Stationary Off-Equilibrium Magnetization in Ferrofluids under Rotational and Elongational Flow

Abstract

The magnetization of a ferrofluid, which is exposed to a flow, was recently proposed to depend on the symmetric velocity gradients (elongational flow). This is demonstrated by an experimental setup, which allows one to evaluate the transport coefficient associated with the elongational flow contribution.

On the microscopic interpretation of the coupling of the symmetric velocity gradient to the magnetization relaxation

“…[1] the flow between two concentric cylinders can be used to determine l 2 : The rotation of the cylinders will create an off-equilibrium component of magnetization M x perpendicular to the direction of the applied magnetic field. It can be determined by measuring the x-component of the magnetic field in the center of the inner cylinder H x by means of a Hall sensor.…”

“…It can be determined by measuring the x-component of the magnetic field in the center of the inner cylinder H x by means of a Hall sensor. Taking into account the demagnetization factor of the fluid layer one obtains [1] …”

“…[1] this phenomenon has been related to a significant contribution of interparticle interaction forming elongated structures in the fluid.…”

“…It has been found [1] that the magnetization dynamics in ferrofluids does not only couple to the vorticity of an existing flow but also to the symmetric velocity gradient v ij ¼ 1 2 ðr i v j À r j v i Þ: In Ref. [1] this phenomenon has been related to a significant contribution of interparticle interaction forming elongated structures in the fluid.…”

Magnetic Soret effect: Application of the ferrofluid dynamics theory

“…This approach, called ferrofluid dynamics (FFD), sparked an impassioned discussion [10,11] about which theory explains better the experimental facts for the reduced viscosity of a MF in an ac magnetic field [12] or for the magnetovortical resonance [13,14]. That new theory also triggered an experiment [15] confirming a proposed nonzero transport coefficient which is zero in the microscopic approach [4,5]. Other proposed effects as shear-excited sound [16,17] await their confirmation yet.…”