Oscillatory Diffusion - A New Diffusion Mechanism for Particulates in Porous Media

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A bacterial biofilm is formed of bacteria embedded in a network of a secreted material called extracellular polymeric substance (EPS). Bacteria living in biofilms are responsible for many human infectious diseases, including cystic fibrosis, tuberculosis, sinusitis, tooth decay, heart disease, urinary tract infections, and infections from contaminated medical devices and implants. Bacterial biofilms play a central role in food contamination, water purification, greenhouse gas generation, and breakdown of environmental contaminants. Methods for treating biofilms involve nanoparticles or antibiotic liquids that can be encapsulated in small protein capsules called liposomes, which can penetrate into the biofilm and release a chemical onto the bacterial colonies. The research team has found that exposure to ultrasound can enhance the rate of penetration of nanoparticles into a biofilm. This award will support a detailed experimental and theoretical investigation of the underlying hydrodynamic mechanisms responsible for enhanced penetration. The investigators propose that the combination of oscillatory flow produced by the ultrasound plus hindered motion of the particles by the porous medium through which they travel results in a novel type of diffusive transport termed oscillatory diffusion. The award will support experiments to measure the effect of ultrasound on diffusion enhancement of nanoparticles in a hydrogel, which is a model of a biofilm, and to examine the detailed motion of individual particles. A mathematical model will be developed to connect the results of these two experiments. The goal of this project is to improve understanding of the oscillatory diffusion phenomenon, in which acoustic excitation is observed to enhance diffusion of particulates in a porous medium. The target application involves the use of ultrasound to enhance diffusion of nanoparticles and liposomes in a biofilm. The primary mechanism for transport of particles and chemicals in biofilms is diffusion, which regulates supply of minerals and nutrients to bacterial colonies and bacteria gene regulation response via cell population sensing (quorum sensing). The oscillatory diffusion phenomenon was recently discovered by the investigators using moderate-intensity ultrasound to inject liposomes filled with antibiotic chemicals into a biofilm for biofilm mitigation. Ultrasound was also shown to enhance penetration of nanoparticles into an algar hydrogel, which models a biofilm protein matrix. This award will support two experimental components and one modeling component. A nanoparticle bulk diffusion experiment will measure ultrasound-induced nanoparticle diffusion coefficient and acoustic streaming velocity in a hydrogel. An individual particle tracking experiment will track motion of individual particles in a porous bed subject to an oscillating flow field to better understand the phenomenon at the individual particle level. The data from these experimental studies will be used to improve and validate a stochastic model of oscillating diffusion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.