Journal
SOFT MATTER
Volume 18, Issue 12, Pages 2414-2421Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/d2sm00142j
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Funding
- Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India [CRG/2020/003643]
- IIT Madras through the Institutions of Eminence (IoE) Scheme, the Ministry of Education, Government of India
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This article investigates the inward drift of a coffee stain, a phenomenon caused by the descent of the water-air interface of a drying drop due to solvent evaporation. The study examines the effect of particle diameter, substrate wettability, and particle concentration on the inward drift of the coffee stain through experiments and model predictions. Additionally, a novel method to estimate the coefficient of friction between particles and the substrate is proposed.
The coffee ring effect, which refers to the formation of a ring-like deposit along the periphery of a dried particle laden sessile drop, is a commonly observed phenomenon. The migration of particles from the interior to the edge of a drying drop as a result of evaporation driven flow directed outwards, is well studied. In this article, we document the inward drift of a coffee stain, which is governed by the descent of the water-air interface of the drying drop due to solvent evaporation. A combination of experimental study and model predictions is undertaken to elucidate the effect of the diameter of particles in the drying drop, the wettability of the substrate on which the drop resides, and the concentration of particles on the inward drift of the coffee stain. This work also suggests a novel method to estimate the coefficient of friction between the particles and the substrate.
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