Freezing of Heavy Water (D₂O) Nanodroplets

We follow the freezing of heavy water (D₂O) nanodroplets formed in a supersonic nozzle apparatus using position resolved pressure trace measurements, Fourier transform infrared spectroscopy, and small-angle X-ray scattering. For these 3–9 nm radii droplets, freezing starts between 223 and 225 K, at volume based ice nucleation rates <i>J</i>_ice,V on the order of 10²³ cm^–3 s^–1 or surface based ice nucleation rates <i>J</i>_ice,S on the order of 10¹⁶ cm^–2 s^–1. The temperatures corresponding to the onset of D₂O ice nucleation are higher than those reported for H₂O by Manka et al. [Manka, A.; Pathak, H.; Tanimura, S.; Wölk, J.; Strey, R.; Wyslouzil, B. E. <i>Phys. Chem. Chem. Phys.</i> <b>2012</b>, <i>14</i>, 4505]. Although the values of <i>J</i>_ice,S scale somewhat better with droplet size than values of <i>J</i>_ice,V, the data are not accurate enough to state that nucleation is surface initiated. Finally, using current estimates of the thermophysical properties of D₂O and the theoretical framework presented by Murray et al. [Murray, B. J.; Broadley, S. L.; Wilson, T. W.; Bull, S. J.; Wills, R. H.; Christenson, H. K.; Murray, E. J. <i>Phys. Chem. Chem. Phys.</i> <b>2010</b>, <i>12</i>, 10380], we find that the theoretical ice nucleation rates are within 3 orders of magnitude of the measured rates over an ∼15 K temperature range

Isothermal Nucleation Rates of n-Propanol, n-Butanol, and n-Pentanol in Supersonic Nozzles: Critical Cluster Sizes and the Role of Coagulation

We follow the nucleation of n-alcohols, n-propanol through n-pentanol, in two sets of supersonic nozzles having differing linear expansion rates. Combining the data from static pressure trace measurements with small-angle X-ray scattering we report the experimental nucleation rates and critical cluster sizes. For n-propanol, position resolved measurements clearly confirm that coagulation of the 2-10 nm size (radius) droplets occurs on the time scale of the experiment but that the effect of coagulation oh the results is minimal. Under the conditions of the current experiments, our results suggest that alcohols have critical clusters that range from the dimer (n-pentanol) to the hexamer (n-propanol). We then compare the experimental results with classical nucleation theory (CNT), the Girshick-Chiu variant of CNT (GC), and the mean field kinetic nucleation theory (MKNT). Both CNT and MKNT underestimate the nucleation rates by up to 5 and 7 orders of magnitude, respectively, while GC theory predicts rates within 2 orders of magnitude. Correspondingly, the critical cluster size for all alcohols is overpredicted by factors of 2-9 with agreement improving with increasing chain length. An interesting byproduct of our experiments is that we find that the coagulation rate is enhanced by a factor of 3 over the value one would calculate for the free molecule regime

Isothermal Nucleation Rates of <i>n</i>‑Propanol, <i>n</i>‑Butanol, and <i>n</i>‑Pentanol in Supersonic Nozzles: Critical Cluster Sizes and the Role of Coagulation

We follow the nucleation of <i>n</i>-alcohols, <i>n</i>-propanol through <i>n</i>-pentanol, in two sets of supersonic nozzles having differing linear expansion rates. Combining the data from static pressure trace measurements with small-angle X-ray scattering we report the experimental nucleation rates and critical cluster sizes. For <i>n</i>-propanol, position resolved measurements clearly confirm that coagulation of the 2–10 nm size (radius) droplets occurs on the time scale of the experiment but that the effect of coagulation on the results is minimal. Under the conditions of the current experiments, our results suggest that alcohols have critical clusters that range from the dimer (<i>n</i>-pentanol) to the hexamer (<i>n</i>-propanol). We then compare the experimental results with classical nucleation theory (CNT), the Girshick-Chiu variant of CNT (GC), and the mean field kinetic nucleation theory (MKNT). Both CNT and MKNT underestimate the nucleation rates by up to 5 and 7 orders of magnitude, respectively, while GC theory predicts rates within 2 orders of magnitude. Correspondingly, the critical cluster size for all alcohols is overpredicted by factors of 2–9 with agreement improving with increasing chain length. An interesting byproduct of our experiments is that we find that the coagulation rate is enhanced by a factor of 3 over the value one would calculate for the free molecule regime