• M. A. Alkhadra


Try to imagine the properties and behavior of a liquid that can be controlled by magnets. Such liquids exist and are called ferrofluids. (The name is a portmanteau of ferromagnetic and fluid; ferrum is Latin for iron, Fe.) A ferrofluid is a colloid of microscopically dispersed magnetic particles suspended in a liquid medium. Each of these tiny particles is completely coated with a surfactant—an organic compound that contains both hydrophobic (water repelling) and hydrophilic (water attracting) groups—that inhibits magnetic clumping and agglomeration. The coupling of liquid and magnetic behavior means that a ferrofluid may be manipulated by applying an external magnetic field, as shown in this video. In fact, magnetic inks are currently used in printing paper currency in the US, which can be observed in the attraction of a genuine dollar bill to a magnet.

In the 1960s, S. Pappell at NASA first developed and classified ferrofluids to control fluids in outer space. These fluids comprise nanoscale particles (diameter ~10 nm) of iron that are small enough to spontaneously and evenly disperse within a carrier fluid. For a ferrofluid to function properly, the magnetic nanoparticles must be chemically stable in the carrier, which entails no aggregation of the particles and no phase separation (unmixing of solid and liquid). Stability is therefore imparted on the mixture by using a surfactant to create a colloid, the composition of which is typically 5% magnetic solids, 10% surfactant, and 85% carrier fluid by volume. Surfactants are compounds that facilitate the dispersion and suspension of particles in a liquid by adhering to the particles and creating a net electrostatic repulsion between them, as depicted in Figure 1. This repulsion raises the energy barrier that particles must overcome to agglomerate, and thus stabilizes the colloid. Indeed, the development of ferrofluids requires the proper choice of surfactant.

Figure 1. Idealized illustration of an aqueous ferrofluid stabilized by a surfactant called tetramethylammonium hydroxide (TMAOH). Surfactant molecules adhere to the surface of magnetite particles, a rock mineral and one of the oxides of iron; the net electrostatic repulsion between charged particles stabilizes the colloid. In principle, TMAOH coats particles of magnetite with hydroxide anions that attract tetramethylammonium cations, which in turn form a diffuse shell around each particle to establish repulsion. Figure is not to scale.

You may have wondered from the image and video above why ferrofluids, when subjected to a magnetic field, consistently display an intricate pattern of peaks and valleys at the surface. These spikes are the result of the normal-field instability discovered and described by R. E. Rosensweig. This magnetically driven instability can be explained by considering the configuration of the fluid that maximizes entropy—a measure of chaos and disorder of the system. (Recall my earlier blog posts on hydrodynamic stability; the effect here is similar in that the fluid seeks to satisfy the second law of thermodynamics.) As far as thermodynamics is concerned, spikey peaks and valleys are favorable: in this configuration, the strength of the external magnetic field is concentrated in the peaks. Spikes of fluid, as a result, follow the field lines outwards into space until the magnetic force is balanced by gravity and surface tension—the elastic property of a fluid surface caused by the attraction of liquid molecules to each other. In other words, the strength of an external magnetic field must overcome gravity and surface tension to produce the beautifully corrugated pattern we see.

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