Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


The future of inks, paints and coatings takes shape

Researchers determine that particle shape affects the 'coffee ring effect'

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!
Further information:

More articles from Materials Sciences:

nachricht From ancient fossils to future cars
21.10.2016 | University of California - Riverside

nachricht Study explains strength gap between graphene, carbon fiber
20.10.2016 | Rice University

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: New 3-D wiring technique brings scalable quantum computers closer to reality

Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.

"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...

Im Focus: Scientists develop a semiconductor nanocomposite material that moves in response to light

In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.

A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...

Im Focus: Diamonds aren't forever: Sandia, Harvard team create first quantum computer bridge

By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.

"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...

Im Focus: New Products - Highlights of COMPAMED 2016

COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.

In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...

Im Focus: Ultra-thin ferroelectric material for next-generation electronics

'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.

Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...

All Focus news of the innovation-report >>>



Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

Agricultural Trade Developments and Potentials in Central Asia and the South Caucasus

14.10.2016 | Event News

World Health Summit – Day Three: A Call to Action

12.10.2016 | Event News

Latest News

Resolving the mystery of preeclampsia

21.10.2016 | Health and Medicine

Stanford researchers create new special-purpose computer that may someday save us billions

21.10.2016 | Information Technology

From ancient fossils to future cars

21.10.2016 | Materials Sciences

More VideoLinks >>>