Viscoelastic properties of ferrofluids

Abstract

The paper deals with theoretical study of non linear viscoelastic phenomena in ferrofluids placed in magnetic field. Our attention is focused on the study of nonstationary flow and Maxwell-like relaxation of the macroscopical viscous stress after alternation of the shear rate. We propose that these phenomena can be explained by finite rate of evolution of chainlike aggregates, consisting of the ferrofluid particles. Statistical model of the chains growth-disintegration is suggested. In this model the chain-single particle mechanism of the chains evolution is considered, the effects of the chain-chain interaction are ignored. The proposed model allows us to estimate the time-dependent function of distribution over number of particles in the chain. Having determined this function and using methods of hydromechanics of ferrofluids with chainlike aggregates, we have studied evolution of the ferrofluid viscosity after stepwise alternation of the fluid shear rate. The estimated time of relaxation is in a reasonable agreement with experimental results. Thus, our analysis shows that the observed macroscopical viscoelastic phenomena in ferrofluids can be provided by evolution of the chain ensemble.

On the theory of transport phenomena in ferrofluids. Effect of chain-like aggregates

“…[14]. In spite of simplifications, this model has allowed us to describe rheological properties of various magnetic fluids with the single-domain and cluster particles as well [15][16][17][18].…”

Diffusion and magnetotransport in ferrofluids containing chain-shaped aggregates

“…As in [14][15][16][17][18], the energy of the directed dipoledipole interaction between adjacent particles in a chain is noticeably higher than kT. Note that this is the necessary condition for the formation of aggregates in a ferrofluid.…”

“…Here, in order to obtain physically reasonable estimates, we shall simulate a chain by an ellipsoid of revolution with major and minor axes nd and d, respectively. Note that the same model was used in [14][15][16][17][18] to determine the rheologi cal characteristics of a ferrofluid.…”

“…Despite its simplicity, use of this model has resulted in the adequate description of the rheological properties of different magnetic fluids formed from both single domain and clustered parti cles [15][16][17][18].…”

“…Within the framework of this model [14][15][16][17][18], parti cles are considered to be identical to ferromagnetic beads with diameter d and magnetic moment m frozen into the bulk particle. It is assumed that the particles can unite to form chains, which are considered spe cific heterofluctuations of density.…”