The deformation of hemispherical sessile bubbles made of ferrofluid soap under vertical uniform magnetic fields was studied using Helmholtz coils. The deformation and the shape of the bubbles were monitored according to the amplitude of the magnetic field, the initial volume of the bubbles and the ferrofluid volume used to create them. The meniscus was found to bear most of the deformation, reshaping into a cylinder, with the remainder of the bubble forming a spherical cap, mainly adapting to the meniscus transformation. The growth of the meniscus height was rationalised using a simple model. More precisely, the meniscus shape depends on the competition between capillary, gravity and magnetic effects. These three ingredients can be rewritten to highlight two characteristic lengths of the system: the capillary and the magnetic lengths. Depending on the magnetic field intensity, the shape of the meniscus is described by one of the two lengths, thus revealing the existence of two distinct regimes.

When a horizontal resistive wire is heated up to the boiling point in a subcooled liquid bath, some vapor bubbles nucleate on its surface. The traditional nucleate boiling theory predicts that bubbles generated from active nucleate sites grow up and depart from the heating surface due to buoyancy and inertia. However, we observed here a different behavior: the bubbles slide along the heated wire. In this situation, unexpected regimes are observed; from the simple sliding motion to bubble clustering. We noticed that bubbles could rapidly change their moving direction and may also interact. Finally, we propose an interpretation for both the attraction between the bubbles and the wire and for the motion of the bubbles on the wire in terms of Marangoni effects. Copyright (C) EPLA, 2017

The flow of air bubbles in a two-dimensional silo is studied experimentally. A particle tracking technique has been used to bring out the main properties of th e flow: displacements of the bubbles, transverse and axial velocities. Air bubbles behave like solid grains i a granular flow. Correlations between the bubble velocities and their deformations have been observed.

In this work, the interaction between electrically charged droplets in microgravity is considered. During the 22 s of microgravity brought by a parabolic flight, water droplets with a radius r ∈ [0.41 − 0.97] mm were released one in front of the other. A high-speed camera allowed studying their interaction in the focal plane. The trajectories of the droplets are well adjusted by a punctual charge model. In some experiments, a physical contact between the charged droplets was observed. These collisions are studied via a phase diagram comparing the droplet Weber number, We, and the collision parameter, χ. By comparing these collisions to experiments involving neutral droplets, we deduce how the collision diagram is affected by electric charges. In particular, we show that the criterion for an impact between two droplets is no more χ < 1.

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form of N-S equations is solved to obtain the detailed flow features through path line profiles, isotherm contours and velocity vectors. The effect of heater aspect ratio (x/L) on the rate of heat transfer is studied in terms of the average Nusselt number (Nu(ave)). The results indicate that effective heat transfer enhancement occurs for a small heater length, resulting in an efficient cooling. Increasing the heater length will favor heat transfer through conduction over convection. The maximum temperature difference across the fluid and the velocity magnitude are found to decrease with heater length. 3D and 2D results are in agreement for short heater lengths, but vary for higher heater lengths, presumably due to the essential effect of the heater width. Further analysis on different types of coolant reveals a constant correlation between Nuave and the Rayleigh number (Ra), with Nu(ave) similar to Ra-0.374 . Benchmark validation for natural convection in a square enclosure is found to be satisfactory against the reported results.

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural convection. In one recent experimental investigation (Dubois et al., 2016) carried on silicone oils and liquid nitrogen, we have demonstrated that the velocity field satisfies specific scaling laws with respect to the temperature increase in the liquid pool. In this work, we pursue the analysis by modeling the heat transfer in a parallelepiped enclosure for a steady laminar flow regime. The Navier-Stokes equations are solved using a finite volume approach to obtain the detailed three-dimensional flow and heat transfer characteristics. A quantitative analysis of the velocity field over the temperature field shows that the experimental power laws are reproduced in simulations. Following Dubois and Berge (1978), a theoretical law originally introduced in the context of the classical Rayleigh-Benard experiment is shown to be satisfied in the simulations over a wide range of Rayleigh numbers (Ra), assuming the definition of the characteristic convection length is adapted to the investigated geometry. Moreover, the simulations are shown to correctly reproduce the main features of the flow, including the characteristic convection length, for different heater lengths. (C) 2017 Elsevier Ltd. All rights reserved.

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been pointed out that ferromagnets can help in reducing the crossed field effect, namely the magnetization decay that is observed under the application of AC transverse magnetic fields. In this work, we pursue a detailed study of the influence of the geometry of the ferromagnetic part on both trapped fields and crossed field effects. The magnetic properties of the hybrid superconducting/soft ferromagnetic structures are characterized by measuring the magnetic moment with a bespoke magnetometer and the local magnetic field density with Hall probes. The results are interpreted by means of 2D and 3D numerical models yielding the distribution of the superconducting currents as a function of the ferromagnet geometry. We examine in details the distortion of the shielding superconducting currents distribution in hybrid structures subjected to crossed magnetic fields. These results confirm the existence of an optimum thickness of the ferromagnet, which depends on the saturation magnetization of the ferromagnetic material and the current density of the superconductor. A hybrid structure providing an efficient protection against the crossed magnetic field while maintaining the magnetic induction along the axis of the structure is suggested. The limitations of the 2D modelling in this configuration are discussed.

We report experiments concerning the melting of ice disks (85 mm in diameter and 14 mm in height) at the surface of a thermalized water bath. During the melting, the ice disks undergo translational and rotational motions. In particular, the disks rotate. The rotation speed has been found to increase with the bath temperature. We investigated the flow under the bottom face of the ice disks by a particle image velocimetry technique. We find that the flow goes downwards and also rotates horizontally, so that a vertical vortex is generated under the ice disk. The proposed mechanism is the following. In the vicinity of the bottom face of the disk, the water eventually reaches the temperature of 4 °C for which the water density is maximum. The 4 °C water sinks and generates a downwards plume. The observed vertical vorticity results from the flow in the plume. Finally, by viscous entrainment, the horizontal rotation of the flow induces the solid rotation of the ice block. This mechanism seems generic: any vertical flow that generates a vortex will induce the rotation of a floating object.

We present an experimental study of a laminar convective phenomenon induced by a centimetric heater totally immersed in a liquid pool (Rayleigh number ranging from 10(4) to 10(7)). This local heating is observed to induce a laminar convection that differs from the classical Rayleigh-Bénard cells created by heating the whole bottom of the fluid: the convection pattern is no more periodic. In order to obtain a complete map of the velocity field, we use Particle Image Velocimetry technique. The vertical velocity between the counter-rotating convective cells is used as the relevant physical parameter to describe the phenomenon. The potential cooling applications of this problem lead us to choose liquid nitrogen as an experimental fluid. We thus compare the results obtained for various temperature gradients in liquid nitrogen with experiments performed at room temperature with silicone oils of various viscosities. The theoretical law for the maximal vertical velocity from classical Rayleigh-Bénard experiments is adapted to the specific geometry investigated by using a new definition for the characteristic wavelength. This length is studied and appears to be dependent on the liquid properties. We finally obtain a remarkable agreement between theory and experimental data.

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When such superconducting (SC) "trapped field magnets" are combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to that of the superconductor alone. In the present work, we study experimentally the magnetic behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field that are orthogonal to their permanent magnetization, i.e. a "crossed-field" configuration. Experimental results show that the usual "crossed-field demagnetization" caused by the cycles of transverse field is strongly reduced in the presence of the ferromagnet.

The ascent motion of an air bubble beneath an inclined plane is experimentally studied. The effects of the surrounding liquid viscosity and surface tension, the bubble radius and the tilt angle are investigated. A dynamical model is proposed. It opposes the buoyant driving force to the hydrodynamical pressure arising from the bubble motion and the capillary meniscus generated in front of the bubble in order to create a lubrication film between the bubble and the plate. This model is compared to experimental data and discussed. Copyright (C) EPLA, 2016

This paper is associated with a video winner of a 2015 APS/DFDMilton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available from the Gallery of Fluid Motion.

Rotating drums are usually used to study the flowing properties of granular materials. Here, we report experiments performed in a drum made of a circular Hele–Shaw cell filled with a monodisperse foam and its surfactant solution. The cell is placed vertically and rotated around its center with an angular velocity ω. The rotation of the cell modifies the foam properties and therefore influences its response to the external strain. The observed foam behavior is similar to granular media in several aspects. Using tools developed for granular materials, we characterize the foam evolution in terms of a dimensionless viscous number If and we compare the foam rheology to the theory developed for the immersed granular material rheology.

Antifoams are widely used to control or to avoid foam production. In order to work, antifoam particles need to break foam films efficiently, which many antifoams do very well. However, once they have broken a film, to continue to be effective they need to be transported to the next film. We show, for the first time, that buoyancy has an important part in the transport of the antifoam particles. In microgravity, where buoyancy and gravitational drainage are strongly slowed down, diffusion leads to poor antifoam performance. The foam is stable for the duration of the experiment, whereas on Earth the foam starts to disappear immediately. Indeed, microgravity renders highly efficient antifoam practically useless.

When placed onto a vibrating liquid bath, a droplet may adopt a permanent bouncing behavior, depending on both the forcing frequency and the forcing amplitude. The relationship between the droplet deformations and the bouncing mechanism is studied experimentally and theoretically through an asymmetric and dissipative bouncing spring model. Antiresonance phenomena are evidenced. Experiments and theoretical predictions show that both resonance at specific frequencies and antiresonance at Rayleigh frequencies play crucial roles in the bouncing mechanism. In particular, we show that they could be exploited for bouncing droplet size selection.

Surface tension profiles in vertical soap films are experimentally investigated. Measurements are performed by introducing deformable elastic objets in the films. The shape adopted by those objects once set in the film is related to the surface tension value at a given vertical position by numerically solving the adapted elasticity equations. We show that the observed dependency of the surface tension versus the vertical position is predicted by simple modeling that takes into account the mechanical equilibrium of the films coupled to previous thickness measurements.

Liquid foams are omnipresent in everyday life, but little is understood about their properties. On Earth, the liquid rapidly drains out of the foam because of gravity, leading to rupture of the thin liquid films between bubbles. Several questions arise: are liquid foams more stable in microgravity environments? Can pure liquids, such as water, form stable foams in microgravity whereas they do not on Earth? In order to answer these questions, we performed experiments both in parabolic flights and in the International Space Station.