Transition to turbulence in the free convection boundary layers on an inclined heated plate

An experimental study has been made of transition to turbulence in the free convective flows on a heated plate. Observations have been made with the plate vertical and inclined at angles up to about 50° to the vertical, both above and below the plate. A fibre anemometer was used to survey the region of intermittent turbulence. Information has thus been obtained about the range of Grashof numbers over which transition takes place. Even when the plate is vertical the region of intermittent turbulence is long. When it is inclined, this region becomes still longer in the flow below the plate as a result of the stabilizing stratification, a Richardson number effect. It is possible to have a whole flow such that it should be described as transitional, not laminar or turbulent. It was noticed that in this flow and the vertical plate one, the velocity during the laminar periods could be either of two characteristic values, one of them close to zero. The behaviour above an inclined plate could be interpreted largely as a trend towards the behaviour described in a preceding paper.

A dynamical instability due to fluid–wall coupling lowers the transition Reynolds number in the flow through a flexible tube

A flow-induced instability in a tube with flexible walls is studied experimentally. Tubes of diameter 0.8 and 1.2 mm are cast in polydimethylsiloxane (PDMS) polymer gels, and the catalyst concentration in these gels is varied to obtain shear modulus in the range 17–550 kPa. A pressure drop between the inlet and outlet of the tube is used to drive fluid flow, and the friction factor $f$ is measured as a function of the Reynolds number $Re$. From these measurements, it is found that the laminar flow becomes unstable, and there is a transition to a more complicated flow profile, for Reynolds numbers as low as 500 for the softest gels used here. The nature of the $f$–$Re$ curves is also qualitatively different from that in the flow past rigid tubes; in contrast to the discontinuous increase in the friction factor at transition in a rigid tube, it is found that there is a continuous increase in the friction factor from the laminar value of $16\ensuremath{/} Re$ in a flexible tube. The onset of transition is also detected by a dye-stream method, where a stream of dye is injected into the centre of the tube. It is found that there is a continuous increase of the amplitude of perturbations at the onset of transition in a flexible tube, in contrast to the abrupt disruption of the dye stream at transition in a rigid tube. There are oscillations in the wall of the tube at the onset of transition, which is detected from the laser scattering off the walls of the tube. This indicates that the coupling between the fluid stresses and the elastic stresses in the wall results in an instability of the laminar flow.

Freezing of a colloidal liquid subject to shear flow

A nonequilibrium generalization of the density-functional theory of freezing is proposed to investigate the shear-induced first-order phase transition in colloidal suspensions. It is assumed that the main effect of a steady shear is to break the symmetry of the structure factor of the liquid and that for small shear rate, the phenomenon of a shear-induced order-disorder transition may be viewed as an equilibrium phase transition. The theory predicts that the effective density at which freezing takes place increases with shear rate. The solid (which is assumed to be a bcc lattice) formed upon freezing is distorted and specifically there is less order in one plane compared with the order in the other two perpendicular planes. It is shown that there exists a critical shear rate above which the colloidal liquid does not undergo a transition to an ordered (or partially ordered) state no matter how large the density is. Conversely, above the critical shear rate an initially formed bcc solid always melts into an amorphous or liquidlike state. Several of these predictions are in qualitative agreement with the light-scattering experiments of Ackerson and Clark. The limitations as well as possible extensions of the theory are also discussed.

The study of pressure induced structural phase transition in spin-frustrated Yb2Ti2O7 pyrochlore

Our in situ x-ray diffraction and Raman measurements of Yb2Ti2O7 pyrochlore show that it undergoes a reversible structural phase transition from cubic pyrochlore to a monoclinic phase at similar to 28.6 GPa. Analysis of the x-ray data shows the transition to be thermodynamically first order and the high pressure phase to be substitutionally disordered. These experimental results are supported by our first principles calculations. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681300]

Effective Mass-Driven Structural Transition in a Mn-Doped ZnS Nanoplatelet

Mn doping in ZnS nanoplatelets has been shown to induce a structural transition from the wurtzite to the zinc blende phase. We trace the origin of this transition to quantum confinement effects, which shift the valence band maximum of the wurtzite and zinc blende polyrnorphs of ZnS at different rates as a function of the nanocrystal size, arising from different effective hole masses in the two structures. This modifies the covalency associated with Mn incorporation and is reflected in the size-dependent binding energy difference for the two structures.

Direct hexagonal transition of amorphous (Ge2Sb2Te5)(0.9)Se-0.1 thin films

Ge2Sb2Te5 (GST) is well known for its phase change properties and applications in memory and data storage. Efforts are being made to improve its thermal stability and transition between amorphous and crystalline phases. Various elements are doped to GST to improve these properties. In this work, Se has been doped to GST to study its effect on phase change properties. Amorphous GST film crystallized in to rock salt (NaCl) type structure at 150 degrees C and then transformed to hexagonal structure at 250 degrees C. Interestingly, Se doped GST ((GST)(0.9)Se-0.1) film crystallized directly into hexagonal phase and the intermediate phase of NaCl is not observed. The crystallization temperature (T-c) of (GST)(0.9)Se-0.1 is around 200 degrees C, which is 50 degrees C higher than the T-c of GST. For (GST)(0.9)Se-0.1, the threshold switching occurs at about 4.5V which is higher than GST (3 V). Band gap (E-opt) values of as deposited films are calculated from Tauc plot which are 0.63 eV for GST and 0.66 eV for (GST)(0.9)Se-0.1. The E-opt decreases for the films annealed at higher temperatures. The increased T-c, E-opt, the contrast in resistance and the direct transition to hexagonal phase may improve the data readability and thermal stability in the Se doped GST film. (C) 2014 AIP Publishing LLC.

A composition-dependent ``re-entrant'' crystallographic phase transition in the substitutional metal acetylacetonate complex (Cr1-xGax)(acac)(3)

The formation of a complete solid solution between acetylacetonate (acac) complexes of chromium and gallium, (Cr1-x,Ga-x)(acac)(3) for 0.1 to form for each nominal value of x, with thermal analysis confirming each composition to have a congruent melting point, making it a substitutional complex. Whereas the pure complexes (i.e. the end members of the solid solution, x = 0 and x = 1) are both centro-symmetric, a composition-dependent crystallographic phase transition to a non-centrosymmetric structure is found to occur for compositions with 0.4 < x < 0.9. Such a ``re-entrant'' crystallographic transition is interpreted to be due to the drive to overcome the disorder present in certain centrosymmetric chromium-rich compositions, by going over to a non-centrosymmetric structure with a doubling of the unit cell. The substitutional complex is shown to lead to a substitutional oxide with the beta-gallate structure. (C) 2014 Elsevier Ltd. All rights reserved.

Texture transition in cold-rolled nickel-40 wt.% cobalt alloy

The micromechanical aspects of rolling texture development in Ni-40 wt.% Co alloy during very large reductions (up to epsilon(t) = 3.9) have been studied. The alloy showed a typical Cu-type texture up to a true strain of epsilon(t) = 3; however, the texture undergoes an abrupt transition to Bs-type on further rolling to epsilon(t) approximate to 4. (The Bs-type texture, here, comprises almost equal fractions of Goss and Bs components.) Microstructural observations, at early stages, show that deformation is accommodated entirely by slip, and very little presence of deformation twinning is observed to explain the texture transition. However, at much higher reduction levels, micrographs show a high fraction of Cu-type shear bands. These bands are predominantly found in Cu-oriented grains and the crystallites inside the shear bands are preferentially oriented towards Goss, which could explain the final texture evolution. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys

Amorphous Ge2Sb2Te5 (GST) alloy, upon heating crystallize to a metastable NaCl structure around 150 degrees C and then to a stable hexagonal structure at high temperatures (>= 250 degrees C). It has been generally understood that the phase change takes place between amorphous and the metastable NaCl structure and not between the amorphous and the stable hexagonal phase. In the present work, it is observed that the thermally evaporated (GST)(1-x)Se-x thin films (0 <= x <= 0.50) crystallize directly to the stable hexagonal structure for x >= 0.10, when annealed at temperatures >= 150 degrees C. The intermediate NaCl structure has been observed only for x, 0.10. Chemically ordered network of GST is largely modified for x >= 0.10. Resistance, thermal stability and threshold voltage of the films are found to increase with the increase of Se. The contrast in electrical resistivity between the amorphous and crystalline phases is about 6 orders of magnitude. The increase in Se shifts the absorption edge to lower wavelength and the band gap widens from 0.63 to 1.05 eV. Higher resistance ratio, higher crystallization temperature, direct transition to the stable phase indicate that (GST)(1-x)Se-x films are better candidates for phase change memory applications.

Effect of ultra-fast mixing in a microchannel due to a soft wall on the room temperature synthesis of gold nanoparticles

The room-temperature synthesis of mono-dispersed gold nanoparticles, by the reduction of chlorauric acid (HAuCl4) with tannic acid as the reducing and stabilizing agent, is carried out in a microchannel. The microchannel is fabricated with one soft wall, so that there is a spontaneous transition to turbulence, and thereby enhanced mixing, when the flow Reynolds number increases beyond a critical value. The objective of the study is to examine whether the nanoparticle size and polydispersity can be modified by enhancing the mixing in the microchannel device. The flow rates are varied in order to study nanoparticle formation both in laminar flow and in the chaotic flow after transition, and the molar ratio of the chlorauric acid to tannic acid is also varied to study the effect of molar ratio on nanoparticle size. The formation of gold nanoparticles is examined by UV-visual spectroscopy and the size distribution is determined using scanning electron microscopy. The synthesized nanoparticles size decreases from a parts per thousand yen6 nm to a parts per thousand currency sign4 nm when the molar ratio of chlorauric acid to tannic acid is increased from 1 to 20. It is found that there is no systematic variation of nanoparticle size with flow velocity, and the nanoparticle size is not altered when the flow changes from laminar to turbulent. However, the standard deviation of the size distribution decreases by about 30% after transition, indicating that the enhanced mixing results in uniformity of particle size.

Structure and magnetotransport properties of the layered manganites REl.2Srl.8Mn,07(R E = La, Pr, Nd)

We have examined the magnetotransport properties and the structure, by Rietveld refinement of powder X-ray data, of the phases RE(1.2)Sr(1.8)Mn(2)O(7) (RE = La, Pr, Nd). We find that on cooling, La1.2Sr1.8Mn2O7 undergoes a transition to a nearly perfect ferromagnet with 90% magnetization at 1.45 T, as reported by earlier workers, but the Pr and Nd phases show only a small magnetization that grows gradually as the temperature is decreased. There seems to be significant correlation between electrical transport and the Jahn-Teller elongation of the apical Mn-O bonds in these systems. The elongation of the apical Mn-O bonds forces the nine-coordinate rock-salt site to be occupied preferentially by the smaller rare-earth-metal cations. This preferential occupation is reliably obtained from the X-ray refinement. All three title phases show a magnetoresistance ratio of about 4(corresponding to a magnetoresistance, [R(0)-R(H)]/R(0), of about 75%) at a field of 7 T and temperatures around 100 K.

Left-Handed DNA in Synthetic and Topologically Constrained Form V-DNA

We have investigated structural transitions in Poly(dG-dC) and Poly(dG-Me5dC) in order to understand the exact role of cations in stabilizing left-handed helical structures in specific sequences andthe biological role, if any, of these structures. From a novel temperature dependent transition it has been shown that a minor fluctuation in Na+ concentration at ambient temperature can bring about Β to Ζ transition. Forthe first time, wehave observed a novel double transition in poly(dG-Me5dC) as the Na+ concentration is gradually increased. This suggests that a minor fluctuation in Na+ concentration in conjunction with methylation may transform small stretches of CG sequences from one conformational state to another. These stretches could probably serve as sites for regulation. Supercoiled formV DNA reconstituted from pBR322 and pßG plasmids have been studied as model systems, in order to understand the nature and role of left-handed helical conformation in natural sequences. A large portion of DNA in form V, obtained by reannealing the two complementary singlestranded circles is forced to adopt left-handed double helical structure due to topological constraints (Lk = 0). Binding studies with Z-DNA specific antibody and spectroscopic studies confirm the presence of left-handed Z-structure in the pßG and pßR322 form V DNA. Cobalt hexamine chloride, which induces Z-form in Poly(dG-dC) stabilizes the Z-conformation in form V DNA even in the non-alternating purine-pyrimidine sequences. A reverse effect is observed with ethidium bromide. Interestingly, both topoisomerase I and II (from wheat germ) act effectively on form V DNA to give rise to a species having an electrophoretic mobility on agarose gel similar to that of open circular (form II) DNA. Whether this molecule is formed as a result of the left-handed helical segments of form V DNA undergoing a transition to the right-handed B-form during the topoisomerase action remains to be solved.

Constraints in relativistic Hamiltonian mechanics

We propose a new scheme for the use of constraints in setting up classical, Hamiltonian, relativistic, interacting particle theories. We show that it possesses both Poincaré invariance and invariance of world lines. We discuss the transition to the physical phase space and the nonrelativistic limit.