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Suppression of the coffee-ring effect by shape-dependent capillary interactions

Nature volume 476, pages 308–311 (2011)Cite this article

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Abstract

When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect1,2,3, is familiar to anyone who has observed a drop of coffee dry. During the drying process, drop edges become pinned to the substrate, and capillary flow outward from the centre of the drop brings suspended particles to the edge as evaporation proceeds. After evaporation, suspended particles are left highly concentrated along the original drop edge. The coffee-ring effect is manifested in systems with diverse constituents, ranging from large colloids1,4,5 to nanoparticles6 and individual molecules7. In fact—despite the many practical applications for uniform coatings in printing8, biology9,10 and complex assembly11—the ubiquitous nature of the effect has made it difficult to avoid6,12,13,14,15,16. Here we show experimentally that the shape of the suspended particles is important and can be used to eliminate the coffee-ring effect: ellipsoidal particles are deposited uniformly during evaporation. The anisotropic shape of the particles significantly deforms interfaces, producing strong interparticle capillary interactions17,18,19,20,21,22,23. Thus, after the ellipsoids are carried to the air–water interface by the same outward flow that causes the coffee-ring effect for spheres, strong long-ranged interparticle attractions between ellipsoids lead to the formation of loosely packed or arrested structures on the air–water interface17,18,21,24. These structures prevent the suspended particles from reaching the drop edge and ensure uniform deposition. Interestingly, under appropriate conditions, suspensions of spheres mixed with a small number of ellipsoids also produce uniform deposition. Thus, particle shape provides a convenient parameter to control the deposition of particles, without modification of particle or solvent chemistry.

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Figure 1: Deposition of spheres and ellipsoids.
Figure 2: Transportation of particles over time.
Figure 3: High-magnification images of particles near the drop contact line.
Figure 4: Behaviour of spheres, ellipsoids, and mixtures of spheres and ellipsoids in drying liquid drops.

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Acknowledgements

We thank K. B. Aptowicz, A. Basu, E. Buckley, J. C. Crocker, P. Habdas, T. Lubensky and K. J. Stebe for discussions and critical reading of the manuscript. We thank J. Crassous and H. Dietsch for the hydrophilic ellipsoids and spheres (see Supplementary Fig. 4). We acknowledge financial support from the National Science Foundation through DMR-0804881, the PENN MRSEC DMR-0520020 and NASA NNX08AO0G. T.S. thanks DAAD for a postdoctoral fellowship.

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Authors and Affiliations

  1. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, 19104, Pennsylvania, USA

    Peter J. Yunker, Tim Still, Matthew A. Lohr & A. G. Yodh

  2. Complex Assemblies of Soft Matter, CNRS-Rhodia-University of Pennsylvania UMI 3254, Bristol, 19007, Pennsylvania, USA

    Tim Still

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Contributions

P.J.Y. initiated the research, designed and performed the experiments, synthesized the colloidal particles, and analysed the data with support from A.G.Y., T.S. and M.A.L.; P.J.Y. and A.G.Y. wrote the manuscript, with support from T.S. and M.A.L.

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Correspondence to Peter J. Yunker.

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Patent applications by P.J.Y. and A.G.Y. may be affected by the paper.

Supplementary information

Supplementary Information

The file contains Supplementary Figures 1-6 with legends, Supplementary Discussion and additional references. (PDF 2283 kb)

Supplementary Movie 1

The movie shows an evaporating suspension of spheres is shown under bright field microscopy. Spheres are transported to the drop contact line, resulting in a ring-like deposit. This drop exhibits the coffee ring effect. (MOV 8050 kb)

Supplementary Movie 2

The movie shows an evaporating suspension of ellipsoids is shown under bright field microscopy. Ellipsoids adsorb to the air-water interface where they form loosely-packed structures. The final deposition is relatively uniform. This drop does not exhibit the coffee ring effect. (MOV 4363 kb)

Supplementary Movie 3

The movie shows an evaporating suspension of spheres is shown under bright field microscopy at a higher magnification. Individual spheres are seen as they are transported to the drop contact line, resulting in a ring-like deposit. This drop exhibits the coffee ring effect. (MOV 21396 kb)

Supplementary Movie 4

The movie shows a an evaporating suspension of ellipsoids is shown under bright field microscopy at a higher magnification. Individual ellipsoids are seen adsorbing to the air-water interface where they form loosely-packed structures. The final deposition is relatively uniform. This drop does not exhibit the coffee ring effect. (MOV 6279 kb)

Supplementary Movie 5

The movie shows a region near the contact line in an evaporating suspension of spheres is shown under bright field microscopy. Individual spheres are seen packing closely at the contact line, resulting in a ring-like deposit. This drop exhibits the coffee ring effect. (MOV 16162 kb)

Supplementary Movie 6

The movie shows a region near the contact line in an evaporating suspension of ellipsoids is shown under bright field microscopy at a higher magnification. Individual ellipsoids are seen adsorbing to the air-water interface where they form loosely-packed structures. These structures prevent ellipsoids from reaching the contact line. This drop does not exhibit the coffee ring effect. (MOV 9728 kb)

Supplementary Movie 7

The movie shows a region near the contact line in an evaporating suspension of ellipsoids with added surfactant is shown under bright field microscopy at a higher magnification. The added surfactant decreases the surface tension. As a result, individual ellipsoids are seen packing closely at the contact line. Unlike suspensions of ellipsoids absent surfactant, this drop exhibits the coffee ring effect. (MOV 3494 kb)

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Yunker, P., Still, T., Lohr, M. et al. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature 476, 308–311 (2011). https://doi.org/10.1038/nature10344

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