Ferrofluid emulsions

The preparation of a ferrofluid emulsions is quite similar to that described for double emulsions. The starting material is a ferrofluid oil made of small iron oxide grains (Fe203) of typical size equal to 10 nm, dispersed in oil in the presence of an oil-soluble surfactant. The preparation of ferrofluid oils was initially described in a US patent [169]. Once fabricated, the ferrofluid oil is emulsifled in a water phase containing a hydrophilic surfactant. The viscosity ratio between the dispersed and continuous phases is adjusted to lie in the range in which monodisperse fragmentation occurs (0.01-2). The emulsification leads to direct emulsions with a typical diameter around 200 nm and a very narrow size distribution, as can be observed in Fig. 1.33. 

Figure 1.33. Scanning electron microscopy image of a ferrofluid emulsion (Courtesy of Ademtech Company.)...

Rgure 2.21. Force vs. distance profiles for ferrofluid emulsions stabilized with /3-casein at different concentrations (points). The ionic strength is 3 10 mol/1, pH = 6.2. The lines are the best fits (see the text for details). (Adapted from [78].)... 

Figure 2.22. (a) Disjoining pressure vs. thickness isotherm for an emulsion film stabilized by 0.1% BSA, ionic strength of 10 mol/1 NaCl, oil phase = hexadecane. The dots are the experimental data, dashed line is the double-layer contribution to the total disjoining pressure, and the solid line is the best fit done supposing additivity of the double-layer and steric forces, (b) Force vs. distance profiles for ferrofluid emulsions stabilized with mixed BSA-Tween-20 adsorption layers. The total concentration of the Tween-20 is kept constant = 5CMC, pH = 5.8. (Adopted from [78].)... 

Acknowledgement. The authors would like to thank C. Mabille, T.G. Mason, K. Pays, P. Gorria, C. Goubault, D. Olea for their contributory work and Ademtech company (Pessac, France) for providing the ferrofluid emulsions. 

J. Liu, M. Lawrence, A. Wu, M.L. Ivey, G.A. Flores, K. Javier, J. Bibette, and J. Richard Field-Induced Structures in Ferrofluid Emulsions. Physical Review Letters 74, 2828-2831 (1995)... 

Similar complex structmes, characterized as folded sheets or bent walls, have been observed in optical microscopy experiments of ferrofluid emulsions at high ramping rates of the magnetic field [288]. When viewed along the field direction they form a labyrinthine pattern whose complexity increases with ceU thickness, particle volume fraction and ramping rate [288]. 

When chains are flexible, as in real systems, thermal fluctuations and defect formation can further increase the escape distance. Direct measurement through optical trap micromanipulation [300] of the lateral interaction of two dipolar chains in a ferrofluid emulsion taking into account their fluctuation behaviour show that long flexible chains are attractive over a rather long-range ( = 8ct at X = 340) and that in stiffer chains the escape distance is reduced ( = 3ct at X = 610). These experiments also reveal that at short separations (4- 5ct) the force can suddenly shift from attraction to repulsion and back [300]. 

Figure 10.3 Submicronic magnetic latex obtained by emulsion polymerization of styrene and divinylben-zene onto a ferrofluid emulsion stabilized by polyacrylic acid based amphiphilic surfactant and initiated by potassium persulate (Montagne, 2002).

Montague, R, Mondain-Monval, O., Pichot, C., and Elaissari, A. 2006. Highly magnetic latexes from submicrometer oU in water ferrofluid emulsion. J. Polym. Sci. Part A 44 2642—2656. 

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