Malinowski, Robert Alexander; (2020) Dynamic Control of Evaporating Droplets Using a Point Source of Vapour. Doctoral thesis (Ph.D), UCL (University College London).

Preview	Text Dynamic control of Evaporating Droplets Using a Point Source of Vapour.pdf Download (104MB) | Preview
	Video A.1 A centered ethanol vapour source.avi - Supplemental Material Download (21MB)
	Video A.2 The Coffee Ring Effect.avi - Supplemental Material Download (31MB)
	Video A.3 Flattening of the central particle deposit by the droplet surface.avi - Supplemental Material Download (27MB)
	Video A.4 Lateral offset of an ethanol vapour source.avi - Supplemental Material Download (32MB)
	Video A.5 Patterning “UCL” with an ethanol vapour source.avi - Supplemental Material Download (34MB)
	Video A.6 Attraction of a binary droplet to a water source.avi - Supplemental Material Download (59MB)
	Video A.7 Repulsion of a binary dropelt from a water source.avi - Supplemental Material Download (59MB)
	Video A.8 Linear deposition of poly(vinyl alcohol).avi - Supplemental Material Download (6MB)
	Video A.9 Sinuous deposition of poly(vinyl alcohol).avi - Supplemental Material Download (7MB)
	Video A.10 Patterning “ucl” from a poly(vinyl alcohol) droplet.avi - Supplemental Material Download (5MB)
	Video A.11 Directing a phase change in a polyethylene glycol deposit.avi - Supplemental Material Download (11MB)
	Video A.12 Mixing methyl red and bromothymol blue via combination of mobile droplets.avi - Supplemental Material Download (10MB)
	Video A.13 Mixing HCl and NaOH via combination of pH indicating mobile droplets.avi - Supplemental Material Download (7MB)

Abstract

Over the past two decades, evaporating droplets have received growing attention owing to their ubiquity and hidden complexities. With inkjet printing becoming a more prominent manufacturing technique, due to its scalability and low cost, understanding and controlling droplet behaviour is becoming increasingly valuable. Here, a novel technique is presented, where a local vapour source is used to manipulate the local composition of an evaporating droplet's surface, either inducing surface tension driven flows, or influencing flows already present. In the former case, these flows can both guide material precisely within a droplet and control larger scale droplet phenomena, such as depinning. Ethanol vapours are shown to effectively combat the Coffee Ring Effect, a common hindrance in processes where a solid is deposited from a droplet, as well as provide additional control over deposition. Evaporating droplets of binary liquid mixtures can move freely over surfaces due to inherent surface tension gradients that form within them. Vapour sources are shown to influence and direct their motion. This new method of controlling motion is used to perform a variety of tasks, including deposition of material, with control over its position and morphology, and creating millimetre scale reactors with complex internal flows. The vapour sources used rely on a small amount of solvent, so have minimal impact on droplets beyond the induced flows. The technique is also simple, meaning it easily accessible for further research, and for scaling to manufacturing processes.

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