Abstract
The ISO standard protocol for determining size of droplets suspended between two layers of silicone oils of differing viscosity was evaluated in experiments using a monosize droplet generator. Cross-sectional areas of 0.27-nl droplets captured in silicone oils decreased linearly with time until droplets disappeared at 26 min after application. Rates of decrease in cross-sectional area of droplets were independent of droplet size (range 0.27–269 nl), but decreased as the gradient in water vapour concentration between droplet and ambient atmosphere decreased. Saturating silicone oils with water reduced rates of decrease in cross-sectional areas of droplets (−37%). Diffusion studies established that the amount of water diffusing through silicone oil increased linearly with time. Permeance to water vapour was inversely related to thickness of the oil layer. Permeability coefficients averaged 45.8×10 −9 and 29.5×10 −9 m 2 s −1 for low- (9.6 cSt) and high-viscosity (10 350 cSt) silicone oil and were up to 52-fold higher than for mineral oils of comparable viscosity. Our data suggest that the high water vapour permeability of silicone oil and the solubility of water in silicone oil accounted for the decrease in size of droplets captured in silicone oil. Based on our findings, the ISO standard protocol should be modified to correct for these effects.
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