Featured Research

from universities, journals, and other organizations

Physicists undo the 'coffee ring effect'

Date:
August 17, 2011
Source:
University of Pennsylvania
Summary:
A team of physicists has shown how to disrupt the "coffee ring effect" -- the ring-shaped stain of particles leftover after coffee drops evaporate -- by changing the particle shape. The discovery provides new tools for engineers to deposit uniform coatings.

Penn physicists recently shown that simply changing particle shape can eliminate the ring-shaped stain that is left behind when drops of certain liquids dry. Video microscopy footage from their experiments shows spherical particles getting swept to the edges, while oblong particles are distributed consistently (see: http://www.youtube.com/watch?v=ZaCGoSTMHyc).
Credit: Kurtis Sensenig, University of Pennsylvania

A team of University of Pennsylvania physicists has shown how to disrupt the "coffee ring effect" -- the ring-shaped stain of particles leftover after coffee drops evaporate -- by changing the particle shape. The discovery provides new tools for engineers to deposit uniform coatings.

Related Articles


The research was conducted by professor Arjun Yodh, director of the Laboratory for Research on the Structure of Matter; doctoral candidates Peter Yunker and Matthew Lohr; and postdoctoral fellow Tim Still, all of the department of Physics and Astronomy in Penn's School of Arts and Sciences.

Their research will be published in the journal Nature on August 18.

"The coffee ring effect is very common in everyday experience," Yunker said. "To avoid it, scientists have gone to great lengths designing paints and inks that produce an even coating upon evaporation. We found that the effect can be eliminated simply by changing the shape of the particle."

The edges of a water drop sitting on a table or a piece of paper, for example, are often "pinned" to the surface. This means that when the water evaporates, the drop can't shrink in circumference but instead flattens out. That flattening motion pushes water and anything suspended in it, such as coffee particles, to its edges. By the time the drop fully evaporates, most of the particles have reached the edge and are deposited on the surface, making a dark ring.

University of Chicago physicists Sidney Nagel, Thomas Witten and their colleagues wrote an influential paper about this process in 1997, which focused mainly on suspended spherical particles, but it was not until the Yodh team's recent experiments that the surprising role played by suspended particle shape was discovered.

Yodh's team used uniformly sized plastic particles in their experiments. These particles were initially spherical but could be stretched into varying degrees of eccentricity, to ensure the experiments only tested the effect of the particle's shape on the drying pattern.

The researchers were surprised at how big an effect particle shape had on the drying phenomenon.

"Different particle geometries change the nature of the membrane at the air-water interface," Yodh said. "And that has big consequences."

Spherical particles easily detach from the interface, and they flow past one another easily because the spheres do not substantially deform the air-water interface. Ellipsoid particles, however, cause substantial undulation of the air-water interface that in turn induces very strong attractions between the ellipsoids. Thus the ellipsoids tend to get stuck on the surface, and, while the stuck particles can continue to flow towards the drop's edges during evaporation, they increasingly block each other, creating a traffic jam of particles that eventually covers the drop's surface.

"Once you stretch the spherical particles by about 20 percent," Yunker said, "the particles deposit uniformly."

After experimenting with suspended particle shape, the researchers added a surfactant, essentially soap, into the drops to show that interactions on the drop's surface were responsible for the effect. With the surfactant lowering the drop's surface tension, ellipsoid particles did not get stuck at the interface and flowed freely to the edge.

They also tested drops that had mixtures of both spherical and oblong particles. When the spheres were much smaller than the ellipsoids, the spheres flowed to the edge, but, at a certain size, they became similarly trapped.

"We were thinking it would be useful if you could just sprinkle in a few of these ellipsoid particles to remove the coffee ring effect," Yodh said, "and we found that sometimes this idea works and sometimes it doesn't."

Understanding the impact of particle shape on drop drying could have applications in printing and painting. The principles could also be relevant in biological and medical contexts.

"In many cases, the way we make coatings involves hazardous chemicals," Yunker said. "If you need something that's bio-compatible, it's more difficult."

"There are a lot of situations where you want uniform coatings," he said. "This work will stimulate people to think about new ways of doing it."

This research was supported by the National Science Foundation, including its Materials Research Science and Engineering Center; NASA; and the CNRS-Rhodia-UPenn Complex Assemblies of Soft Matter collaboration.


Story Source:

The above story is based on materials provided by University of Pennsylvania. Note: Materials may be edited for content and length.


Journal Reference:

  1. Peter J. Yunker, Tim Still, Matthew A. Lohr, A. G. Yodh. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature, 2011; 476 (7360): 308 DOI: 10.1038/nature10344

Cite This Page:

University of Pennsylvania. "Physicists undo the 'coffee ring effect'." ScienceDaily. ScienceDaily, 17 August 2011. <www.sciencedaily.com/releases/2011/08/110817135353.htm>.
University of Pennsylvania. (2011, August 17). Physicists undo the 'coffee ring effect'. ScienceDaily. Retrieved December 19, 2014 from www.sciencedaily.com/releases/2011/08/110817135353.htm
University of Pennsylvania. "Physicists undo the 'coffee ring effect'." ScienceDaily. www.sciencedaily.com/releases/2011/08/110817135353.htm (accessed December 19, 2014).

Share This


More From ScienceDaily



More Matter & Energy News

Friday, December 19, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Navy Unveils Robot Fish

Navy Unveils Robot Fish

Reuters - Light News Video Online (Dec. 18, 2014) The U.S. Navy unveils an underwater device that mimics the movement of a fish. Tara Cleary reports. Video provided by Reuters
Powered by NewsLook.com
3D Printed Cookies Just in Time for Christmas

3D Printed Cookies Just in Time for Christmas

Reuters - Innovations Video Online (Dec. 18, 2014) A tech company in Spain have combined technology with cuisine to develop the 'Foodini', a 3D printer designed to print the perfect cookie for Santa. Ben Gruber reports. Video provided by Reuters
Powered by NewsLook.com
First Etihad Superjumbo Flight in December

First Etihad Superjumbo Flight in December

AFP (Dec. 18, 2014) The first flight of Etihad Airways' long-awaited Airbus A380 superjumbo will take place later in December, the Abu Dhabi carrier said Thursday, also announcing its first Boeing 787 Dreamliner route. Duration: 01:09 Video provided by AFP
Powered by NewsLook.com
Ford Expands Air Bag Recall Nationwide

Ford Expands Air Bag Recall Nationwide

Newsy (Dec. 18, 2014) The automaker added 447,000 vehicles to its recall list, bringing the total to more than 502,000. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

More Coverage


Future of Inks, Paints and Coatings Takes Shape: Researchers Determine That Particle Shape Affects the 'coffee Ring Effect'

Aug. 17, 2011 If you've ever spilled a drop of coffee on a surface, you might have noticed the curious way the color concentrates at the edges when the coffee dries. This is known as the "coffee ring ... read more

Strange & Offbeat Stories


Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins