Evaporation of alcohol droplets on surfaces in moist air

Yang, Lisong; Pahlavan, Amir A; Stone, Howard A; Bain, Colin D

Abstract

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 isopropyl alcohol (IPA) deposited on a surface under controlled humidity. Condensation of water vapour in the ambient environment onto alcohol droplets leads to unexpectedly complex drying behaviour. 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 suppress the coffee-ring effect, and the formation of water-rich films around an alcohol-rich drop. We developed a lubrication model that accounts for the coupling between the flow field within the drop, the shape of the drop, and the vapor 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-gradient-driven flows arising from water condensation/evaporation at the surface of the droplet. One unexpected feature from the simulation is that water can evaporate and condense concurrently in different parts of the drop, providing fundamental insights that simpler models based on average fluxes lack. We further observed rim instabilities at higher RH that are well-described by a model based on the Rayleigh-Plateau instability. Our findings have implications for the testing and use of alcohol-based disinfectant sprays and printing inks.

Citation

Yang, L., Pahlavan, A. A., Stone, H. A., & Bain, C. D. (2023). Evaporation of alcohol droplets on surfaces in moist air. Proceedings of the National Academy of Sciences, 120(38), Article e2302653120. https://doi.org/10.1073/pnas.2302653120

• Journal Article Type: Article
• Acceptance Date: Aug 1, 2023
• Online Publication Date: Sep 11, 2023
• Publication Date: Sep 19, 2023
• Deposit Date: Aug 11, 2023
• Publicly Available Date: Sep 27, 2023
• Journal: Proceedings of the National Academy of Sciences
• Print ISSN: 0027-8424
• Electronic ISSN: 1091-6490
• Publisher: National Academy of Sciences
• Peer Reviewed: Peer Reviewed
• Volume: 120
• Issue: 38
• Article Number: e2302653120
• DOI: https://doi.org/10.1073/pnas.2302653120
• Public URL: https://durham-repository.worktribe.com/output/1715856