Acoustic wave in a suspension of magnetic nanoparticle with sodium oleate coating.

The ultrasonic propagation in the water-based magnetic fluid with doubled layered surfactant shell was studied. The measurements were carried out both in the presence as well as in the absence of the external magnetic field. The thickness of the surfactant shell was evaluated by comparing the mean size of magnetic grain extracted from magnetization curve with the mean hydrodynamic diameter obtained from differential centrifugal sedimentation method. The thickness of surfactant shell was used to estimate volume fraction of the particle aggregates consisted of magnetite grain and surfactant layer. From the ultrasonic velocity measurements in the absence of the applied magnetic field, the adiabatic compressibility of the particle aggregates was determined. In the external magnetic field, the magnetic fluid studied in this article becomes acoustically anisotropic, i.e., velocity and attenuation of the ultrasonic wave depend on the angle between the wave vector and the direction of the magnetic field. The results of the ultrasonic measurements in the external magnetic field were compared with the hydrodynamic theory of Ovchinnikov and Sokolov (velocity) and with the internal chain dynamics model of Shliomis, Mond and Morozov (attenuation).

Structuring from nanoparticles in oil-based ferrofluids.

The effect of magnetic field on the structure formation in an oil-based magnetic fluid with various concentrations of magnetite particles was studied. The evaluation of the experimental data obtained from small-angle X-ray scattering and ultrasonic attenuation indicates the formation of chain-like aggregates composed of magnetite particles. The experimental data obtained from ultrasonic spectroscopy fit well with the recent theoretical model by Shliomis, Mond and Morozov but only for a diluted magnetic fluid. In this model it is assumed that a dimer is the main building block of a B -field-induced chain-like structure, thus the estimation of the nematic order parameter does not depend on the actual length of the structure. The scattering method used reveals information about the aggregated structure size and relative changes in the degree of anisotropy in qualitative terms. The coupling constant [Formula: see text] , concentrations [Formula: see text] , average particle size d and its polydispersity [Formula: see text] were initially obtained using the vibrating sample magnetometry and these results were further confirmed by rheometry and scattering methods. Both the particles' orientational distribution and the nematic order parameter S were inferred from the ultrasonic measurements. The investigation of SAXS patterns reveals the orientation and sizes of aggregated structures under application of different magnetic-field strengths. In addition, the magnetic-field-dependent yield stress was measured, and a relationship between the yield stress and magnetic-field strength up to 0.5 T was established.

The effect of temperature on the electric conductivity of poly(dimethyl siloxane) ferromagnetic gel.

In this paper the influence of temperature on the electrical conductivity of a ferromagnetic gel is investigated. The material used was poly(dimethyl siloxane) (PDMS) gel which contained randomly distributed magnetite nanosized particles. The electrical conductivity was measured by means of the two-point dc method. During the heating of the PDMS in the temperature range of 295-460 K the electrical conductivity increased from about 2 × 10(-12) to 2 × 10(-8) S m(-1). A study of the current-temperature dependence obtained during subsequent heating runs revealed two subranges of temperature characterized by different activation energies. The presence of these subranges could be explained either by the liberation of two different types of charge carrier or by the increase in the degree of polymer cross-linking. In the upper temperature subrange (420-460 K) both types of charge carrier probably contribute to the electrical conductivity of PDMS ferromagnetic gel.

Application of the ultrasonic waves in structural investigation of ferrofluid.

It has been established that structural changes in a ferrofluid (formation of clusters) upon the effect of an external magnetic field can be effectively studied by ultrasound spectroscopy. In particular, the resonance absorption of ultrasonic waves enables detection and determination of the size of spherical clusters. This paper reports results of a study of changes of the size of clusters formed in a magnetic field as a function of the rate of the magnetic field intensity changes, the ferrofluid temperature, the frequency of the ultrasound wave and the angle between the magnetic field vector and the direction of ultrasound wave propagation.

Investigation of magnetic fluids by ultrasonic and magnetic methods

The influence of magnetic field on the acoustic and magnetic properties of magnetic liquids is discussed. By fitting the curve of Taketomi's theory to the experimental data of the anisotropy of ultrasonic attenuation, the values of quantities describing the structure of magnetic liquids have been determined. Moreover, the dependence of magnetic susceptibility on frequency has been measured. It shows that two processes of magnetization, based on the mechanisms proposed by Brown and Neel, contribute to the magnetization of the magnetic liquid.

The effect of the rate of magnetic field and temperature changes on the ultrasonic wave absorption coefficient in a magnetic fluid

The rate of external magnetic field changes as well as temperature influences the structure of a magnetic fluid. This work presents experimental results on changes in the ultrasonic wave absorption coefficient of EMG-605 water-based magnetic fluid as a function of the external magnetic field intensity for different rates of magnetic field changes, at different temperatures. On the basis of these studies, radii of spherical clusters formed in the fluid under the influence of an external magnetic field have been determined.

The analysis of frequency of occlusal sounds in patients with periodontal diseases and gnathic dysfunction.

A study is presented of occlusosonograms obtained from healthy subjects and patients with periodontal diseases and gnathic problems. Gnathosonic examinations were carried out by means of a set of equipment whose basic elements were two piezoceramic transducers mounted on a specially made holder and an IBM PC with an ADC card. The evaluation of occlusosonograms was by means of mathematical analysis of the frequency pattern. In pathological stages, the occlusosonograms exhibit more high frequency components. In the case of healthy subjects, the recorded signal took the form of a strong impact of low frequencies and its subsequent exponential decay.

Gnathosonic study of occlusion in patients wearing complete dentures.

This study aimed to analyse occlusiosonograms from the same edentulous patients (a) wearing old complete dentures and (b) after adjustment of the occlusion of new dentures. Occlusiosonograms of subjects with a complete natural dentition and of edentulous patients wearing complete dentures were compared. Gnathosonic examination was carried out by means of piezoceramic transducers linked to an IBM PC/AT computer. Evaluation of occlusiosonograms was by mathematical analysis of frequency. Differences in the characteristic shape of occlusiosonograms and their frequency between the groups were demonstrated. Lack of stabilisation of old dentures was visible in some cases.