The dynamic behavior of magnetic colloids in suspension

Abstract

The dynamic behavior of magnetic colloidal particles in suspension is investigated. The particles of the core-shell colloid consist of a cobalt ferrite core embedded in a silica shell and are stabilized by surface charges. As the suspension is strongly opaque to visible light, it can not be probed by dynamic light scattering techniques as a result of strong multiple scattering as well as absorption effects. Therefore, the static and dynamic behavior is probed with small-angle X-ray scattering and X-ray photon correlation spectroscopy (XPCS), respectively. Using XPCS, we are able to study the diffusion coefficient of an opaque colloidal system as a function of the scattering vector. In this paper, we report on the behavior of the colloidal system in the absence of an external magnetic field, meaning that the magnetic moments of the particles are oriented randomly. We find no evidence for magnetic interactions in the static data, while the dynamic XPCS data deviate very significantly from the predictions of model calculations.

Hydrodynamic and electrokinetic effects on the dynamics of charged colloids and macromolecules

“…For systems with added salt, the values of these short-time properties are located in between the zero-salt and infinite-salt (zero-charge) limits. Whereas the majority of experimentally available scattering data on charge-stabilized suspensions is in agreement with the ordering relations in equations (8) [22,23], a violation of these relations has been purportedly observed in certain scattering experiments [25,26]. Colloidal hard spheres have a common static and hydrodynamic length scale given by the particle radius a.…”

Structure and short-time dynamics in suspensions of charged silica spheres in the entire fluid regime

“…No signature of any unusually small H͑q͒ values was found, even for low-salt systems close to the freezing line, as reported by other authors. 10,11 Extensive calculations of the peak values H͑q m ͒ of the hydrodynamic function of fluid systems, characterized by the empiric criterion S͑q m ͒ Ͻ 3.1, in a broad range of and Z values have revealed a characteristic region where all values for H͑q m ͒ can be found. This type of analysis can be of help in the interpretation of existing experimental data and in identifying conditions where either hydrodynamic hindrance or enhancement of collective diffusion may be found.…”

“…The majority of earlier work was concerned with systems showing peak heights in H͑q͒ not far from the corresponding equal-concentration hard-sphere values. 5,6 In some earlier work, strikingly low peak values of H͑q͒ have been reported to occur in dense low-salt systems, [7][8][9][10] an observation which was interpreted initially in terms of hydrodynamic screening. 11 We show that all our experimental short-time diffusion data, obtained for three different series of samples, can be consistently explained by the renormalized density fluctuation expansion method of Beenakker and co-workers, spheres ͑HSs͒ only, but in later work its zeroth-order version was used to make predictions also for charge-stabilized systems.…”

Dynamics of hard sphere suspensions using dynamic light scattering and X-ray photon correlation spectroscopy: Dynamics and scaling of the intermediate scattering function

“…A number of authors have applied XPCS to study the dynamics of suspensions of colloidal particles 2,[7][8][9][10][11] . This builds on extensive studies using light scattering techniques 12,13 , and, more recently, microscopy techniques [14][15][16] .…”