Table 1 Comparison of aerosol viscosity measurement techniques and capabilities

Fluorescence lifetime imaging (FLIM)

10⁻³–10³

0.5–100

Ambient

T range can be extended by using a cooled/heated microscope stage. The upper viscosity limit could be extended using different molecular rotors

^35,43,44,45

Particle rebound

10⁰–10²

<0.4

Ambient

RH dependence of rebound fraction measured to identify moisture content that gives rise to viscosity range specified

^46,47

Dimer relaxation

5 × 10⁵–2 × 10⁷

~0.05–0.2

−15 to +80 °C

Identify T/RH conditions for transition in viscosity. Extrapolation to find T_g possible. Exact size and temperature ranges depend on many parameters

^48,49

Light scattering

~10⁷

~0.1–0.9

−38 to +10 °C

Transition in depolarisation ratio (minimum size limit specified) from non-spherical to spherical shape on measurement timescale occurs at the viscosity indicated

⁵⁰

Shape relaxation

10⁵–10¹¹

<0.2

Ambient

Combination of particle diameter and mass measurements used to estimate shape factor

⁵¹

SEM imaging

10⁻¹–10¹²

0.3–2

Ambient

Qualitative distinction between ‘solid’, ‘semi-solid’ and ‘liquid’ particles for complex field and laboratory-generated samples

⁵²

Aerosol optical tweezers

10⁻³–10⁹

5–20

Ambient

Requires mL of sample. Measurable coalescence timescales span 10⁻⁶ to 10⁵ s. RH dependence of viscosity

^53,54,55,56

Bead mobility

10⁻³–10³

30–50

Ambient

Samples collected with impactors can be analysed. RH dependence on viscosity can be determined

⁵⁷

Poke flow

10³–10⁷

25–75

Ambient

Samples collected with impactors can be analysed. Uncertainties in the measured viscosities are large due to assumptions used when determine viscosity

^58,59

Bulk viscometry/rheology

10⁻³–10⁸ typical

Bulk

−40 to 200 ^oC typical

Crossover in storage and loss modulus allows identification of any phase state change

⁶⁰