Measurement of droplet properties at atmospheric temperature using a containerless method*

Ambient aerosol particles can be organic and inorganic, with a size range of 10nm to 10 micrometers. These particles impact air quality, clouds, climate, and life on earth by affecting the scattering of sunlight, serving as cloud and ice nucleation nuclei. Classically, these atmospheric aerosol particles are considered a liquid phase; consequently, the viscosity is assumed to be low. Recently, it has been reported that aerosol particles also exist in the form of semi-solid or even amorphous solid-like particles depending on their physical or chemical properties. The information on the particle phase stage is crucial for many atmospheric processes like ice nucleation, water transport, gas-particle interactions, Chemical aging, formation and partitioning of aerosols, and multiphase and heterogeneous reactions.  There are several experimental approaches, such as  Particle rebound, Dimer relaxation, Light scattering, and Shape relaxation, have been developed to probe the viscosity of aerosol particles. However, none of the available techniques is sufficiently versatile to determine aerosol viscosity at atmospherically relevant conditions for various particle sizes, chemical compositions, and sample sizes. Here we present a novel way to measure the viscosity of droplets suspended in an electrodynamic balance under atmospheric conditions. In the present technique, we have used thermodynamically metastable and kinetically stagnant states of Lithium chloride and water droplets at microscopic volumes. The technique involves induced capillary oscillations on the charged droplet using externally applied excitation frequency, and the viscosity is determined by correctly identifying the phase lag between droplet surface oscillations and applied excitation signal.