Journal
LANGMUIR
Volume 32, Issue 45, Pages 11899-11906Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.langmuir.6b03221
Keywords
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Funding
- Div Of Civil, Mechanical, & Manufact Inn
- Directorate For Engineering [1401438] Funding Source: National Science Foundation
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Ellipsoidal particles have previously been shown to suppress the coffee-ring effect in millimeter-sized colloidal droplets. Compared to their spherical counterparts, ellipsoidal particles experience stronger adsorption energy to the drop surface where the anisotropy-induced deformation of the liquid-air interface leads to much greater capillary attractions between particles. Using inkjet-printed colloidal drops of varying drop size, particle concentration, and particle aspect ratio, the present work demonstrates how the suppression of the coffee ring is not only a function of particle anisotropy but rather a competition between the propensity for particles to assemble at the drop surface via capillary interactions and the evaporation-driven particle motion to the contact line. For ellipsoidal particles on the drop surface, the capillary force (F-gamma) increases with the particle concentration and aspect ratio, and the hydrodynamic force (F-mu) increases with the particle aspect ratio but decreases with drop size. When F-gamma/F-mu > 1, the surface ellipsoids form a coherent network inhibiting their migration to the drop contact line, and the coffee-ring effect is suppressed, whereas when F-gamma/F-mu < 1, the ellipsoids move to the contact line, resulting in coffee-ring deposition.
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