Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies

Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies

 

Michele Pugnetti*, Yi Zhou*, Andrea R. Biedermann*

* Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland (michele.pugnetti@geo.unibe.ch)

 

In AMS (anisotropy of magnetic susceptibility)-based pore fabric studies, the role of ferrofluid impregnation is crucial to ensure significant magnetic measurements. However, no standard methods to test the ferrofluid impregnation of porous media have been proposed so far. The details of fluid behaviour in porous media are important in many fields of natural sciences, but nanoparticle distribution in the fluid is particularly important for magnetic measurements. In this study methods to test the impregnation efficiency of ferrofluid in porous media, and nanoparticle distribution are proposed, using different materials: wood, agarose and TEOS (tetraethylorthosilicate) gel, and synthetic samples of given composition and grain size, as well as natural rocks. Magnetic pore fabric measurements are normally performed on natural porous samples to correlate the direction of maximum magnetic susceptibility with the direction of preferred pore elongation, and preferred flow direction. The advantage of using artificial samples is the possibility to control and adjust some physical parameters, including porosity and pore size, to keep them more uniform or fix them to a given value. This allows investigating the nanoparticle distribution in ideal samples without the influence of additional heterogeneities inherent to natural samples and to determine the lowest porosity value and smallest pore size that is possible to impregnate with ferrofluid. In particular, the agarose and TEOS gel have a uniform porous structure controlled by the gel concentration or chemical agents used in sample preparation. The wood has a wider range of porosity compared to rocks and a known intrinsically anisotropic structure. The synthetic samples have a uniform grain size, mineralogy and structure. First the porosity of the samples was measured, then to impregnate the samples different methods were developed and tested, (1) percolation, (2) standard vacuum impregnation, (3) flow-through impregnation, (4) diffusion process in gel structure. Impregnation efficiency was evaluated both optically and magnetically. Different impregnation methods provide different impregnation efficiency depending also on the investigated material; in particular porosity plays an important role in limiting the impregnation efficiency. Initial experiments indicate that in general, flow-through impregnation is more efficient than vacuum impregnation because it combines the effect of vacuum with the pressure applied to the fluid that is pushed through the sample. The best results on natural samples were obtained using calcarenites with relatively high porosity. These results and the methods proposed here will help advance magnetic pore fabrics studies and impregnation processes in general.