If a person pushes a broken-down car alone, there is a certain effect. If another person helps, the result is the sum of their efforts. If two micro-particles are pushing another microparticle, however, the resulting effect may not necessarily be the sum their efforts. A recent study published in Nature Communications, measured this odd effect that scientists call “many body.”
In the microscopic world, where the modern miniaturized machines at the new frontiers of technology operate, as long as we are in the presence of two particles, things are relatively simple. When other particles are added, however, the situation becomes more complicated than common sense would suggest.
Colloids confined by laser beams
Soft Matter Lab @ Bilkent University
Imagine there are two people pushing a broken-down car: the total force is the sum of their forces. Similarly, if there are three people, it would be the sum of the force of three people, and so on. Now imagine a solid particle of a few thousandths of a millimeter, a colloid, immersed in fluid. Just ahead there is a similar particle.
If there are “critical” thermal fluctuations in the fluid that separates them, they will either repel or attract each other without even touching: the fluctuations are responsible for it. In other words, an interaction force, or "critical Casimir" force emerges, as if the particles were connected by an invisible spring. To obtain critical fluctuations, we only need one of many transparent liquids composed of a mixture of two fluids which gradually separate like oil and water when their temperature is raised.
What happens when a third colloid comes in? “Something counterintuitive,” explains SISSA professor, Andrea Gambassi, one of the authors of the study and long-standing collaborator of Prof. Siegfried Dietrich, Director at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart, “the total force that one of the particles ‘perceives’ is different from the sum of the interactions with the other two when they are present separately.”
For Dietrich and Gambassi critical Casimir forces are nothing new: in 2008, they published together a study in Nature, where these forces, which had been predicted theoretically since 1978, were directly measured for the first time in collaboration with the experimental group of Prof. Clemens Bechinger, Head of the 2nd Institute of Physics at the University of Stuttgart and Max Planck fellow at the MPI-IS. “In simple words” Gambassi continues, “the forces do not add up linearly like they do in our daily life. Here we are dealing with what physicists call a many-body effect, which is typical of fluctuation-induced forces.”
The new study measured this effect for the first time in a system made up of glass (silica) microspheres immersed in fluid. By reconstructing critical Casimir forces with only two particles and then with three, the researchers demonstrated the nonadditivity of these forces.
“The knowledge of these effects is very important from the point of view of both fundamental and applied research, especially for scientists who design micro-machines to perform a variety of tasks. Each micro-machine is made up of several mechanical components in relative motion and in order to understand how the different ‘gears’ interact with each other, the knowledge of many-body interaction is crucial, especially in the presence of fluids,” explains Gambassi.
Laser beams, optical tweezers, and critical mixtures
The experiment, conducted by the group led by Professor Giovanni Volpe at the University of Bilkent in Turkey, starts with colloids immersed in a mixture of water and lutidine (an oily substance). Below 34°C, this mixture is similar to water, but when the temperature is raised, a transition occurs: first the fluid becomes opaque because of the effects of critical fluctuations, after which the oil begins to separate, floating on the water. “It is around this phase transition that we observe the many-body effects,” ex-plains Volpe.
The colloids immersed in fluid, however, move randomly and diffuse with Brownian motion, the typical movement of microscopic objects immersed in a liquid, as explained theoretically by Einstein. In order to “confine” them, the fluid was illuminated by thin laser beams focussed on one point: when the particles entered the beam, they tended to stay where the light was most intense. In this way, the laser acted as a sort of optical tweezers. By keeping two colloids close together using two laser beams, it was possible to accurately measure their random motions with a video taken from the microscope. Then, using statistical methods, the forces at play were reconstructed. With the help of another optical tweezers, the researchers then added a third particle.
“On approaching the phase transition, when comparing the experiment with two and three colloids, we observed that there was no linear addition of the forces and that many-body effects were present,” explains Dietrich. “Of course, if we added more colloids, the situation would become even more complicated and interesting.” And Volpe concludes: “In this way we demonstrated that the many-body effect is real and we succeeded in measuring it with unexpected accuracy, especially when we consider that we are dealing with forces of one-thousandth of a millionth of a gram. Now we would like to use them to design and develop new micro-machines.”
Annette Stumpf | Max-Planck-Institut für Intelligente Systeme
Riddle of matter remains unsolved: Proton and antiproton share fundamental properties
19.10.2017 | Johannes Gutenberg-Universität Mainz
Space radiation won't stop NASA's human exploration
18.10.2017 | NASA/Johnson Space Center
University of Maryland researchers contribute to historic detection of gravitational waves and light created by event
On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...
Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.
Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....
Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).
When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...
Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.
How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...
Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.
It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...
17.10.2017 | Event News
10.10.2017 | Event News
10.10.2017 | Event News
19.10.2017 | Life Sciences
19.10.2017 | Interdisciplinary Research
19.10.2017 | Earth Sciences