Strong Shear Field Research Articles

Mutual and self-diffusion in fluids undergoing strong shear.

We derive a Green-Kubo relation for the mutual diffusion tensor (MDT) of a binary fluid mixture subject to a strong Couette shear field. We calculate the MDT of a Lennard-Jones (LJ) mixture and the self-diffusion tensor (SDT) of a pure LJ fluid as functions of state point and strain rate. The qualitative dependence of the diffusion tensors upon these variables is explained in greater detail by analyzing the velocity and mass current autocorrelation functions. Finally, we compare the zero-color field color conductivity with the SDT. For nonzero shear rates we find that the color conductivity and the SDT are different, confirming the prediction by Evans, Baranyai, and Sarman [Mol. Phys. (to be published)], according to which these two transport coefficients should only be related at zero shear rate.

The role of transport phenomena in protein crystal growth

The idiosyncrasies of protein crystal growth are widely known: crystals often stop growing before reaching a size adequate for X-ray diffraction studies; sometimes their quality is poor. The reasons for this behavior are not well understood. We investigated several transport mechanisms to ascertain whether they might play significant roles in protein crystal growth. Scale analyses were used to establish the order-of-magnitude of forces arising from natural convection. They show that shear forces are probably too small to dislodge molecules from the crystal surface or alter protein structure by unfolding. Strong shear fields can align molecules, but we find that the shear arising from natural convection is much too weak to alter the orientation distribution of protein molecules. Crystal growth rates are set by a balance between attachment kinetics and transport to the crystal surface and, according to our analysis, crystals prepared in growth rate studies are usually too small to be affected by either diffusion or convective transport. The role of salt rejection at the crystal-fluid interface was also investigated. Salt rejection could influence growth by changing the transport velocity normal to the interface or by changing the driving force for diffusion. The magnitude of each effect appears too small to be significant.

The rheological properties of concentrated cetyltrimethylammonium bromide-salicylic acid solutions in water

Data on the rheological properties of the hexadecyl-trimethylammonium salicylate system (CTAB-SA) in water are reported. Three concentrations were used (0.1, 0.01, and 0.001 M). For the highest concentration, the effect of temperature on the rheology was studied in detail. The rheology of the 0.1 M CTAB-SA solution indicates a very uniform micellar size. By contrast with concentrated polymethyl methycrylate dispersions studied by the author, there was a strong divergence between the viscosity-shear rate and viscosity-frequency data, although the plateau low shear rate and frequency values agreed over a wide range of temperature. This effect could be explained by a shear rate dependent diffusion constant. The large temperature variation of the plateau viscosity and elasticity modulus values could be explained by a combination of micellar number concentration and flexibility changes as the temperature varies. At lower concentrations, the rheological data shows evidence of polydispersity in micellar size. Strong shear thickening and extensional viscosity effects are also evident, probably due to micellar overlap and cluster formation in strong shear fields and the alignment of the very long micelles in elongational flow. The shear thickening effects take some 200 s to relax (0.01 M solution). Recovery of the elasticity after shearing the 0.1 M solution is rapid (a few hundred milliseconds).

Rotation of fragments in catastrophic impact

The rotation of fragments produced by catastrophic impacts into basalt targets was investigated using framing camera records taken by Fujiwara and Tsukamoto (1980, Icarus 44, 142–153). Most of the cores have low rotation rates, of the order 1 rev · sec −1 or less. Many spall fragments have high rotation rates in the strong shear field produced in the target material by impact.

On the Core Structure of the Glide‐Set 90° and 30° Partial Dislocations in Silicon

AbstractBy considering valence force potentials the cores of the glide‐set 30° and 90° partial dislocations in silicon are investigated. It is shown that the 30° partial probably does not contain dangling bonds, while on the other hand dangling bonds at the 90° partial are more likely to exist as pairing of bonds across its core is opposed by a strong shear field. The implications of the results on the electron states of the partials are briefly discussed.