Average size and size distribution of large droplets produced in a free-jet expansion of a liquid

The experimental parameters and fluid properties affecting the average size (N) over bar and the size distribution P(N) of droplets formed by fragmentation of a liquid after expansion into a vacuum are investigated. The mean droplet size is found to be a function of the surface tension of the liquid, the nozzle diameter, and a characteristic flow speed. The size distribution is found to be a linear exponential distribution; measurements deviate from this distribution at small sizes if a factor which is a function of the cluster size is included in the measuring process. Good agreement with measured distributions of both positive and negative droplet ions formed from neutral He-4 droplets by electron impact is found. The strong dependence of mean droplet size on source-orifice diameter found in the present analysis indicates that earlier correlations of droplet size with specific entropy in the source were useful at best only for a fixed nozzle size. (C) 1999 American Institute of Physics. [S0021-9606(99)51505-5].