Cross-sectional tracking of particle motion in evaporating drops: flow fields and interfacial accumulation.

Trantum JR, Eagleton ZE, Patil CA, Tucker-Schwartz JM, Baglia ML, Skala MC, Haselton FR
Langmuir. 2013 29 (21): 6221-31

PMID: 23611508 · PMCID: PMC4915780 · DOI:10.1021/la400542x

The lack of an effective technique for three-dimensional flow visualization has limited experimental exploration of the "coffee ring effect" to the two-dimensional, top-down viewpoint. In this report, high-speed, cross-sectional imaging of the flow fields was obtained using optical coherence tomography to track particle motion in an evaporating colloidal water drop. This approach enables z-dimensional mapping of primary and secondary flow fields and changes in these fields over time. These sectional images show that 1 μm diameter polystyrene particles have a highly nonuniform vertical distribution with particles accumulating at both the air-water interface and the water-glass interface during drop evaporation. Particle density and relative humidity are shown to influence interfacial entrapment, which suggests that both sedimentation rate and evaporation rate affect the dynamic changes in the cross-sectional distribution of particles. Furthermore, entrapment at the air-water interface delays the time at which particles reach the ring structure. These results suggest that the organization of the ring structure can be controlled based on the ratio of different density particles in a colloidal solution.

MeSH Terms (7)

Air Colloids Particle Size Polystyrenes Surface Properties Volatilization Water

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