(Press-News.org) A puzzling observation, pursued through hundreds of experiments, has led Stanford researchers to a simple yet profound discovery: under certain circumstances, droplets of fluid will move like performers in a dance choreographed by molecular physics.
"These droplets sense one another, they move and interact, almost like living cells," said Manu Prakash, an assistant professor of bioengineering and senior author of an article published (today) in Nature.
The unexpected findings may prove useful in semiconductor manufacturing and self-cleaning solar panels, but what truly excites Prakash is that the discovery resulted from years of persistent effort to satisfy a scientific curiosity.
It all began in 2009 when Nate Cira, then an undergraduate at University of Wisconsin, was doing an unrelated experiment. In the course of that experiment Cira deposited several droplets of food coloring onto a sterilized glass slide and was astonished when they began to move.
Cira replicated and studied this phenomenon alone for two years until he became a graduate student at Stanford where he shared this curious observation with Prakash. The professor soon became hooked by the puzzle, and recruited a third member to the team: Adrien Benusiglio, a postdoctoral scholar in the Prakash Lab.
Together they spent three years performing increasingly refined experiments to learn how these tiny droplets of food coloring sense one other and move. In living cells these processes of sensing and motility are known as chemotaxis.
"We've discovered how droplets can exhibit behaviors akin to artificial chemotaxis," Prakash said.
As the Nature article explains, the critical fact was that food coloring was a two-component fluid. In such fluids, two different chemical compounds coexist while retaining separate molecular identities--imagine liquid marbles and ball bearings.
The droplets in this experiment consisted of two simple molecular compounds found naturally in food coloring, water and propylene glycol.
The researchers discovered how the dynamic interactions of these two molecular components enabled inanimate droplets to mimic some of the behaviors of living cells.
Surface tension and evaporation
Essentially, the droplets danced because of a delicate balance between surface energy and evaporation.
Evaporation is easily understood. On the surface of any liquid, some molecules convert to a gaseous state and float away.
Surface tension is what causes liquids to bead up. It arises from how tightly the molecules in a liquid bind together..
Water evaporates more quickly than propylene glycol. Water also has a higher surface tension. These differences create a tornado like flow inside the droplet, which not only allows them to move but also allows a single droplet to sense its neighbors.
To understand the molecular forces involved imagine shrinking down to size and diving inside a little droplet.
There, water and polypropylene glycol molecules try to remain evenly distributed but differences in evaporation and surface tension create turmoil within the droplet.
On the curved dome of each droplet, water molecules become gaseous and float away faster than their evaporation-averse propylene glycol neighbors.
This evaporation happens more readily on the lower thin edges of the domed droplet, leaving excess of propylene glycol there. Meanwhile, the peak of the dome has a higher concentration of water.
The water at the top exerts its higher surface tension to pull the droplet tight so it doesn't flatten out. This tugging causes a tumbling molecular motion inside the droplet. Thus surface tension gets the droplet ready to roll.
Evaporation determines the direction of that motion. Each droplet sends aloft gaseous molecules of water like a radially emanating signal announcing the exact location of any given droplet. The droplets converge where the signal is strongest.
So evaporation provided the sensing mechanism and surface tension the pull to move droplets together in what seemed to the eye to be this careful dance.
Rule for two-component fluids
The researchers experimented with varied proportions of water and propylene glycol. Droplets that were one-percent propylene glycol (PG) to 99 percent water exhibited much the same behavior as droplets that were two-thirds PG to just one-third water.
Based on these experiments the paper describes a "universal rule" to identify any two-component fluids that will demonstrate sensing and motility.
Adding colors to the mixtures made it easier to tell how the droplets of different concentrations behaved and created some visually striking results (see video).
In one experiment, a droplet with more propylene glycol seems to chase a droplet with more water. In actuality, the droplet with more water exerts a higher surface tension tug, pulling the polypropylene droplet along.
In another experiment, researchers showed how physically separated droplets could align themselves using ever-so-slight signals of evaporation to break symmetry and finally converge.
In a third experiment they used Sharpie pens to draw black lines on glass slides. The lines changed the surface of the slide and created a series of catch basins. They filled each basin with fluids of different concentrations to create a self-sorting mechanism. Droplets bounced from reservoir to reservoir until they sensed the fluid that matched their concentration and merged with that pool.
Ramifications
What started as a curiosity driven project may also have many practical implications.
The deep physical understanding of two component fluids allows the researchers to predict which fluids and surfaces will show these unusual effects. The effect is present on a large number of common surfaces and can be replicated with a number of chemical compounds.
Among other potential uses, they plan to apply this understanding to finding better ways to clean the glass and silicon surfaces used in semiconductor chips and create self-cleaning solar panels, an outstanding problem in large scale solar energy implementation.
"If necessity is the mother of invention, then curiosity is the father," observed Prakash.
INFORMATION:
Media contact:
Tom Abate, Associate Director of Communications, Stanford Engineering, tabate@stanford.edu, 650-736-2245
DURHAM, N.C. -- The molecular machines that copy DNA in a living cell are amazingly fast and accurate at pairing up the correct bases -- G with C and A with T -- into each new double helix.
They work by recognizing the shape of the right base pair combinations, and discarding those -- such as a G and a T -- that don't fit together correctly. Yet for approximately every 10,000 to 100,000 bases copied, these machines make a mistake that if uncorrected will be immortalized in the genome as a mutation.
For decades, researchers have wondered how these seemingly random ...
Bacteria that live on iron were found for the first time at three well-known vent sites along the Mid-Atlantic Ridge, one of the longest undersea mountain ranges in the world. Scientists report that these bacteria likely play an important role in deep-ocean iron cycling, and are dominant members of communities near and adjacent to sulfur-rich, black-smoker hydrothermal vents prevalent along the Mid-Atlantic Ridge. These unique chemosynthetic communities live off the chemical components in the vent fluid, rather than sunlight used by their photosynthetic counterparts. This ...
The human-dominated geological epoch known as the Anthropocene probably began around the year 1610, with an unusual drop in atmospheric carbon dioxide and the irreversible exchange of species between the New and Old Worlds, according to new research published today in Nature.
Previous epochs began and ended due to factors including meteorite strikes, sustained volcanic eruptions and the shifting of the continents. Human actions are now changing the planet, but are we really a geological force of nature driving Earth into a new epoch that will last millions of years?
Scientists ...
The first 3D reconstruction of the skull of a 360 million-year-old near-ancestor of land vertebrates has been created by scientists from the Universities of Bristol and Cambridge, UK. The 3D skull, which differs from earlier 2D reconstructions, suggests such creatures, which lived their lives primarily in shallow water environments, were more like modern crocodiles than previously thought.
The researchers applied high-resolution X-ray computed tomography (CT) scanning to several specimens of Acanthostega gunnari, one of the 'four-footed' vertebrates known as tetrapods ...
Mutations in the methyl CpG binding protein 2 gene (MECP2) are the cause of the devastating childhood neurological disorder Rett Syndrome. Despite intense efforts spanning several decades the precise function of MECP2 has been difficult to pin down. Research primarily funded by the Rett Syndrome Research Trust (RSRT) and the National Institutes of Neurological Disease and Stroke (NINDS), and published today in the journal Nature reveals important information that could lead to new treatment approaches. The study, led by Michael Greenberg, Ph.D., Chairman of the Department ...
WINSTON-SALEM, N.C. - March 11, 2015 - Acute respiratory distress syndrome (ARDS) is a life-threatening lung condition that affects approximately 200,000 people a year in the United States and has a higher mortality rate than breast and prostate cancer combined. The condition most often occurs in people who are critically ill or who have significant injuries; those who do survive it often experience profound skeletal muscle weakness.
Over the past 30 years, efforts to fight ARDS with various drug therapies aimed at the lungs have failed. However, doctors at Wake Forest ...
(Santa Barbara, Calif.) - Foreign born graduate students in STEM (science, technology, engineering and math) disciplines who wish to pursue a career in industry or NGOs are much more likely to stay in the U.S. than those who wish to pursue a career in academia or government concludes a study by researchers at UC Santa Barbara's Center for Nanotechnology in Society. Published on March 11, 2015 in the open access journal PLOS ONE, the study provides new insight into why foreign-born graduate students in STEM fields choose to remain in the United States or return to their ...
Ask people what it takes to live a long life, and they'll say things like exercise, take Omega-3s, and see your doctor regularly.
Now research from Brigham Young University shows that loneliness and social isolation are just as much a threat to longevity as obesity.
"The effect of this is comparable to obesity, something that public health takes very seriously," said Julianne Holt-Lunstad, the lead study author. "We need to start taking our social relationships more seriously."
Loneliness and social isolation can look very different. For example, someone may be surrounded ...
A study of the brains of rats exposed to lead has uncovered striking similarities with what is known about the brains of human schizophrenia patients, adding compelling evidence that lead is a factor in the onset of schizophrenia.
Results of the study by scientists at Columbia University's Mailman School of Public Health appear in the journal Translational Psychiatry.
The researchers found that lead had a detrimental effect on cells in three brain areas implicated in schizophrenia: the medial prefrontal cortex, the hippocampus, and the striatum of rats exposed to lead ...
Storing Ebola-infected sewage for a week at 86° Fahrenheit or higher should allow enough time for more than 99.99 percent of the virus to die, though lower ambient temperatures may require a longer holding period, according to a new study by researchers at Georgia State University's School of Public Health.
The study co-authored by Lisa M. Casanova, assistant professor of environmental health, and Scott R. Weaver, research assistant professor in Epidemiology and Biostatistics, used bacteriophage Φ6, a type of virus, as a stand-in to study how long Ebola and similar ...
