamental understanding is needed. Hence, in this work we attempt at complementing our earlier studies by measuring the form factor of single stars in a sea of linear homopolymers of different molar masses, and give a theoretical description of the observed osmotic star shrinkage.…”

Effect of cohesion on shear banding in quasistatic granular materials

“…When they yield under slow shear, the relative motion is confined to narrow regions (between large solid-like parts) called shear bands [1][2][3]. Shear bands are observed in many complex materials, which range from foams [4] and emulsions [5,6] to colloids [7] and granular matter [1,2,[8][9][10][11][12][13][14][15][16][17]. There has been tremendous effort to understand the shear banding in flow of non-cohesive grains [1,2,[8][9][10][11][12][13][14][15][16][17][18][19].…”

Confocal microscopy of colloidal dispersions in shear flow using a counter-rotating cone–plate shear cell

“…Extension of these methods to colloidal dispersions under shear is expected to lead to new insights into shear-induced phase transitions that have been investigated in the past and new phenomena that cannot be so easily inferred from a k-space analysis. The former include the formation of crystalline order under oscillatory shear [8], shear melting of colloidal crystals [9], shear banding in dispersions of spherical [10,11] and rod-like colloids [12].…”