Lee Hansol D, Ray Kamal K, Tivanski Alexei V
Department of Chemistry, University of Iowa , Iowa City, Iowa 52242, United States.
Anal Chem. 2017 Dec 5;89(23):12720-12726. doi: 10.1021/acs.analchem.7b02755. Epub 2017 Nov 22.
Currently, the impact of various phase states of aerosols on the climate is not well understood, especially for submicrometer sized aerosol particles that typically have extended lifetime in the atmosphere. This is largely due to the inherent size limitations present in current experimental techniques that aim to directly assess the phase states of fine aerosol particles. Herein we present a technique that uses atomic force microscopy to probe directly for the phase states of individual, submicrometer particles by using nanoindentation and nano-Wilhelmy methodologies as a function of relative humidity (RH) and ambient temperature conditions. When using these methodologies for substrate deposited individual sucrose particles, Young's modulus and surface tension can be quantified as a function of RH. We show that the force profiles collected to measure Young's modulus and surface tension can also provide both qualitative and quantitative assessments of phase states that accompany solid, semisolid, and liquid particle phases. Specifically, we introduce direct measurements of relative indentation depth and viscoelastic response distance on a single particle basis at a given applied force to quantitatively probe for the phase state as a function of RH and corresponding viscosity. Thus, we show that the three phase states and phase state transitions of sucrose can be identified and ultimately propose that this technique may also be used to study other atmospherically relevant systems.
目前，气溶胶的各种相态对气候的影响尚未得到充分理解，尤其是对于通常在大气中具有较长寿命的亚微米级气溶胶颗粒。这主要是由于当前旨在直接评估细颗粒气溶胶相态的实验技术存在固有的尺寸限制。在此，我们提出一种技术，该技术利用原子力显微镜，通过使用纳米压痕和纳米威尔海姆方法，作为相对湿度（RH）和环境温度条件的函数，直接探测单个亚微米颗粒的相态。当将这些方法用于沉积在基底上的单个蔗糖颗粒时，杨氏模量和表面张力可以作为RH的函数进行量化。我们表明，为测量杨氏模量和表面张力而收集的力曲线还可以对伴随固体、半固体和液体颗粒相的相态提供定性和定量评估。具体而言，我们引入了在给定外力作用下基于单个颗粒的相对压痕深度和粘弹性响应距离的直接测量，以定量探测作为RH和相应粘度函数的相态。因此，我们表明可以识别蔗糖的三相态和相态转变，并最终提出该技术也可用于研究其他与大气相关的系统。
