"Understanding micro and nano-particle transportation within evaporating liquid droplets has great potential for several technological applications, including nanostructure self-assembly, lithography patterning, particle coating, and biomolecule concentration and separation," said Professor Chih-Ming Ho of the UCLA Henry Samueli School of Engineering and Applied Science.

"However, before we can engineer biosensing devices to do these applications, we need to know the definitive limits of this phenomenon."

"So our research turned to physical chemistry to find the lowest limits of coffee-ring formation."

"If we consider human blood, or saliva, it has a lot of micro and nano-scale molecules or particles that carry important health information," said Dr Tak-Sing Wong of UCLA Engineering's department of mechanical and aerospace engineering.

"If you put this blood or saliva on a surface, and then it dries, these particles will be collected in a very small region in the ring."

"By doing so, we can quantify these biomarkers by various sensing techniques, even if they are very small and in a small amount in the droplets."

As water evaporates from a droplet, particles suspended inside the liquid move to the edges.

Once all the water has evaporated, the particles are concentrated in a ring around the stain left behind.

But if a droplet is small enough, the water will evaporate faster than the particles move.

Instead there will be a relatively uniform concentration in the stain, as the particles have not had enough time to move to the edges.

"It is the competition between the timescale of the evaporation of the droplet and the timescale of the movement of the particles that dictates coffee-ring formation," said Xiaoying Shen, the paper's lead author and a senior microelectronics major at Peking University in China.

"Knowing the minimum size of this so-called coffee ring will guide us in making the smallest biosensors possible," Dr Wong said.

"This means that we can pack thousands, even millions, of small micro-biosensors onto a lab-on-a-chip, allowing one to perform a large number of medical diagnostics on a single chip."

"This may also open the doors to potentially detecting multiple diseases in one sitting."

"There's another important advantage -- this whole process is very natural, it's just evaporation."

"We don't need to use additional devices, such as an electrical power source or other sophisticated instruments to move the particles."

"Evaporation provides a very simple way of concentrating particles and has potential in medical diagnosis."

"For example, researchers at Vanderbilt University were recently awarded a Gates Foundation Research Fund for proposing the use of the coffee-ring phenomenon for malaria detection in developing countries."

Image credit: http://www.flickr.com/photos/free-photos/ / CC BY 2.0