Gravitational effects on coffee-ring formation during the evaporation of sessile droplets

Journal article

The evaporation of multiple droplets is an interesting and abundant phenomena in nature and industry and as a result has many applications. Most research to this point has focused on studying the isolated case but in reality it is rare for droplets to evaporate on their own. Droplets evaporating in close proximity to each other interact and their dynamics are influenced by this. To study them the evaporation rates must be extracted from each droplet in the array. Traditionally for isolated droplets the volume and evaporation rate has been established by imaging the droplet from the side and measuring its profile, however for multiple sessile droplets in two dimensional arrays not all droplets can be seen simultaneously. A solution to this is to image the arrays from the top but without knowing the droplet heights or contact angles, the volumes cannot be established. In addition to this, it is difficult to dispense droplets repeatably and accurately before they evaporate. Both these challenges have prevented authors from conducting careful experimental investigations of the evaporation. Because of this, theoretical work has moved ahead of experimental work, with several recent analytical models for the evaporation being proposed [1, 2]. In this thesis a technique is developed, validated and optimised to image the droplets simultaneously (chapter 2). Experimental data is then taken for multiple droplets and compared to these analytical models evaluating their performance and demonstrating their limitations. It is found that the theory works well for small arrays of droplets (chapter 3) but cannot capture the dynamics of large arrays or arrays on heated substrates (chapter 4). Exhaled breath droplets are then investigated and it is found that whilst the array is evaporating the larger droplets grow due to the Kelvin effect [3]. The diffusive theory is modified to account for this and a simple mean field model is proposed, capturing the experimental array dynamics well (chapter 5). Finally, direct numerical simulations (DNS) of multiple droplets are conducted and it is shown that the model qualitatively captures the droplet evaporation rates (chapter 6). These simulations make future investigations of the vapour and thermal dynamics of the arrays possible, which are challenging to image and measure experimentally

Authors: Moore, MR; Wray, AW

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Cambridge University Press
• Journal: Journal of Fluid Mechanics
• Volume: 967
• Pages: a26
• Article number: A26
• Publication date: 2023-07-19
• DOI: 10.1017/jfm.2023.493
• EISSN: 1469-7645
• ISSN: 0022-1120
• Language: English
• Keywords: Chemistry; Surface tension; Thermodynamics; Advection; Evaporation; Ring (chemistry); Gravitation; Materials science; Diffusion; Physics; Mass transfer; Coffee ring effect; Classical mechanics; Mechanics