Phonon and Elastic Instabilities in MoC and MoN

We present several results related to the instability of MoC and MoN in the B1 (sodium chloride) structure. These compounds were proposed as potential superconductors with moderately high transition temperatures. We show that the elastic instability in B1-structure MoN, demonstrated several years ago, persists at elevated pressures, thus offering little hope of stabilizing this material without chemical doping. For MoC, another material for which stoichiometric fabrication in the B1-structure has not proven possible, we find that all of the cubic elastic constants are positive, indicating elastic stability. Instead, we find X-point phonon instabilities in MoC (and in MoN as well), further illustrating the rich behavior of carbo-nitride materials. We also present additional electronic structure results for several transition metal (Zr, Nb and Mo) carbo-nitride systems and discuss systematic trends in the properties of these materials. Deviations from strict electron counting dependencies are apparent.Comment: 5 pages and 4 trailing figures. Submitted to PR

Elastic instabilities in planar elongational flow of monodisperse polymer solutions

We investigate purely elastic flow instabilities in the almost ideal planar stagnation point elongational flow field generated by a microfluidic optimized-shape cross-slot extensional rheometer (OSCER). We use time-resolved flow velocimetry and full-field birefringence microscopy to study the behavior of a series of well-characterized viscoelastic polymer solutions under conditions of low fluid inertia and over a wide range of imposed deformation rates. At low deformation rates the flow is steady and symmetric and appears Newtonian-like, while at high deformation rates we observe the onset of a flow asymmetry resembling the purely elastic instabilities reported in standard-shaped cross-slot devices. However, for intermediate rates, we observe a new type of elastic instability characterized by a lateral displacement and time-dependent motion of the stagnation point. At the onset of this new instability, we evaluate a well-known dimensionless criterion M that predicts the onset of elastic instabilities based on geometric and rheological scaling parameters. The criterion yields maximum values of M which compare well with critical values of M for the onset of elastic instabilities in viscometric torsional flows. We conclude that the same mechanism of tension acting along curved streamlines governs the onset of elastic instabilities in both extensional (irrotational) and torsional (rotational) viscoelastic flows

Elastic Instabilities within Antiferromagnetically Ordered Phase in the Orbitally-Frustrated Spinel GeCo2_2O4_4

Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$ reveal unusual elastic instabilities due to the phonon-spin coupling within the antiferromagnetic phase. Shear moduli exhibit anomalies arising from the coupling to short-range ferromagnetic excitations. Diplike anomalies in the magnetic-field dependence of elastic moduli reveal magnetic-field-induced orbital order-order transitions. These results strongly suggest the presence of geometrical orbital frustration which causes novel orbital phenomena within the antiferromagnetic phase.Comment: 5 pages, 3 figure

Role of surface roughness in superlubricity

We study the sliding of elastic solids in adhesive contact with flat and rough interfaces. We consider the dependence of the sliding friction on the elastic modulus of the solids. For elastically hard solids with planar surfaces with incommensurate surface structures we observe extremely low friction (superlubricity), which very abruptly increases as the elastic modulus decreases. We show that even a relatively small surface roughness may completely kill the superlubricity state.Comment: 11 pages, 17 figures, format revte

High performance microfluidic rectifiers for viscoelastic fluid flow

The flow of Newtonian and non-Newtonian fluids within microfluidic rectifiers with a hyperbolic shape was investigated to assess the effect of the bounding walls on the diodicity of the microfluidic device and achieve high flow anisotropy. Three microchannels were used, with different depths and the same geometrical configuration, which creates a strong extensional flow and generates high anisotropic flow resistance between the two flow directions. The Newtonian fluid, de-ionized water, was used as a reference fluid. The viscoelastic fluid used was an aqueous solution of polyethylene oxide (0.1% w/w) with high molecular weight. The flow patterns were visualized using streak photography and the velocity field was investigated using micro-particle image velocimetry. Moreover, pressure drop measurements were performed in order to compare the diodicity achieved in the microfluidic rectifiers. For the Newtonian fluid flow, the experimental results are compared with numerical predictions obtained using a finite-volume method and good agreement was found between both approaches. For the viscoelastic fluid, significant anisotropic flow resistance can be achieved. The effect of the bounding walls was analysed and found to be qualitatively similar for all microchannels. Nevertheless, in quantitative terms, the diodicity is enhanced when the wall effect is reduced, i.e. when the channels are deeper. A maximum diodicity above six was found for the deeper channel, a value well beyond those previously reported

Spectra of turbulence in dilute polymer solutions

We investigate turbulence in dilute polymer solutions when polymers are strongly stretched by the flow. We establish power-law spectrum of velocity, which is not associated with a flux of a conserved quantity, in two cases. The first case is the elastic waves range of high Reynolds number turbulence of polymer solutions above the coil-stretch transition. The second case is the elastic turbulence, where chaotic flow is excited due to elastic instabilities at small Reynolds numbers.Comment: 14 pages, RevTe