Evaporation of alcohol droplets on surfaces in moist air [dataset]

Droplets of alcohol-based formulations are common in applications from sanitising sprays to printing inks. However, our understanding of the drying dynamics of these droplets on surfaces and the influence of ambient humidity is still very limited. Here, we report the drying dynamics of picoliter droplets of isopropanol alcohol (IPA) deposited on a surface under controlled humidity. As relative humidity (RH) increases, we observed a variety of phenomena including enhanced spreading, non-monotonic changes in the drying time, the formation of pancake-like shapes that suppresses the coffee-ring effect, and the formation of water-rich films around an alcohol-rich drop at higher RH. We hypothesize that water vapour in the ambient environment can condense onto alcohol droplets, leading to this unexpectedly complex drying behaviour. We developed a lubrication model that accounts for the coupling between the flow field within the drop, the shape of the drop, and the vapour concentration field. The model reproduces many of the experimentally observed morphological and dynamic features, revealing the presence of unusually large spatial compositional gradients within the evaporating droplet and surface-tension driven flows due to water condensation into the droplet. We further observe rim instabilities at higher RH that are well-described by a model based on the Rayleigh-Plateau instability. Our findings have potential implications for the testing and use of alcohol-based disinfectant sprays and printing inks.

醇基制剂液滴在消毒喷雾、印刷油墨等诸多应用场景中十分常见。然而，目前学界对这类液滴在固体表面的干燥动力学过程以及环境湿度的影响机制的认知仍十分有限。本研究报告了受控湿度条件下，沉积于表面的异丙醇（isopropanol alcohol, IPA）皮升液滴的干燥动力学过程。随着相对湿度（relative humidity, RH）升高，我们观测到一系列现象：液滴铺展程度增强、干燥时间呈现非单调变化、形成可抑制咖啡环效应的薄饼状形貌，以及在高湿度条件下于富酒精液滴周围形成富水薄膜。我们提出假说：环境中的水蒸气可在酒精液滴表面凝结，进而引发此类超出预期的复杂干燥行为。我们构建了一套润滑理论模型，该模型可描述液滴内部流场、液滴形貌与蒸汽浓度场之间的耦合关系。该模型复现了实验中观测到的多数形貌与动力学特征，揭示出蒸发液滴内部存在异常显著的空间组分梯度，以及因水蒸气在液滴内凝结引发的表面张力驱动流。我们还在高湿度条件下观测到液滴边缘不稳定性，该现象可通过基于瑞利-普拉托不稳定性（Rayleigh-Plateau instability）的模型得到良好解释。本研究结果可为醇基消毒喷雾与印刷油墨的测试及应用提供潜在参考。