Jump and boundary conditions at interfaces
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The conservation equations introduced above can be applied within each phase and up to an interface. However, they are not valid across the interface, where sharp changes in various properties occur. Appropriate boundary conditions at the interface must be specified in order to solve the governing equations for heat, mass, and momentum transfer in the two adjoining phases. The interface conditions will serve as boundary conditions for the transport equations in the adjacent phases. Jump conditions at the interface can be obtained by applying the basic laws (conservation of mass, momentum, energy, and the second law of thermodynamics) at the interface. It is the objective of this subsection to specify mass, momentum, and energy balance at a non-flat liquid-vapor interface (see figure), as well as species balance in solid-liquid-vapor interfaces. For solid-liquid or solid-vapor interfaces, these jump conditions can be significantly simplified.
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Conservation of mass at interface
See Main Article Conservation of mass at interface.
Conservation of momentum at interface
See Main Article Conservation of momentum at interface.
Conservation of energy at interface
See Main Article Conservation of energy at interface.
Conservation of mass species at interface
For a general interface between phases k and j in a multi-component system, a local balance in mass flux of species i must be upheld. The total species mass flux, 
, at an interface is:
See Main Article Conservation of mass species at interface.
Rarefied vapor self-diffusion model
Neglecting the presence of noncondensable gases, the rarefied vapor flow can be simulated by a self-diffusion model. The term self-diffusion here means the interdiffusion of particles of the same mass due to a gradient in density.
See Main Article Rarefied vapor self-diffusion model.