Thermoreversible Ferrogels

Abstract

Summary: For the first time, thermoreversible ferrogels (FG) by physical gelation of ferrofluids have been described. Finavestan A80B paraffin oil in a concentration range of the gelator KRATON G‐1650 with Cgelator = 3–10 wt.‐%, was used to obtain stable and homogeneous FGs. TEM micrographs revealed that the magnetite particles are preferably located in the ‘free’ paraffin phase between micellar domains of the gelator. So, the magnetite nanoparticles make visible a ‘negative’ picture of the structure of the micellar domains of the gel, which is observed in the pure gel as a ‘positive’ image. The mean diameter of the polystyrene cores is $\overline d$ = 17 nm, and that of the magnetite particles is $\overline d$ = 7 nm.

Measuring the deformation of a ferrogel sphere in a homogeneous magnetic field

“…The ferrofluid was prepared in the classical way [20] with 24.5 w% magnetite per paraffin oil Finavestan A 50 B (Total). The shear viscosity at 20 C of 11 mPa s and the molar mass of 280 (manufacturer information) are lower than that of the earlier used paraffin oil Finavestan A 80 B [20]. Using A 80 B, the resulting ferrogels are too stiff to be remarkably deformed by the magnetic field.…”

Surface instabilities and magnetic soft matter

“…Due to their superparamagnetic behaviour, they can be controlled by externally applied magnetic fields. The same is true for ferrogels [4][5][6][7], with the difference that here the magnetic nano particles are embedded in an elastic (polymer) matrix. This is handy for many upcoming applications, such as soft actuators, magnetic valves, magnetoelastic mobile robots [8], artificial muscles [9] or magnetic controlled drug delivery [10][11][12][13].…”

“…For both the di-and triblock copolymer the styrene content amounts to 30 wt.% (manufacturer information). In comparison with the pure triblock copolymers used in earlier studies [33], the diblock acts here as a plasticiser and lowers the softening temperature to 25 C, according to the falling ball method [6]. Both the ferrofluid and the ferrogel reveal no structure in optical micrographs, i.e.…”

“…Also in our ferrogels, we observe glassy PS micelles arranged in clusters, which are interconnected by bridging chains of the ethylene-butylene middle block of the triblock copolymer gelator used. The size of these domains varies in the range from 60 to 120 nm [6,33]. Next, we apply (3) to our relaxation data.…”

Rosensweig instability of ferrogel thin films or membranes

“…The case of hydrodynamically equivalent bulk fluids, where the gravitational influence on the film or membrane is reduced, will also be treated briefly. The present treatment can also be applied to thermoreversible ferrogels [7] as long as the temperature is below the transition value, and it also contains the case of a simple ferrofluid or a viscoelastic ferrofluid (magnetorheological) film. The complete set of dynamic equations describing isotropic and uniaxial ferrogels has been given recently in [8] and [9], respectively, using the method of generalized hydrodynamics.…”