Liquids ferrofluids

FIGURE 16.43 When a magnet is pulled up from this viscous ferrofluid, the particles of iron(lll) oxide align themselves with the magnetic field. Because strong attractions exist between the particles and the detergent molecules in the oil, the liquid is pulled into the field along with the particles. 

In TFB technique, the thin film radius is typically of the order of 100 pm, far larger than the contact film radius likely to be formed when two micron-sized droplets approach. The magnetic chaining technique overcomes this limitation, allowing the direct measurement of force-distance profiles between liquid colloidal droplets. This technique exploits the properties of monodisperse ferrofluid... 

Liquid magnets ferrofluids) exhibit interesting properties of value to technology (Rosensweig, 1982 Popplewell, 1984). Since cooperative magnetism is a property of the... 

For ferrofluids, the usual form of the Bernoulli equation which relates the pressure of an incompressible liquid to its kinetic and potential energy must be modified to include the magnetic pressure induced by an applied magnetic field. The modified equation reads ... 

Nematic liquid crystals prepared from 35.9% sodium dodecyl sulfate, 7.2% decanol, and 56.9% water (H20 or D20) Ferrofluid (1%) dispersed in the liquid crystal Stable nematic II liquid crystalline ferrofluid was produced diameter = 154 9.4 A number of grains/cm3 = 1.9 x 10 magnetization at saturation = 2.20 g/cm3 785, 786... 

The mixer discussed below is part ofa ferrofluidic microsystem [159], Ferrofluids can be used for fluid displacement by magnetic action using an external permanent magnet source. Ferrofluids are superparamagnetic liquids. 

Assays of enzymatic activity of polymer degrading enzymes are often difficult to perform using traditionally immobilized systems. Using a liquid-liquid phase system makes it easier to establish contact between the catalyst and the substrate. Products may then partition differendy as compared to the substrate. This forms the basis for the assay of dextranase using a dye substrate, blue dex-tran. The liberated color is preferentially extracted into the more hydrophobic top phase (103). Adding a ferrofluid that is a submicron magnetic preparation to an aqueous two-phase extraction system makes it possible to substantially facilitate phase separation by magnetic force (104). 

In brief, the investigation of ferrofluids through Raman spectroscopy permits to access the physical and chemical properties of both solid and liquid phases. Comparison between the Raman spectra obtained from liquid water and the Raman spectra obtained from the uncoated and coated nanoparticles dispersed as ferrofluid provides information about the interface nanoparticle surface-carrier liquid [63]. The onset of the hematite phase in core magnetite dispersed as magnetic fluids was followed by Raman spectroscopy, and the results showed that the laser intensity at which the phase-transition takes place was higher for nanoparticles in the colloid than that for the same core as powder samples [69]. 

Ferrofluids have recently been combined with other complex fluids to produce fluids with very interesting behavior. When ionic ferrofluids are doped into nematic liquid crystals (Brochard and de Gennes 1970), the resulting nematic liquid orients under a magnetic field 10 times lower than that required in the absenceof the dopant (Chen and Amer 1983 Bacri... 

The fluid dynamic properties of continuous magnetic media, including liquid ferromagnetic media (ferrofluids), are described by the Navier-Stokes equations with the addition of terms describing the interaction with the magnetic field and are the subject of a considerable number of studies (for a review, see Ref. 1). The motion of discrete magnetic particles in a very... 

The picture in terms of the decoupled Eangevin equations (98) and (99) (omitting the inertial term in Eq. (98) is that the orientational correlation functions of the longitudinal and transverse components of the magnetization in the axially symmetric potential, Kvm sin" . are simply multiplied by the liquid state factor, exp(—t/T ), of the Brownian (Debye) relaxation of the ferrofluid stemming from Eq. (99). As far as the fenomagnetic resonance is concerned, we shall presently demonstrate that this factor is irrelevant. 

Stepanov and Shliomis [80] have proposed an egg model akin to a three-dimensional form of the itinerant oscillator model [72]. (We recall [72] that in the itinerant oscillator, one relaxation mechanism occurs due to rotation of the oscillator as a whole, the other due to jumping of the inner rotator over a potential barrier) whereby the interdependence of the two relaxation mechanisms is taken into account. In the egg model [80], the ferrofluid particle is represented as an egg of volume V (the hydrodynamic volume) embedded in the liquid with viscosity 17. The magnetic moment of the ferroparticle is modeled by the yolk of volume V magnetic torque acts directly on the yolk but due to the viscosity of the white it is also transmitted to the eggshell. Such a model allows one to take into account the coupling between the two mechanisms in a quantitative way so that its influence on and may be estimated. 

The stable colloidal dispersion of iron oxide nanoparticles in a liquid carrier medium is known as magnetic ferrofluid. The iron oxides mostly used are magnetite Fe304 and maghemite y-Fe203. The carrier medium can be chosen to be aqueous or organic, depending on the application. The stability of the ferrolluids is the main... 

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