C.-Y. D. Lu

Johnson–Segalman model with a diffusion term in cylindrical Couette flow

We study the Johnson–Segalman (JS) model as a paradigm for some complex fluids which are observed to phase separate, or “shear band” in flow. We analyze the behavior of this model in cylindrical Couette flow and demonstrate the history dependence inherent in the local JS model. We add a simple gradient term to the stress dynamics and demonstrate how this term breaks the degeneracy of the local model and prescribes a much smaller (discrete, rather than continuous) set of banded steady state solutions. We investigate some of the effects of the curvature of Couette flow on the observable steady state behavior and kinetics, and discuss some of the implication for metastability.

Coexistence and phase separation in sheared complex fluids

We demonstrate how to construct dynamic phase diagrams for complex fluids that undergo transitions under flow, in which the conserved composition variable and the broken-symmetry order parameter (nematic, smectic, crystalline, etc.) are coupled to shear rate. Our construction relies on a selection criterion, the existence of a steady interface connecting two stable homogeneous states. We use the (generalized) Doi model of lyotropic nematic liquid crystals as a model system, but the method can be easily applied to other systems, provided nonlocal effects are included.

Steric Zipper of the Amyloid Fibrils Formed by Residues 109–122 of the Syrian Hamster Prion Protein

We report the results of atomic force microscopy, Fourier-transform infrared spectroscopy, solid-state nuclear magnetic resonance, and molecular dynamics (MD) calculations for amyloid fibrils formed by residues 109-122 of the Syrian hamster prion protein (H1). Our data reveal that H1 fibrils contain no more than two beta-sheet layers. The peptide strands of H1 fibrils are antiparallel with the A117 residues aligned to form a linear chain in the direction of the fibril axis. The molecular structure of the H1 fibrils, which adopts the motif of steric zipper, is highly uniform in the region of the palindrome sequence AGAAAAGA. The closest distance between the two adjacent beta-sheet layers is found to be about 5 A. The structural features of the molecular model of H1 fibrils obtained by MD simulations are consistent with the experimental results. Overall, our solid-state NMR and MD simulation data indicate that a steric zipper, which was first observed in the crystals of fibril-forming peptides, can be formed in H1 fibrils near the region of the palindrome sequence.

Non-linear rheology of lamellar liquid crystals

Abstract.We measure the non-linear relation between the shear stress and shear rate in the lyotropic lamellar phase of C12E5 /water system. The measured shear thinning exponent changes with the surfactant concentration. A simple rheology theory of a lamellar or smectic phase is proposed with a prediction

Smectic rheology close to the smectic-nematic transition

\dot{{\gamma}}

Uniform Mesoporous Silica Hexagon and Its Two‐Dimensional Colloidal Crystal

∼ σ3/2 , where

A density functional theory of chiral block copolymer melts

\dot{{\gamma}}

The dielectric spectrum of flexible polyelectrolyte solutions

is the shear rate and σ is the shear stress. We consider that the shear flow passed through the defect structure causes the main dissipation. As the defect line density varies with the shear rate, the shear thinning arises. The defect density is estimated by the dynamic balance between the production and annihilation processes. The defect production is caused by the shear-induced layer undulation instability. The annihilation occurs through the shear-induced defect collision process. Further flow visualization experiment shows that the defect texture correlates strongly with the shear thinning exponent.

The static dielectric constant of a colloidal suspension

We study the rheological properties of a thermotropic liquid crystal, 8CB, in the smectic phase close to the smectic-nematic (Sm-N) transition temperature. Three different regimes were identified in the flow curves at different temperatures: i) appearance of the yield stress at low stresses, ii) power law behavior at intermediate shear stresses, and iii) Newtonian at higher shear stresses. The vanishing of the yield stress at the Sm-N transition temperature is correlated with a rapid growth of focal conic domains. The constructed dynamic phase diagram exhibits the two different smectic phases together with the flow-induced Sm-N transition.

Smectic Edge Dislocations under Shear

Well-ordered mesoporous silica nanoparticles with uniform hexagonal disk shapes are synthesized under dilute alkaline conditions with a two-step process, separating the nucleation and growth process. The resulting monodisperse hexagons can be arranged in a 2-dimensional (2D) ordered periodical super-structure. The hexagonal symmetry is similar in both scales. A statistical mechanical cell model is applied to analyze consequences of the interesting packing structure, including osmotic bulk modulus, phase separation and defects.