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

细菌生物膜是由嵌入细胞外聚合物质（EPS）的分泌物质网络中的细菌形成的。生活在生物膜中的细菌是许多人类传染病的罪魁祸首，包括囊性纤维化、肺结核、鼻窦炎、蛀牙、心脏病、尿路感染以及受污染的医疗设备和植入物引起的感染。细菌生物膜在食品污染、水净化、温室气体产生和环境污染物分解中发挥着核心作用。处理生物膜的方法包括纳米颗粒或抗生素液体，它们可以被封装在称为脂质体的小蛋白质胶囊中，脂质体可以渗透到生物膜中，并向细菌菌落释放化学物质。研究小组发现，暴露在超声波下可以提高纳米粒子进入生物膜的渗透速度。该合同将支持对提高侵彻能力的潜在流体动力机制进行详细的实验和理论研究。研究人员提出，超声波产生的振荡流动加上粒子在多孔介质中受阻的运动，导致了一种称为振荡扩散的新型扩散传输。该奖项将支持测量超声波对水凝胶（一种生物膜模型）中纳米颗粒扩散增强效果的实验，并研究单个颗粒的详细运动。将建立一个数学模型来把这两个实验的结果联系起来。这个项目的目标是提高对振荡扩散现象的理解，在振荡扩散现象中，观察到声波激励可以增强多孔介质中颗粒的扩散。目标应用包括使用超声波来增强纳米颗粒和脂质体在生物膜中的扩散。生物膜中颗粒和化学物质的主要运输机制是扩散，扩散调节细菌菌落的矿物质和营养物质供应，细菌基因通过细胞群体感应（quorum sensing）进行调控反应。振荡扩散现象最近被研究人员发现，使用中等强度的超声波将充满抗生素化学物质的脂质体注入生物膜，以缓解生物膜。超声波也被证明可以增强纳米粒子对海藻水凝胶的渗透，这是一种模拟生物膜蛋白质基质的水凝胶。该合同将支持两个实验组件和一个建模组件。纳米颗粒体扩散实验将测量超声诱导纳米颗粒在水凝胶中的扩散系数和声流速度。单个颗粒跟踪实验将跟踪受振荡流场影响的多孔床中单个颗粒的运动，以便更好地了解单个颗粒水平上的现象。这些实验研究的数据将用于改进和验证振荡扩散的随机模型。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.