Nuestros investigadores

Wenceslao González Viñas

Física y Matemática Aplicada
Facultad de Ciencias. Universidad de Navarra
Facultad de Ciencias. Universidad de Navarra
Líneas de investigación
Transiciones coloidales fuera del equilibrio, Patrones de condensación (Breath Figures), Cosmología en el laboratorio, Ondas Superficiales, Materia blanda, Redes complejas
Índice H
15, (Google Scholar, 14/12/2018)
14, (WoS, 14/12/2018)
14, (Scopus, 14/12/2018)

Publicaciones científicas más recientes (desde 2010)

Autores: González-Viñas, W, (Autor de correspondencia)
ISSN 0022-3727  Vol. 52  Nº 34  2019  págs. 344001
We report experimental results on the patterns that are formed during spin-coating of magnetic colloids at moderate concentrations and compare them with results obtained in diluted colloids. We show that, for moderate concentrations, the magnetic interaction between the (ferro)magnetic particles and with the external field is strong enough to overcome the centrifugal force. We study two different configurations for the magnetic field. The first one consists on an axial uniform field, where we obtain spikes perpendicular to the substrate with a well defined order which decreases as rotation rate increases. The second one consists on a radial non-uniform field, where we obtain elongated deposits radially disposed on the substrate. The effect of magnetic fields at moderate concentrations on the effective viscosity is confirmed to be much more important in the case of a uniform magnetic field, by increasing the hydrodynamic time-scale which gives the ferromagnetic particles enough time to strongly interact to form the spikes.
Autores: González-Viñas, W;
ISSN 0927-7757  Vol. 532  2017  págs. 530 - 534
Spin-coating technique is very fast, cheap, reproducible, simple and needs less material to fabricate films of particulate systems/colloids. Their thickness and uniformity may be controlled by means of external fields. We apply magnetic fields during the spin-coating of a moderately concentrated superparamagnetic colloid (made of silica coated magnetite particles). We study the influence of different magnetic field configurations (homogeneous and inhomogeneous) on the resulting spin-coated deposits and compare experimental results under various conditions. Superparamagnetic colloids behave as, non-Newtonian, magnetorheological fluids. Their viscosity vary significantly under applied magnetic fields. We measure and compare the effect of uniform and non-uniform magnetic fields on their relative effective viscosity, using the spin-coated deposits and a previously existing model for simple colloids. The mechanisms involved in the deposits formation under different experimental conditions are also discussed. In particular, we show that the magnetophoretic effect plays an important role in the spin-coating of magnetic colloids subjected to non-uniform magnetic fields. We characterize an effective magnetoviscosity in non-uniform magnetic fields that is largely influenced by the magnetophoretic effect that enhances the flow of the magnetic fluid.
Autores: Ardanza-Trevijano, Sergio; Poduska, K. M.; et al.
ISSN 2470-0045  Vol. 95  Nº 3  2017  págs. 032607
Polycrystals of thin colloidal deposits, with thickness controlled by spin-coating speed, exhibit axial symmetry with local 4-fold and 6-fold symmetric structures, termed orientationally correlated polycrystals (OCPs). While spin-coating is a very facile technique for producing large-area colloidal deposits, the axial symmetry prevents us from achieving true long-range order. To obtain true long-range order, we break this axial symmetry by introducing a patterned surface topography and thus eliminate the OCP character. We then examine symmetryindependent methods to quantify order in these disordered colloidal deposits. We find that all the information in the bond-orientational order parameters is well captured by persistent homology analysis methods that only use the centers of the particles as input data. It is expected that these methods will prove useful in characterizing other disordered structures.
Autores: Seco Gudina, R. ; Guadarrama , José Manuel; González-Viñas, W;
ISSN 1951-6355  Vol. 226  Nº 6  2017  págs. 1189 - 1198
We report experimental results of drop-wise condensation on a wettability patterned substrate. It consists of a 2-d array of hydrophilic patches/spots on a macroscopically hydrophobic surface. We show that in this kind of system, there is not a relevant humidity sink, but the scale and the closeness of the different patches/spots affect the mechanisms which are important during the experiment. These results may provide clues to obtain higher dew yields in arid or semi-arid regions as a way to obtain potable water.
Autores: González-Viñas, W;
ISSN 0168-1923  Vol. 217  Nº S1  2016  págs. 472 - 473
Autores: Sharivar, K.; de Vicente, J.; et al.
ISSN 0964-1726  Vol. 25  Nº 7  2016  págs. 075036
Hybrid colloids composed of micron-sized ferromagnetic (carbonyl iron) and diamagnetic (silica) particles suspended in cyclohexanone, behave as, non-Newtonian, magnetorheological fluids. We measure and compare the magnetic field-dependent viscosity of hybrid diluted colloids using spin-coating and conventional magnetorheometry. We extend a previously developed model for simple colloids to this kind of hybrid colloids. As in the previous model, the viscosity of the colloidal suspension under applied fields can be derived from the surface coverage of the dry spin-coated deposits for each type of particles, and from the viscosity of the colloid at zero field. Also, our results allow us to obtain the evaporation rate of the solvent as a function of the rotation speed. Finally, we also measure the viscosity of the same suspension with a torsional parallel plate magnetorheometer under uniaxial DC magnetic fields aligned in the velocity gradient direction of a steady shearing flow. The experimental results under different conditions and the effect of each component on the magnetorheological properties of the resulting colloid are discussed. Standard spin-coating technique can be used both to characterize smart materials - complex fluids as well as to fabricate films with arbitrary solvents by tuning their viscosity by means of external fields.
Autores: Anand, S.; Rykaczewski, K.; et al.
ISSN 0743-7463  Vol. 31  Nº 19  2015  págs. 5353 - 5363
Water droplets condensing on solidified phase change materials such as benzene and cyclohexane near their melting point show in-plane jumping and continuous "crawling" motion. The jumping drop motion has been tentatively explained as an outcome of melting and refreezing of the materials surface beneath the droplets and can be thus considered as an inverted Leidenfrost-like effect (in the classical case vapor is generated from a droplet on a hot substrate). We present here a detailed investigation of jumping movements using high-speed imaging and static cross-sectional cryogenic focused ion beam scanning electron microscope imaging. Our results show that drop motion is induced by a thermocapillary (Marangoni) effect. The in-plane jumping motion can be delineated to occur in two stages. The first stage occurs on a millisecond time scale and comprises melting the substrate due to drop condensation. This results in droplet depinning, partial spreading, and thermocapillary movement until freezing of the cyclohexane film. The second stage occurs on a second time scale and comprises relaxation motion of the drop contact line (change in drop contact radius and contact angle) after substrate freezing. When the cyclohexane film cannot freeze, the droplet continuously glides on the surface, resulting in the crawling motion.
Autores: Aslam, R. ; González-Viñas, W;
ISSN 1951-6355  Vol. 224  Nº 2  2015  págs. 435 - 445
Pattern formation in colloids by weak ac fields in vertical deposition-like configuration at different temperatures has been studied experimentally. At low evaporation (room temperature), the effect of the field leads to the evolution of a one-dimensional array of clusters along the contact line and columnar colloidal dried deposits are obtained at higher evaporation. We investigate the flow dynamics involved in this pattern formation. Homogeneous variation of the contact angle by electrowetting effect becomes unstable and breaks the translational symmetry at the meniscus. Electrokinetic forces together with capillary forces result in the accumulation of particles for pattern formation. The movement of electrically charged colloidal particles is controlled by weak ac electric field even at higher temperatures. We observe the effect of increasing initial particle concentration on the behavior of the clusters for various field frequencies. The average distance between clusters increase monotonically with an increase in the initial particle concentration. We also observe that the average width of columns increases according to the applied field strength.
Autores: Guadarrama , José Manuel; Mongruel, A. ; González-Viñas, W; et al.
Revista: EPL
ISSN 0295-5075  Vol. 110  Nº 5  2015  págs. 56002
The formation of frost in presence of salt (NaCl) crystal is experimentally investigated on a hydrophobic surface. It presents several remarkable features due to the interplay of salty-water saturation pressure evolution, initially lower than the saturation pressure of ice and water, and the percolating propagation of ice dendrites from defects throughout the supercooled water droplet pattern. In particular, it is remarkable that nucleation of supercooled water and/or ice is prevented around the salty drop in a region of inhibited condensation where the substrate remains dry. As condensation proceeds, salt concentration decreases to eventually become lower than ice's, allowing ice dendrites to hit the salty drop. Salty water then melts ice but eventually freezes as an effect of dilution.
Autores: Guadarrama , José Manuel; D.A. Beysens; et al.
ISSN 1539-3755  Vol. 89  Nº 1  2014  págs. 012402
We describe the evolution of a water drop saturated with NaCl and the growth of pure water droplets in a breath figure pattern (BF) condensing around it. This salty drop acts as a humidity sink, inhibiting the BF inside a ring at a distance r = delta from the sink center and slowing down BF growth outside the ring. The initial salty drop is taken either from a salt-saturated solution (type I experiment) or by placing an NaCl crystal on the substrate (type II experiment). The results are similar, provided that the initial time for type II evolution is taken at the end of the crystal dissolution. The evolution of the salty drop radius R is deduced from the establishment of a three-dimensional hyperbolic concentration profile around the salty drop. This profile scales with r/delta. Accounting for the salt concentration decrease with salty drop growth, R is seen to grow as t(5). In the region r > delta, water droplets nucleate and grow. The rate of evolution of the water droplets at constant r/delta can be used to determine the local water pressure. The corresponding data reasonably agree with a hyperbolic water vapor profile around the salty drop. These results can be applied to the growth of BF patterns to determine whether hyperbolic or linear water vapor profiles apply.
Autores: Guadarrama , José Manuel; Mongruel, A.; Medici, M.-G. ; et al.
ISSN 1292-8941  Vol. 37  Nº 11  2014  págs. 109
Some tenebrionind beetles inhabiting the Namib desert are known for using their body to collect water droplets from wind-blown fogs. We aim to determine whether dew water collection is also possible for desert insects. For this purpose, we investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of Physasterna cribripes (Tenebrionidæ) beetle, where the macro-structure appears as a series of ¿bumps¿, with ¿valleys¿ between them. Dew formation experiments were carried out in a condensation chamber. The surface properties (infra-red emissivity, wetting properties) were dominated by the wax at the elytra surface and, to a lower extent, its micro-structure. We performed scanning electron microscope on histological sections and determined the infra-red emissivity using a scanning pyrometer. The emissivity measured (0.95±0.07 between 8¿14¿m) was close to the black body value. Dew formation occurred on the insect¿s elytra, which can be explained by these surface properties. From the surface coverage of the condensed drops it was found that dew forms primarily in the valleys between the bumps. The difference in droplet nucleation rate between bumps and valleys can be attributed to the hexagonal microstructure on the surface of the valleys, whereas the surface of the bumps is smooth. The drops can slide when they reach a critical size, and be collected at the insect's mouth.
Autores: M. Pichumani; P. Bagheri; K.M. Poduska; et al.
ISSN 1744-683X  Vol. 9  Nº 12  2013  págs. 3220-3229
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.
Autores: Miranda, Montserrat Ana; Burguete, Javier; Mancini, Héctor Luis; et al.
ISSN 1539-3755  Vol. 87  Nº 3  2013  págs. 032902-1 - 032902-7
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.
Autores: Miranda, Montserrat Ana; Laroze, D.; González-Viñas, W;
ISSN 0953-8984  Vol. 25  Nº 40  2013  págs. 404208
We present a study of the freezing dynamics of topological defects in a subcritical system by testing the Kibble¿Zurek (KZ) mechanism while crossing a tri-stable region in a one-dimensional quintic complex Ginzburg¿Landau equation. The critical exponents of the KZ mechanism and the horizon (KZ-scaling regime) are predicted from the quasistatic study, and are in full accordance with the quenched study. The correlation length, in the KZ freezing regime, is corroborated from the number of topological defects and from the spatial correlation function of the order parameter. Furthermore, we characterize the dynamics to differentiate three out-of-equilibrium regimes: the adiabatic, the impulse and the free relaxation. We show that the impulse regime shares a common temporal domain with a fast exponential increase of the order parameter.
Autores: Guadarrama , José Manuel; Mongruel, A.; González-Viñas, W; et al.
Revista: EPL
ISSN 0295-5075  Vol. 101  Nº 1  2013  págs. 16009
We report the observation of an unconventional mechanism for frost formation. On a smooth hydrophobic surface cooled much below the water freezing temperature (-9 degrees 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. Copyright (c) EPLA, 2013
Autores: González-Viñas, W;
ISSN 1744-683X  Vol. 9  Nº 8  2013  págs. 2506-2511
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.
Autores: Guadarrama , José Manuel; González-Viñas, W;
ISSN 1539-3755  Vol. 87  Nº 5  2013  págs. 054401-1 - 054401-5
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.
Autores: Miranda, Montserrat Ana; Burguete, Javier; González-Viñas, W; et al.
ISSN 0218-1274  Vol. 22  Nº 7  2012  págs. 1250165-1 - 1250165-10
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 = d epsilon/dt vertical bar(epsilon=0) of the quenched transition. From the analysis of fronts, we show that these mechanisms follow different correlation length scales xi similar to 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.
Autores: Bartlett, A.; Giuliani, M.; et al.
ISSN 0743-7463  Vol. 28  2012  págs. 3067 - 3070
Autores: Miranda, Montserrat Ana; Burguete, Javier; González-Viñas, W; et al.
ISSN 2174-6036  Vol. 1  2011  págs. 60 - 64
Autores: Giuliani, Maximiliano Andrés; González-Viñas, W;
ISSN 1539-3755  Vol. 83  2011  págs. 047301
Autores: Giuliani, Maximiliano Andrés; González-Viñas, W;
Revista: The European physical journal. Special topics
ISSN 1951-6355  Vol. 192  Nº 1  2011  págs. 121 - 128
Autores: González-Viñas, W;
Revista: Magnetohydrodynamics
ISSN 0024-998X  Vol. 47  Nº 2  2011  págs. 191 - 199
Autores: Criado, R.; Romance, M.; Mancini, Héctor Luis; et al.
Revista: International Journal of Bifurcation and Chaos
ISSN 0218-1274  Vol. 20  Nº 3  2010  págs. 749 - 752
Autores: Giuliani, Maximiliano Andrés; González-Viñas, W; Poduska, K.M.; et al.
ISSN 1948-7185  Vol. 1  Nº 9  2010  págs. 1481 - 1486
Autores: Narhe, R.D.; González-Viñas, W; Beysens, D.A.;
Revista: Applied Surface Science
ISSN 0169-4332  Vol. 256  Nº 16  2010  págs. 4930 - 4933
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 75 degrees to 150 degrees, corresponding to changing the surface properties from hydrophobic to ultrahydrophobic and super-hydrophobic. 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%.