The Aerochop company is based on a know-how of nearly thirty years in applications of aerodynamics to chemical physics and spectroscopy.
The technical solutions provided by Aerochop have been developed in the framework of fundamental research directed toward Astrochemistry. The study of chemical reactions and physical processes at extremely low temperature (down to 10 K) is mandatory for the understanding of interstellar medium where about 150 more or less complex molecules have been detected. The so-called “dense interstellar clouds » where the density is in the range 104-105 cm-3 and temperature close to 10K are the cradle of stars and their cortege of planets, the formation of which is a central problem in astrophysics. Very expensive programs are therefore partly devoted to their observation as the ALMA observatory.
Although less cold, the understanding of planetary atmospheres, like the Titan’s one, requires also a large amount of rate coefficients and branching ratios in the relevant temperature range.
From the experimental point of view the study of chemical reactions below liquid nitrogen temperature started in the early eighties, especially with the design by Bertrand Rowe of an apparatus known as CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) which allows to generate very low temperature (presently down to 6 K) by expanding a buffer gas through a Laval nozzle. Since no cryogenic cooling is required to achieve ultra low temperatures, heterogeneous condensation is avoided. Homogeneous nucleation is most often inhibited for thermodynamic and kinetic reasons. First devoted to ion-molecule reactions the technique has been extended, in collaboration with Ian Smith then at Birmingham University, to radical molecule reactions and to a variety of relaxation processes. These achievements have been awarded the Descartes prize (link) of the European community.
The technique has however a technical drawback. For fluid dynamics reasons uniformity of the flow is maintained along a large distance only for sufficiently large flow rates (typically 50 l/min CNTP) and avoiding problems with homogeneous nucleation requires a low pressure. Therefore large pumping speeds (~25000 m3/h) are necessary, at least to obtain the lowest temperatures. The CRESU facility is in fact designed as a rarefied wind tunnel (link ??) with very large pumps. To overcome this problem in the early 90’s, Mark Smith, at Arizona University, has developed a pulsed version of the CRESU allowing a significant reduction of the apparatus size and of the gas consumption. Various versions of this method have then been used at Berkeley (Steve Leone), Göttingen (Bernd Abel) and Leeds (Mike Pilling). The technique makes use of pulsed electromagnetic valve at the entrance of the resevoir nozzle. Up to now the lowest temperature achieved with these systems is 53 K.
At Rennes (France) another way of pulsing the flow has been developed by the “Astrochimie Expérimentale “team. It makes use of an « aerodynamic chopper » set on the diverging part of the Laval nozzle (see products) and the system can be used in a pseudo-continuous mode. The quality of the flow is as good as in the pure continuous mode and temperatures down to 23 K have been presently achieved. Remarkable economy (presently a factor of ten) in gas flow rate and pumping capacity can be achieved in this way.
The Aerodynamic chopper has been patented and a variety of technical development achieved in the framework of this fundamental research. Therefore the Aerochop company has been created in order to promote these technologies and to offer services with a unique range of skills and competences.