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Sea spray aerosols contain a large array of organic compounds that contribute to high viscosities at low relative humidity and temperature thereby slowing translational diffusion of water. The Stokes-Einstein equation describes how viscosity is inversely correlated with the translational diffusion coefficient of the diffusing species. However, recent studies indicate the Stokes-Einstein equation breaks down at high viscosities achieved in the particle phase (>10 12 Pa·s), underestimating the predicted water diffusion coefficient by orders of magnitude and revealing the need for directly studying the diffusion of water in single aerosol. A new method is reported for measuring the water diffusion coefficient in single suspended charged sucrose-water microdroplets in the 30-60 micron diameter range. The translational water diffusion coefficient is quantified using H 2 O/D 2 O isotope exchange technique between 26-54% relative humidity (RH) with a recently developed mobile electrodynamic balance apparatus. The results are in good agreement with literature, particularly the Vignes-type parameterization from experiments using isotope exchange and optical tweezers. This mobile electrodynamic balance will allow future studies of atmospherically relevant chemical systems, including field studies.
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Manifestations of static and dynamic heterogeneity in single molecule translational measurements in glassy systems
Rotational–translational decoupling in systems near T g , in which translational diffusion is apparently enhanced relative to rotation, has been observed in ensemble and single molecule experiments and has been linked to dynamic heterogeneity. Here, simulations of single molecules experiencing homogeneous diffusion and static and dynamic heterogeneous diffusion are performed to clarify the contributions of heterogeneity to such enhanced translational diffusion. Results show that time-limited trajectories broaden the distribution of diffusion coefficients in the presence of homogeneous diffusion but not when physically reasonable degrees of static heterogeneity are present. When dynamic heterogeneity is introduced, measured diffusion coefficients uniformly increase relative to input diffusion coefficients, and the widths of output distributions decrease, providing support for the idea that dynamic heterogeneity can drive apparent translational enhancement. Among simulations with dynamic heterogeneity, when the frequency of dynamic exchange is correlated with the initial diffusion coefficient, the measured diffusion coefficient behavior as a function of observation time matches that seen experimentally, the only set of simulations explored in which this occurs. Taken together with experimental results, this suggests that enhanced translational diffusion in glassy systems occurs through dynamic exchange consistent with wide underlying distributions of diffusion coefficients and exchange coupled to local spatiotemporal dynamics.
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- Award ID(s):
- 1954803
- PAR ID:
- 10427342
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 157
- Issue:
- 18
- ISSN:
- 0021-9606
- Page Range / eLocation ID:
- 184506
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0118892
