114 Journal of Magnetism and Magnetic Materials 85 ( 1990) 114- 120 North-Holland zyxwvutsrqp PHYSICAL PROPERTIES OF MAGNETIZABLE STRUCTURE-REVERSIBLE MEDIA V.I. KORDONSKY, Z.P. SHULMAN, S.R. GORODKIN, S.A. DEMCHUK, I.V. PROKHOROV, E.A. ZALTSGENDLER and B.M. KHUSID Heat and Mass TransJer Institute, BSSR Academy of Sciences, 220728 Minsk, USSR Consideration is made of the effect of applied magnetic fields on the rheological, magnetic and thermophysical properties of magnetorheological suspensions (MRS). In a field, the shear stress highly increases. and the thermal conductivity grows and becomes anisotropic. Also, specific features of magnetization due to the mechanical mobility of the magnetic moment media are shown. An invariant dependence of viscosity is obtained, the effects of mechanical memory and reversing of shear stresses in a rotating field are revealed. The results are presented on the development of the magnetorheological effect model, on the structuring kinetics and on the methods of controlling the flow, heat and mass transfer. The possibility is shown of the purposeful impact on turbulent flow in an immersed jet and in a channel. Also, the results are given of theoretical and experimental studies of the rheology of systems with a nonmagnetic dispersed phase, i.e. of suspensions of nonmagnetic particles in a magnetic fluid and of particles of high-temperature superconducting ceramic. In both cases. the increase of shear stress in a field can be compared with that in MRS. 1. Introduction A magnetizable structure-revesible medium is a new material with controlled rheological proper- ties. It is a stable mixture of noncolloid magnetic suspended particles (about 1 rJ.m) in the fluid, which are not subjected to the Brownian motion and form rather a stable structure [l] when an applied magnetic field is present. In this case, in a dynamic system consisting of a field-formed struc- ture and of a moving medium interacting with it, each combination of prescribed parameters (vis- cosity of the medium, magnetic properties, volume concentration of particles, field strength, shear rate) is consistent with a certain set of structure elements and with a quite definite morphology of the system, i.e. mutual orientation of elements in space. Such a system induced by an applied field is manifest in nonlinear rheological effects of a system, i.e. in a sharp and controlled increase of effective viscosity, plasticity and viscoelasticity in a field. This property was later called the magne- torheological effect (MRE) and ferrosuspensions which display it, magnetorheological suspensions (MRS). In addition to the changes of mechanical properties, in a magnetic field other properties such as thermophysical, magnetic, optical, acous- 0304~8X53/90/$03.50 B 1990 - Elsevier Science Publishers B.V (North-Holland) tic, etc. sensitive to the state of the structure of the MRS undergo substantial variations. Thus, MRE allows direct electric signal control of the flow, heat and mass transfer, and the electric and mag- netic characteristics of a fluid medium, which, in its turn, provides new processes and equipment. 2. Maguetorheological suspensions Depending on the type and concentration of the dispersed phase, the aggregation and sedimen- tation stability of MRSs is provided both by their direct structuring due to the interaction of the molecular forces of particles and by injection of protective colloid-type substances into a dispersed medium that form structural-mechanical barriers between particles. In what follows, as a rule. the entire medium must be structurized until it attains viscoplastic properties. Provision of stability is a necessary but not yet sufficient condition for a high-effective MRS to exist. Its capability to re- versible tixotropic conversions outside a field is still important. The MRS viscosity must decrease sharply under shear flow conditions. The last situation completely specified by physical and chemical properties of protective colloid is espe-