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In physics and engineering, **mass flux** is the rate of mass flow. Its SI units are kg m^{−2} s^{−1}. The common symbols are *j*, *J*, *q*, *Q*, *φ*, or Φ (Greek lower or capital Phi), sometimes with subscript *m* to indicate mass is the flowing quantity. Mass flux can also refer to an alternate form of flux in Fick's law that includes the molecular mass, or in Darcy's law that includes the mass density.^{[1]}

Sometimes the defining equation for mass flux in this article is used interchangeably with the defining equation in mass flow rate. For example, *Fluid Mechanics, Schaum's et al* ^{[2]} uses the definition of mass flux as the equation in the mass flow rate article.

Mathematically, mass flux is defined as the limit

For mass flux as a vector **j**_{m}, the surface integral of it over a surface *S*, followed by an integral over the time duration *t*_{1} to *t*_{2}, gives the total amount of mass flowing through the surface in that time (*t*_{2} − *t*_{1}):

The area required to calculate the flux is real or imaginary, flat or curved, either as a cross-sectional area or a surface.

For example, for substances passing through a filter or a membrane, the real surface is the (generally curved) surface area of the filter, macroscopically - ignoring the area spanned by the holes in the filter/membrane. The spaces would be cross-sectional areas. For liquids passing through a pipe, the area is the cross-section of the pipe, at the section considered.

The vector area is a combination of the magnitude of the area through which the mass passes through, *A*, and a unit vector normal to the area, . The relation is .

If the mass flux **j**_{m} passes through the area at an angle θ to the area normal , then

Consider a pipe of flowing water. Suppose the pipe has a constant cross section and we consider a straight section of it (not at any bends/junctions), and the water is flowing steadily at a constant rate, under standard conditions. The area *A* is the cross-sectional area of the pipe. Suppose the pipe has radius *r* = 2 cm = 2 × 10^{−2} m. The area is then

Substituting the numbers gives:

Using the vector definition, mass flux is also equal to:^{[3]}

where:

- ρ = mass density,
**u**= velocity field of mass elements flowing (i.e. at each point in space the velocity of an element of matter is some velocity vector**u**).

Sometimes this equation may be used to define **j**_{m} as a vector.

In the case fluid is not pure, i.e. is a mixture of substances (technically contains a number of component substances), the mass fluxes must be considered separately for each component of the mixture.

When describing fluid flow (i.e. flow of matter), mass flux is appropriate. When describing particle transport (movement of a large number of particles), it is useful to use an analogous quantity, called the **molar flux**.

Using mass, the mass flux of component *i* is

The **barycentric mass flux** of component *i* is

- ρ = mass density of the entire mixture,
*ρ*= mass density of component_{i}*i*,**u**_{i}= velocity of component*i*.

The average is taken over the velocities of the components.

If we replace density ρ by the "molar density", concentration c, we have the **molar flux** analogues.

The molar flux is the number of moles per unit time per unit area, generally:

So the molar flux of component *i* is (number of moles per unit time per unit area):

Mass flux appears in some equations in hydrodynamics, in particular the continuity equation:

Molar flux occurs in Fick's first law of diffusion:

**^**"Thesaurus: Mass flux". Retrieved 2008-12-24.^{[permanent dead link]}**^**Fluid Mechanics, M. Potter, D.C. Wiggart, Schuam's outlines, McGraw Hill (USA), 2008, ISBN 978-0-07-148781-8**^**Vectors, Tensors, and the basic Equations of Fluid Mechanics, R. Aris, Dover Publications, 1989, ISBN 0-486-66110-5