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The Warsaw Voice » Special Sections » March 31, 2015
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Mysterious Behavior of Fluids
March 31, 2015   
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For several years, it has been known that superfluid helium housed in reservoirs located next to each other behaves like a single body, even when the channels connecting the reservoirs are too narrow and too long to allow a substantial flow between them. A new theoretical model developed by an international team of scientists including Polish researchers reveals that the phenomenon of mysterious communication between fluid reservoirs at some distance from each other is much more common than previously thought.

This phenomenon was discovered recently and has only been observed in helium cooled to a very low temperature. The new theoretical model suggests that this effect may also be present in other liquids—and in much more typical conditions.

The first report of reservoirs of liquid acting in this way was published in 2010 in Nature Physics. A team from the University at Buffalo and the State University of New York created an array of tens of millions of tiny reservoirs for liquid helium on a silicon plate. Each minute reservoir was a cube whose sides were two microns (millionths of a meter) in size, with the centers of adjacent reservoirs six microns apart. The prepared plate was covered with another full silicon plate. This was done so as to leave a very narrow gap above the reservoirs, of only 32 nanometers (billionths of a meter).

The size of the gap was thousands of times smaller than both the size of the reservoirs themselves and the distance between adjacent reservoirs. Such compact dimensions made significant flow virtually impossible. It was thus expected that after pouring in liquid helium it would “do its own thing” in each reservoir, regardless of what was happening in adjacent reservoirs. In the experiment, the temperature of the liquid helium was measured in a single reservoir, and in the entire system. If the reservoirs were truly independent, the temperature of the whole system would equal the temperature of a single reservoir multiplied by the total number of reservoirs. However, this was not the case: a clear excess of temperature was observed in the system. The superfluid helium, apparently divided into millions of independent reservoirs, was inexplicably acting as if it was still a physical whole.

“Let’s change the scale for a moment and imagine cubic containers, each with sides two meters in size,” says Anna Maciołek, Ph.D., of the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw, who also works at the Max Planck Institute for Intelligent Systems and the University of Stuttgart in Germany. “Each pair of containers is connected by a tube four meters long with a diameter of three millimeters. According to existing theories, such a small channel should not synchronize the phenomena occurring in the containers. And yet, in the microworld it does.”

Superfluid helium is a liquid whose properties are to a large extent the result of quantum phenomena, so initially it seemed that it was those that were responsible for the outcome of the experiment. In cooperation with Prof. Douglas Abraham from Oxford University, Maciołek has developed a theory describing the observed phenomenon. The new theory, confirmed by computer simulations carried out by Oleg Vasilyev from the Max Planck Institute, proves that the effect of “action at a distance” does not require a quantum physics explanation and can also occur in classical single-component fluids, as well as in mixtures.

Analysis of the new theoretical model has revealed that the phenomenon occurs under certain conditions. For it to occur, in the initial experiment the liquid helium needed to be at a state close to the appearance (or disappearance) of superfluidity. For other fluids, however, low temperatures are not required. Water and lutidine—a model mixture of water with oil—mix only in a certain range of temperatures and the “action at a distance” phenomenon only appears within this range. The most important requirement therefore turns out to be the proximity of the phase transition, that is, the state in which the two different forms (phases) of the liquid can occur simultaneously. The dimensions of the reservoirs and the connecting channels are also important: the phenomenon ceases to exist when the distances are significantly larger than the size of human cells.

“Physics in the microworld, even classical physics, is turning out, not for the first time, to be different from the physics we all know from everyday life,” Maciołek says.

The results of this research, presented recently in the physics journal Physical Review Letters, can be applied in microfluidic systems. Systems of this type are constructed to carry out chemical or biological experiments on individual droplets. The sizes of the containers and channels in microfluidic systems are so small that the “action at a distance” phenomenon can appear as an unintended effect, distorting the results of experiments, or as an intentionally introduced factor increasing the functionality of the system.
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