# Jump and boundary conditions at interfaces

Shape of the liquid-vapor interface near a vertical wall.

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.

## Conservation of mass at interface

${\dot m''_\delta } = {\rho _\ell }\left( {{V_{\ell ,{\mathbf{n}}}} - {V_{I,{\mathbf{n}}}}} \right) = {\rho _v}\left( {{V_{v,{\mathbf{n}}}} - {V_{I,{\mathbf{n}}}}} \right)$

See Main Article Conservation of mass at interface.

## Conservation of momentum at interface

${p_v} - {p_\ell } = \sigma (T)\left( {\frac{1}{{{R_I}}} + \frac{1}{{{R_{II}}}}} \right) - {p_d}$

See Main Article Conservation of momentum at interface.

## Conservation of energy at interface

$\left( {{k_v}\nabla {T_v} - {k_\ell }\nabla {T_\ell }} \right) \cdot {\mathbf{n}} = {\dot m''_\delta }{h_{\ell v}}$

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, ${\dot m''_i}$, at an interface is:

${\dot m''_i} = {\rho _{k,i}}\left( {{{\mathbf{V}}_{k,i}} - {{\mathbf{V}}_I}} \right) \cdot {\mathbf{n}} = {\rho _{j,i}}\left( {{{\mathbf{V}}_{j,i}} - {{\mathbf{V}}_I}} \right) \cdot {\mathbf{n}}$

See Main Article Conservation of mass species at interface.

## Supplementary Conditions

See Main Article Supplementary Conditions.