Université de Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON UMR 5256, 69626 Villeurbanne, France.
Université de Toulouse, CNRS, IMFT INPT-UPS, 31400 Toulouse, France.
Langmuir. 2023 Feb 28;39(8):2957-2965. doi: 10.1021/acs.langmuir.2c02700. Epub 2023 Feb 16.
The formation of liquid cloud droplets from aerosol particles in the Earth atmosphere is still under debate particularly because of the difficulties to quantify the importance of bulk and surface effects in these processes. Recently, single-particle techniques have been developed to access experimental key parameters at the scale of individual particles. Environmental scanning electron microscopy (ESEM) has the advantage to provide in situ monitoring of the water uptake of individual microscopic particles deposited on solid substrates. In this work, ESEM was used to compare droplet growth on pure ammonium sulfate (NH)SO and mixed sodium dodecyl sulfate/ammonium sulfate (SDS/(NH)SO) particles and to explore the role of experimental parameters, such as the hydrophobic-hydrophilic character of the substrate, on this growth. With hydrophilic substrates, the growth on pure salt particles was strongly anisotropic, but this anisotropy was suppressed by the presence of SDS. With hydrophobic substrates, it is the wetting behavior of the liquid droplet that is impacted by the presence of SDS. The wetting behavior of the pure (NH)SO solution on a hydrophobic surface shows a step-by-step mechanism that can be attributed to successive pinning-depinning phenomena at the triple-phase line frontier. Unlike the pure (NH)SO solution, the mixed SDS/(NH)SO solution did not show such a mechanism. Therefore, the hydrophobic-hydrophilic character of the substrate plays an important role in the stability and dynamics of the liquid droplets' nucleation by water vapor condensation. In particular, hydrophilic substrates are not suited for the investigation of the hygroscopic properties (deliquescence relative humidity (DRH) and hygroscopic growth factor (GF)) of particles. Using hydrophobic substrates, data show that the DRH of (NH)SO particles is measured within 3% accuracy on the RH and their GF could indicate a size-dependent effect in the micrometer range. The presence of SDS does not seem to modify the DRH and GF of (NH)SO particles. This study shows that the water uptake on deposited particles is a complex process but, once carefully taken into account, ESEM is a suitable technique to study them.
从地球大气中的气溶胶颗粒形成液体云滴仍然存在争议，特别是因为难以量化这些过程中体相和表面效应的重要性。最近，已经开发出了单颗粒技术来获取单个颗粒尺度上的实验关键参数。环境扫描电子显微镜（ESEM）具有提供在固体基底上沉积的单个微观颗粒水吸收原位监测的优势。在这项工作中，ESEM 被用于比较纯硫酸铵（NH 4 ）SO 和混合十二烷基硫酸钠/硫酸铵（SDS/（NH 4 ）SO）颗粒上液滴的生长，并探索实验参数的作用，例如基底的疏水-亲水特性，对这种生长的影响。对于亲水基底，纯盐颗粒上的生长具有强烈的各向异性，但 SDS 的存在抑制了这种各向异性。对于疏水基底，受 SDS 存在影响的是液滴的润湿行为。纯（NH 4 ）SO 溶液在疏水表面上的润湿行为显示出逐步的机制，可归因于三相线前沿的连续钉扎-去钉扎现象。与纯（NH 4 ）SO 溶液不同，混合 SDS/（NH 4 ）SO 溶液没有表现出这种机制。因此，基底的疏水-亲水特性在水蒸气冷凝导致的液滴成核的稳定性和动力学中起着重要作用。特别是，亲水基底不适合研究颗粒的吸湿性（吸湿相对湿度（DRH）和吸湿生长因子（GF））。使用疏水基底，数据表明，（NH 4 ）SO 颗粒的 DRH 在 RH 上以 3%的精度测量，其 GF 可以在微米范围内指示尺寸依赖性效应。SDS 的存在似乎不会改变（NH 4 ）SO 颗粒的 DRH 和 GF。本研究表明，沉积颗粒上的水分吸收是一个复杂的过程，但一旦仔细考虑，ESEM 是研究这些过程的合适技术。
