Physics of Fluids - Publications

Evaporating droplets on oil-wetted surfaces: Suppression of the coffee-stain effect

Open Access
Proceedings of the National Academy of Sciences 117, 16756–16763 (2020)

Authors

	Yaxing Li
	Christian Diddens
	Tim Segers
	Herman Wijshoff
	Michel Versluis
	Detlef Lohse

BibTeΧ

@article {Li16756,
	author = {Li, Yaxing and Diddens, Christian and Segers, Tim and Wijshoff, Herman and Versluis, Michel and Lohse, Detlef},
	title = {Evaporating droplets on oil-wetted surfaces: Suppression of the coffee-stain effect},
	volume = {117},
	number = {29},
	pages = {16756--16763},
	year = {2020},
	doi = {10.1073/pnas.2006153117},
	publisher = {National Academy of Sciences},
	abstract = {Evaporating suspension droplets in general leave ring-like stains. For many applications requiring homogeneous deposition, the stains imply a major problem. This in particular holds for inkjet printing, surface patterning, self-assembly, and 3D printing technologies. Here we offer a robust method to suppress the coffee-stain effect by letting particle-laden droplets evaporate on oil-wetted surfaces. Through tuning the surface energy of the droplets by adding surfactants, we can manipulate the contact line behavior, the flow structure, and the wetting state of the thin film surrounding the droplet and accomplish a controlled particle deposition. Our findings may open perspectives for uniform coatings in the abovementioned applications, but are also relevant for other complex liquid droplet evaporation, e.g., for disease transmission or in agriculture.The evaporation of suspension droplets is the underlying mechanism in many surface-coating and surface-patterning applications. However, the uniformity of the final deposit suffers from the coffee-stain effect caused by contact line pinning. Here, we show that control over particle deposition can be achieved through droplet evaporation on oil-wetted hydrophilic surfaces. We demonstrate by flow visualization, theory, and numerics that the final deposit of the particles is governed by the coupling of the flow field in the evaporating droplet, the movement of its contact line, and the wetting state of the thin film surrounding the droplet. We show that the dynamics of the contact line can be tuned through the addition of a surfactant, thereby controlling the surface energies, which then leads to control over the final particle deposit. We also obtain an analytical expression for the radial velocity profile which reflects the hindering of the evaporation at the rim of the droplet by the nonvolatile oil meniscus, preventing flow toward the contact line, thus suppressing the coffee-stain effect. Finally, we confirm our physical interpretation by numerical simulations that are in qualitative agreement with the experiment.},
	issn = {0027-8424},
	URL = {https://www.pnas.org/content/117/29/16756},
	eprint = {https://www.pnas.org/content/117/29/16756.full.pdf},
	journal = {Proceedings of the National Academy of Sciences}
}