We report temperature (T = +22.5 ∼ −57.0 °C)-controlled optical trapping of single dimethylsulfoxide (DMSO) droplets with the diameter (d) of 7−15 μm in air. Optically levitated DMSO microdroplets containing 0.1 mol/dm 3 (=M) potassium iodide (KI) as an additive for reducing the vapor pressure of DMSO in air have been suggested to take supercooled liquid states even below the freezing temperature (f p ) of the bulk DMSO liquid (f p = +18.4 °C in the presence of 0.1 M KI) as seen in bright-field microscopic observations of the droplet. Clear evidence for supercooling of an aerosol DMSO microdroplet below f p has been obtained by in situ optical trapping−polarized Raman microspectroscopy of the droplet down to −14.9 °C. Analysis of the polarized Raman spectral data of an aerosol DMSO droplet (d = ∼10 μm) has demonstrated that the droplet at +22.5, +0.2, or −14.9 °C is characterized by the rotational relaxation time (τ rot ) of a DMSO molecule in the droplet being 1.95, 2.58, or 3.90 ps, respectively. On the basis of the τ rot values and the Stokes−Einstein equation (τ rot = 8πa 3 η/k B T where a, η, k B are the radius (1.883 Å) of a DMSO molecule, the viscosity in DMSO, and the Boltzmann constant, respectively), the η values in the DMSO microdroplet in air at +22.5, +0.2, or −14.9 °C have been estimated to be 2.39, 2.94, or 4.20 cP, respectively, while that of bulk DMSO liquid at +20.5 °C is 1.98 cP. We also report the T-dependence (+22.5 > T > −14.9 °C) of the viscosity in a single aerosol DMSO microdroplet (d = ∼10 μm) and the effects of aerosolization in air on the viscosity in DMSO.

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/N2 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 (τrot) 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 (τvib) in the aerosol and bulk EtOH systems were almost comparable with one another: 0.86–0.98 ps. Furthermore, although the τvib value of an aerosol EtOH microdroplet was almost unchanged irrespective of d as described above, the τrot 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 τrot 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.