Departament de F�sica Aplicada

28- I. Sendina-Nadal, D. Roncaglia, D. Vives, V.Perez-Munuzuri, M. Gomez-Gesteira, V. Perez-Villar, J. Echave, J. Casademunt, L. Ramirez-Piscina and F. Sagues.

Percolation thresholds in chemical excitable media.

Phys Rev E 58, R1183-R1186 (1998).

Abstract: The behavior of chemical waves advancing through a disordered excitable medium is investigated in terms of percolation theory and autowave properties in the framework of the light-sensitive Belousov-Zhabotinsky reaction. By controlling the number of sites with a given illumination, different percolation thresholds for propagation are observed, which depend on the relative wave transmittances of the two-state medium considered.

29- R.Gonzalez-Cinca, L.Ramirez-Piscina, J.Casademunt, A.Hernandez-Machado, T.T.Katona, T.Borzsonyi and A. Buka

Heat diffusion anisotropy in dendritic growth: Phase-field simulations and experiments in liquid crystals

Journal of Crystal Growth 193, 712-719 (1998).

Abstract: An anisotropic heat diffusion coefficient is introduced in order to study some interfacial growth phenomena. This anisotropy has been incorporated in a phase field model which has been studied numerically to reproduce some fundamental solidification situations (needle crystal growth) as well as the dynamics of a nematic–smectic-B interface. As a general result, we find that dendrites grow faster in the lower heat diffusion direction. Simulation results are compared with experiments with remarkable qualitative agreement.

30- J. Armero, J. Casademunt, L. Ramirez-Piscina and J.M. Sancho.

Ballistic and diffusive corrections to front propagation in the presence of multiplicative noise

Phys Rev E 58, 5494-5500 (1998).

Abstract: We study the dynamics of reaction-diffusion fronts under the influence of multiplicative noise. An approximate theoretical scheme is introduced to compute the velocity of the front and its diffusive wandering due to the presence of noise. The theoretical approach is based on a multiple scale analysis rather than on a small noise expansion and is confirmed with numerical simulations for a wide range of the noise intensity. We report on the possibility of noise sustained solutions with a continuum of possible velocities, in situations where only a single velocity is allowed without noise.

31- A. Penaranda, C.E. Auguet and L. Ramirez-Piscina

Transitions in disordered suspensions of superconducting granules under external magnetic field

Solid State Communications 109, 227 (1999).

Abstract: We perform simulations of transitions in three-dimensional suspensions of superconducting granules, placed completely at random in a sample, when an external magnetic field is slowly increased. The results show that these transitions induce order in the system. This effect appears as advantageous in applications for Superheated Superconducting Granule detectors.

32- A. Penaranda, C.E. Auguet and L. Ramirez-Piscina

Surface field in an ensemble of superconducting spheres under external magnetic field

Nuclear Instruments and Methods in Physics Research A 424, 512-522 (1999)

Abstract: We perform calculations of the magnetic field on the surface of an ensemble of superconducting spheres when placed into an external magnetic field, which is the configuration employed in superheated superconducting granule detectors. The Laplace equation is numerically solved with appropriate boundary conditions by means of an iterative procedure and a multipole expansion.

33- A.M. Lacasta, I.R. Cantalapiedra, C.E. Auguet, A. Penaranda and L. Ramirez-Piscina

Modelling of spatio-temporal patterns in bacterial colonies

Physical Review E 59, 7036 (1999)

Abstract: A diffusion-reaction model for the growth of bacterial colonies is presented. The often observed cooperative behavior developed by bacteria which increases their motility in adverse growth conditions is here introduced as a nonlinear diffusion term. The presence of this mechanism depends on a response which can present hysteresis. By changing only the concentrations of agar and initial nutrient, numerical integration of the proposed model reproduces the different patterns shown by Bacillus subtilis OG-01.

34- R. Folch, J. Casademunt, A. Hernandez-Machado and L. Ramirez-Piscina

Phase-field model for Hele-Shaw flows with arbitrary viscosity contrast I: theoretical approach

Physical Review E 60, 1724 (1999)

Abstract: We present a phase-field model for the dynamics of the interface between two inmiscible fluids with arbitrary viscosity contrast in a rectangular Hele-Shaw cell. With asymptotic matching techniques we check the model to yield the right Hele-Shaw equations in the sharp-interface limit, and compute the corrections to these equations to first order in the interface thickness. We also compute the effect of such corrections on the linear dispersion relation of the planar interface. We discuss in detail the conditions on the interface thickness to control the accuracy and convergence of the phase-field model to the limiting Hele-Shaw dynamics. In particular, the convergence appears to be slower for high viscosity contrasts.

35- R. Folch, J. Casademunt, A. Hernandez-Machado and L. Ramirez-Piscina

Phase-field model for Hele-Shaw flows with arbitrary viscosity contrast II: numerical study

Physical Review E 60, 1734 (1999)

Abstract: We implement a phase-field simulation of the dynamics of two fluids with arbitrary viscosity contrast in a rectangular Hele-Shaw cell. We demonstrate the use of this technique in different situations including the linear regime, the stationary Saffman-Taylor fingers, and the multifinger competition dynamics, for different viscosity contrasts. The method is quantitatively tested against analytical predictions and other numerical results. A detailed analysis of convergence to the sharp interface limit is performed for the linear dispersion results. We show that the method may be a useful alternative to more traditional methods.

36.- I. Sendina-Nadal, V.Perez-Munuzuri, M. Gomez-Gesteira, A.P. Perez-Munuzuri, V. Perez-Villar, D. Vives, F. Sagues, J. Casademunt, J.M. Sancho and L. Ramirez-Piscina.

Effects of a quenched disorder on wave propagation in excitable media.

International Journal of Bifurcations and Chaos 9, 2353-2362 (1999).

Abstract: The behavior of autowaves under the effect of a quenched disorder is studied in the framework of the light-sensitive Belousov–Zhabotinsky reaction. This allows us to introduce spatial disorder on the excitability by projecting patterns of light transmittance. In particular, we have selected a dichotomic random distribution of levels of transmittance. If the two values of transmittance are equally probable and allows wave propagation without breaking the waves, we find an opposite effect on the wave front velocity and shape depending on the considered dimension. On the other hand, if one of the two values of the transmittance distribution is set on the nonexcitable region, percolation phenomena can arise by changing the number of excitable sites. The different addressed situations are analytically interpreted giving theoretical predictions for the experimental and numerical results.

37- I. Sendina-Nadal, S. Alonso, V.Perez-Munuzuri, M. Gomez-Gesteira, V. Perez-Villar, L. Ramirez-Piscina, J. Casademunt, J.M. Sancho and F. Sagues.

Brownian motion of spiral waves driven by spatio-temporal structured noise.

Phys. Rev. Lett. 84, 2734 (2000).

Abstract: Spiral chemical waves subjected to a spatiotemporal random excitability are experimentally and numerically investigated in relation to the light-sensitive Belousov-Zhabotinsky reaction. Brownian motion is identified and characterized by an effective diffusion coefficient which shows a rather complex dependence on the time and length scales of the noise relative to those of the spiral. A kinematically based model is proposed whose results are in good qualitative agreement with experiments and numerics.

38- T.T.Katona, T.Borzsonyi, A. Buka, R.Gonzalez-Cinca, L.Ramirez-Piscina, J.Casademunt, A.Hernandez-Machado and L. Kramer

Pattern forming Instabilities of the Nematic Smectic-B interface

Physics Reports 337, 37-65 (2000)

Abstract: Free growth properties of the smectic B liquid crystalline phase into the supercooled nematic have been investigated in quasi-two-dimensional geometry. Different orientation combinations of the two phases have been achieved experimentally and the interfacial patterns have been studied and analysed as a function of undercooling. The angular dependence of the surface tension has been deduced from the shape of the interface in thermal equilibrium. The experimentally determined surface tension anisotropy has been incorporated into computer simulations based on the phase-field model. The simulations have reproduced qualitatively the rich variety of morphologies (extending from the faceted shape to fully developed dendrites) observed in the experiments for a given set of undercoolings in three geometries. Anisotropic heat diffusion on the nematic side, relevant to our experimental system has also been introduced. Both in the experiments and in the simulations we find that the growth is faster in the lower heat diffusion direction.

39- R. Gonzalez-Cinca, L. Ramirez-Piscina, J. Casademunt, y A. Hernandez-Machado.

Sidebranching induced by external noise in solutal dendritic growth

Physical Review E 63, 051602 (2001)

Abstract: We have studied sidebranching induced by fluctuations in dendritic growth. The amplitude of sidebranching induced by internal (equilibrium) concentration fluctuations in the case of solidification with solutal diffusion is computed. This amplitude turns out to be significantly smaller than values reported in previous experiments. The effects of other possible sources of fluctuations (of an external origin) are examined by introducing nonconserved noise in a phase-field model. This reproduces the characteristics of sidebranching found in experiments. Results also show that sidebranching induced by external noise is qualitatively similar to that of internal noise, and it is only distinguished by its amplitude.

40- R. Gonzalez-Cinca, L. Ramirez-Piscina, J. Casademunt, y A. Hernandez-Machado.

Sidebranching in solutal dendritic growth

in Branching in Nature, Dynamics and Morphogenesis of Branching Structures, from Cell to River Networks.

Les Houches School, Vol. 14, pp 403-408

Eds. V. Fleury, J.-F. Gouyet and M. Leonetti (Springer-Verlag and EDP-Sciences, Les Ulis, 2001).

41- I.R. Cantalapiedra, A.M. Lacasta, C.E. Auguet, A. Penaranda and L. Ramirez-Piscina.

Pattern Formation Modeling of Bacterial Colonies

in Branching in Nature, Dynamics and Morphogenesis of Branching Structures, from Cell to River Networks.

Les Houches School, Vol. 14, pp 359-364

Eds. V. Fleury, J.-F. Gouyet and M. Leonetti (Springer-Verlag and EDP-Sciences, Les Ulis, 2001).

42- A. Hernandez-Machado, J. Soriano, A. M. Lacasta, M.A. Rodriguez, L. Ramirez-Piscina and J. Ortin

Interface roughening in Hele--Shaw flows with quenched disorder: experimental and theoretical results

Europhysics Letters 55, 194-200 (2001)

Abstract: We study the forced fluid invasion of an air-filled model porous medium at constant flow rate, in 1+1 dimensions, both experimentally and theoretically. We focus on the nonlocal character of the interface dynamics, due to liquid conservation, and its effect on the scaling properties of the interface upon roughening. Specifically, we study the limit of large flow rates and weak capillary forces. Our theory predicts a roughening behaviour characterized at short times by a growth exponent β1 = 5/6, a roughness exponent α1 = 5/2, and a dynamic exponent z1 = 3, and by β2 = 1/2, α2 = 1/2, and z2 = 1 at long times, before saturation. This theoretical prediction is in good agreement with the experiments at long times. The ensemble of experiments, theory, and simulations provides evidence for a new universality class of interface roughening in 1+1 dimensions.

43- X. Ruiz, L. Ramirez-Piscina and J. Casademunt

Numerical studies of fluid flow in microgravity conditions for confined crystal growth

Astrophysics and Space Science 276, 135-140 (2001)

Abstract: We study the convective ﬂow induced by residual accelerations in microgravity conditions for different geometric arrangements which are relevant to crystal growth experiments. We consider both constant and oscillating acceleration and focus mostly on the transient relaxation dynamics. Results are relevant to estimate impact of more realistic residual accelerations in crystal growth experiments.

44- A. Penaranda, C.E. Auguet and L. Ramirez-Piscina

Diamagnetic Interactions in Disordered Suspensions of Metastable Superconducting Granules

European Physical Journal B 25, 155-165 (2002)

Abstract. The simulation of the transition sequence of superheated Type I superconducting granules (SSG) in disordered suspensions when an external magnetic ﬁeld is slowly increased from zero has been studied. Simulation takes into account diamagnetic interactions and the presence of surface defects. Results have been obtained for the transition sequence and surface ﬁelds distribution covering a wide range of densities. These results are compared with previous analytical perturbative theory, which provides qualitative infor- mation on transitions and surface magnetic ﬁelds during transitions, but with a range of validity apparently limited to extremely dilute samples. Simulations taking into account the complete diamagnetic interactions between spheres appear to be a promising tool in interpreting SSG experiments, in applications such as particle detectors, and in some fundamental calculations of Solid State Physics.

45- A. Rocco, L. Ramirez-Piscina and J. Casademunt

Kinematic reduction of reaction-diffusion fronts with multiplicative noise: Derivation of stochastic sharp-interface equations

Physical Review E 65, 056116 (2002)

Abstract: We study the dynamics of generic reaction-diffusion fronts, including pulses and chemical waves, in the presence of multiplicative noise. We discuss the connection between the reaction-diffusion Langevin-like field equations and the kinematic (eikonal) description in terms of a stochastic moving-boundary or sharp-interface approximation. We find that the effective noise is additive and we relate its strength to the noise parameters in the original field equations, to first order in noise strength, but including a partial resummation to all orders which captures the singular dependence on the microscopic cutoff associated with the spatial correlation of the noise. This dependence is essential for a quantitative and qualitative understanding of fluctuating fronts, affecting both scaling properties and nonuniversal quantities. Our results predict phenomena such as the shift of the transition point between the pushed and pulled regimes of front propagation, in terms of the noise parameters, and the corresponding transition to a non-Kardar-Parisi-Zhang universality class. We assess the quantitative validity of the results in several examples including equilibrium fluctuations and kinetic roughening. We also predict and observe a noise-induced pushed-pulled transition. The analytical predictions are successfully tested against rigorous results and show excellent agreement with numerical simulations of reaction-diffusion field equations with multiplicative noise.

46- A. Penaranda, C.E. Auguet, L. Ramirez-Piscina and T.A. Girard

Diamagnetic interactions in superhead-superconducting microgranules under an external magnetic field

Contributions to Science 2, 197-207 (2002)

The study of the phase transitions produced in ensembles of metastable superconducting granules by magnetic eld variations holds interest for fundamental physics and for applications in particle detectors. It is a problem whose theoretical study has been long hampered by the di culty in treating diamagnetic interactions between granules. In this review we describe the behaviour of such systems, develop the numerical procedures to deal with them, and present some experimental and numerical results.

47- A. Penaranda and L. Ramirez-Piscina

Dynamical ordering induced by preferential transitions in Planar Arrays of Superheated Superconducting granules

Solid State Communications 127, 395-400 (2003)

Abstract: We perform simulations of planar arrays of superheated superconducting granules (PASS) under an external magnetic field, analyzing transitions undergone by the system when the external field is slowly increased from zero. We observe, for high concentrations, the existence of an interval of external fields for which no transitions are induced. This effect is analogous to a ‘hot border zone’ identified in the response of superheated superconducting granule detectors. We explain such behaviour as produced by a geometrical ordering dynamically induced in the system by transitions in preferential sites due to diamagnetic interactions.

48- R. Benitez and L. Ramirez-Piscina

Initial Transients in the Symmetric Model for Directional Solidification

in 'Interface and Transport Dynamics, Computational Modelling',

Lecture Notes in Computational Science and Engineering, Vol. 32, p. 160-165

Eds. Heike Emmerich, Britta Nestler and Michael Schreckenberg (Springer-Verlag, Berlin, 2003)

Abstract: We study the initial transient during the directional solidification of a dilute mixture in the symmetrical constant-gap approximation. We perform phase-field simulations of the transient recoil stages and compare the results with predictions obtained from the sharp-interface model. In particular, we focus in the evolution of the front position and of the transient dispersion relation, obtaining quantitative agreement between theory and simulations. Results are applied to the destabilization of the front by fluctuations.

49- R. Gonzalez-Cinca, R. Folch, R. Benitez, L. Ramirez-Piscina, J. Casademunt, A. Hernandez-Machado

Phase-field models in interfacial pattern formation out of equilibrium

in Advances in Condensed Matter and Statistical Mechanics, ed. by E. Korucheva and R. Cuerno, p. 203-236, Nova Science Publishers (2004), arXiv:cond-mat/0305058

Review article on phase-field models, with application examples.

50- A. Penaranda, C.E. Auguet and L. Ramirez-Piscina

Simulations of Transitions in Superheated Superconducting Granules

Nuclear Instruments and Methods in Physics Research A 520, 201-204 (2004)

Abstract: We perform simulations of both colloidal suspensions and planar arrays systems of superheated superconducting granules. Transitions induced by an increasing external field appear in both situations as an ordering mechanism. Concentrated planar arrays present an interval of external field for which no transitions are produced. These effects can be interesting in applications for superheated superconducting granule detectors.

51- E. Meca, I. Mercader, O. Batiste and L. Ramirez-Piscina

Blue sky catastrophe in double-diffusive convection

Physical Review Letters 92, 234501 (2004)

Abstract: A global bifurcation of the blue sky catastrophe type has been found in a small Prandtl number binary mixture contained in a laterally heated cavity. The system has been studied numerically applying the tools of bifurcation theory. The catastrophe corresponds to the destruction of an orbit which, for a large range of Rayleigh numbers, is the only stable solution. This orbit is born in a global saddle-loop bifurcation and becomes chaotic in a period-doubling cascade just before its disappearance at the blue sky catastrophe.

52- R. Benitez and L. Ramirez-Piscina

Stochastic Phase-Field Simulations of Symmetric Alloy Solidification

Fluctuation and Noise Letters 4, L505-L510 (2004)

Abstract: We study initial transient stages in directional solidification by means of a non-variational phase field model with fluctuations. This model applies for the symmetric solidification of dilute binary solutions and does not invoke fluctuation-dissipation theorem to account for the fluctuation statistics. We devote our attention to the transient regime during which concentration gradients are building up and fluctuations act to destabilize the interface. To this end, we calculate both the temporally dependent growth rate of each mode and the power spectrum of the interface evolving under the effect of fluctuations. Quantitative agreement is found when comparing the phase-field simulations with theoretical predictions.

53- E. Meca, I. Mercader, O. Batiste and L. Ramirez-Piscina

Complex dynamics in double-diffusive convection

Theoretical and Computational Fluid Dynamics 18, 231-238 (2004)

Abstract: The dynamics of a small Prandtl number binary mixture in a laterally heated cavity is studied numerically. By combining temporal integration, steady state solving and linear stability analysis of the full PDE equations, we have been able to locate and characterize a codimension-three degenerate Takens–Bogdanov point whose unfolding describes the dynamics of the system for a certain range of Rayleigh numbers and separation ratios near S=-1.

54- R. Gonzalez-Cinca and L. Ramirez-Piscina

Numerical study of the shape and integral parameters of a dendrite

Physical Review E 70, 051612 (2004)

Abstract: We present a numerical study of sidebranching of a solidifying dendrite by means of a phase-field model. Special attention is paid to the regions far from the tip of the dendrite, where linear theories are no longer valid. Two regions have been distinguished outside the linear region: a first one in which sidebranching is in a competition process and a second one further down where branches behave as independent of each other. The shape of the dendrite and integral parameters characterizing the whole dendrite (contour length and area of the dendrite) have been computed and related to the characteristic tip radius for both surface tension and kinetic dominated dendrites. Conclusions about the different behaviors observed and comparison with available experiments and theoretical predictions are presented.

55- A. Penaranda and L. Ramirez-Piscina

Hot-border effects, ordering, and avalanches in planar arrays of superheated superconducting granules

Nuclear Instruments and Methods in Physics Research A 540, 188-199 (2005)

Abstract: We present results of simulations of transitions in planar arrays of superheated superconducting granules (PASS) immersed in an external magnetic field parallel to the system. Analysis of the behaviour of the system when the external field is slowly increased from zero shows the existence of an interval of external fields for which no transitions are produced. This gap is found to be a consequence of ordering induced by transitions. However, this ordering is different for diluted and for concentrated systems. Avalanches of different sizes with distributions depending on concentration are observed.

56- R. Benitez and L. Ramirez-Piscina

Sharp-Interface Projection of a Fluctuating Phase-Field Model

Physical Review E 71, 061603 (2005)

Abstract: We present a derivation of the sharp-interface limit of a generic fluctuating phase-field model for solidification. As a main result, we obtain a sharp-interface projection which presents noise terms in both the diffusion equation and in the moving boundary conditions. The presented procedure does not rely on the fluctuation-dissipation theorem, and can therefore be applied to account for both internal and external fluctuations in either variational or nonvariational phase-field formulations. In particular, it can be used to introduce thermodynamical fluctuations in nonvariational formulations of the phase-field model, which permit to reach better computational efficiency and provide more flexibility for describing some features of specific physical situations. This opens the possibility of performing quantitative phase-field simulations in crystal growth while accounting for the proper fluctuations of the system.

57- I. Mercader, O. Batiste, L. Ramirez-Piscina, X. Ruiz, S. Rudiger and J. Casademunt

Bifurcations and chaos in single-roll natural convection with low Prandtl number

Physics of Fluids 17, 104108 (2005)

Abstract: Convective flows of a small Prandtl number fluid contained in a two-dimensional cavity subject to a lateral thermal gradient are numerically studied by using different techniques. The aspect ratio (length to height) is kept at around 2. This value is found optimal to make the flow most unstable while keeping the basic single-roll structure. Two cases of thermal boundary conditions on the horizontal plates are considered: perfectly conducting and adiabatic. For increasing Rayleigh numbers we find a transition from steady flow to periodic oscillations through a supercritical Hopf bifurcation that maintains the centrosymmetry of the basic circulation. For a Rayleigh number of about ten times that of the Hopf bifurcation the system initiates a complex scenario of bifurcations. In the conductive case these include a quasiperiodic route to chaos. In the adiabatic one the dynamics is dominated by the interaction of two Neimark-Sacker bifurcations of the basic periodic solutions, leading to the stable coexistence of three incommensurate frequencies, and finally to chaos. In all cases, the complex time-dependent behavior does not break the basic, single-roll structure.

58- C. Corbella, B. Echebarria, L. Ramirez-Piscina, E. Pascual, J. L Andujar, and E. Bertran

Spontaneous formation of nanometric multilayers of metal-carbon films by up-hill diffusion during growth

Applied Physics Letters 87, 213117 (2005)

Abstract: We report the spontaneous formation of multilayer structures with nanometric periodicity during Ti–C thin-film growth by reactive magnetron sputtering. Their characterization was performed by transmission electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and secondary ion mass spectrometry. We discuss film structure and morphology as a function of metal content, and propose surface-directed spinodal decomposition as the mechanism responsible for the segregation of species in separated layers by up-hill diffusion.

59- J. Carrera, X. Ruiz, L. Ramirez-Piscina, J. Casademun and M. Dreyer

Generation of a Monodisperse Microbubble Jet in Microgravity

AIAA Journal 46, 2010-2019 (2008)

Abstract: A new method to create a jet of a virtually monodisperse microbubble suspension of prescribed bubble size into a quiescent cavity is proposed. The method is insensitive to gravity and is based on the creation of a slug ﬂow at a T junction in a capillary tube before injection. We develop a theoretical analysis that establishes the validity and efﬁciency of the method, as controlled by the crossﬂow Weber number, and yields a simple explicit prediction for the bubble size in terms of the injection parameters. The method operates efﬁciently for small Weber numbers, yet it generates small bubbles of very uniform size. The reduced size dispersion is also explained within the theoretical model. The method of bubble formation, injection, and spreading by the resulting turbulent jet is validated experimentally in 4.7 s of free fall in the drop tower at the University of Bremen. Experiments demonstrate the physical principle behind the method of bubble formation and allow us to explore the dynamics of the resulting bubble jet after injection of the slug ﬂow into a quiescent cavity in microgravity conditions as an efﬁcient method of bubble spreading and transport. Detailed measurements of average local velocities show that bubbles are essentially passive with respect to the carrier mean ﬂow, and the inherent turbulence of the ﬂow is crucial for optimal spreading of the bubble distribution and reduction of bubble coalescence. The shape of the bubble jet is studied as a function of the Reynolds number. Finally, the degree of coalescence is also characterized and found to be remarkably small.

60- S. Arias, X. Ruiz, J. Casademunt, L. Ramirez-Piscina and R. Gonzalez-Cinca

Experimental Study of a Microchannel Bubble Injector for Microgravity Applications

Microgravity - Science and Technology 21, 107-111 (2009)

Abstract: We perform a quantitative characterization of a microbubble injector in conditions relevant to microgravity. The injector pregenerates a slug flow by using a capillary T-junction, whose operation is robust to changes in gravity level. We address questions regarding the performance under different injection conditions. In particular we focus on the variation of both gas and liquid flow rates. The injection performance is characterized by measuring bubble injection frequency and bubble sizes. We obtain two distinct working regimes of the injector and identify the optimal performance as the crossover region between them.

61- C. Corbella, B. Echebarria, L. Ramirez-Piscina, E. Pascual, J.L. Andujar, E. Bertran

Growth kinetics of nanometric dendrites in metal-carbon thin films

Acta Materialia 57, 4948-4956 (2009)

Abstract: Tungsten-carbon films deposited by pulsed-DC reactive magnetron sputtering show the formation of a dendritic structure at the nanometric scale. The structure is formed by a combination of a polycrystalline β–W phase together with a non-stoichiometric WC1-x phase. The nanodendrites coincide with W-rich zones, whereas C-rich regions are located at the interstices. The characteristics of this nanostructure have been modulated by varying the metal concentration of the films. Composition, structure and morphology were characterized by X-Ray Photoelectron Spectroscopy, Electron Probe Micro-Analysis, Transmission Electron Microscopy, X-Ray Diffraction and Atomic Force Microscopy, whereas mechanical and tribological properties were evaluated by profilometry, nanoindentation and microscratch. The observed growth pattern is interpreted as the result of nucleation and growth of a W phase into a W-C amorphous matrix, whose growth is controlled by diffusion of carbon. A simulation model based in phase field modelling presenting similar morphologies is formulated. This special structure combines properties of W and diamond-like carbon films, which enlarges the scope of applications towards self-lubricant hard and low-friction coatings with improved stability.

62- S. Arias, R. Gonzalez-Cinca, X. Ruiz, L. Ramirez-Piscina and J. Casademunt

Characterization of the performance of a minibubble generator in conditions relevant to microgravity

Colloids and Surfaces A 365, 52-55 (2010).

Abstract: We perform a characterization of a recently reported minibubbles (bubbles with a diameter of the order of 10^-3 m) generator in microgravity related conditions. Generation of bubbles is based on the generation of a slug flow in a capillary T-junction, whose operation is robust to changes in the gravity level. We address questions regarding the performance under different working regimes. In particular, we focus on the regimes found within a large range of gas and liquid injection flow rates. The injection performance is characterized by measuring bubble generation frequency. We propose curves obtained empirically for the behaviour of generation frequency and crossover between regimes.

63- E. Meca and L. Ramirez-Piscina

Transient convective instabilities in directional solidification

Physics of Fluids 22, 114110 (2010)

Abstract: We study the convective instability of the melt during the initial transient in a directional solidification experiment in a vertical configuration. We obtain analytically the dispersion relation, and perform an additional asymptotic expansion for large Rayleigh number that permits a simpler analytical analysis and a better numerical behavior. We find a transient instability, i.e. a regime in which the system destabilizes during the transient whereas the final unperturbed steady state is stable. This could be relevant to growth mode predictions in solidification.

64- P. Bitlloch, X. Ruiz, L. Ramirez-Piscina and J. Casademunt

Spatial structure and velocity fluctuations in turbulent bubble jets in microgravity

International Journal of Transport Phenomena 12, 189-197 (2011)

We study the dynamics of turbulent jets of bubbles created by the injection of a slug flow into a quiescent cavity, both theoretically and experimentally using data from drop tower experiments. The generated bubble jets exhibit a remarkably low degree of coalescence leading to virtually monodisperse suspensions of spherical bubbles. The turbulence of the jet is responsible for the spatial dispersion of the bubble distribution in roughly conical jets. We propose a stochastic model for the dispersion of bubbles in the jet, in which the statistics of bubble concentration is computed within a k-epsilon model of turbulence, coupled to a Langevin dynamics with an inhomogeneous diffusivity. Numerical integration is compared to experimental data, focussing on the spatial structure of the bubble jet. Results show that bubble interactions can be neglected as far as spatial dispersion is concerned, even though bubbles are not strictly passive tracers.

65- X. Ruiz, P. Bitlloch, L. Ramirez-Piscina, J. Casademunt

Impact of stochastic accelerations on dopant segregation in microgravity semiconductor crystal growth

Journal of Crystal Growth 355, 88-100 (2012).

The residual accelerations that are typically present in microgravity environments (g-jitters) contain a broad spectrum of frequencies and may be modeled as stochastic processes. Their effects on the quality of the semiconductor crystals are analyzed here quantitatively with direct numerical simulation. In particular we focus on the dopant segregation effects due to thermosolutal convection as a function of the parameters characterizing the statistics of the stochastic force. The numerical simulation is specified for material parameters of two doped semiconductors (Ge:Ga and GaAs:Se) in realistic conditions of actual microgravity environments. As a general result, we show that the segregation response is strongly dominated by the low-frequency part of the g-jitter spectrum. In addition, we develop a simplified model of the problem based on linear response theory that projects the dynamics into very few effective modes. The model captures remarkably well the segregation effects for an arbitrary time-dependent acceleration of small amplitude, while it implies an enormous reduction of computer demands. This model could be helpful to analyze results from real accelerometric signals and also as a predictive tool for experimental design.

66- A.M. Lacasta, L. Ramirez-Piscina, J.M. Sancho, K. Lindenberg

Speeding chemical reactions by focusing

Journal of Chemical Physics 138, 144502 (2013)

We present numerical results for a chemical reaction of colloidal particles which are transported by a laminar fluid and are focused by periodic obstacles in such a way that the two components are well mixed and consequently the chemical reaction is speeded up. The roles of the various system parameters (diffusion coefficients, reaction rate, and obstacles sizes) are studied. We show that focusing speeds up the reaction from the diffusion limited rate ∼t−1/2 to very close to the perfect mixing rate, ∼t−1.

67- M.J. Skaug, A.M. Lacasta, L. Ramirez-Piscina, J.M. Sancho, K. Lindenberg and D.K. Schwartz

Single-molecule di!usion in a periodic potential at a solid-liquid interface

Soft Matter 10, 753 (2014)

We used single-molecule tracking experiments to observe the motion of small hydrophobic fluorescent molecules at the interface between water and a solid surface that exhibited periodic chemical patterns. The dynamics were characterized by non-ergodic, continuous time random walk statistics. The step-size distributions displayed enhanced probability of steps to periodic distances, consistent with theoretical predictions for diffusion in an atomic/molecular scale periodic potential. Surprisingly, this general behavior was observed here for surfaces exhibiting characteristic length scales three orders of magnitude larger than atomic/molecular dimensions, and may provide a new way to understand and control solid–liquid interfacial diffusion for molecular targeting applications.

68- L. Ramirez-Piscina and J.M. Sancho

Molecular Na-channel excitability from statistical physics

Europhysics Letters 108, 50008 (2014).

The excitable properties of the neural cell membrane is the driving mechanism of the neural pulses. Coordinated ionic fluxes across Na and K channels are the devices responsible of this function. Here we present a simple microscopic physical scenario which accounts for this phenomenology. The main elements are ions and channel doors that obey the standard formulation of statistical physics (overdamped Langevin equations) with appropriate nonlinear interacting potentials. From these equations we obtain the ionic flux and the dynamics of the membrane potential. We show that the excitable properties of the membrane are present in a single and simple Na channel. From this framework, additional microscopic information can be obtained, such as statistics of single-channels dynamics or the energetics of action potential events.

69- E. Meca, I. Mercader and L. Ramirez-Piscina

Transitions between symmetric and nonsymmetric regimes in binary-mixture convection

Physica D 303, 39-49 (2015)

We present here a comprehensive picture of the different bifurcations found for small to moderate Rayleigh number in binary-mixture convection with lateral heating and negative separation ratio (SS). The present work connects the symmetric regime found for pure fluid (S=0S=0) (Mercader et al., 2005) with the fundamentally nonsymmetric regime found for S=−1S=−1 (Meca et al., 2004) [2,3]. We give a global context as well as an interpretation for the different associations of bifurcations found, and in particular we interpret an association of codimension-two bifurcations in terms of a higher codimension bifurcation never found, to our knowledge, in the study of an extended system.

70- Pau Bitlloch, Xavier Ruiz, Laureano Ramirez-Piscina, Jaume Casademunt

Turbulent Bubble Jets in Microgravity. Spatial Dispersion and Velocity Fluctuations

Microgravity Science and Technology 27, 207-220 (2015)

A detailed statistical analysis of bubble dispersion in turbulent jets based on data from drop tower experiments is presented here. A stochastic model is also introduced in order to capture these statistics to a large extent, treating bubbles as passive tracers with a local diffusivity given by a k- ε description of the turbulence. Bubble-bubble and bubble-flow interactions are neglected. Simple scaling analysis suggests that this approach is justified sufficiently far downstream. It is also found that, although interactions cannot be neglected very close to the inlet, the model predictions for the overall spatial distribution of the bubble ensemble are compatible with data within experimental uncertainty, and within the limited statistics of the experiments. In addition, the velocity fluctuations from the same experiments are analyzed, obtaining the local standard deviation of bubble velocities. We also find good agreement between experimental data and the effective model. Slight deviations between the model predictions and the experimental data are found at the jet margins, concerning the dependence on Reynolds number of jet angle and the relative velocity fluctuations. Consequently, significant bubble-flow interactions seem to be confined at the boundaries of the jets.

Nonlinear Physics and Far from Equilibrium Systems