Review of Singh et al. 2023

This paper describes a method for characterizing the surface tension and viscosity of levitated particles in an electrodynamic balance by analysis of the phase offset of the shape oscillations of the particle with respect to an external driver. This method builds off existing “single-shot” methods for characterizing these properties that induce these oscillations via coalescence. The authors provide data for model systems that show reasonable agreement to literature, and provide an extensive supporting information document detailing the method development and analysis procedures. Overall this is a very compelling method to probe important physicochemical properties of particles. 

In my responses below I highlight some information that could be provided more clearly, in addition to some other queries and suggestions.

1. What is the driver voltage amplitude and how is the appropriate magnitude of this chosen?
2. What is the uncertainty in the particle size, and how much of this translates to uncertainty in viscosity and surface tension? Equations 6 and 7 both show dependencies on R^2 or higher, indicating some significant propagation of error.  The authors chose to use the phase function approach, which for large particles can yield reasonable accuracy when comparing measured spectra to Mie theory simulations. There are several approaches to sizing using these data, as described in the SI. Thus, it would be convenient to describe how the angular scattering pattern is used in the main text along with the estimated uncertainty.
3. I would suspect that when the droplet is driven at a frequency resonant with a mode of oscillation that the amplitude of the oscillation would increase significantly, leading to a higher scattered signal from the droplet. Using a dual-phase lock-in, the amplitude of the scattering and the phase of the signal, both as a function of frequency, could be analyzed as the driver frequency is swept. Has amplitude information been measured and/or analyzed to identify the resonant frequency? This would presumably directly yield omega_0 for an underdamped oscillation, but perhaps not yield sufficient information on the damping constant for viscosity analysis. 
4. What are the limits of viscosity that can be probed with this method. The viscosity values reported here are all many orders of magnitude lower than what is typically considered to be a viscous particle. Is there a limit on the viscosity that can be measured due to the limited response of a more viscous particle to any kind of shape deformation? Is this connected with the deviation of the measure viscosity in this approach from the literature range (Figure 3 for example)? 
5. In the same vane, what are the limits of surface tension that can be measured? Would surfactant coated particles be accessible?
6. Were measurements performed on particles that spanned a range of charge states? When calculating the charge according to the SI, no gas flow drag factor is reported. How does the gas flow in the chamber affect the force balance? How big is the uncertainty in the charge and how much does this contribute to the calculation of surface tension and viscosity?
7. What is the timescale for evaporation in the measurements shown in Figure 6? Does the decrease in surface tension arise from the adsorption of know contaminants from an external source (i.e. are there lubricant oils that might be outgassing, low purity gas cylinders, etc.), or are these present in the particle and become more concentrated at the surface as evaporation occurs? 
8. How large are the oscillations in the particle shape? 
9. What size range of particles can be explored using this technique? Being able to access particles that span a wide range of surface-volume ratios would facilitate an exploration of surface partitioning and the influence of surfactant depletion etc. 

Review on Singh et al.: “The viscosity and surface tension of supercooled levitated droplets determined by excitation of shape oscillations”.

The authors develop a new technique for measuring viscosity and surface tension of levitated droplets in an electrodynamic balance (EDB) by superimposing a high frequency, high voltage field across the DC electrodes of the EDB. This field stimulates shape oscillations of the droplet, which can be detected by monitoring light scattering. The phase shift of these oscillations relative to the excitation field - when scanning the excitation field over a sufficient frequency range - allows deducing viscosity and surface tension of the droplet.

The new technique is a very interesting alternative to methods previously used; the paper is well written and supported by adequate figures and is definitely of interest to the readership of AMT.

I am very pleased recommending publication as is.

The only comment I would like to ask the authors to consider is that they explain/estimate in more detail the limitations of the technique in both upper viscosity values and range of surface tension are.

Technical comments:

I was particularly interested in understanding the upper limit for the viscosity. In order to gain a better feeling for the behavior of the phase shift, I put some numbers into eq. (7) to estimate the natural angular frequency, ω₀, namely (R=50 µm, σ=70 mN/m, ρ=1000 kg/m³, Q= 0.8 pC) and came up with 47 kHz. Using eq. (6) with a viscosity of 2 mPa s, I calculate a γ of roughly 4000. Putting those numbers into eq. (5), I cannot reproduce something similar to what is shown in Fig. 2. Most likely this is a mistake on my side, but the authors could provide in the SI some numbers on, ω₀ and provide a plot where they show the expected phase shift assuming log spaced viscosity data keeping all other parameters constant.

Fig. S4: Please explain what is causing the apparent decrease in droplet charge after 70 s in a bit more detail. What is the size of the particle at this time of evaporation? Is this reaching the stability limit of the EDB or is the DC-feedback loop to keep the droplet in the center of the EDB no longer working?

Connected to the data shown in Fig. S4: How is the flow in the EDB affecting the determination of Q based on the applied DC-field? Does the drag force cause a systematic uncertainty here?