Tuning colloidal gels by shear

Using a powerful combination of experiments and simulations we demonstrate how the microstructure and its time evolution are linked with mechanical properties in a frustrated, out-of-equilibrium, particle gel under shear. An intermediate volume fraction colloid–polymer gel is used as a model system, allowing quantification of the interplay between interparticle attractions and shear forces. Rheometry, confocal microscopy and Brownian dynamics reveal that high shear rates, fully breaking the structure, lead after shear cessation to more homogeneous and stronger gels, whereas preshear at low rates creates largely heterogeneous weaker gels with reduced elasticity. We find that in comparison, thermal quenching cannot produce structural inhomogeneities under shear. We argue that external shear has strong implications on routes towards metastable equilibrium, and therefore gelation scenarios. Moreover, these results have strong implications for material design and industrial applications, such as mixing, processing and transport protocols coupled to the properties of the final material

The use of X-ray tomography in the study of water repellents and consolidants

To reduce the weathering rate of natural building stones, a wide variety of water repellents and consolidants are commercially available. Although a lot of research is performed on these products, it remains difficult to determine which product is appropriate to use for a certain type of building stone. Each type of building stone has its own petrographic characteristics (mineralogy, texture, ...), leading to typical physical and technical properties which influence its rate of decay. The localisation of the products inside a stone type is not only depending on the properties of the products themselves, but also linked to the texture and structure of the stone. The impregnation depth of the products strongly influences their efficiency and is therefore a key issue in the determination if a product is functional for a certain type of stone. X-ray micro-CT has recently been introduced as a non-destructive material evaluation technique for engineering and geology purposes. The fact that micro-CT can provide information about the internal structure and properties of natural building stones, is a major advantage in the study of their conservation. Firmly linked with more classical research techniques, this non-destructive technique offers an extra dimension to the cultural heritage research. In this paper, non-destructive X-ray computed micro-tomography (micro-CT) turns out to be a powerful tool as it can visualise the presence of water repellents and consolidants inside the stone and can help to detect the influence that these treatments exert on porosity and pore-size distribution. For the visualization of the product its atomic number and density and the amount of product inside the natural building stone is crucial. Besides the contrast in attenuation, the resolution of the micro-CT also needs to be taken into account to obtain a good contrast between stone and product. By doping with 3-bromopropyltrimethoxysilane, more contrast occurs between the stone material and the conservation product. When the amount of product inside the samples is very low and the product is homogeneously spread inside the sample, the doped product is difficult to distinguish from the stone material due to the resolution of the micro-CT. When the amount of doped product inside the stone material reaches a certain threshold value, the dope will appear in the 2D cross-sections derived from micro-CT, creating a good visualization of the products inside the stone samples. (C) 2008 Elsevier B.V. All rights reserved

X-ray micro-CT applied to natural building materials and art objects

In recent years, x-ray micro-CT has become an important tool for investigating all kinds of materials. The technique uses x-ray transmission images of a sample to create a 3D representation of the specimen. Owing to its nondestructive nature, it is especially suited to investigate rare or expensive samples. This article aims to point out the potential x-ray computed tomography (CT) has to offer to the preservation of cultural heritage and the world of art. Research was performed at the UGCT facility of the Ghent University (Belgium), using a 'state of the art' x-ray microtomograph. In this article, the results for two applications of x-ray CT are shown and discussed. The first application is the localization of salts in sandstone and limestone, as a contribution to the European Saltcontrol project. As for the second application, several bronze artifacts were scanned and reconstructed. This allows one to look at the interior composition of the sample and detect possible signs of reparation

Virtual histology by means of high-resolution X-ray CT

Micro-CT is a non-destructive technique for 3D tomographic investigation of an object. A 3D representation of the internal structure is calculated based on a series of X-ray radiographs taken from different angles. The spatial resolution of current laboratory-used micro-CT systems has come down over the last years from a few tens of microns to a few microns. This opens the possibility to perform histological investigations in 3D on a virtual representation of a sample, referred to as virtual 3D histology. The advantage of micro-CT based virtual histology is the immediate and automated 3D visualization of the sample without prior slicing, sample preparation like decalcification, photographing and aligning. This not only permits a drastic reduction in preparation time but also offers the possibility to easily investigate objects that are difficult to slice. This article presents results that were obtained on punch biopsies of horse skin, (dental) alveolus of ponies and chondro-osseous samples from the tarsus of foals studied with the new high resolution micro-CT set-up (HRXCT) at the Ghent University (Belgium) (http://www.ugct.ugent.be). This state-of-the-art set-up provides a 1 micron resolution and is therefore ideally suited for a direct comparison with standard light microscopy-based histology

A combination of synchrotron and laboratory X-ray techniques for studying tissue-specific trace level metal distributions in Daphnia magna

In the field of environmental toxicology, the study on the effects of the presence of transition metals such as Cu, Ni and Zn on the health of pelagic and benthic invertebrates is an important research topic. Fast dynamic 2D micro-XRF scanning and dynamic XRF micro-CT experiments on D. magna, a frequently used ecotoxicological model organism, allow the quantitative investigation of the accumulation of metals within specific organs with microscopic resolution. K-means clustering allows comparison of the concentrations of the elements of interest between different tissues/organs. Principal component analysis allows most of the significant statistical variations in these large and complex multivariate data sets to be explained. Also, by combining SR-XRF and absorption CT data sets using appropriate software packages, it becomes possible to unravel the tissue-specific 2D/3D distribution of metals in-situ within delicate organic samples on the 3-15 mu m resolution level in an essentially non-destructive manner