Pressure trace measurements and small angle neutron scattering (SANS) were used to probe the binary condensation of D2O-H2O mixtures in a supersonic nozzle. Each expansion started from the same initial pressure (carrier gas and condensible vapor) and temperature. The partial pressures of D2O and H2O were adjusted so that the onset of condensation always occurred at the same position in the nozzle. Under these conditions, the total pressure of condensible at onset varied linearly between the pressures of the pure components. Furthermore, the partial pressure at onset for pure H2O was 29–34% higher than that for pure D2O. The SANS scattering signals also varied systematically with the mixture composition. As the mixtures became progressively richer in H2O, the scattering intensity dropped rapidly because the scattering length of H2O is much smaller in magnitude and of opposite sign to that of D2O. Further analysis of the scattering spectra showed that the particle size of the aerosol was increasing as the mixtures became more water rich. The increase in particle size was consistent with the increase in condensible molar flowrate needed to maintain onset at a fixed position in the nozzle as the mixture becomes richer in H2O.

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