Anisotropy of the magnetoviscous effect in a ferrofluid with weakly interacting magnetite nanoparticles.

The anisotropy of the magnetoviscous effect of a ferrofluid has been studied in a specially designed slit die viscometer, which allows three distinct orientations of the magnetic field with respect to the fluid flow. The corresponding Miesowicz viscosity coefficients were determined in dependence of the shear rate and the magnetic field intensity to gain a comprehensive magnetorheological characterization of the fluid. The particles in the fluid have a mean diameter of 13 nm corresponding to an interaction parameter of λ ≈ 1.3 for magnetite. Thus, the fluid can be expected to show a transition from non-interacting individual particles to microstructures with chain-like associated particles when the magnetic field intensity is increased and the shear rate is decreased. The observed field and shear dependent anisotropy of the magnetoviscous effect is explained coherently in terms of these microstructural changes in the fluid.

The influence of hydrodynamic diameter and core composition on the magnetoviscous effect of biocompatible ferrofluids.

Suspensions of magnetic nanoparticles have received increasing interest in the biomedical field. While these ferrofluids are already used for magnetic resonance imaging, emerging research on cancer treatment focuses, for example, on employing the particles as drug carriers, or using them in magnetic hyperthermia to destroy diseased cells by heating of the particles. To enable safe and effective applications, an understanding of the flow behaviour of the ferrofluids is essential. Regarding the applications mentioned above, in which flow phenomena play an important role, viscosity under the influence of an external magnetic field is of special interest. In this respect, the magnetoviscous effect (MVE) leading to an increasing viscosity if an external magnetic field of a certain strength is applied, is well-known for singlecore ferrofluids used in the engineering context. In the biomedical context, multicore ferrofluids are preferred in order to avoid remanence magnetization and to enable a deposition of the particles by the organism without complications. This study focuses on a comparison of the MVE for three ferrofluids whose composition is identical except in relation to their hydrodynamic diameter and core composition-one of the fluids contains singlecore particles, while the other two feature multicore particles. This enables confident conclusions about the influence of those parameters on flow behaviour under the influence of a magnetic field. The strong effects found for two of the fluids should be taken into account, both in future investigations and in the potential use of such ferrofluids, as well as in manufacturing, in relation to the optimization of flow behaviour.

Anisotropy of the magnetoviscous effect in ferrofluids containing nanoparticles exhibiting magnetic dipole interaction.

The aim of this work has been the investigation of the anisotropy of the viscosity of a ferrofluid with magnetically interacting particles which are able to form structures in an applied magnetic field. The results of the experiments show a significant deviation from the case of a fluid without strong dipolar interactions. Furthermore, we have determined the dependence of the ratio of the viscosity coefficients on shear rate providing an insight into the microstructural reasons for the observed effects.

X-ray microcomputed tomography as a tool for the investigation of the biodistribution of magnetic nanoparticles.

Computed tomography is a widely used technique to study the inner structure of opaque samples using the material-dependent attenuation of x-rays. Microcomputed tomography follows the same principles used for conventional medical CT scanners, but improves the spatial resolution to a few micrometers. As an example for the application of x-ray microtomography, the study of the 3D biodistribution of magnetic nanoparticles in tumoral tissue after minimal invasive cancer therapy, which is one of the crucial factors for this kind of therapy, is presented in this article. In particular, the possibilities and problems resulting from the use of different sources of radiation--synchrotron and x-ray tubes, respectively--will be discussed.

Magnetic measurements on frozen ferrofluids as a method for estimating the magnetoviscous effect.

Magnetic measurements on frozen ferrofluids with and without significant structure formation in an applied magnetic field have been performed. The results of these investigations were compared with the magnetic field dependent rheological properties for two different kinds of ferrofluids. Magnetic experiments performed similarly to conventional field cooled-field warming magnetic tests show the contribution of magnetic domain blocking and structure reorganization to the rheology of ferrofluids. Our efforts have shown the possibility of giving an estimate of the magnetoviscous effect by considering the temperature dependence of the magnetization of a frozen sample.

Parametric modulation of thermomagnetic convection in magnetic fluids.

Previous theoretical investigations on thermal flow in a horizontal fluid layer have shown that the critical temperature difference, where heat transfer changes from diffusion to convective flow, depends on the frequency of a time-modulated driving force. The driving force of thermal convection is the buoyancy force resulting from the interaction of gravity and the density gradient provided by a temperature difference in the vertical direction of a horizontal fluid layer. An experimental investigation of such phenomena fails because of technical problems arising if buoyancy is to be changed by altering the temperature difference or gravitational acceleration. The possibility of influencing convective flow in a horizontal magnetic fluid layer by magnetic forces might provide us with a means to solve the problem of a time-modulated magnetic driving force. An experimental setup to investigate the dependence of the critical temperature difference on the frequency of the driving force has been designed and implemented. First results show that the time modulation of the driving force has significant influence on the strength of the convective flow. In particular a pronounced minimum in the strength of convection has been found for a particular frequency.

Capillary viscosimetry on ferrofluids.

Experiments performed for different ferrofluids under shear flow have shown that an increase of the magnetic field strength applied to the sample yields an increase of the fluid's viscosity, the so called magnetoviscous effect. It has been shown that the magnitude of the effect is strongly related to the modification of the microstructure of ferrofluids and can be influenced by varying both the dipole-dipole interaction between the particles and the concentration of large particles within the fluid. This result has been further used to synthesize new ferrofluids which, on one hand, are more compatible for technical applications but, on the other hand, led to difficulties for the experimenters in measuring the viscous behavior in the presence of a magnetic field. To overcome this problem, a specially designed ferrofluid-compatible capillary viscometer has been developed. Within this paper, the experimental setup as well as experimental results concerning the investigation of the magnetoviscous effect in both diluted and concentrated cobalt-based ferrofluids are presented.

Microcomputed tomography analysis of ferrofluids used for cancer treatment.

In order to reduce the side effects generated by the most common cancer treatment therapies, chemo- and radiotherapy, two new approaches are being investigated. These new approaches are magnetic drug targeting (MDT) and magnetic hyperthermia, and are based on the use of magnetic nanoparticles. In the first one, these magnetic nanoparticles are used as drug carriers and the success of the treatment depends on the correct distribution of the drug within the tumour tissue. Computed tomography analysis has been performed on tumour tissue after MDT in order to find out the distribution of the nanoparticles. The measurements have been carried out in two different laboratories, one based on a synchrotron beamline and another one with a cone x-ray source. First results show that the drug carriers form clusters within the tumour tissue.

In situ observation and numerical calculations of the evolution of metallic foams.

During the past two decades the drainage behaviour and temporal evolution of aqueous foams has been subjected to intensive research activities. For the in situ monitoring of liquid metallic foams, it is not possible to use the established drainage observation methods employed for aqueous foams. The generally high melting point and conductivity as well as the opaque nature of these systems require the use of high temperature furnaces in combination with radiography techniques based on x-rays or neutrons. Due to these experimental difficulties, the data from a direct in situ observation of the material redistribution in liquid metallic foams has not been tested quantitatively with numerical solutions from any of the existing drainage models. In two recent studies the density profiles of solidified aluminium foam columns at different stages of the ageing process have been compared with corresponding numerical solutions of the Plateau channel-dominated drainage equation. However, due to the stochastic nature of the foam structure and its development, it was not possible to achieve a reliable quantitative comparison. In our paper we will show a direct quantitative comparison between experimental observations performed by means of x-ray radioscopy and numerical solutions of an adapted version of the channel-dominated drainage equation. Our results indicate that this theory can in principle be used to predict the temporal evolution during the ageing of metallic foams. By incorporating coalescence effects into the numerical description it can be shown that the material redistribution strongly depends upon the rate at which the pore structure evolves. This is in good agreement with our experimental observations.

Role of interleukin-1beta in the pathogenesis of diabetic retinopathy.

AIM: To examine the role of cytokine interleukin-1beta (IL-1beta) in retinal capillary cell death in diabetes. METHODS: The effect of glucose on the expression of IL-1beta was measured in the bovine retinal endothelial cells. The role of IL-1beta in the accelerated endothelial cell loss was determined by investigating the effect of human recombinant IL-1beta on their apoptosis in normal and high glucose conditions, and was confirmed using interleukin-1 receptor antagonist (IL-1ra). RESULTS: High glucose increased IL-1beta expression by 60% compared with cells incubated in 5 mM glucose (p<0.05). Incubation of cells with IL-1beta increased NO levels by about 80% and activated NF-kappaB by 40%. In the same cells apoptosis was increased by 70% and caspase-3 activity was increased by 40%. Supplementation of IL-1beta in 20 mM glucose medium further increased nitric oxide and NF-kappaB, and accelerated apoptosis, and addition of IL-1ra significantly decreased glucose induced abnormalities and apoptosis. CONCLUSIONS: IL-1beta accelerates apoptosis of retinal capillary cells via activation of NF-kappaB, and the process is exacerbated in high glucose conditions. These studies suggest a possible role of IL-1beta in the development of retinopathy in diabetes, and offer a possible rationale to test IL-1beta receptor antagonists to inhibit the development of diabetic retinopathy.

Invalidation of the Kelvin force in ferrofluids.

Direct and unambiguous experimental evidence for the magnetic force density being of the form MnablaB in a certain geometry-rather than being the Kelvin force MnablaH--is provided for the first time. ( M is the magnetization, H is the field, and B is the flux density.)

Direct detection of domains in phospholipid bilayers by grazing incidence diffraction of neutrons and atomic force microscopy.

The geometry of domains in phospholipid bilayers of binary (1:1) mixtures of synthetic lecithins with a difference in chain length of four methylene groups has been studied by two independent, direct and complementary methods. Grazing incidence diffraction of neutrons provided gel domain sizes of less than 10 nm in both the gel and the coexistence phase of the mixture, while no domains were detected for the fluid phase. For the coexistence region, the neutron data suggest that domains grow in number rather than in size with decreasing temperature. Atomic force microscopy was used to study gel phase size and shape of the domains. The domains imaged by atomic force microscopy exhibit a rather irregular shape with an average size of 10 nm, thus confirming the neutron results for this phase. The good agreement between atomic force microscopy and neutron results, despite the completely different nature of their observables, has potential for the future development of refined models for the interpretation of neutron data from heterogeneous membranes in terms of regularly spaced and spatially extended scatterers.