TRANSPORT-PROPERTIES OF GAS IN SILICA AEROGEL

The motion of gas molecules in silica aerogel is restricted by the solid silica matrix. As a result, the mean free path, velocity distribution function, diffusivity, viscosity and thermal conductivity are changed. In this work, relations for these quantities are derived for a gas in silica aerogel using an approach similar to that used for a gas in free space, Results for the mean free path predict that, for p greater than or equal to 10 bar, the mean free path of the gas molecules in aerogel will bs almost the same as in free space. However, as the pressure is reduced, the mean free path reaches a constant finite value instead of increasing as in free space. The thermal conductivity of a gas in aerogel starts to decrease It p = 10 bar and is almost negligible at 0.01 bar, while the thermal conductivity of a gas between parallel walls 1 cm apart starts to decrease at p = 10(-1) bar and is almost negligible at p = 10(-7) bar. The predicted thermal conductivity of a gas in aerogel is in good agreement with experimental results.