by Staff Writers
Vienna, Austria (SPX) Oct 26, 2017
In 1827, the English botanist Robert Brown made an observation of seemingly little importance that would turn out to play a central role in the development of the atomic theory of matter.
Looking through the objective of a microscope, he noticed that pollen grains floating in water were constantly jiggling around as if driven by an invisible force, a phenomenon now known as Brownian motion. It was later understood that the irregular motion of the pollen particle is caused by the incessant buffeting of the water molecules surrounding the pollen particle.
Albert Einstein's theoretical analysis of this phenomenon provided crucial evidence for the existence of atoms. The collisions of the pollen grain with the water molecules have two important effects on the motion of the grain.
On one hand, they generate friction that slows the particle down and, at the same time, their thermal agitation keeps the particle moving. Brownian motion results from the balance of these competing forces.
Friction and thermal motion caused by the environment also deeply affect transitions between long-lived states for example phase transitions such as freezing or melting. The long-lived states, e.g. different phases of a material or distinct chemical species, are separated by a high energy barrier as depicted schematically in the illustration.
The barrier between the wells prevents the physical system from rapidly interconverting between the two states. As a consequence, the system spends most of its time rattling around in one of the wells and only rarely jumps from one well to the other. Such transitions are important for many processes in nature and technology, ranging from phase transitions to chemical reactions and the folding of proteins.
Friction's unexpected influence on transitions
More surprisingly, Kramers predicted that the transition rate also depends on the friction in a very interesting way. For strong friction, the system moves sluggishly leading to a small transition rate. As the friction is decreased, the system moves more freely and the transition rate grows.
At sufficiently low friction, however, the transition rate starts to decrease again because in this case it takes a long time for the system to acquire sufficient energy from the environment to overcome the barrier. The resulting maximum of the transition rate at intermediate friction is called the Kramers turnover.
Measuring Kramers' prediction with laser-trapped nanoparticles
Just like the pollen grain observed by Brown, the nanoparticle constantly collides with the molecules surrounding it and these random interactions occasionally push the nanoparticle over the barrier.
By monitoring the motion of the nanoparticle over time, the scientists determined the rate at which the nanoparticle hops between the wells for a wide range of frictions, which can be accurately tuned by adjusting the pressure of the gas around the nanoparticle. The rate obtained from their experiment clearly confirms the turnover predicted by Kramers almost 80 years ago.
"These results improve our understanding of friction and thermal motion at the nanoscale and will be helpful in the design and construction of future nanodevices", says Christoph Dellago, one of the authors of the study.
Loic Rondin, Jan Gieseler, Francesco Ricci, Romain Quidant, Christoph Dellago and Lukas Novotny, "Direct measurement of Kramers turnover with a levitated nanoparticle", in Nature Nanotechnology 2017.
Providence RI (SPX) Oct 20, 2017
Brown University researchers have demonstrated a way to bring a powerful form of spectroscopy - a technique used to study a wide variety of materials - into the nano-world. Laser terahertz emission microscopy (LTEM) is a burgeoning means of characterizing the performance of solar cells, integrated circuits and other systems and materials. Laser pulses illuminating a sample material cause t ... read more
University of Vienna
Nano Technology News From SpaceMart.com
Computer Chip Architecture, Technology and Manufacture
|The content herein, unless otherwise known to be public domain, are Copyright 1995-2017 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. All articles labeled "by Staff Writers" include reports supplied to Space Media Network by industry news wires, PR agencies, corporate press officers and the like. Such articles are individually curated and edited by Space Media Network staff on the basis of the report's information value to our industry and professional readership. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement|