Droplet sizing using silicone oils

The ISO standard protocol for determining size of droplets suspended betwee n two layers of silicone oils of differing viscosity was evaluated in exper iments using a monosize droplet generator. Cross-sectional areas of 0.27-nl droplets captured in silicone oils decreased linearly with time until drop lets disappeared at 26 min after application. Rates of decrease in cross-se ctional area of droplets were independent of droplet size (range 0.27-269 n l), but decreased as the gradient in water vapour concentration between dro plet and ambient atmosphere decreased. Saturating silicone oils with water reduced rates of decrease in cross-sectional areas of droplets (- 37%). Dif fusion studies established that the amount of water diffusing through silic one oil increased linearly with time. Permeance to water vapour was inverse ly related to thickness of the oil layer. Permeability coefficients average d 45.8x10(-9) and 29.5 x 10(-9) m(2) s(-1) for low- 19.6 cSt) and high-visc osity (10 350 cSt) silicone oil and were up to 52-fold higher than for mine ral oils of comparable viscosity. Our data suggest that the high water vapo ur permeability of silicone oil and the solubility of water in silicone oil accounted for the decrease in size of droplets captured in silicone oil. B ased on our findings, the ISO standard protocol should be modified to corre ct for these effects. (C) 2001 Elsevier Science Ltd. All rights reserved.