Optical Trapping–Polarized Raman Microspectroscopy of Single Ethanol Aerosol Microdroplets: Droplet Size Effects on Rotational Relaxation Time and Viscosity

摘要

Optical trapping–polarized Raman microspectroscopy of single ethanol (EtOH) microdroplets with a diameter (d ) of 6.1–16.5 μm levitated in an EtOH vapor-saturated air/N_(2) gas atmosphere has been explored to elucidate the vibrational and rotational motions of EtOH in the droplets at 22.0 °C. The Raman spectral bandwidth of the C–C stretching vibrational mode observed for an aerosol EtOH microdroplet was narrower than that of bulk EtOH, suggesting that the vibrational/rotational motions of EtOH in the aerosol system were restricted compared to those in the bulk system. In practice, polarized Raman microspectroscopy demonstrated that the rotational relaxation time (τ_(rot)) of EtOH in an aerosol microdroplet with d = 16. 5 μm was slower (2.33 ps) than that in a bulk EtOH (1.65 ps), while the vibrational relaxation times (τ_(vib)) in the aerosol and bulk EtOH systems were almost comparable with one another: 0.86–0.98 ps. Furthermore, although the τ_(vib) value of an aerosol EtOH microdroplet was almost unchanged irrespective of d as described above, the τ_(rot) value increased from 2.33 to 3.57 ps with a decrease in d from 16.5 to 6.1 μm, which corresponded to the increase in EtOH viscosity (η) from 1.33 to 2.04 cP with the decrease in d . The droplet size dependences of τ_(rot) and η in an aerosol EtOH microdroplet were discussed in terms of the gas/droplet interfacial molecular arrangements of EtOH and Laplace pressure experienced by a spherical EtOH microdroplet in the gas phase.