CAREER:Revealing the Complex Fluid Dynamics of Conventional Liquids Using Vibrating Nanoparticles

非技术摘要科学家们已经对水等传统液体在普通时间和长度尺度上的行为有了广泛的了解，并对组成液体的单个分子有了了解。然而，在这两个极端之间，液体的行为仍然知之甚少，阻碍了分子传感器等技术的发展，并限制了对生物分子功能的科学理解。造成这一认识差距的主要原因是缺乏实验方法来探索所涉及的短时间尺度和快时间尺度。首席研究人员最近开发了一种直接访问这一区域的新方法，使用短激光脉冲来监测漂浮在液体中的纳米级金属颗粒的振动。这个项目应用这种实验方法，结合严格的理论模型，对普通液体在纳米尺度和皮秒时间尺度上的不寻常和复杂的性质有了基本的了解。这项研究与教育活动相结合，其总体目标是改善未被充分代表的少数群体的招募、留住和成功，并在自然科学领域转学学生。这涉及三个相互关联的活动：(1)对即将到来的少数族裔和转学物理学生的推广；(2)本科生参与研究项目；以及(3)现代物理实验室课程的重建，以涉及现代教育方法和当代物理主题。该项目的目标是提供对传统液体(如水)与快速移动的固体纳米结构相互作用时产生的非牛顿效应的定量了解。在更大尺度上用于流体动力学问题的简化假设在纳米尺度上被打破了。特别是，传统液体具有纯粘性响应的常见假设不再成立，因为纳米级物体运动的特征时间与液体中的分子弛豫时间相当。本项目使用首席研究员最近开发的一种方法，基于悬浮在液体中的振动金属纳米颗粒的超快激光光谱，对这种复杂的响应进行了实验研究。测量金属纳米颗粒在粘性液体中的千兆赫兹尺度的机械振动，以获得对传统液体的线性、剪切粘弹性响应的定量、唯象描述。实验的扩展涉及到粘弹性响应的更复杂的方面，包括可压缩性、太赫兹频率响应和非线性粘弹性。将实验结果与分子动力学模拟结果进行了比较，以解释微观相互作用如何产生连续体粘弹性响应。通过提供微观的、分子水平的描述和液体性质的整体流体动力学描述之间的联系，该项目解决了理解组成液体的分子之间的微观相互作用如何产生液体中的连续介质行为的巨大挑战。

Non-technical abstractScientists have developed an extensive understanding of how conventional liquids, such as water, behave on ordinary time and length scales, and have also developed an understanding of the individual molecules that make up the liquids. However, the behavior of liquids remains poorly understood between these two extremes, impeding the development of technologies such as molecular sensors, and limiting scientific understanding of the function of biological molecules. The main reason for this gap in knowledge has been the lack of experimental methods to probe the short length scales and fast time scales involved. The principal investigator has recently developed a novel method to access this regime directly, using short laser pulses to monitor the vibrations of nanometer-scale metal particles floating in the liquids. This project applies this experimental method, together with rigorous theoretical models, to develop a fundamental understanding of the unusual and complex properties of ordinary liquids at nanometer length scales and picosecond time scales. The research is integrated with educational activities that have the overall objective of improving the recruitment, retention, and success of underrepresented minorities and transfer students in the physical sciences. This involves three interconnected activities: (1) outreach to incoming minority and transfer physics students; (2) involvement of undergraduate students in the research project; and (3) a rebuilding of the Modern Physics Laboratory course to involve modern educational approaches and contemporary physics topics.Technical abstractThe objective of this project is to provide a quantitative understanding of the non-Newtonian effects that arise in conventional liquids, such as water, when they interact with a rapidly moving solid nanostructure. Simplifying assumptions that are used for fluid-dynamics problems at larger scales break down at the nanometer scale. In particular, the common assumption that conventional liquids have a purely viscous response no longer holds, because the characteristic times for the motion of nanoscale objects are comparable to molecular relaxation times in the liquids. This project investigates this complex response experimentally using a method, recently developed by the principal investigator, based on ultrafast laser spectroscopy of vibrating metal nanoparticles suspended in the liquids. Measurements of the gigahertz-scale mechanical vibrations of metal nanoparticles in viscous liquids are used to obtain a quantitative, phenomenological description of the linear, shear viscoelastic response of conventional liquids. Extensions of the experiments access more complex aspects of the viscoelastic response, including compressibility, terahertz-frequency response, and nonlinear viscoelasticity. The experimental results are compared to molecular-dynamics simulations in order to explain how the continuum viscoelastic response emerges from microscopic interactions. By providing a link between microscopic, molecular-level descriptions and bulk, fluid-dynamical descriptions of liquid properties, the project addresses the grand challenge of understanding how continuum behavior in liquids emerges from microscopic interactions among the molecules that make up the liquid.