International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan.
Phys Chem Chem Phys. 2019 Jan 17;21(3):1046-1058. doi: 10.1039/c8cp04504f.
The study of vapor absorption into liquid desiccant droplets is of general relevance to a better understanding and description of vapor absorption phenomena occurring at the macroscale as well as for practical optimization of dehumidification and refrigeration processes. Hence, in the present work, we provide the first systematic experimental study on the fundamentals of vapor absorption into liquid desiccant at the droplet scale, which initiates a novel avenue for the research of hygroscopic droplet growth. More specifically we address the behavior of lithium bromide-water droplets on hydrophobic PTFE and hydrophilic glass substrates under controlled ambient conditions. Driven by the vapor pressure difference between the ambient air and the droplet interface, desiccant droplets absorb water vapor and increase in volume. To provide further insights on the vapor absorption process, the evolution of the droplet profile is recorded using optical imaging and relevant profile characteristics are extracted. Results show that, even though the final expansion ratio of droplet volume is only a function of relative humidity, the dynamics of contact line and the absorption rate are found to differ greatly when comparing data with varying substrate wettability. Droplets on hydrophilic substrates show higher absorption kinetics and reach equilibrium with the ambient much faster than those on hydrophobic substrates. This is attributed to the absorption process being controlled by solute diffusion on the droplet side and to the shorter characteristic length for the solute diffusion on hydrophilic substrates. Moreover, the apparent droplet spreading process on hydrophilic substrates when compared to hydrophobic ones is explained based on a force balance analysis near the triple contact line, by the change of liquid-vapor surface tension due to the increase in water concentration, and assuming a development of a precursor film.
研究蒸汽在液体干燥剂液滴中的吸收对于更好地理解和描述宏观尺度上发生的蒸汽吸收现象以及实际优化除湿和制冷过程具有普遍意义。因此，在本工作中，我们首次在液滴尺度上对蒸汽吸收到液体干燥剂的基本原理进行了系统的实验研究，这为吸湿液滴生长的研究开辟了新的途径。更具体地说，我们研究了在控制环境条件下，在疏水 PTFE 和亲水玻璃基底上的溴化锂-水液滴的行为。在环境空气和液滴界面之间的蒸气压差的驱动下，干燥剂液滴吸收水蒸气并增加体积。为了进一步了解蒸汽吸收过程，使用光学成像记录液滴轮廓的演化，并提取相关轮廓特征。结果表明，尽管最终的液滴体积膨胀比仅取决于相对湿度，但当比较具有不同基底润湿性的数据时，接触线的动力学和吸收速率却有很大差异。亲水基底上的液滴表现出更高的吸收动力学，并且比疏水基底上的液滴更快地达到与环境的平衡。这归因于吸收过程受溶质在液滴侧的扩散控制，并且在亲水基底上溶质扩散的特征长度较短。此外，与疏水基底相比，亲水基底上的液滴表观扩展过程可以通过在三相接触线附近的力平衡分析来解释，这是由于水浓度增加导致的液体-蒸汽表面张力的变化，并假设形成了一个前导膜。
