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Tuesday 16 October 2018, 4:00pm to 5:00pm
Non-isothermal physic-chemical processes in superfluid helium
Abstract: Due to Bernoulli forces, quantized vortices in superfluid helium tend to capture into their core any particles suspended in He-II.
In view of the one-dimensionality of the motion of particles trapped in the vortex core, the probability of their coalescence there is much higher than in bulk liquid, so that the condensation process should predominantly occur in the vortices. This effect was used by us to create a universal method for producing thin nanowires. The metals were introduced into the He-II by laser ablation of targets immersed in liquid.
In order to explain the thicknesses of experimentally grown nanowires from different metals and the wires dense-packing structure, it was necessary to assume the melting of metallic particles during their condensation in He II. This sounded paradoxical, since He-II has a record high thermal conductivity and it was considered impossible for the existence of noticeable local overheating there.
First of all, it was necessary to suppose that the threshold for breakdown of high quantum thermal conductivity at a heat flux of several W cm −2 is valid for nano-scale bodies as well. Then, assuming that the coagulation process was adiabatic, a formula was derived for the diameter of the resulting nanowire.
But anymore the most convincing proof of the existence of huge overheating during condensation would be the detection of intense thermal radiation from the condensation zone. Optical studies carried out in the visible range showed:
1. Coagulation of metals in He II leads to intense luminescence, whose brightness temperature correlates with the melting point of a given metal.
2. The time dependences of the intensity of luminescenceand brightness temperature indicate the adiabatic coagulation and the radiative character of the metal cooling.
3. Coagulation kinetics and the value of overheating both indicate that the coagulation process (fusion of particles of a close radius)prevails over accretion (adherence of small particles to the heavy centre) in He II.
Prof. Eugene Gordon
Russian Academy of Sciences