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 effect," and recently, researchers have determined that the shape of the particles in the liquid is an important factor in creating this pattern. The research results could eventually translate into new techniques or formulations for product coatings, or better inks and paints.
This illustration represents a how a dried drop would appear if it contained round particles (red) or elongated particles (blue). When a drop of coffee or tea dries, its particles (which are round) leave behind a ring-like stain called the "coffee ring effect" (upper left). But if you change the shape of the particles, the coffee stain behavior changes too. Elongated particles (blue) do not exhibit the coffee ring effect, rather they are deposited across the entire area of the drop, resulting in a uniformly dark stain (lower right). Credit: Felice Macera, University of Pennsylvania
This work, published in the August 18 issue of the journal Nature was performed by Arjun Yodh and colleagues at the University of Pennsylvania.
"We found that if you change the shape of the particles in the solution, the coffee ring effect goes away, and you end up with a uniform coating," said Peter Yunker, a graduate student in Yodh's lab.
First, a little fluid dynamics: As the liquid in a droplet evaporates, the edges remain fixed, so as the volume decreases, fluid flows outward from the middle of the droplet to its edges. This flow carries particles to the edges, and round particles at the edge will pack closely. By the time all of the liquid in the droplet evaporates, most of the particles will be at the edge, producing the coffee ring effect.
Both the shape that liquid droplets take, and the way the shape changes as the droplets evaporate, is greatly influenced by surface tension at the air-liquid interface. This tension is a property of the interface, based on how the molecules in the liquid interact with one another versus the air. For example, liquids with a high surface tension, like water, may form a raised droplet, because the molecules are very attracted to one another and not so attracted to the air. In contrast, liquids with lower surface tension, like alcohols, are more likely to form flat spots instead of curved droplets.
The Yodh group found that elongated particles in a liquid behave differently than round ones because of the way they are affected by the surface tension of the air-liquid interface. The forces at work are even observable in a common breakfast cereal.
"If you make the particles elongated or ellipsoidal, they deform the air-water interface, which causes the particles to strongly attract one another. You can observe this effect in a bowl of cheerios-if there are only a few left they clump together in the middle of the bowl, due to the surface tension of the milk," explained Yunker.
This clumping changes the way the particles distribute themselves within the droplet. Even if the clumped ellipsoidal particles reach the edge of the droplet, they do not pack as closely as round particles. The loosely packed clumps eventually spread to cover the entire surface, filling it so an even coating of particles is deposited when evaporation is complete.
"This work gives us a new idea about how to make a uniform coating, relatively simply. If you change the particle shape, you can change the way a particle is deposited. You can also make mixtures. In some cases, even just a small amount of ellipsoids can change the way the particles deposit when they dry," said Yodh.
In future studies, the research team will explore drying and deposition of different types of fluids. They will also investigate different particle sizes and shapes, and the interplay of particle mixtures.
"This is an exciting scientific result with potential commercial applications, which was in part enabled by support of the Materials Research Science and Engineering Center at the University of Pennsylvania," said Mary Galvin, program director for the division of materials research at the National Science Foundation, which partially funded the research. The centers program, recently renamed Materials Research Centers and Teams, provides support for interdisciplinary materials research and education while addressing fundamental problems in science and engineering.
Lisa Van Pay | EurekAlert!
Electron tomography technique leads to 3-D reconstructions at the nanoscale
24.05.2018 | The Optical Society
These could revolutionize the world
24.05.2018 | Vanderbilt University
The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.
Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
25.05.2018 | Event News
02.05.2018 | Event News
13.04.2018 | Event News
25.05.2018 | Event News
25.05.2018 | Machine Engineering
25.05.2018 | Life Sciences