Instabilities of a Ferrofluid Droplet in Alternating and Rotatong Magnetic Fields
- PDF / 349,678 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 6 Downloads / 201 Views
UNSTABOLOTOES OF A FERROFLUDD DROPLET ON•ALTERNATONG AND ROTATONG MAGHIETOC FOELDS J.-C. BacrO (*), R. PerZynski and D. Salin LaboratoiredAcoustique et Optique de la Matiare Condensge (**), Universitg Pierreet Marie Curie, Tour 13, bofte 78, 4 place Jussieu, 75252 ParisCedex 05, France
A. 0.
6
ebers
Institute of Phycics, Latvian Academy of Sciences, 229021 Riga, Salaspils,Republic of Latvia
ABSTRACT For the first time, response of magnetic fluid microdrops to time dependent magnetic fields are tested in two different experimental conditions (rotating field, crossed static and alternating fields). In both cases, a new shape instability is observed leading to cog-wheels either rotating either motionless. A simple model account for the stability range of oblate ellipsoidal shapes and for the lateral peak instability leading to the cogs which crown the cog-wheel.
INTRODUCTIfON A magnetic fluid (MF) is a colloidal suspension of magnetic particles [ 1,2]. If these particles are large enough, they acts as rigid dipoles: their magnetic moment is locked in the particle easyaxis of magnetization. Due to fluid viscous-drag torque, rapid variations in magnetic field orientation induce a slow rotating response of the particles [3]. A body couple appears, meaning that the viscous stress tensor is no longer symmetric. New phenomena occurs in M.F. owing to this asymmetric stress tensor. Such phenomena have been already observed in some experiments performed with bulk MF, such as immersion of a MF beaker in a rotating magnetic field or any ME flow with some vorticity in a constant magnetic field [4,51. Bulk effects compensate each others and experimentally it is clear that the resulting effect is a surface one. We report here some basic experiments on a magnetic fluid microdrop submitted to a time dependent magnetic field: the first experiment is the drop response to a rotating magnetic field, the second one shows the behavior of such a droplet in two crossed magnetic fields (one is alternating, the other field being steady static). These experiments are performed with MF microdrops to enlarge both magnetic and surface effects. The microdrops are: - spherical in zero field, a few 10 pgm in radius, which allows to neglect gravity, - made of the highly magnetic phase of a demixing ionic ferrofluid, well known for its large magnetic susceptibility (20 X 5 40) and its very low surface tension (a few 10-6 J.m- 2). In both experiments, as magnetic field is increased to a few 100 A/m, the microdrop exhibits spectacular shape instabilities leading whatever the frequency to unexpected cog-wheels. These experimental results are presented in the framework of magneto-hydrodynamic theory of ME.
(*) Affiliation : Universit6 Paris 7 (**) Associated with the Centre National de la Recherche Scientifique Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Materials Research Society
242
A.
EXPERIMENTAL STUDY
An ionic ferrofluid is a colloidal dispersion of monodomain magnetic particles, of typical size 10 nm, in an aqueous solution. The colloidal
Data Loading...
