Minimal Size of Coffee Ring Structure

A macroscopic evaporating water droplet with suspended particles on a solid surface will form a ring-like structure at the pinned contact line due to induced capillary flow. As the droplet size shrinks, the competition between the time scales of the liquid evaporation and the particle movement may i...

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Bibliographic Details
Published in:The journal of physical chemistry. B 2010-04, Vol.114 (16), p.5269-5274
Main Authors: Shen, Xiaoying, Ho, Chih-Ming, Wong, Tak-Sing
Format: Article
Language:English
Subjects:
B: Macromolecules, Soft Matter
Hydrophobic and Hydrophilic Interactions
Microscopy, Electron, Scanning
Nanoparticles - chemistry
Particle Size
Volatilization
Water - chemistry
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Summary:A macroscopic evaporating water droplet with suspended particles on a solid surface will form a ring-like structure at the pinned contact line due to induced capillary flow. As the droplet size shrinks, the competition between the time scales of the liquid evaporation and the particle movement may influence the resulting ring formation. When the liquid evaporates much faster than the particle movement, coffee ring formation may cease. Here, we experimentally show that there exists a lower limit of droplet size, D c, for the successful formation of a coffee ring structure. When the particle concentration is above a threshold value, D c can be estimated by considering the collective effects of the liquid evaporation and the particle diffusive motion within the droplet. For suspended particles of size ∼100 nm, the minimum diameter of the coffee ring structure is found to be ∼10 μm.
ISSN:1520-6106
1520-5207
DOI:10.1021/jp912190v