Heterogeneity of porous materials and elastomers at different length scales
Final Report Abstract
Although apparently made from the same material, most objects and products are not uniform upon close inspection. In fact it is hard to produce uniform materials. While the structure of wood and rock reveals itself in color changes on the macroscopic scale there are invisible changes hidden on the microscopic scale, for example the variations in the size of the pores which take up water when wetting the object. Similar variations in material properties are found in nearly all objects made from construction materials including polyethylene pipes, where the crystallinity varies across the pipe wall from the temperature gradient prevailing when the pipe is cooled down after extrusion from of the polymer melt. This variation in properties changes during the lifetime of a product, affects its function and ultimately also defines the service life. To understand and improve material properties it is therefore important to be able to look inside objects and image their structures on different length scales, the macroscopic length scale accessible to the human eye and the microscopic length scale related to function. Diagnostic medicine uses X-ray tomography and magnetic resonance imaging to image patients. The same techniques are used in nondestructive materials testing, whereby magnetic resonance is more suitable to study soft matter and fluids in porous media. Also, magnetic resonance is unique in its ways of providing image contrast, that is, in making hidden features visible. In this project new image contrast was made accessible, which assigns details of molecular transport by diffusion restricted in different-size microscopic pores to macroscopic positions within the object. An example, where this is important is in the quest of optimizing the oil recovery from stone saturated with oil and water, the situation most frequently encountered in oil wells. Oil and water in the pores of rock can uniquely be assigned in so-called diffusion-relaxation correlation maps. We have designed methods capable of identifying these different regions in a rock sample by this method in a time saving way by averaging the diffusion-relaxation correlation maps from pixels in the image with similar contrast. Originally we thought of studying polymer samples, but ended up finding a way, which is far more interesting for analyzing drill cores from oil wells. In our studies on porous material models from glass beads, mortar and ceramic we found a way of imaging fluid-filled macroscopic objects with functional microscopic contrast that can be applied to cylindrical samples without the conventional need of pulsing magnetic gradient fields but by rotating the sample instead. This means that we expect that images with diffusionrelaxation correlation contrast can be measured in the future with compact instruments that do not require bulky and heavy gradient amplifiers but only a motor to rotate the cylindrical sample inside the magnet, a simple way well suited for rugged service on a drilling platform or in a geophysical testing laboratory.
Publications
- Relaxation exchange NMR and T2 weighted imaging of porous ceramics, Poster P33, International Conference on Magnetic Resonance Microscopy, Beijing, 14th – 18th August 2011
Y. Zhang, B. Blümich
- Spatially resolved D-T2 correlation NMR of porous media, Poster P63, International Conference on Magnetic Resonance Microscopy, Cambridge, 25th – 29th August 2013
Y. Zhang, B. Blümich
