How robust is the ring stain for evaporating suspension droplets?

MSAMBWA, Y., FAIRHURST, D.J. and OUALI, F., 2013. How robust is the ring stain for evaporating suspension droplets? Interfacial Phenomena and Heat Transfer, 1 (3), pp. 207-214. ISSN 2169-2785

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Abstract

The ring stain is commonly seen when droplets containing particles, such as coffee, are left to dry on a surface: a pinned contact line leads to outward radial flow, which is enhanced by the diverging evaporative flux at the contact line. As shown by Deegan et al. (1997) particles are swept outwards in this flow and create a ring which grows according to a simple power law with time. The final dried width and height of the ring should also be given by power laws of concentration, with both exponent equal to 0.5 provided all particles are in the ring, and the packing factor and ring profile are constant. We use suspensions of polystyrene particles in water with sizes ranging from 200 to 500 nm and initial concentrations c 0 from 0.009% to 1% deposited on glass substrates to investigate these scaling predictions. We vary the drying rate from 0.5 to 5 nl/s using humidity and reduced pressure, use a range of substrates to vary the initial contact angle between 5° and 35°, and invert the droplets to change the direction of gravity. We find that for all but the very lowest pressures, the ring height follows the predicted power law, with exponent equal to 0.50 ± 0.04 and the ring width having an exponent of 0.33 ± 0.05. The discrepancy between the measured and predicted width exponent is accounted for by an observed variation of droplet radius with concentration, and the presence of particles in the center of the droplet. In addition, for low pressures (fast evaporation) the scaling laws no longer hold: the ring is much narrower and there is significant deposition in the center of the droplet, possibly due to reduced particle-enhanced pinning.

Item Type: Journal article
Publication Title: Interfacial Phenomena and Heat Transfer
Creators: Msambwa, Y., Fairhurst, D.J. and Ouali, F.
Publisher: Begell House
Date: 2013
Volume: 1
Number: 3
ISSN: 2169-2785
Identifiers:
NumberType
10.1615/InterfacPhenomHeatTransfer.2013010209DOI
Rights: © Begell House Inc. 2013
Divisions: Schools > School of Science and Technology
Depositing User: EPrints Services
Date Added: 09 Oct 2015 10:15
Last Modified: 09 Jun 2017 13:23
URI: http://irep.ntu.ac.uk/id/eprint/10101

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