Testing the structure of magnetic paints with and without superimposed shear

Abstract

SynopsisThe structure development in dispersions of magnetic barium ferrite particles in cyclohexanone with polyvinylchloride wetting resin was tested by oscillatory rheological measurements and orthogonal superposition of steady and oscillatory shear. The optimum dispersion is achieved at the resin concentration c ϭ c 0 , which corresponds to a minimum in the viscoelastic modulii. At c Ͻ c 0 the system is highly elastic, brittle ͑G Ј drops sharply and G Љ goes through a strong maximum at high strain͒, thixotropic and its relaxation spectrum, H(), is dominated by long relaxation times, , which indicates the existence of a strong network. At c Ͼ c 0 the system becomes less elastic, more flexible, less thixotropic and H() is dominated by short , which indicates that the structure formation in this case is dominated by small aggregates rather than a network. By superimposing steady and oscillatory shear we show that the steady shear dramatically break the structure and shifts H() to very short .

New insights on carbon black suspension rheology -- anisotropic thixotropy and anti-thixotropy

“…Orthogonal superposition (OSP), in which a small amplitude oscillation is superposed in the direction perpendicular to that of the primary shear flow, is valuable in measuring shear-controlled structures 29,[51][52][53] , probing anisotropic structures 53,54 , and studying nonlinear viscoelasticity using linear concepts 48,54,55 . While scattering, microscopy, and other methods mentioned earlier can characterize the structure of CB under flow, OSP is useful for studying the transient behavior and investigating the anisotropy in the mechanical properties of materials.…”

Characterization of Concentrated Magnetic Colloids by Measurements of Frequency Dependence of AC Magnetic Susceptibility

“…These are the magnetic analogues of the Cole-Cole and the Cole-Davidson models for the low frequency dependence of the dielectric constant (10,11). In the Cole-Cole model the frequency dependence of the susceptibility is given as (7) ͑͒ ϭ ϱ ϩ 0 Ϫ ϱ 1 ϩ ͑i 0 ͒ ␣ , [4] where the empirical parameter ␣ can assume values between 0 and 1. This empirical equation was originally derived to explain the experimentally observed frequency dependence of the dielectric constant of liquid and solid dielectrics (10).…”

“…cannot be used for studying these magnetic colloids. Only a few experimental techniques such as rheology (4), electroacoustics (5), and transverse susceptibility measurements (6) provide an adequate view inside the magnetic paints, but none of these techniques alone provides a complete picture. The present work is the first attempt to study the properties of concentrated magnetic colloids by measuring the frequency dependence of the AC magnetic susceptibility.…”