http://hdl.handle.net/10171/122 Sat, 25 May 2013 21:35:17 GMT 2013-05-25T21:35:17Z http://hdl.handle.net/10171/29204 Title: Breath figures of two immiscible substances on a repellent surface Author(s) : Guadarrama-Cetina, J. (José); González-Viñas, W. (Wenceslao) Abstract: The understanding of the competition between different substances while condensing on a cold surface is of high interest in situations in which it is desirable to control their condensation rates and the formed morphologies. We do the experiments for mixtures of water and hexamethyldisiloxane vapors at several concentrations. The dropwise condensation of the vapors forms breath figures on a substrate that is repellant to both substances. We report the average radius of the drops for each specie as a function of time. Also, we pay attention to the evolution of the corresponding morphologies and the appearance of hybrid clusters. Mon, 31 Dec 2012 23:00:00 GMT http://hdl.handle.net/10171/29204 2012-12-31T23:00:00Z http://hdl.handle.net/10171/29157 Title: Brittle-to-ductile transition in a fiber bundle with strong heterogeneity Author(s) : Kovács, K. (Kornél); Cruz-Hidalgo, R. (Raúl); Pagonabarraga, I. (Ignacio); Kun, F. (Ferenc) Abstract: We analyze the failure process of a two-component system with widely different fracture strength in the framework of a fiber bundle model with localized load sharing. A fraction 0 α 1 of the bundle is strong and it is represented by unbreakable fibers, while fibers of the weak component have randomly distributed failure strength. Computer simulations revealed that there exists a critical composition αc which separates two qualitatively different behaviors: Below the critical point, the failure of the bundle is brittle, characterized by an abrupt damage growth within the breakable part of the system. Above αc, however, the macroscopic response becomes ductile, providing stability during the entire breaking process. The transition occurs at an astonishingly low fraction of strong fibers which can have importance for applications. We show that in the ductile phase, the size distribution of breaking bursts has a power law functional form with an exponent μ = 2 followed by an exponential cutoff. In the brittle phase, the power law also prevails but with a higher exponent μ = 9 2. Thetransition between the two phases shows analogies to continuous phase transitions. Analyzing the microstructure of the damage, it was found that at the beginning of the fracture process cracks nucleate randomly, while later on growth and coalescence of cracks dominate, which give rise to power law distributed crack sizes. Sun, 28 Apr 2013 22:00:00 GMT http://hdl.handle.net/10171/29157 2013-04-28T22:00:00Z http://hdl.handle.net/10171/28167 Title: Inverse cascades sustained by the transfer rate of angular momentum in a 3D turbulent flow Author(s) : Lopez-Caballero, M. (Miguel); Burguete, J. (J.) Abstract: The existence of energy cascades as signatures of conserved magnitudes is one of the universal characteristics of turbulent flows. In homogeneous 3D turbulence, the energy conservation produces a direct cascade from large to small scales, although in 2D, it produces an inverse cascade pointing towards small wave numbers. In this Letter, we present the first evidence of an inverse cascade in a fully developed 3D experimental turbulent flow where the conserved magnitude is the angular momentum. Two counterrotating flows collide in a central region where very large fluctuations are produced, generating a turbulent drag that transfers the external torque between different fluid layers. Tue, 19 Mar 2013 23:00:00 GMT http://hdl.handle.net/10171/28167 2013-03-19T23:00:00Z http://hdl.handle.net/10171/28116 Title: Impact of time-dependent nonaxisymmetric velocity perturbations on dynamo action of von Kármán-like flows Author(s) : Giesecke, A. (A.); Stefani, F. (F.); Burguete, J. (J.) Abstract: We present numerical simulations of the kinematic induction equation in order to examine the dynamo efficiency of an axisymmetric von K´arm´an–like flow subject to time-dependent nonaxisymmetric velocity perturbations. The numerical model is based on the setup of the French von K´arm´an-sodium dynamo (VKS) and on the flow measurements from a water experiment conducted at the University of Navarra in Pamplona, Spain. The principal experimental observations that are modeled in our simulations are nonaxisymmetric vortexlike structures which perform an azimuthal drift motion in the equatorial plane. Our simulations show that the interactions of these periodic flow perturbations with the fundamental drift of the magnetic eigenmode (including the special case of nondrifting fields) essentially determine the temporal behavior of the dynamo state.We find two distinct regimes of dynamo action that depend on the (prescribed) drift frequency of an (m = 2) vortexlike flow perturbation. For comparatively slowly drifting vortices we observe a narrow window with enhanced growth rates and a drift of the magnetic eigenmode that is synchronized with the perturbation drift. The resonance-like enhancement of the growth rates takes place when the vortex drift frequency roughly equals the drift frequency of the magnetic eigenmode in the unperturbed system. Outside of this small window, the field generation is hampered compared to the unperturbed case, and the field amplitude of the magnetic eigenmode is modulated with approximately twice the vortex drift frequency. The abrupt transition between the resonant regime and the modulated regime is identified as a spectral exceptional point where eigenvalues (growth rates and frequencies) and eigenfunctions of two previously independent modes collapse. In the actual configuration the drift frequencies of the velocity perturbations that are observed in the water experiment are much larger than the fundamental drift frequency of the magnetic eigenmode that is obtained from our numerical simulations. Hence, we conclude that the fulfillment of the resonance condition might be unlikely in present day dynamo experiments. However, a possibility to increase the dynamo efficiency in the VKS experiment might be realized by an application of holes or fingers on the outer boundary in the equatorial plane. These mechanical distortions provoke an anchorage of the vortices at fixed positions thus allowing an adjustment of the temporal behavior of the nonaxisymmetric flow perturbations. Thu, 06 Dec 2012 23:00:00 GMT http://hdl.handle.net/10171/28116 2012-12-06T23:00:00Z http://hdl.handle.net/10171/28105 Title: Frozen dynamics and synchronization through a secondary symmetry-breaking bifurcation Author(s) : Miranda, M.A. (Montserrat A.); Burguete, J. (J.); Mancini, H.L. (Héctor Luis); González-Viñas, W. (Wenceslao) Abstract: We show evidence of the frozen dynamics (Kibble-Zurek mechanism) at the transition one-dimensional (1D) front of an extended 1D array of convective oscillators that undergo a secondary subcritical bifurcation. Results correspond to a global synchronization process from nonlocal coupling between the oscillating units. The quenched dynamics exhibits defect trapping at the synchronization front according to the Kibble-Zurek mechanism, predicted for condensed matter systems. A stronger subcriticality prevents the fronts from freezing defects during the quenched transitions. A synchronization model of supercritical oscillating units is proposed to explain differentiation mechanisms in morphogenesis above a critical crossing rate when the frequency of the individual oscillators becomes coherent. The phases of such oscillators are spatially coupled through a Kuramoto-Battogtokh term that leads to the experimentally observed subcriticality. As a consequence, we show that the Kibble-Zurek mechanism overcomes non-locality of a geometrical network above a critical crossing rate. Mon, 31 Dec 2012 23:00:00 GMT http://hdl.handle.net/10171/28105 2012-12-31T23:00:00Z http://hdl.handle.net/10171/27799 Title: Dynamics, crystallization, and structures in colloid spin coating Author(s) : Pichumani, M. (Moorthi); Bagheri, P. (P.); Poduska, K.M. (K. M.); González-Viñas, W. (Wenceslao); Yethiraj, A. (Anand) Abstract: Spin coating is an out-of-equilibrium technique for producing polymer films and colloidal crystals quickly and reproducibly. In this review, we present an overview of theoretical and experimental studies of the spin coating of colloidal suspensions. The dynamics of the spin coating process is discussed first, and we present insights from both theory and experiment. A key difference between spin coating with polymer solutions and with monodisperse colloidal suspensions is the emergence of long range (centimeter scale) orientational correlations in the latter. We discuss experiments in different physical regimes that shed light on the many unusual partially ordered structures that have long-range orientational order, but no long-range translational order. The nature of these structures can be tailored by adding electric or magnetic fields during the spin coating procedure. These partially ordered structures can be considered as model systems for studying the fundamentals of poorly crystalline and defect-rich solids, and they can also serve as templates for patterned and/or porous optical and magnetic materials. Mon, 31 Dec 2012 23:00:00 GMT http://hdl.handle.net/10171/27799 2012-12-31T23:00:00Z http://hdl.handle.net/10171/27798 Title: Spatiotemporal phase synchronization in a large array of convective oscillators Author(s) : Miranda, M.A. (Montserrat A.); Burguete, J. (J.) Thu, 31 Dec 2009 23:00:00 GMT http://hdl.handle.net/10171/27798 2009-12-31T23:00:00Z http://hdl.handle.net/10171/27768 Title: Dynamics of crystal structure formation in spincoated colloidal films Author(s) : Giuliani, M. (Maximiliano); González-Viñas, W. (Wenceslao); Poduska, K.M. (K. M.); Yethiraj, A. (Anand) Abstract: The spin-coating of colloidal suspensions is an inherently nonequilibrium process that gives rise to highly reproducible, but polycrystalline, films with different symmetries depending on experimental parameters. In this study, we explore the transient dynamics of evaporative colloid spin-coating for the first time, via a combination of high-speed imaging, atomic force microscopy, static photography, and scanning electron microscopy. As the wet colloidal film thins and dries, we observe several symmetry transitions, while at the same time remarkably, the thinning rate (in nondimensional time units) collapses to one universal curve for all rotation rates. We correlate static and dynamic measures of crossovers in ordering regimes, and obtain an estimate of the evaporation rate in the late stage of drying. We conclude that the thinning dynamics controls the local volume fraction and stress profiles, which in turn drives the structural transitions. Thu, 31 Dec 2009 23:00:00 GMT http://hdl.handle.net/10171/27768 2009-12-31T23:00:00Z http://hdl.handle.net/10171/27762 Title: Water Condensation on Zinc Surfaces Treated by Chemical Bath Deposition Author(s) : Narhe, R.D. (Ramchandra D.); González-Viñas, W. (Wenceslao); Beysens, D.A. (Daniel A.) Abstract: Water condensation, a complex and challenging process, is investigated on a metallic (Zn) surface, regularly used as anticorrosive surface. The Zn surface is coated with hydroxide zinc carbonate by chemical bath deposition, a very simple, low-cost and easily applicable process. As the deposition time increases, the surface roughness augments and the contact angle with water can be varied from 75º to 150º , corresponding to changing the surface properties from hydrophobic to ultrahydrophobic and superhydrophobic. During the condensation process, the droplet growth laws and surface coverage are found similar to what is found on smooth surfaces, with a transition from Cassie-Baxter to Wenzel wetting states at long times. In particular, it is noticeable in view of corrosion effects that the water surface coverage remains on order of 55%. Thu, 31 Dec 2009 23:00:00 GMT http://hdl.handle.net/10171/27762 2009-12-31T23:00:00Z http://hdl.handle.net/10171/27758 Title: Magnetorheology from surface coverage of spin-coated colloidal films Author(s) : Pichumani, M. (Moorthi); González-Viñas, W. (Wenceslao) Abstract: In magnetorheological fluids, the viscosity usually increases with the field and the non-Newtonian character of these complex fluids may vary significantly. We provide a new method to measure the relative viscosity of a superparamagnetic colloid, by applying a magnetic field during a spin-coating process, which involves evaporation of the solvent. We define the compact equivalent height to take into account the discrete nature of the suspension, and we compare experimental results under different conditions. We extend the model of Cregan et al. (J. Colloid Interface Sci., 2007, 314, 324) to turn it into an evaporation rate independent one. The generality of the resulting model facilitates measurement of the magnetic field dependent viscosity. We also discuss the morphologies of the final dried colloidal deposits and the possible mechanisms involved in their formation. Mon, 31 Dec 2012 23:00:00 GMT http://hdl.handle.net/10171/27758 2012-12-31T23:00:00Z http://hdl.handle.net/10171/27687 Title: Percolation-induced frost formation Author(s) : Guadarrama-Cetina, J. (José); Mongruel, A. (Anne); González-Viñas, W. (Wenceslao); Beysens, D.A. (Daniel A.) Abstract: We report the observation of an unconventional mechanism for frost formation. On a smooth hydrophobic surface cooled much below the water freezing temperature (−9 °C), we find that, instead of the classical freezing of individual supercooled condensed droplets, frost can occur through a multi-step 2-dimensional percolation-driven mechanism. This in-plane propagation process provides a model to investigate more complex bulk phase transformations such as those occurring in atmospheric supercooled clouds. It can also lead to a new method to control and design in-plane solidification at a nanoscale level. Mon, 31 Dec 2012 23:00:00 GMT http://hdl.handle.net/10171/27687 2012-12-31T23:00:00Z http://hdl.handle.net/10171/27663 Title: Spin-coating of dilute magnetic colloids in a magnetic field Author(s) : Pichumani, M. (Moorthi); González-Viñas, W. (Wenceslao) Abstract: Spin-coating of colloids is a versatile method in fabricating films of colloidal particles at a short span of time. Controlling key parameters like the rate of rotation, the initial concentration and the nature of the continuum phase is easy to perform experimentally. However, the effects of these parameters are not fully understood yet. To enhance the understanding of the coating process, we study the spin-coating of dilute magnetic colloids. The coating process is controlled externally by means of a magnetic field and we investigate its effects by studying the morphology of the dried coating obtained after the experiments. Morphological transitions are explained through the mean area and the number density of clusters, together with anisotropy properties. Further, we relate the occupation factor of clusters to the non-planarization conditions observed in this kind of experiments. Fri, 31 Dec 2010 23:00:00 GMT http://hdl.handle.net/10171/27663 2010-12-31T23:00:00Z http://hdl.handle.net/10171/19226 Title: Exploring the Kibble-Zurek mechanism in a secondary bifurcation Author(s) : Miranda, M.A. (Montserrat A.); Burguete, J. (J.); González-Viñas, W. (Wenceslao); Mancini, H.L. (Héctor Luis) Abstract: We present new experimental results on the quenching dynamics of an extended thermo-convective system (a network array of approximately 100 convective oscillators) going through a secondary subcritical bifurcation. We characterize a dynamical phase transition through the nature of the domain walls (1D-fronts) that connect the basic multicellular pattern with the new oscillating one. Two different mechanisms of the relaxing dynamics at the threshold are characterized depending on the crossing rate $\mu=\left.\frac{d\varepsilon}{dt}\right|_{\varepsilon=0}$ of the quenched transition. From the analysis of fronts, we show that these mechanisms follow different correlation length scales $\xi \sim \mu^{-\sigma}$. Below a critical value $\mu_c$, a slow response dynamics yields a spatiotemporal coherent front with weak coupling between oscillators. Above $\mu_c$, for rapid quenches, defects are trapped at the front with a strong coupling between oscillators, similarly to the Kibble-Zurek mechanism in quenched phase transitions. These defects, pinned to the fronts, yield a strong decay of the correlation length. Sat, 31 Dec 2011 23:00:00 GMT http://hdl.handle.net/10171/19226 2011-12-31T23:00:00Z http://hdl.handle.net/10171/23264 Title: Instabilities of conducting fluid layers in cylindrical cells under the external forcing of weak magnetic fluids Author(s) : Burguete, J. (J.); Miranda, M.A. (Montserrat A.) Abstract: In this work, we analyze recent results concerning the instabilities created in a layer of liquid metal by the action of time-dependent magnetic fields. The experimental setup allows the characterization of different patterns very close to the threshold. For very low frequencies of the forcing field, the axisymmetric fluid layer destabilizes with different azimuthal wavenumbers. An improved analysis allows the characterization of different patterns for interaction parameter values as low as 5 · 10 −3. Sat, 31 Mar 2012 22:00:00 GMT http://hdl.handle.net/10171/23264 2012-03-31T22:00:00Z http://hdl.handle.net/10171/23263 Title: Modified Spin-coating Technique to Achieve Directional Colloidal Crystallization Author(s) : Bartlett, A.P. (Andrew P.); Pichumani, M. (Moorthi); Giuliani, M. (Maximiliano); González-Viñas, W. (Wenceslao); Yethiraj, A. (Anand) Abstract: Fabricating large single crystals with colloidal spheres as building blocks is challenging and of competitive interest. Spin-coating of colloids offers a robust technique, which is highly reproducible in obtaining colloidal crystals even at fast dynamical regimes; however, these crystals are intrinsically polycrystalline due to the axial symmetry of spin-coating. We report a new method that applies a nonuniform electric field during the spin-coating process. By arranging the field direction to be stationary in the rotating frame, we are able to break the axial symmetry and to orient the colloids along one predefined direction. By regulating the applied field strength, we demonstrate local control over the orientation of the crystallites, and thus, the orientation is determined by the applied field strength. Sat, 31 Dec 2011 23:00:00 GMT http://hdl.handle.net/10171/23263 2011-12-31T23:00:00Z