Zhang Kaiqi, David Grégory, Zhao Yue, Xiao Hua-Yun, Signorell Ruth, Li Chenxi
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 200240 Shanghai, China.
Laboratory of Physical Chemistry, ETH Zürich, CH-8093 Zürich, Switzerland.
Research (Wash D C). 2023 Aug 5;8:0813. doi: 10.34133/research.0813. eCollection 2025.
Optical levitation provides a powerful platform for probing the physicochemical properties of nano- and microparticles. In optical levitation experiments involving nonreacting droplets, metastable states, or so-called "thermally locked" states, can emerge. However, there has been no report on thermal locking induced by chemical reactions or the impact of thermal locking on the reaction mechanisms or rates. Herein, we investigate the growth of optically levitated aqueous droplets in which sulfate forms through the SO-NO and the SO-Mn-O heterogeneous reactions-2 environmentally important sulfate formation pathways. We observe (semi-)discrete droplet growth occurring via consecutive thermally locked states, which result from the competition between water vapor condensation driven by sulfate formation and evaporation driven by droplet heating through resonant absorption of the trapping laser. By combining Mie theory-based analysis of the stimulated Raman scattering and droplet thermodynamics, we develop an approach to retrieve the key properties (e.g., temperature, pH, and molality) of thermally locked droplets and demonstrate that chemistry-driven thermal locking results in a signature particle growth pattern. Comparison of sulfate formation rates in locked versus unlocked droplets further reveals that thermal locking can accelerate chemical reactions or even change the dominant mechanism by promoting photoinduced reaction pathways. As light intensity enhancement within the droplet is localized near the droplet surface, the photoinduced reactions lead to droplet growth patterns similar to those driven by surface reactions. This work uncovers a novel phenomenon emerging from light-droplet interactions, offering a mechanistic framework for leveraging thermal locking to probe droplet properties and study chemical reactions under resonant conditions.
光学悬浮为探测纳米和微米颗粒的物理化学性质提供了一个强大的平台。在涉及不发生反应的液滴、亚稳态或所谓“热锁定”状态的光学悬浮实验中，可能会出现这些状态。然而，尚无关于化学反应引起的热锁定或热锁定对反应机理或速率影响的报道。在此，我们研究了通过SO-NO和SO-Mn-O非均相反应（两种对环境重要的硫酸盐形成途径）形成硫酸盐的光学悬浮水滴的生长情况。我们观察到通过连续的热锁定状态出现（半）离散的液滴生长，这是由硫酸盐形成驱动的水蒸气凝结与通过捕获激光的共振吸收使液滴加热驱动的蒸发之间的竞争导致的。通过结合基于米氏理论的受激拉曼散射分析和液滴热力学，我们开发了一种方法来获取热锁定液滴的关键性质（例如温度、pH值和质量摩尔浓度），并证明化学驱动的热锁定会导致独特的颗粒生长模式。比较锁定和未锁定液滴中硫酸盐的形成速率进一步表明，热锁定可以加速化学反应，甚至通过促进光诱导反应途径改变主导机制。由于液滴内的光强增强集中在液滴表面附近，光诱导反应导致的液滴生长模式类似于由表面反应驱动的模式。这项工作揭示了一种由光 - 液滴相互作用产生的新现象，为利用热锁定来探测液滴性质和研究共振条件下的化学反应提供了一个机理框架。
