Optical Trapping-Microspectroscopy of Single Aerosol Microdroplets in Air: Supercooling of Dimethylsulfoxide...

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/dm³ (=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 = 8πa³η/k_BT 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.