Gonzalez-Viñas, W. (Wenceslao)
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- Breath figures of two immiscible substances on a repellent surface(APS, 2013) Gonzalez-Viñas, W. (Wenceslao); Guadarrama-Cetina, J. (Jose)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.
- Electrophoretic deposition of colloidal crystals assisted by hydrodynamic flows(2006) Yoldi, M. (María); Sirera-Bejarano, R. (Rafael); Arcos, M.C. (M. Cristina); Gonzalez-Viñas, W. (Wenceslao)Latex-based colloidal crystals have been grown by electrophoretic deposition. The deposition has been assisted by hydrodynamic von Kármán-like flows, which lead to quantitative improvements. It has been studied the influence of applied voltage, deposition time and flow rate on the number of deposited layers and on the mean domain size. The samples were studied with microscopy and precision weight measurement. It has been found that there is a critical time after which the deposition mechanism changes, and the behavior of the system before and after this critical time is considered. The mean domain size and the deposition time were reduced to non-dimensional forms which show the collapse of the data for different applied voltages and flow rates into one curve.
- Unified description over time of heterogeneous condensation with quenched disorder(American Physical Society, 2023-12-27) Gonzalez-Viñas, W. (Wenceslao); Urbina, R. (Ruddy)We report experimental results on breath figures (BFs) observed on substrates with quenched disorder. The evolution of BFs is found to be primarily influenced by global parameters associated with boundary conditions. We investigate classical statistical measures and explore topological properties using persistent homology techniques based on a modified Vietoris-Rips complex. Our findings reveal that the evolution of the number surface density of condensed droplets plays a crucial role in determining various condensation stages previously considered distinct. This evolution is significantly influenced by the distribution of nucleation sites and the individual growth law governing water droplets when coalescence does not occur. Ultimately, we demonstrate the capability to predict coalescence events based on the topological characteristics of BFs at a given point in time.
- Modified Spin-coating Technique to Achieve Directional Colloidal Crystallization(American Chemical Society, 2012) Giuliani, M. (Maximiliano); Yethiraj, A. (Anand); Pichumani, M. (Moorthi); Bartlett, A.P. (Andrew P.); Gonzalez-Viñas, W. (Wenceslao)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.
- Magnetorheology of hybrid colloids obtained by spin-coating and classical rheometry(IOP, 2016-06-16) Shahrivar, K. (Keshvad); Gonzalez-Viñas, W. (Wenceslao); Muhammad-Aslam, R. (Raheema); Vicente, J. (Juan) deHybrid 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.
- Frozen dynamics and synchronization through a secondary symmetry-breaking bifurcation(APS, 2013) Burguete-Mas, F.J. (Francisco Javier); Gonzalez-Viñas, W. (Wenceslao); Miranda, M.A. (Montserrat A.); Mancini-Maza, H. L. (Hector Luis)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.
- Orientationally correlated colloidal polycrystals without long-range positional order(2008) Yethiraj, A. (Anand); Kumar, K. (K.); Sirera-Bejarano, R. (Rafael); Poduska, K.M. (Kristin M.); Arcos, M.C. (M. Cristina); Gonzalez-Viñas, W. (Wenceslao)We probe the local and global structure of spin-coated colloidal crystals via laser diffraction measurements and scanning electron and atomic force microscopies, and find that they are unique three-dimensional orientationally correlated polycrystals, exhibiting short-range positional order but long-range radial orientational correlations, reminiscent of—but distinct from—two-dimensional colloidal hexatic phases. Thickness and symmetries are controllable by solvent choice and spin speed. While the polycrystallinity of these colloidal films limits their applicability to photonics, we demonstrate their feasibility as templates to make crack-free magnetic patterns.
- Frost formation with salt(EPL Association, European Physical Society, 2015) Beysens, D.A. (Daniel A.); Gonzalez-Viñas, W. (Wenceslao); Mongruel, A. (Anne); Guadarrama-Cetina, J. (Jose)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.
- Percolation-induced frost formation(IOP, 2013) Beysens, D.A. (Daniel A.); Gonzalez-Viñas, W. (Wenceslao); Mongruel, A. (Anne); Guadarrama-Cetina, J. (Jose)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.
- Dynamics, crystallization, and structures in colloid spin coating(2013) Yethiraj, A. (Anand); Pichumani, M. (Moorthi); Poduska, K.M. (Kristin M.); Bagheri, P. (P.); Gonzalez-Viñas, W. (Wenceslao)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.