In film-wise condensation, the surface over which the steam condenses is wet-able and hence, as the steam condenses, a film of condensate is formed. Generally, film-wise condensation results in low heat transfer rates as the film of condensate impedes the heat transfer. The thickness of the film formed depends on many parameters including orientation of the surface, viscosity, rate of condensation etc.
Drop-wise condensation takes place when the surface over which condensation takes place is non wet-able. In this mode, when steam condenses, the droplets are formed. When the drops become bigger, they simply fall under the gravity. In drop-wise condensation, high heat transfer rates are achieved and hence, many times, chemicals are used to ensure that condensation takes place drop-wise.