Stokes flow paths separation and recirculation cells in X-junctions of varying angle

Fluid and solute transfer in X-junctions between straight channels is shown to depend critically on the junction angle in the Stokes flow regime. Experimentally, water and a water-dye solution are injected at equal flow rates in two facing channels of the junction: Planar Laser Induced fluorescence (PLIF) measurements show that the largest part of each injected fluid "bounces back" preferentially into the outlet channel at the lowest angle to the injection; this is opposite to the inertial case and requires a high curvature of the corresponding streamlines. The proportion of this fluid in the other channel decreases from 50% at 90\degree to zero at a threshold angle. These counterintuitive features reflect the minimization of energy dissipation for Stokes flows. Finite elements numerical simulations of a 2D Stokes flow of equivalent geometry con rm these results and show that, below the threshold angle 33.8\degree recirculation cells are present in the center part of the junction and separate the two injected flows of the two solutions. Reducing further leads to the appearance of new recirculation cells with lower flow velocities

Granular flow through an aperture: Influence of obstacles near the outlet

We study how the presence of obstacles in a confined system of monodisperse disks affects their discharge through an aperture. The disks are driven by a horizontal conveyor belt that moves at constant velocity. The mean packing fraction at the outlet decreases as the distance between the obstacles and the aperture decreases. The obstacles organize the dynamics of the stagnant zones in two characteristic behaviors that differ mainly in the magnitude of the fluctuations of the fraction of stagnant disks in the system. It is shown that the effective aperture is reduced by the presence of obstacles.Fil: Areán, M. G.. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Boschan, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires; ArgentinaFil: Cachile, Mario Andres. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Aguirre, Maria Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentin

Evaporation induced flow inside circular wells

Flow field and height averaged radial velocity inside a droplet evaporating in an open circular well were calculated for different modes of liquid evaporation.Comment: 5 page, 3 figures, submitted to European Physical Journal

Transport of non tracer particles in simple flows

Se realizó un estudio experimental del comportamiento de partículas transportadas en un flujo simple. Para ello se utilizaron pares de canales milimétricos con un ángulo de intersección de 0°, y un segmento de comunicación entre ellos, por los cuales se inyectan simultáneamente dos fluidos miscibles. Uno de los fluidos es una solución de agua que contiene un colorante (trazador pasivo) y microesferas (partículas no trazadoras) de flotabilidad neutra, el otro fluido está formado por la misma solución, pero sin ningún tipo de trazador. Para el análisis sistemático de las trayectorias de las partículas, se implementó un dispositivo experimental con técnicas de rastreo de partículas. Los experimentos fueron filmados con alta resolución temporal y espacial utilizando una cámara de alta resolución acoplada a una lupa de laboratorio. Las imágenes se analizaron con un programa de procesamiento desarrollado en el laboratorio. Se analizaron las trayectorias de las partículas para distintos número de Reynolds, obteniéndose la distribución, las velocidades y la concentración de microesferas a la salida de cada canal en función de los parámetros estudiados.An experimental study on the behaviour of transported particles in a flow of simple geometry has been developed. A pair of millimetric channels that cross at 0°, with a shunt between them where two miscible fluids are inyected at constant flow rate. One of the fluids is a solution of dye (passive tracer) as well as microbeads (non passive tracer) of neutral buoyancy. The other fluid is made of the same solution but without any tracer. For the systematic study of the particle trajectories an experimental tracking method has been set up. The experiments have been recorded with a high spatial and temporal resolution camera coupled to a low power microscope. The images have been analysed with a coded processing program developed in our laboratory. The trajectories of the particles, with different Reynolds number, have been analysed, in order to obtain the distribution, speed and bead concentration at each outlet as a function of the used parameters and have been compared with previous results on similar geometries and miscible fluids.Fil: Brabat, M.. École Nationale Supérieure D'électronique, Informatique, Télécommunications, Mathématiques et Mécanique de Bordeaux; FranciaFil: Freytes, Verónica Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Cachile, Mario Andres. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: D'angelo, María Verónica. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentin

Spreading Dynamics of Polymer Nanodroplets

The spreading of polymer droplets is studied using molecular dynamics simulations. To study the dynamics of both the precursor foot and the bulk droplet, large drops of ~200,000 monomers are simulated using a bead-spring model for polymers of chain length 10, 20, and 40 monomers per chain. We compare spreading on flat and atomistic surfaces, chain length effects, and different applications of the Langevin and dissipative particle dynamics thermostats. We find diffusive behavior for the precursor foot and good agreement with the molecular kinetic model of droplet spreading using both flat and atomistic surfaces. Despite the large system size and long simulation time relative to previous simulations, we find no evidence of hydrodynamic behavior in the spreading droplet.Comment: Physical Review E 11 pages 10 figure

Mechanical tuning of the evaporation rate of liquid on crossed fibers

We investigate experimentally the drying of a small volume of perfectly wetting liquid on two crossed fibers. We characterize the drying dynamics for the three liquid morphologies that are encountered in this geometry: drop, column and a mixed morphology, in which a drop and a column coexist. For each morphology, we rationalize our findings with theoretical models that capture the drying kinetics. We find that the evaporation rate depends significantly on the liquid morphology and that the drying of liquid column is faster than the evaporation of the drop and the mixed morphology for a given liquid volume. Finally, we illustrate that shearing a network of fibers reduces the angle between them, changes the morphology towards the column state, and so enhances the drying rate of a volatile liquid deposited on it

The relation of steady evaporating drops fed by an influx and freely evaporating drops

We discuss a thin film evolution equation for a wetting evaporating liquid on a smooth solid substrate. The model is valid for slowly evaporating small sessile droplets when thermal effects are insignificant, while wettability and capillarity play a major role. The model is first employed to study steady evaporating drops that are fed locally through the substrate. An asymptotic analysis focuses on the precursor film and the transition region towards the bulk drop and a numerical continuation of steady drops determines their fully non-linear profiles. Following this, we study the time evolution of freely evaporating drops without influx for several initial drop shapes. As a result we find that drops initially spread if their initial contact angle is larger than the apparent contact angle of large steady evaporating drops with influx. Otherwise they recede right from the beginning