Ultrasound-enhanced diffusion and streaming of colloids in porous media

Abstract

Diffusion of colloidal particles in porous media is critical for the development of specially engineered colloids used to aid the recovery of oil from a petroleum reservoir. However, shielded structures, such as dead-end pores, prevent these colloids from reaching the available oil. This article examines the use of ultrasound fields as a mechanism for increasing the mobility of colloidal particles in porous media. Particle tracking of optical video microscopy experiments performed in micromodels of varying porosity are used to construct mean square displacement, from which effective diffusion coefficients and average particle velocities can be measured. We introduce an acoustic field by sweeping a phase space that consists of multiple voltages, frequencies and porosities. Our experiments show that diffusion scales linearly with voltage, while average particle velocity scales quadratically. We found that the diffusion and average velocity peaked near a frequency of ~40 kHz for porosities of 0.62, 0.76 and 0.9, suggesting the presence of a natural frequency. A second natural frequency with a value close to ~90 kHz was also observed in experiments conducted at 0.62 and 0.76. The effective diffusion at 40 kHz has a maximum value ~0.4 μm2/s, which is a factor of 5.5 times greater than in the absence of an acoustic field. Outside of the 40 kHz resonance peak, diffusion remains enhanced, with a measured coefficient of ~0.15 μm2/s. This is still a factor of 2 greater than when no acoustic field is present. Overall, our results demonstrate that ultrasound enhances the transport of colloidal particles through porous media. Future work aims to use these results as the foundation for improved modelling of acoustically stimulated transport through porous media as well as applying these results to problems related to particulate separation.