Constant Stress Flows Research Articles

Grazing Incidence X-ray Diffraction Studies of Lipid-Peptide Mixed Monolayers during Shear Flow.

Grazing incidence X-ray diffraction (GIXD) studies of monolayers of biomolecules at an air–water interface give quantitative information of in-plane packing, coherence length of crystalline domains, etc. Rheo-GIXD measurements can reveal quantitative changes in the nanocrystalline domains of a monolayer under shear. Here, we report GIXD studies of monolayers of alamethicin peptide, DPPC lipid, and their mixtures at an air–water interface under steady shear stress. The alamethicin monolayer and the mixed monolayer show a flow jamming transition. On the other hand, the pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer under constant stress flows steadily with a notable enhancement of the area/molecule and coherence lengths, suggesting the fusion of nanocrystallites during flow. The DPPC–alamethicin mixed monolayer shows no significant change in the area/DPPC molecule, but the coherence lengths of the individual phases (DPPC and alamethicin) increase, suggesting that the crystallites of individual phases grow bigger by merging of domains. More phase separation occurs in the system during flow. Our results show that rheo-GIXD has the potential to explore in situ molecular structural changes under rheological conditions for a diverse range of confined biomolecules at interfaces.

A novel technique for conducting creep experiments in equibiaxial elongation

A recently developed rheological technique known as continuous lubricated squeezing flow (CLSF) is adapted to perform constant stress, or creep, experiments in equibiaxial elongation flows of polymer melts. By modifying the CLSF technique, which was developed for constant strain rate deformations, we demonstrate that the technique can also be used to generate constant stress flows. Measured steady state viscosities are compared to constant rate elongation results for polymer melts having different molecular characteristics. Linear polymers show strain softening and compare well in constant stress and constant strain rate deformations. The branched polymer shows strain hardening and a viscosity that is slightly higher in constant stress for low rates. Limitations of the current version of the CLSF technique for creep flows are also briefly discussed.

Atomistic simulation of plasticity in silicon nanowires

We study the tensile deformation of polycrystalline Si nanowires by means of molecular dynamics simulations. The initial microstructure is composed by a network of nanocrystals glued together by a thin layer of amorphous material. Atomistic simulations could clearly identify liquidlike flow in the constrained amorphous Si as the responsible for the observed elongation. After this first stage of nearly constant-stress flow, a necking instability sets in, eventually leading to fracture, at the point when the nanowire diameter becomes comparable to the size of the nanocrystals.

Deformation behavior of Mo 5Si 3 single crystal at high temperatures

Compressive deformation behavior of D8 m -type Mo 5Si 3 single crystals was investigated at temperatures between 1473 and 1723 K in an argon atmosphere. Four compression axes, namely [001], near-[111], near-[101] and [100] were chosen. Plasticity occurred at and above 1573 K, whereas at 1473 K the crystals failed by brittle fracture before yielding. After high-temperature yielding, all crystals except the [001] crystal exhibited a large yield drop, followed by an apparent steady state flow. Slip traces of {110} and other relatively low index planes were observed on crystal surfaces. Dislocations were characterized on (001) slip plane by TEM, and 〈110〉(001) slip was identified. Considering the constant-stress flow behavior as a steady-state creep process, it was estimated that a stress component is about 6 and an apparent activation energy of deformation is approximately 490 kJ mol −1.

Equibiaxial extensional flow of polymer melts via lubricated squeezing flow. II. Flow modeling

A model for lubricated squeezing flow of a viscoelastic fluid is developed in order to study the viability of this flow as a rheological technique for generating equibiaxial extensional deformations in polymer melts. In this simple flow model, the melt, described by an upper-convected Maxwell fluid, is squeezed between thin films of a Newtonian fluid. Comparisons of the model predictions for constant strain rate and constant stress flows are made with experimental results presented in the first paper. Predictions from the model are able to describe the effects of lubricant viscosity and experimental configuration and indicate the technique fails for these flows at Hencky strains of approximately one. The cause for this failure is lubricant thinning, which leads to significant errors in both the measured stress difference and the strain.

Equibiaxial extensional flow of polymer melts via lubricated squeezing flow. I. Experimental analysis

The technique of lubricated squeezing flow is evaluated using experiments in both constant strain rate and constant stress flows of two low-density polyethylene melts. Experimental parameters that include the lubricant viscosity and sample aspect ratio are systematically varied to examine their effect on the viability of the technique in the linear viscoelastic regime by comparing measured quantities to those predicted by finite linear viscoelastic theory. An evaluation is also made by comparing viscosities measured using the lubricated squeezing flow technique and Meissner's rotating clamp (MAD) rheometer in the non-linear regime. In both cases, deviations between the expected and measured viscosities were observed, indicating that the technique is not applicable to large strains for constant strain rate and constant stress flows. It is suggested that this limit is the result of lubricant thinning.