ANNUAL REPORT 2011 - Instituto de Estructura de la Materia

pure oxygen from stagnation pressuresabout 10 times higher (~ 2 bar); with these pumps we expect the jet to reachrotational temperatures between 4 and 10 K, as well as to observe the formation of dimers and small O 2 clusters. Wehave purchased and installed a new controlled evaporator-mixer to produce jets of gaseous mixtures of H 2 O dilutedin He or H 2 , since their collisions, of astrophysical interest, are the subject of current research projects.We have also built an ortho-para hydrogencatalytic converter, improving an original design from the Max-Planck-Institut für Strömungsforschung in Göttingen (Germany). This new converter can operate with steady flow rates upto 5.4 g/hour, yielding less than 0.4% residual ortho-H 2 . Such a high purity is critical for the experiments oncondensation, since small amounts of impurities can significantly affect the nucleation rate. A second"uncatalyzed"line through the convertercan beused as a cryogenic trap for preparation of mixtures. It must bestressed that much of the necessary high precision parts have been machined in the Mechanical Workshop ofCFMAC, whose support hasbeen essential. Our laboratory has now two converters, which allowed conducting theexperiments on mixtures of para-H 2 and ortho-D 2 described below.Liquid microjets (filaments) of para-H 2 and ortho-D 2 mixtures at 2%, 20% and 50% were produced on instrument(A), in a joint project with the University of Frankfurt (Germany). These filaments, 2 to 5 microns in diameter, arecooled by surface evaporation in vacuum, producing liquid samples highly undercooled below their melting point,until they solidify or break into droplets. Series of Raman spectra at different axial distances have been recorded onthese filaments, monitored by laser shadowgraphy, allowing us to track the crystallization with a time resolution of ~10 ns. It was found that small amounts of ortho-D 2 impurity delays significantly the crystallization rate of para-H 2 , aquantum effect not previously observed.On instrument (B) we have measured series of supersonic jets of pure H 2 O from a 350 micron nozzle at stagnationtemperature T0 = 398 K and pressures from p0 = 40 mbar to 400 mbar, in order to study the H 2 O:H 2 O inelasticcollisions. These measurements eventually showed that H 2 O condensation is always present for p0> 40 mbar,disturbing the thermal evolution of the jet and preventing the quantitative analysis of collisional kinetics. We havealso measured condensation-free mixtures of 5% H 2 O in He, reaching rotational temperatures of 36 K, the lowest sofar. These experiments are expected to yield valuable quantitative information onthe H 2 O:He collisions. To thisregard, some preliminary values of transfer rates by H 2 O:He inelastic collisions have been calculated by the group ofTheoretical Molecular Interactions and Dynamics of theInstituto de Física Fundamental CSIC, which works in closecollaboration with us.In the methodology section, we have confirmed the inadequacy of current models (based on the isentropicapproximation of an ideal gas) for an accurate description of the fluid dynamics of supersonic jets. Therefore, wehave developed "ex novo" a fluid dynamics theory, based on the rigorous physical principles of mass, momentumand energy conservation, treating the expanding gas as a real gas, with no idealization. This treatment can bridgefrom continuum mechanics to quantum mechanics in a rigorous way, and describes the gas media at the molecularlevel more adequately than through the intractable generalized Boltzmann equation. The theory reveals twodominant regimes in the jets, depending on whether the transfer of energy by inelastic collisions is "easy" or"difficult". The resulting fluid dynamic equations allow to interpret in a natural way the gradual breakdown ofthermal equilibrium along the jet, quantifying its properties in terms of collisions through the novel concept of gasdynamicheat capacity, which only at equilibrium converges into the conventional one. The developed theory alsoshows unambiguously that supersonic jets are not isentropic, although this approach may be useful under certainconditions, which are now clearly defined.69