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We investigate the physical properties of soap films and soap foams, the liquid phase of which contains a stable colloidal suspension of magnetic particles. The physical properties of such systems result from the equilibrium between the capillary forces, the gravity and the local magnetic forces. Therefore, they depend on the strength of an applied magnetic field or force, which makes it possible to act from outside on the system. We present the effect of the external field on the magnetic soap foams and films through two studies. First, we show that the dynamics of drainage in a freely suspended magnetic soap film is modified by an external magnetic field. Second, we present the control, via an external magnetic force, of the structure adopted by a monodisperse foam in a cylindrical tube. Those studies open possible applications of the ferrofluid foam, in the field of technological applications, but also as a model experimental system to study the drainage, the dynamical rheology, and the stability of soap foams.

We present here new experimental results about the waves at the horizontal free surface of a magnetic fluid submitted to a normal magnetic field. The waves are generated by a small modulation at frequency of the vertical field . Using a shadowgraph method, we are able to measure the wavevector k of the 2D waves for a given value of and . The dispersion relation of the surface waves is established experimentally. On the other hand, we propose a theoretical derivation of the dispersion equation which includes a more complete treatment of the magnetic term than the previous works. Finally, we conclude that a linear and inviscid analysis is sufficient to fit well the experimental data, except in the vicinity of the critical field where a surface instability occurs.

Viscous fingering phenomenon in a linear channel is studied for a magnetic fluid subjected to an external magnetic field. The competition between the hydrodynamic effects and the capillary effects leads to the formation of an interface between the air and the fluid which has a finger shape. It is the so-called Saffman–Taylor instability (STI). The influence of the magnetic effects depends on the direction of the applied field: it is possible either to enhance or to reduce the destabilizing phenomena. We study the onset of the STI and compare the experimental results with the linear analysis including the magnetic contribution. In the nonlinear regime, the measurement of the width of the finger as a function of the direction and the amplitude of the magnetic field is understood using a phenomenological approach.

Two-dimensional magnetic fluid foams are cellular structures whose framework is made of magnetic fluid. The features of these equilibrated patterns are driven by a control parameter: the amplitude of the applied magnetic field. When the latter is rapidly increased, an instability occurs: the walls between cells undulate. Such an instability has also been observed in other 2D cellular structures, which exist for instance in Langmuir monolayers or in magnetic garnets thin films. In this paper we give a theoretical analysis of this instability, the issues of which are shown to be well confirmed by experiments and numerical simulations.

We present an experimental study of a cellular system which may be formed in a two-dimensional ~2D! layer made of an immiscible mixture of a magnetic fluid and an oil. We obtain a wet or a dry 2D froth, the characteristics of which are determined by the strength of an externally applied magnetic field. The froth is formed in an equilibrium state; the topological and geometrical features of the froth are fixed by the value of the applied field. The cellular pattern of the froth is stable in time, and a coarseninglike behavior is observed on decreasing the amplitude of the field. This cellular system can be used as a model to study topological processes in the equilibrium state, contrary to soap froths which are nonequilibrium systems because gas diffusion occurs. We show that the topological characteristics are statistically reversible after a magnetic-field cycle, and we present a statistical study of the froth features as a function of the amplitude of the field. The topological correlations between neighboring cells are well described by the Aboav-Weaire law. Finally, we consider the area of an n-sided cell, and show that its growth rate ~magnetic field dependent! is, in a surprising manner, proportional to n26, similarly to the Von Neumann law. @S1063-651X~97!10009-5#

We present some experimental results concerning the conformational instabilities observed in a thin layer made of an immiscible mixture of a magnetic fluid and another liquid submitted to an external magnetic field. We observed macroscopic domains with stripe or bubble shape. The transition bet~~een the two morphologies is observed and a coexistence of the two phases is visualized in a variable thickness layer. Furthermore, we give other results about the instabilities of the parallel stripe pattern: by increasing the external field the stripe pattern becomes a chevron structure, which also becomes unstable for higher values of the magnetic field and gives a hairpin pattern. Finally, we present a new system: a magnetic liquid froth in two dimensions which is analogous to soap froths with an additional external control parameter, the magnetic field.

Two methods of determination of the surface tension at the interface of a magnetic liquid and another fluid, in a confined two-dimensional geometry, are presented. The first is based upon a surface instability under the action of a vertical magnetic field and the second uses the deformation of a magnetic droplet in plane layer under the influence of a horizontal magnetic field. Theoretical calculations and experimental results are presented in both cases. Both determinations lead to comparable values of the surface tension $\ensuremath{\sigma}\ensuremath{\approx}3$ mN ${\mathrm{m}}^{\ensuremath{-}1}$.

An immiscible two-component mixture, made of a magnetic fluid and an organic liquid, in a Hele-Shaw cell under a magnetic field, can form different patterns like stripes or bubbles. Two-dimensional smectic-like structures are obtained with many parallel stripes. An analogy with a smectic liquid crystal allows us to define elastic moduli of the stripes. These are measured from very simple experiments. From stripe pattern energy calculation, the dependence of these elastic constants on control parameters is established. The main result is that the smectic analogy allows a rather good description of the stripe system.

Patterns of magnetic fluid stripes are experimentally studied and compared to numerical simulations by boundary integral equation technique. Like in amphiphile monolayers and ferromagnetic films, we observe bending and alternated fingering of a single stripe. For bending deformations, a two-dimensional-smectic analogy is presented. This analogy is only valid for quasistatic variations of the external magnetic field strength. A fingering phenomenon occurs for non-quasistatic variations of the fields and for large stripes.

We present experiments showing the Rayleigh-Taylor instability at the interface between a dense magnetic liquid and an immiscible less dense liquid. The liquids are confined in a Hele-Shaw cell and a magnetic field is applied perpendicular to the cell. We measure the wavelength and the growth rate at the onset of the instability as a function of the external magnetic field. The wavelength decreases as the field increases. The amplitude of the interface deformation grows exponentially with time in the early stage, and the growth rate is an increasing function of the field. These results are compared to theoretical predictions given in the framework of linear stability analysis.

We present a new cellular structure, obtained in a pseudo two-dimensional (2D) layer of magnetic fluid (MF) submitted to a perpendicular magnetic field. This froth is analogous to 2D soap froths, because its energy contains an interfacial contribution. Nevertheless, whereas a soap froth coarsens in time to minimize the surface of its interface, the 2D MF froth can be stable in time. Indeed it also has a magnetic energy, which allows the cellular pattern to reach an equilibrium state. The properties of such a froth in its equilibrium state are thus driven by competition between the surface energy and the magnetic energy. The latter depends on the amplitude of the applied magnetic field, which is a control parameter of this system. An evolution of the structure is obtained on decreasing the amplitude of the field: the number of cells decreases so that the pattern turns into a single drop of MF in zero field. We present here a study of this coarsening with decreasing of the field. For high amplitudes of the applied field, we have observed out of equilibrium states of the 2D MF froth: the area of the cells evolves in time, depending on their number of sides. This behaviour looks like a Von Neumann one (which drives the time-evolution of 2D soap froths), but with the opposite sign: 5-sided cells grow in time, 7-sided cells shrink, whereas 6-sided cells do not evolve. In section 6, the 2D MF froths are shown to obey the Aboav and Weaire law which describes the topological interactions between cells, as all the 2D cellular structures. Finally, we present a potential application of the 2D MF froth, for the study of topology in 2D cellular patterns.