Flow and rheology of interfaces at microscopic length scales

粘弹性界面在自然界中普遍存在；如在肺的细胞膜和肺泡中，并可人工构建；例如在乳剂/泡沫和肥皂膜中。因此，确定它们的流变性对于我们理解发生在这些界面上的许多过程的动力学至关重要。此外，这些知识可能有助于为工业应用设计更好的材料，特别是在食品和化妆品领域。在本项目中，pi将研究使用微观颗粒（直径20 nm - 2微米）作为粘弹性探针，并通过观察颗粒的热布朗运动获得表面粘度和弹性。由于探针的大小，这种方法比目前存在的大多数界面流变学技术更敏感。通过将不同距离的粒子对的布朗运动联系起来，他们还将测量依赖于长度尺度的粘弹性。这将把界面的整体特性与其微观结构和动力学联系起来，这是其他技术所缺乏的方法。他们计划将我们的方法应用于：a)表面粘度极低的系统；特别是在空气-水界面的磷脂分子。目前还不存在低于10-8 Pa-s-m的表面粘度的精确测量。b)具有空间异质域的系统，因此表面流变性取决于这些域的大小。他们将通过对磷脂和脂肪酸施加表面压力来创建这些结构域，并测量其长度尺度依赖的流变性。c)皂膜流体力学；探针粒子的运动产生一个流场，影响其他粒子的运动。通过我们的技术将粒子的运动联系起来，我们将能够测量这个流场。此外，PI计划通过对探测粒子施加已知的力来主动扰动这些界面。颗粒对力的响应所经历的阻力是粘弹性的直接度量。通过对粒子施加大的力，他们将获得界面的非线性响应，这将是至关重要的，也将具有工业意义。广泛影响：粘弹性界面在自然界中无处不在；如在肺的细胞膜和肺泡中，并可人工构建；例如在乳剂/泡沫和肥皂膜中。因此，确定它们的流变性对于我们理解发生在这些界面上的许多过程的动力学至关重要。此外，这可能有助于为工业应用设计更好的材料，特别是在食品和化妆品领域。PI每年至少会为小学生组织一次实地考察。我们过去做过这些；这些实地考察使学生能够亲身体验实验室。特别是，他们计划围绕界面开发新的活动，例如肥皂膜（以及一般的泡沫）。这些实地考察通常需要我们实验室小组的每个人一天的时间（包括准备，与学生互动，以及清理）。PI对本科生的研究有着广泛的承诺，在过去的五年里，我们实验室已经指导了18名本科生研究人员。他们计划通过每年至少与一名本科生合作来继续这项研究，可能每年有两名（因为学生经常在学年期间工作以获得学分）。这些本科生和研究生将学习显微镜技术、计算机数据分析和一些湿实验室化学，为他们选择的任何职业方向提供有用的经验。

0853837Weeks Viscoelastic interfaces are ubiquitous in nature; such as in cell membranes and in the alveoli of the lung, and can be artificially constructed; such as in emulsions/foams and soap films. Determining their rheology is, therefore, critical to our understanding of the dynamics of many processes that occur at these interfaces. Furthermore, this knowledge may help in the design of better materials for industrial applications, particularly in the food and cosmetics sector. In this project, the PIs will investigate the use of microscopic particles (diameters 20 nm - 2 microns) as probes of the viscoelasticity, and obtain the surface viscosity and elasticity by observing the thermal Brownian motion of the particles. Because of the size of the probes, this method is more sensitive than most interfacial rheology techniques that currently exist. By correlating the Brownian motion of pair of particles at different separations, they also will measure length-scale dependent viscoelasticity. This will relate the bulk properties of interfaces with their microstructure and dynamics, an approach that is lacking in other techniques. They plan to apply our method to: a) Systems with extremely low surface viscosity; specifically phospholipid molecules at an air water interface. Accurate measurements of surface viscosities below 10-8 Pa-s-m do not currently exist. b) Systems that have spatially heterogeneous domains, and therefore surface rheology that depends on the size of these domains. They will create these domains by applying surface pressure to phospholipids and fatty acids, and measure their length scale dependent rheology. c) Soap film hydrodynamics; the motion of a probe particle creates a flow field affecting the motion of other particles. Correlating the motion of particles by our technique, we will be able to measure this flow field. Further, The PI plans to actively perturb these interfaces by applying a known force to the probe particles. The drag experienced by the particles in response to the force is a direct measure of the viscoelasticity. By applying large forces to a particle, they will obtain the non linear response of the interface, which will be of fundamental importance and will have industrial relevance as well. Broad Impact: Viscoelastic interfaces are ubiquitous in nature; such as in cell membranes and in the alveoli of the lung, and can be artificially constructed; such as in emulsions/foams and soap films. Determining their rheology is therefore critical to our understanding of the dynamics of many processes that occur at these interfaces. Further, this may help in the design of better materials for industrial applications, particularly in the food and cosmetics sector. The PI will conduct at least one field trip each year, for groups of primary school students. We have done these in the past; these field trips give students hands on laboratory experiences. In particular, they plan to develop new activities around interfaces such as soap films (and foams in general). These field trips generally take everybody in our laboratory group one day of time (including preparation, interacting with the students, and cleaning up). The PI has an extensive commitment to undergraduate research, and has mentored 18 undergraduate researchers in our laboratory over the past five years. They plan to continue this by working with at least one undergraduate per year on this research, and possibly two per year (as often students work during the school year for academic credit). These undergraduates, and also the graduate student involved, will learn microscopy skills, computer data analysis, and some wet lab chemistry, providing useful experiences for whatever career directions they choose.