Molecular flux
Definition
Diffusion can be defined as the transport of material due to the motion of the atoms. Whenever there is a concentration gradient in the fluid, the diffusion occurs. The diffusion flux is defined by the Flucks law and it states that whenever there is a concentration gradient, the diffusion occurs and the negative of this change in concentration gradient with respect to position is directly proportional to the diffusion flux. The corresponding mathematical expression is given below as follows:
 J=χ δC/δx
Here,
 J = Diffusion Flux
 χ = Diffusivity
 δC/δx = Concentration Gradient
When motion is down the gradient, the negative sign in the expression makes the flux positive. In the direction of increasing x, the negative gradient drives the flux ^{[1]}.
Diffusive flux describes the transport of a quantity in response to a gradient. This is in contrast to the transport of a quantity due to bulk fluid motion. Diffusion can occur in the presence of bulk fluid motion or in the absence of bulk fluid motion as long as the gradient exists. Two important forms of diffusive flux exist in transport phenomena:
 Fourier's law of conduction: The flux of heat due to a temperature gradient
 Fick's law of diffusion: The flux of a chemical species due to a concentration gradient
Flux has units of per unit area per unit time. If the flux of is represented as the total rate that enters and leaves a control volume due to diffuse flow is

(1) 
 ↑ [Steven L. Jacques, S. A. (n.d.). Diffusion Theory. Retrieved April 24, 2019, from Fick's 1st law of diffusion: https://omlc.org/classroom/ece532/class5/ficks1.html