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

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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.