In a solar thermal distillation still, the condensation occurs on glass cover. The drops grow in size and at a critical size starts sliding down the glass cover. On its way downwards, the droplet grow in size by direct condensation and coalesce by other droplets. However, if the substrate (glass cover) length is too large, the droplet will become heavy and the viscous forces between the droplet and glass will no more be able to hold the droplet, thus, the droplet fall off. This depends on contact angle of droplet on substrate, vapor flux, inclination of substrate etc.
You can use VOF (volume of fluid) multiphase modelling method coupled with level set (LS) algorithm in FLUENT to model the droplet motion. The contact angle on the substrate (wall) can be provided as boundary condition. However if you would like model condensation of vapour to form droplet, appropriate source terms for heat transfer must be added to the continuity equation.
few references to model droplet motion:
http://www.sciencedirect.com/science/article/pii/S000925091300050X
http://www.sciencedirect.com/science/article/pii/S0921883114002702
You might also consider a phase-field model, which can treat two-phase equilibrium while rigorously incorporating surface energy and contact angles of arbitrary shapes. It is a great method for studying coalescence, and it shouldn't be too difficult to add gravity (maybe it's already been done?).
Here is a relevant paper:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.035703
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