Acoustic Streaming Flows Induced by Microbubbles in Viscoelastic Fluids: Fundamentals and Applications to Micro-Rheometry

向水中加入少量的聚合物可以使溶液变成弱弹性流体，既表现出粘性的类液体行为，又表现出弹性的类固体行为。弹性行为是由于聚合物在流动中的拉伸和盘绕。即使是较弱的弹性也会对流体行为产生深远的影响，这在许多工业应用中是必不可少的，从喷洒和农药沉积到管道流动的减阻。为了在这些应用中达到预期的效果，精确测量流体粘度和弹性至关重要。虽然粘度很容易测量，但弱弹性流体的弹性一直是测量的挑战。测量弹性的一种潜在方法是利用由在声场中振动的微型气泡引起的稳定流动。气泡引起的流动模式将根据声波频率和流体性质而变化，从而可以推断流体的弹性。该奖项的主要目的是深入了解粘弹性流体中控制微泡诱导流动的关键因素。该奖项还将纳入重要的教育活动，通过本科研究项目传播微流体的应用以及粘弹性流体的概念，让少数民族和代表性不足的学生参与研究，以及针对初高中学生的外展模块。精确测量低粘度弹性流体的松弛时间仍然是一项具有挑战性的任务。该奖项将通过声学驱动微气泡来解决这一挑战。气泡产生的稳定流动取决于驱动声学和流体弹性，这表明基于微气泡的流变仪测量小松弛时间是可行的。在本研究中，pi将结合实验，模拟和理论建模来阐明弹性与声流之间的相互作用：1)进行系统的实验来表征气泡动力学和流动行为；2)基于渐近方法推导出适当的理论来理解流体弹性和声学振荡的相互作用；3)通过与任务1的结果验证模拟来建立精确预测流动模式的数值模型；4)利用任务3中开发的模型来设计新型的片上流变仪设备。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。

Adding minute amounts of polymers into water can turn the solution into a weakly elastic fluid, which exhibit both viscous, liquid-like, and elastic, solid-like behaviors. The elastic behavior is due to stretching and coiling of polymers under flow. Even weak elasticity can have a profound effect on fluid behaviors, which are essential in many industrial applications, ranging from spraying and pesticide deposition to drag reduction in piping flows. To achieve the desired outcomes in those applications, it is critical to accurately measure the fluid viscosity and elasticity. While viscosity is easy to measure, the elasticity of weakly elastic fluids has been challenging to measure. One potential method to measure elasticity is to use steady flows caused by a micro-sized bubble that is vibrating in an acoustic field. The flow patterns that are induced by the bubble will change depending on the acoustic frequency and the fluid properties, allowing inferring of the fluid elasticity. The principal aim of this award is to provide a deep understanding of the key factors governing the microbubble induced flows in a viscoelastic fluid. This award will also incorporate significant educational activities, which disseminate applications of microfluidics as well as concept of viscoelastic fluids through undergraduate research projects, engagement of minority and underrepresented students into research, and outreach modules that are aimed at middle and high school students.Accurate measurement of relaxation times of low-viscosity elastic fluids is still a challenging task. This award will tackle this challenge by using acoustically driven microbubbles. The steady streaming flows arising from the bubble are shown to depend on the driving acoustics and fluid elasticity, suggesting feasible microbubble-based rheometry devices for measuring small relaxation times. In this research, the PIs will combine experiments, simulations, and theoretical modeling to elucidate the interplay between elasticity and acoustic streaming flows by: 1) conducting systematic experiments to characterize the bubble dynamics and streaming flow behavior, 2) deriving appropriate theory based on the asymptotic method to understand the interaction of the fluid elasticity and acoustic oscillations, 3) establishing numerical models to accurately predict the flow patterns, by validating the simulations with the outcome of Task 1, and 4) engineering novel rheometer-on-a-chip devices using the developed models in task 3.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.