While the behaviour of large amounts of liquid is dictated by gravity, surface forces become dominant at small scales. They have for example the remarkable ability to make droplets stick to their substrates (even if they are inclined), which is a practical issue in many cases (windshields, window panes, greenhouses, or microfluidic devices). Here we describe how this problem can be overcome with super-hydrophobic materials. These materials are often developed thanks to micro-textures, which decorate a solid surface, and we describe the way such textures modify the wettability of that solid. We conclude by showing the unusual dynamics of drops in a super-hydrophobic situation.

https://iopscience.iop.org/article/10.1088/0034-4885/68/11/R01/pdf

Non-sticking drops

John M Wallace and Peter V Hobbs London: Academic 1977 pp xvii + 467 price £12.80 This is a comprehensive textbook for undergraduate courses in atmospheric physics. It contains general physical meteorology (atmospheric hydrostatics, cloud physics, radioactive transfer and thermodynamics), some selected topics of special interest (aerosol physics, aeronomy and physical climatology) and dynamic meteorology describing and interpreting large scale atmospheric motions.

Atmospheric Science: An Introductory Survey

Atmospheric boundary layers with weak stratification are relatively well described by similarity theory and numerical models for stationary horizontally homogeneous conditions. With common strong stratification, similarity theory becomes unreliable. The turbulence structure and interactions with the mean flow and small-scale nonturbulent motions assume a variety of scenarios. The turbulence is intermittent and may no longer fully satisfy the usual conditions for the definition of turbulence. Nonturbulent motions include wave-like motions and solitary modes, two-dimensional vortical modes, microfronts, intermittent drainage flows, and a host of more complex structures. The main source of turbulence may not be at the surface, but rather may result from shear above the surface inversion. The turbulence is typically not in equilibrium with the nonturbulent motions, sometimes preventing the formation of an inertial subrange. New observational and analysis techniques are expected to advance our understanding of...

Stably Stratified Atmospheric Boundary Layers

We experimentally study the formation and evolution of threads containing more viscous liquids surrounded by less viscous, immiscible liquids through hydrodynamic focusing in square microchannels. Over a large range of viscosities and interfacial tensions, five characteristic regimes of flow behavior are identified: threading, jetting, dripping, tubing, and displacement. We locate the boundaries between these regimes on a flow map based on the capillary number of each fluid. In the jetting and the dripping regimes, the droplet size is measured and related to fluid properties, flow parameters, and geometry. The critical thread length before jetting droplets and the critical length of a viscous tail before breakup in dripping are also examined. This study classifies and defines regimes of thread instabilities that can be used to produce supra- and subchannel size viscous droplets in an elementary microfluidic geometry.

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Capillary threads and viscous droplets in square microchannels

When a drop of a viscous fluid is deposited on a bath of the same fluid, it is shown that its coalescence with this substrate is inhibited if the system oscillates vertically. Small drops lift off when the peak acceleration of the surface is larger than $g$. This leads to a steady regime where a drop can be kept bouncing for any length of time. It is possible to inject more fluid into the drop to increase its diameter up to several centimeters. Such a drop remains at the surface, forming a large sunk hemisphere. When the oscillation is stopped, the two fluids remain separated by a very thin air film, which drains very slowly ($\ensuremath{\sim}30\text{ }\text{ }\mathrm{min}$). An analysis using lubrication theory accounts for most of the observations.

From bouncing to floating: noncoalescence of drops on a fluid bath.

This article gives the principle of hydrodynamic lubrication and also presents the new phenomenon of levitating drops over liquid film flow, which is explained using hydrodynamic lubrication theory.

Hydrodynamic lubrication: Experiment with ‘Floating’ drops

Recent experimental results of Bhat and Narasimha (1996) have revealed a dramatic difference in the entrainment between jets and plumes subjected to off-source volumetric heating and their unheated counterparts. Experimental observations show that plumes entrain more rapidly than jets; the greater entrainment by the plume is typically attributed to the presence of buoyancy in the plume. In contrast, the addition of buoyancy away from the source by volumetric heating produces the opposite effect of reduced entrainment. Apart from buoyancy, other factors such as acceleration due to pressure gradients or other body forces can also affect the rate of entrainment. In this paper, we develop a model for entrainment to explain the mechanism by which buoyancy produces contrasting effects on entrainment in volumetrically heated flows in comparison to their unheated counterparts. The model highlights the role of density stratification in the process of vortex sheet roll-up in free shear flows. With this model, we ar...

http://research.me.udel.edu/~prasad/papers/pof_vortex_model.pdf

Vortex-dynamics model for entrainment in jets and plumes

A common injector geometry upstream of a static mixer is the centerline injector. A flow instability can arise due to viscosity differences between the injected core-flow and the outer co-flow. This instability can adversely affect the effectiveness of the mixing operation. An experimental investigation of miscible viscosity-stratified flow in a circular geometry was performed using Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV). The experimental results for the stable region agree with the analytical results. The unstable region exhibits different modes depending on the viscosity ratio, volume flux ratio, and Reynolds number. The modes include wavy core-flow with fissures and wavy core-flow with core breakup. The time-averaged experiment velocity profiles for the unstable core indicate a broadening of the jet at the centerline, which is consistent with the LIF visualization.



L'injecteur centre est une geometrie d'injection commune en amont des melangeurs statiques. Une instabilite de l'ecoulement peut survenir en raison des differences de viscosite entre l'ecoulement piston injecte et le cocourant exterieur. Cette instabilite peut nuire a l'efficacite de l'operation de melange. On a donc mene une etude experimentale pour un ecoulement biphasique miscible stratifie par la viscosite dans une geometrie circulaire basee sur la fluorescence induite par laser (LIF) et la velocimetrie a imagerie de particules (PIV). Les resultats experimentaux pour la region stable concordent avec les resultats analytiques. La region instable montre differents modes qui dependent du rapport de viscosite, du rapport de flux volumique et du nombre de Reynolds. Ces modes comprennent l'ecoulement piston a vagues avec fissures et l'ecoulement piston avec vagues avec rupture du piston. Les profils de vitesse experimentaux moyennes dans le temps pour le piston instable indiquent un elargissement du jet a la ligne de centre qui est coherent avec la visualisation par LIF.

/pdf/instability-due-to-viscosity-stratification-downstream-of-a-o6j1e7lnvw.pdf

Instability due to Viscosity Stratification Downstream of a Centerline Injector

A vertical jet of water impinging on a horizontal surface produces a radial film flow followed by a circular hydraulic jump. We report a phenomenon where fairly large (1 ml) drops of liquid levitate just upstream of the jump on a thin air layer between the drop and the film flow. We explain the phenomenon using lubrication theory. Bearing action both in the air film and the water film seems to be necessary to support large drops. Horizontal support is given to the drop by the hydraulic jump. A variety of drop shapes is observed depending on the volume of the drop and liquid properties. We show that interaction of the forces due to gravity, surface tension, viscosity and inertia produces these various shapes.

Levitation of a drop over a film flow

Double-diffusive finger convection occurs in many natural processes.The theories for double-diffusive phenomena that exist at present consider systems with linear stratification in temperature and salinity. The double-diffusive systems with step change in salinity and temperature are, however, not amenable to simple stability analysis. Hence factors that control the width of the finger, velocity, and fluxes in systems that have step change in temperature and salinity have not been understood so far. In this paper we provide new physical insight regarding factors that influence finger convection in two-layer double-diffusive system through two-dimensional numerical simulations. Simulations have been carried out for density stability ratios (R-rho) from 1.5 to 10. For each density stability ratio, the thermal Rayleigh number (Ra-T) has been systematically varied from 7x10(3) to 7x10(8). Results from these simulations show how finger width, velocity, and flux ratios in finger convection are interrelated and the influence of governing parameters such as density stability ratio and the thermal Rayleigh number. The width of the incipient fingers at the time of onset of instability has been shown to vary as Ra-T-1/3. Velocity in the finger varies as Ra(T)1/3/R-rho. Results from simulation agree with the scale analysis presented in the paper. Our results demonstrate that wide fingers have lower velocities and flux ratios compared to those in narrow fingers. This result contradicts present notions about the relation between finger width and flux ratio. A counterflow heat-exchanger analogy is used in understanding the dependence of flux ratio on finger width and velocity.

K. R. Sreenivas

Papers

Hydrodynamic lubrication: Experiment with ‘Floating’ drops

Vortex-dynamics model for entrainment in jets and plumes

Instability due to Viscosity Stratification Downstream of a Centerline Injector

Levitation of a drop over a film flow

On the relationship between finger width, velocity, and fluxes in thermohaline convection