Induced internal rotation in magnetic fluid composites
M.I. Piso
Romanian Space Agency Research Centre –
GRL
21-25 Mendeleev str., 70168 Bucharest, Romania
piso@rosa.ro
Abstract
Acting with a electromagnetic rotating field on a magnetofluidic composite, rotations of the fluid mass occurred. For appropriate frequencies of the electromagnetic field, the inner rotation of the nonmagnetic component has been observed.
Magnetofluidic composites
Magnetic liquids are as stable suspensions of 50..100 Å monodomain solid magnetic particles in a base liquid whose specific gravity is smaller than the one of the particles. Electrochemical mechanisms ensure sufficient repulsion between the particles to avoid gravitational sedimentation. The physical properties of the magnetic fluids may be even modified and the field of applications enlarged if one deals with the magnetofluidic composites (MFC). MFC are mixtures of magnetic liquid and larger nonmagnetic particles (~100...5000 nm) made of different materials . If the large particles are electroconductive (metal or metal-coated), the resulting macroscopic medium manifests itself as a liquid with special characteristics, especially as an active medium for sensors (Piso, 1990).
Induced rotation
The motion of magnetic fluids in rotating fields has been studied by several authors (for examples, see Journal of Magnetism and Magnetic Materials). A comprehensive review was given by Rosenzweig et al. (1990). Standard experiments (see Rosenzweig et al., 1990) denote rotation ranges up to 200 rpm in fields of 10-3... 10-2 T at external excitation of 100 ... 1000 Hz.
To evidence such rotations, a MFC with 5¸30 μm spherical aluminium inclusions has been used instead of a magnetic fluid. The first experimental setup has derived from a magnetic liquid membrane, held by a permanent annular magnet (barium ferrite, 55 mm mean diameter) and separating two volumes of water, as shown in Figure 1. The system has been excited by means of a rotating magnetic field generated by three coils disposed at 1200 each other. The magnetic liquid employed was based on cyclo-hexanole, 450 Gs saturation magnetization. Exciting the with a variable frequency generator, macroscopic rotations were visually observed in the interval 25 - 55 Hz, with a peak at 38...45 Hz.
To partially clarify the character of inner or macroscopic rotation, the experimental device shown in Figure 2 has been constructed.
Nonmagnetic tungsten particles were levitated by means of two permanent magnets in a container filled with a dielectric magnetic liquid. Applying a variable frequency rotating field and measuring the electromagnetic quality factor of the exciting electromagnets, a minimum has been observed at frequencies of the same order of magnitude as in the previous case. To indirectly evidence the mechanical rotation character of the inner system of non-magnetic inclusions, the inductances which generate the rotating field were connected as components of a parametric circuit. It seemed that, due to the internal loses (viscosity) of the liquid, a permanent effective power consumption appeared. Performing a slight rotation of the container around an axis normal to the rotating field axis, a transient variation of the circuit quality factor appeared.
An explanation has been given, as follows: due to the conservation of the angular momentum, a transient nonparallelism between the particles momenta and the symmetry axis of the magnetic rotating field occurs, and this nonparallelism directs to a variation of the impedance of the sensor. To confirm it, we performed a gravitational calibration of the system, by suspending it as a pendulum, the correlation between the axial angular acceleration and the Q factor has been measured. The results are presented in Figure 3, where an almost linear dependence Q factor by the angular acceleration may be evidenced.
The short conclusion is that, in definite conditions, individual stabile rotations of the inclusions occur in a manner such the liquid seems to acquire a global angular momentum.
The experiment should be performed in precise definite circumstances. One of the main difficulties encountered was to calibrate the system. against mechanical rotation parameters.
It is to point out that such phenomena could be easier studied in microgravity conditions, with durations of more than 3..4 seconds. due to the weaker levitation permanent fields needed. Stronger fields conduct to viscosities which may obscure the phenomena.
References