Occurrence of coexisting dendrite morphologies: immiscible fluid displacement in an anisotropic radial Hele-Shaw cell under a high flow rate regime

Viscous fingering morphologies during the displacement of a high viscosity fluid by a low viscosity immiscible fluid in a radial fourfold anisotropic Hele-Shaw cell are examined. By using the kerosene-glycerin system for which the mu/T ratio (mu being the relative viscosity and T the interfacial tension between the fluids) is about ten times higher than that for the commonly used air-glycerin system, we have been able to access the hitherto unexplored Nca greater than approximately 1 regime (capillary number Nca = U mu/T, U being the advancing fingertip velocity). Within the anisotropy-dominated regime, and when flow rates are significantly high (capillary number well beyond Nca = 1), a new phase is seen to evolve wherein the dendrites grow simultaneously along the channels and along the directions making an angle of 45 degrees with the channels, both being kinetically driven. This new phase resembles the one observed in a miscible fluid system at all flow rates of the displacing fluid. [A. G. Banpurkar et al., Phys. Rev. E 59, 2188 (1999)].

Shape effects in random sequential adsorption of zero-area angled objects on a continuum substrate

Random sequential adsorption (RSA) of zero-area symmetric angled objects on a continuum substrate is studied for the full range (0 degrees -180 degrees ) of values of the arm angle straight phi. The value of exponent alpha [in rho(t) approximately t(alpha), where rho(t) is the number density] exhibits a dramatic crossover feature near straight phi = 0 degrees or 180 degrees. It is shown that the functional dependence of alpha on straight phi is governed by the degree of shape dependent frustration of the adsorption process. The crucial role of the geometrical character of the zero-area object in RSA dynamics is thus established.

Phase transitions in the incoherent lattice fluctuations in YBa2Cu3O(7-delta)

The growing body of experimental evidence for the existence of complex textures of charges and spins in the high-temperature superconductors has drawn attention to the so-called 'stripe-phase' models as a possible basis for the mechanism of superconductivity in these materials. Such observations have until now been restricted to systems where the texture dynamics are slow or suppressed altogether, and do not include the important case of YBa2Cu3O(7-delta). It seems likely that the dynamic behaviour of stripes, which has been suggested to undergo several phase transitions as a function of temperature, should also be reflected in the lattice properties of the host materials, and this forms the motivation for our present experiments. Specifically, we use MeV helium ion channelling, an ultrafast real-space probe of atomic displacements (with sub-picometre resolution), to probe incoherent lattice fluctuations in YBa2Cu3O(7-delta) as a function of temperature and oxygen doping. We detect lattice fluctuations that are larger than the expected thermal vibration component, and which show anomalies characteristic of the phase transitions anticipated for a dynamic stripe phase. Comparison of our lattice results with single-particle-tunnelling and photoemission data highlights the importance of spin-charge separation phenomena in the copper oxide superconductors.