Nav: Home

Coffee-ring phenomenon explained in new theory

December 20, 2016

RENO, Nev. - The formation of a simple coffee stain has been the subject of complex study for decades, though it turns out that there remain some stones still to be turned. Researchers at the University of Nevada, Reno have modeled how a colloidal droplet evaporates and found a previously overlooked mechanism that more accurately determines the dynamics of particle deposition in evaporating sessile droplets, which has ramifications in many fields of today's technological world.

"Understanding and manipulating the dynamics of particle deposition during evaporation of colloidal drops can be used in DNA sequencing, painting, ink jet printing and fabricating ordered micro/nano-structures," Hassan Masoud, assistant professor in the Department of Mechanical Engineering, said. "And now we understand it better than ever before. Our discovery builds on a large body of work; we took an extra step though, modeling the interaction of suspended particles with the free surface of the drop. We believe our findings are going to fundamentally change the common perception on the mechanism responsible for the so-called 'coffee-ring' phenomenon."

When a droplet dries on a surface, the particles suspended in it usually deposit in a ring-like pattern, leaving a stain or residue, called the coffee-ring effect. Until now, the stain was thought to form as a result of the fluid flow inside the drop. Masoud and his team found that the free surface of the droplet, the top layer where it is in contact with the air, plays a critical role in the deposition of the particles.

"When the drop evaporates, the free surface collapses and traps the suspended particles," Masoud said. "Our theory shows that eventually all the particles are captured by the free surface and stay there for the rest of their trip towards the edge of the drop."

Masoud and his team used a less familiar modeling system, known as the Toroidal Coordinate System, that allowed them to reduce the three-dimensional governing equations into a one-dimensional form. Despite a decent amount of server space and speed, the team opted to write out their many equations long-hand, on dozens of very large pieces of news print paper.

"Our innovative approach - and using some ugly-long equations - distinguishes our work from previous research," he said. "No one else has used this coordinate system for this problem, and this allows us to track the motion of particles in the drop in a natural way."

The discovery allows scientists to manipulate the motion of solute particles by altering the surface tension of the liquid-gas interface rather than controlling the bulk flow inside the drop.

"We can use surfactants to tweak the surface tension," Masoud said. "In a simple example, if you are cleaning solar panels, which can lose up to 90 percent of their efficiency when dirty, the preferred method of cleaning is water, but that leaves behind a stain that is hard to wipe out. Changing the flow dynamics during evaporation with a specialized cleaning agent can leave the panels cleaner and more efficient."

Their peer-reviewed paper, "Alternative mechanism for coffee-ring deposition based on active role of free surface," was published Dec. 12 in the American Physical Society's publication Physical Review E.
-end-
Masoud's team includes his doctoral student Saeed Jafari Kang, post-doctoral fellow Vahid Vandadi, and collaborator James D. Felske, who is a professor in the Department of Mechanical and Aerospace Engineering at the University at Buffalo.

Masoud leads the Complex Fluids and Active Matter Group at the University. Before coming to the University of Nevada, Reno's College of Engineering, he was a lecturer and post-doctoral fellow at Princeton University. He received his doctorate from the Georgia Institute of Technology in 2012. His interests include fluid mechanics, ?transport phenomena and hydrodynamics of soft and active matter. He uses the tools of applied mathematics and ?high-performance computing to solve state-of-the-art problems at the intersection of ?engineering, physics and biology.

University of Nevada, Reno

Related Surface Tension Articles:

Artificial cartilage under tension as strong as natural material
Biomedical engineers at the University of California, Davis, have created a lab-grown tissue similar to natural cartilage by giving it a bit of a stretch.
Cholesterol -- a key player at the lung surface
Cholesterol, a naturally occurring compound at the lung surface, has been shown to have a clear effect on the properties of this nanoscale film that covers the inside of our lungs.
Dramatic improvement in surface finishing of 3-D printing
Waseda University researchers have developed a process to dramatically improve the quality of 3-D printed resin products.
Ubiquitous but overlooked, fluid is a source of muscle tension
The cellular fluid in every muscle fiber appears to play a key but previously unacknowledged role in the mechanics of muscle stretch, according to a new study by Brown University biologists.
First detection of boron on the surface of Mars
Boron has been identified for the first time on the surface of Mars, indicating the potential for long-term habitable groundwater in the ancient past.
Deep insights from surface reactions
Using the Stampede supercomputer at the Texas Advanced Computing Center, researchers have developed biosensors that can speed up drug development, designed improved materials for desalinization, and explored new ways of generating energy from bacteria.
How water flows near the superhydrophobic surface
The international scientific team, led by Olga Vinogradova (Professor at the Faculty of Physics, the Lomonosov Moscow State University and the chief of laboratory at Institute of Physical chemistry and Electrochemistry, Russian Academy of Sciences) has managed to characterize theoretically the behavior of water in close vicinity to a superhydrophobic surface.
NASA to map the surface of an asteroid
NASA's OSIRIS-REx spacecraft will travel to near-Earth asteroid Bennu to sample surface material and return it to Earth for study.
Surface tension can sort droplets for biomedical applications
A team led by Colorado State University's Arun Kota has engineered a simple and inexpensive device that can sort droplets of liquid based solely on the liquids' varying surface tensions.
New surface makes oil contamination remove itself
Researchers of Aalto University have developed surfaces where oil transports itself to desired directions.

Related Surface Tension Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Digital Manipulation
Technology has reshaped our lives in amazing ways. But at what cost? This hour, TED speakers reveal how what we see, read, believe — even how we vote — can be manipulated by the technology we use. Guests include journalist Carole Cadwalladr, consumer advocate Finn Myrstad, writer and marketing professor Scott Galloway, behavioral designer Nir Eyal, and computer graphics researcher Doug Roble.
Now Playing: Science for the People

#530 Why Aren't We Dead Yet?
We only notice our immune systems when they aren't working properly, or when they're under attack. How does our immune system understand what bits of us are us, and what bits are invading germs and viruses? How different are human immune systems from the immune systems of other creatures? And is the immune system so often the target of sketchy medical advice? Those questions and more, this week in our conversation with author Idan Ben-Barak about his book "Why Aren't We Dead Yet?: The Survivor’s Guide to the Immune System".