Coffee Ring Stains Inspired a New Type of Biomedical Device
You see coffee ring stains everyday.
How this everyday phenomenon can be turned into devices for biomedical applications?
Coffee ring forms when a colloidal droplet evaporates at a pinned contact line, where the suspended particles are transported and concentrated at the edge of the droplet due to evaporation-induced capillary flow (Deegan et al., Nature 2001). The natural phenomenon possess a novel avenue in accomplishing fundamental steps of particle transport without the need for external power sources.
The minimal size of coffee ring structures
The coffee ring formation is a natural process, and may have a great potential for biomolecular sensing and processing at the micro- and nanoscale. However, before we can take advantage of this phenomenon at smaller length scales, the definitive size limit of this natural process needs to be more fully understood. We studied the influence of droplet size on the coffee ring formation. We found that as the droplet size shrinks, the competition between the time scales of the liquid evaporation and the particle movement will influence the resulting ring formation. When the liquid evaporates much faster than the particle movement, coffee ring formation may cease. For suspended particles of size ~100 nm, the minimum diameter of the coffee ring structure is found to be ~10 μm.
Nanochromatography driven by the coffee-ring effect
The coffee ring phenomenon has long been known for its ability to concentrate particles at the rim of a dried liquid droplet, yet little is known about its particle separation capability. In this work, we studied the physics of particle separation during the coffee ring formation. We found that the governing separation mechanism is attributed to a particle-size selection near the contact line of an evaporating droplet. Based on this mechanism, we demonstrated nano-chromatography of three relevant biological entities (proteins, Escherichia coli, and mammalian cells) in a liquid droplet, with a separation resolution on the order of ~100 nm and a dynamic range from ~10 nm to a few micrometers. These findings have direct implications for developing low-cost technologies for disease diagnostics in resource-poor environments.
1. Xiaoying Shen, Chih-Ming Ho, and Tak-Sing Wong‡, “Minimal Size of Coffee Ring Structure”, The Journal of Physical Chemistry B, vol. 114, pp. 5269 – 5274 (2010).
[Cover Image Article; Top 20 most read articles in June 2010]
2. Tak-Sing Wong*‡, Ting-Hsuan Chen*, Xiaoying Shen, and Chih-Ming Ho‡, “Nano-Chromatography Driven by the Coffee Ring Effect”, Analytical Chemistry, vol. 83, pp. 1871 – 1873 (2011).
‡Corresponding author, *Shared first authors
[Top 20 most read articles in February & March 2011]
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Copyright by Tak-Sing Wong 2013. All rights reserved.
Department of Mechanical Engineering
Materials Research Institute
Huck Institutes of the Life Sciences
The Pennsylvania State University, University Park, PA 16802