Viscosity of erythritol and erythritol-water particles as a function of water activity: New results and an intercomparison of techniques for measuring the viscosity of particles
Abstract. A previous study reported an uncertainty of up to 3 orders of magnitude for
the viscosity of erythritol (1,2,3,4-butanetetrol)–water particles. To help
reduce the uncertainty in the viscosity of these particles, we measured the
diffusion coefficient of a large organic dye (rhodamine B
isothiocyanate–dextran, average molecular weight ∼ 70 000 g mol - 1 ) in an erythritol–water matrix as a
function of water activity using rectangular-area fluorescence recovery after
photobleaching (rFRAP). The diffusion coefficients were then converted to
viscosities of erythritol–water particles using the Stokes–Einstein
equation. In addition, we carried out new viscosity measurements of
erythritol–water particles using aerosol optical tweezers. Based on the new
experimental results and viscosities reported in the literature, we conclude
the following: (1) the viscosity of pure erythritol is 184 - 73 + 122  Pa s (2
standard deviations); (2) the addition of a hydroxyl (OH) functional group to
a linear C4 carbon backbone increases the viscosity on average by a
factor of 27 - 5 + 6 (2 standard deviations); and (3) the increase in
viscosity from the addition of one OH functional group to a linear
 C4 carbon backbone is not a strong function of the number of OH
functional groups already present in the molecule up to the addition of three
OH functional groups, but the increase in viscosity may be larger when the
linear C4 carbon backbone already contains three OH functional
groups. These results should help improve the understanding of the viscosity
of secondary organic aerosol particles in the atmosphere. In addition, these
results show that at water activity>0.4 the rFRAP technique,
aerosol optical tweezers technique, and bead-mobility technique give results in
reasonable agreement if the uncertainties in the measurements are considered.
At water activity , the mean viscosity values determined by
the optical tweezers technique were higher than those determined by the
bead-mobility and rFRAP techniques by 1–2 orders of magnitude. Nevertheless,
the disagreement in viscosity measured using multiple techniques reported in
this paper is smaller than reported previously.