Structure of propagating high-stress fronts in a shear-thickening suspension.

We report direct measurements of spatially resolved stress at the boundary of a shear-thickening cornstarch suspension revealing persistent regions of high local stress propagating in the flow direction at the speed of the top boundary. The persistence of these propagating fronts enables precise measurements of their structure, including the profile of boundary stress measured by boundary stress microscopy (BSM) and the nonaffine velocity of particles at the bottom boundary of the suspension measured by particle image velocimetry (PIV). In addition, we directly measure the relative flow between the particle phase and the suspending fluid (fluid migration) and find the migration is highly localized to the fronts and changes direction across the front, indicating that the fronts are composed of a localized region of high dilatant pressure and low particle concentration.

Shear thickening behavior in dense repulsive and attractive suspensions of hard spheres.

Shear thickening in stable dense colloidal suspensions is a reversible phenomenon and no hysteresis is observed in the flow curve measurements. However, a reduction in the stability of colloids promotes particle aggregation and introduces a time dependent rheological response. In this work, by using a model colloidal system of hard spherical silica particles (average diameter of 415 nm) with varying particle volume fractions 0.

Shear-Induced Ordering of Nanopores and Instabilities in Concentrated Surfactant Mesh Phases.

Mixed surfactant systems with strongly bound counterions show many interesting phases such as the random mesh phase consisting of a disordered array of defects (water-filled nanopores in the bilayers). The present study addresses the non-equilibrium phase transition of the random mesh phase under shear to an ordered mesh phase with a high degree of coherence between nanopores in three dimensions. In situ small-angle synchrotron X-ray study under different shear stress conditions shows sharp Bragg peaks in the X-ray diffraction, successfully indexed to the rhombohedral lattice with 3̅ space group symmetry.

Dynamics and memory of boundary stresses in discontinuous shear thickening suspensions during oscillatory shear.

We report direct measurements of spatially resolved surface stresses of a dense suspension during large amplitude oscillatory shear (LAOS) in the discontinuous shear thickening regime using boundary stress microscopy. Consistent with previous studies, bulk rheology shows a dramatic increase in the complex viscosity above a frequency-dependent critical strain. We find that the viscosity increase is coincident with that appearance of large heterogeneous boundary stresses, indicative of the formation of transient solid-like phases (SLPs) on spatial scales large compared to the particle size.

Role of particle orientational order during shear thickening in suspensions of colloidal rods.

Rheology of dense anisotropic colloidal suspensions often exhibits unsteady flow at constant imposed shear stress and/or shear rate. Using simultaneous high-resolution confocal microscopy and rheology, we find that the temporal behavior arises due to a strong coupling between shear flow and particle orientation. At smaller applied stresses, the orientation of rods fluctuates around the flow direction.

Contact Networks Enhance Shear Thickening in Attractive Colloid-Polymer Mixtures.

Increased shear thinning arising due to strong attractive interactions between colloidal particles is thought to obscure shear thickening. Here, we demonstrate how moderate attractions, induced by adding a nonadsorbing polymer, can instead enhance shear thickening. We measure the rheology of colloidal suspensions at a constant particle volume fraction of ϕ=0.

Localized stress fluctuations drive shear thickening in dense suspensions.

Dense particulate suspensions exhibit a dramatic increase in average viscosity above a critical, material-dependent shear stress. This thickening changes from continuous to discontinuous as the concentration is increased. Using direct measurements of spatially resolved surface stresses in the continuous thickening regime, we report the existence of clearly defined dynamic localized regions of substantially increased stress that appear intermittently at stresses above the critical stress.

Viscoelastic behavior of human lamin A proteins in the context of dilated cardiomyopathy.

Lamins are intermediate filament proteins of type V constituting a nuclear lamina or filamentous meshwork which lines the nucleoplasmic side of the inner nuclear membrane. This protein mesh provides a supporting scaffold for the nuclear envelope and tethers interphase chromosome to the nuclear periphery. Mutations of mainly A-type lamins are found to be causative for at least 11 human diseases collectively termed as laminopathies majority of which are characterised by aberrant nuclei with altered structural rigidity, deformability and poor mechanotransduction behaviour.

Reversible shear-induced crystallization above equilibrium freezing temperature in a lyotropic surfactant system.

We demonstrate a unique shear-induced crystallization phenomenon above the equilibrium freezing temperature (T(K)°) in weakly swollen isotropic (Li) and lamellar (La) mesophases with bilayers formed in a cationic-anionic mixed surfactant system. Synchrotron rheological X-ray diffraction study reveals the crystallization transition to be reversible under shear (i.e.

Re-entrant phase behavior of a concentrated anionic surfactant system with strongly binding counterions.

The phase behavior of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the strongly binding counterion p-toluidine hydrochloride (PTHC) has been examined using small-angle X-ray diffraction and polarizing microscopy. A hexagonal-to-lamellar transition on varying the PTHC to SDS molar ratio (alpha) occurs through a nematic phase of rodlike micelles (Nc) --> isotropic (I) --> nematic of disklike micelles (N(D)) at a fixed surfactant concentration (phi). The lamellar phase is found to coexist with an isotropic phase (I') over a large region of the phase diagram.

Aggregation of a peptide antibiotic alamethicin at the air-water interface and its influence on the viscoelasticity of phospholipid monolayers.

The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water interface have been studied using interfacial rheology and fluorescence microscopy techniques. Fluorescence microscopy of alamethicin monolayers revealed a coexistence of liquid expanded (LE) and solid phases at the surface concentrations studied. Interfacial oscillatory shear measurements on alamethicin monolayers indicate that its viscoelastic properties are determined by the area fraction of the solid domains.