Testing Ferrofluid Impregnation Efficiency in Artificial Samples and Transparent Polymers with Magnetic and Optical Methods: Implications for Magnetic Pore Fabric Studies

Abstract:

Ferrofluid impregnation is important for studying the structure of pores using magnetic pore fabric characterization. Ferrofluids commonly used in this application contain nanoparticles with dimensions of about 10nm in water- or oil-based carrier fluid. Testing the impregnation efficiency in microporous media such as hydrogels gives indication on the lower pore size limit of a material that can be impregnated. Systematic tests for impregnation efficiency indicate which parts of a sample are impregnated to what degree. In this study, the following synthetic samples were prepared to investigate differences in impregnation efficiency related to pore size distribution and porosity: (1) agarose gel samples were synthetized with agarose concentrations of 0.5%, 1%, 1.5%, 2%, (2) TEOS (tetraethoxysilane) aerogel samples were prepared starting from xerogel precursors through CO2 supercritical extraction, (3) synthetic samples made of grains with specific sizes were prepared with different components and different proportions to investigate the influence of those factors on interconnected porosity and on impregnation efficiency. The AMS (anisotropy of magnetic susceptibility) of the unimpregnated samples was measured first using the 15 positions measurement scheme with repeated measurements at 1kHz, 4kHz, 16kHz and field strength of 200A/m and compared later with AMS on ferrofluid-impregnated samples. Impregnation methods include capillary impregnation, vacuum pressure and flowthrough, ferrofluid diffusion into the samples’ pore-water, and diffusion driven with an external permanent magnet, to determine the most convenient impregnation method. The impregnation efficiency in agarose and TEOS samples was tested first by a preliminary visual inspection, which helped detect macroscopic impregnation features, such as cracks. Since the visual technique is nondesctructive, complementary impregnation efficiency measurements were performed by cutting the agarose and TEOS gel samples in small cubes, and visualizing the gradient in impregnation efficiency from the external surface to the core of the cylindrical sample by magnetic susceptibility measurements. This magnetic quantification of impregnation efficiency was the only step applied to non-transparent samples. First results show that (1) ferrofluid diffusion in agarose structure is strongly affected by an external permanent magnet, especially in the direction of strongest field gradient; (2) vacuum enhances the diffusion process, but high vacuum or long impregnation time affect the gel structure; (3) pore sizes close to the ferrofluid nanoparticle size (10nm) can be effectively impregnated by diffusion in pore-water, but in the absence of additional forces the process is relatively slow. This study is important in magnetic pore fabrics applications, because it suggests new impregnation methods (pore-water diffusion, flowthrough and magnet-driven diffusion), provides information on impregnation efficiency and its gradients throughout a sample, and reveals which pore sizes can be impregnated with which method.