The effects of magnetic felds on the properties of ferrofluids and ferrogels

Abstract

Experimental study of the static magnetic properties of ferrofluid emulsions has demonstrated the nonmonotonic field dependence of the emulsion magnetic permeability. In a weak magnetic field, the emulsion permeability rapidly grows and reaches a maximum, and then it decreases slowly in stronger fields. We suggest a theoretical description of the effect based on the following idea. The weak field growth of the emulsion magnetic permeability is caused by the droplet elongation and the resulting reduction of demagnetizing field under the constant value of the ferrofluid magnetic susceptibility. Subsequent decrease of the emulsion magnetic permeability in a stronger magnetic field is explained by the decay of the ferrofluid magnetic susceptibility under an approximately constant degree of droplet elongation.

Magnetic properties of ferrofluid emulsions: the effect of droplet elongation

“…The magnetic permeability of the carrier liquid (oil) is considered to be equal to unity. To describe the magnetic parameters of the ferrofiuid, we use the firstorder modified mean-field model [4], which has been proved as giving excellent agreement with experiments and computer simulations both for moderately concentrated ferrofluids [5][6][7] and for ferrogels [8,9]. In the framework of this model, the ferrofluid magnetization M¿ is given by…”

“…To determine the degree of droplet elongation at each given external field, we use the free energy minimization approach. We assume the droplet free energy as a sum of surface energy and magnetic energy Ho 2/3 (8) where a is the interfacial tension; and the ellipsoid surface area S^ is expressed in terms of the droplet volume V¿ and eccentricity e. Calculating the minimum yields the following expression dS'd(e)/de (9) It is worth noting that the shape of a ferrofluid droplet exposed to an external magnetic field could be also studied within a pressure approach, since the driven force of the droplet elongation is the magnetic pressure. This method was described in [11], and the obtained relation for the case of arbitrary magnetic field is coincident with that of Eq.…”

“…The final set of equations, at each given external field strength Ho determining the internal field Hd, the droplet magnetization Mj, and the droplet eccentricity e, includes Eqs. (1), ( 5) and (9). The ferrofluid emulsion magnetic permeability is calculated based on Eq.…”

Magnetomechanical response of bilayered magnetic elastomers

“…A lot of work has been spent on investigating how such ferrogels mechanically respond to external magnetic fields. In particular, these analyses focused on the nature of the induced shape changes [1,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. It turned out that the spatial distribution of the magnetic particles within a sample can qualitatively influence its response to the external field.…”

Bridging from particle to macroscopic scales in uniaxial magnetic gels

“…The overall deformation occurs on a macroscopic length scale and is characterized, for instance, by the magnitude of the elastic moduli. However, the nature of the mechanical response and other characteristic material properties can strongly depend on the spatial distribution of the magnetic particles within the sample [1,31,[44][45][46][47][48][49][50][51]. Typical particle diameters range from nano-to micrometers and thus involve an intermediate, i.e.…”