Department of Chemistry, McGill University, Montreal, QC H3A 0B8, Canada.
Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, QC H3A 0B9, Canada.
Proc Natl Acad Sci U S A. 2019 Oct 1;116(40):19880-19886. doi: 10.1073/pnas.1907687116. Epub 2019 Sep 16.
Advancements in designing complex models for atmospheric aerosol science and aerosol-cloud interactions rely vitally on accurately measuring the physicochemical properties of microscopic particles. Optical tweezers are a laboratory-based platform that can provide access to such measurements as they are able to isolate individual particles from an ensemble. The surprising ability of a focused beam of light to trap and hold a single particle can be conceptually understood in the ray optics regime using momentum transfer and Newton's second law. The same radiation pressure that results in stable trapping will also exert a deforming optical stress on the surface of the particle. For micron-sized aqueous droplets held in the air, the deformation will be on the order of a few nanometers or less, clearly not observable through optical microscopy. In this study, we utilize cavity-enhanced Raman scattering and a phenomenon known as thermal locking to measure small deformations in optically trapped droplets. With the aid of light-scattering calculations and a model that balances the hydrostatic pressure, surface tension, and optical pressure across the air-droplet interface, we can accurately determine surface tension from our measurements. Our approach is applied to 2 systems of atmospheric interest: aqueous organic and inorganic aerosol.
在设计大气气溶胶科学和气溶胶-云相互作用的复杂模型方面取得进展，迫切需要精确测量微观粒子的物理化学性质。光学镊子是一种基于实验室的平台，可以进行此类测量，因为它能够从混合物中分离出单个颗粒。使用动量传递和牛顿第二定律，可以在射线光学范围内从概念上理解聚焦光束捕获和保持单个颗粒的惊人能力。导致稳定捕获的相同辐射压力也会对颗粒表面施加光学变形应力。对于在空气中的微米级水液滴，变形将在几个纳米或更小的量级，通过光学显微镜显然无法观察到。在这项研究中，我们利用腔增强拉曼散射和一种称为热锁定的现象来测量光学捕获液滴中的小变形。借助光散射计算和一个在空气-液滴界面上平衡静水压力、表面张力和光学压力的模型，我们可以从测量结果中准确确定表面张力。我们的方法适用于 2 种大气感兴趣的系统：有机和无机气溶胶水溶液。
