# What is the exact difference between diffusion, convection and advection?

I have tried to explore the information but still not very clear on the exact difference between diffusion, convection and advection. Can anyone help me out to clear my concept?

• Mar 3, 2015 at 19:34

Convection is the collective motion of particles in a fluid and actually encompasses both diffusion and advection.

• Advection is the motion of particles along the bulk flow
• Diffusion is the net movement of particles from high concentration to low concentration

We typically describe the above two using the partial differential equations: \begin{align} \frac{\partial\psi}{\partial t}+\nabla\cdot\left(\mathbf u \psi\right)&=0\tag{advection}\\ \frac{\partial\psi}{\partial t}&=\nabla\cdot\left(D\nabla\psi\right)\tag{diffusion} \end{align} where $\psi$ is the quantity in consideration, $\mathbf u$ is the fluid velocity and $D$ the diffusion coefficient (sometimes called the diffusivity).

There are some nuances to the combined effect for convection (e.g., forced, natural, gravitational mechanisms), but the general definition for it is the total motion.

• As mentioned in the related question physics.stackexchange.com/q/24489/2451, convection generally involves the idea of a heat transfer in the flow which is responsible of the collective motion. Mar 5, 2015 at 15:41
• @Lalylulelo: You are speaking from a thermodynamics point of view. From a fluid dynamics point of view (which is mine, as it's the focus of my research), convection is as I defined it: the collective motion of a fluid regardless of cause. Mar 5, 2015 at 15:51

That is, convection is the sum of fluid movement due to bulk transport of the media (like the water in a river flowing down a stream - advection) and the brownian/osmotic dispersion of a fluid constituent from high density to lower density regions (like a drop of ink slowly spreading out in a glass of water - diffusion).

• This might be better if you expanded on this some. Mar 3, 2015 at 20:03
• I dunno, the collective answer set has covered it pretty well. Mar 3, 2015 at 20:18

Diffusion is when single particles move about and transports its momentum and energy to other particles. Convection is a large movement (in roughly the same direction) of a large mass of particles. For the difference see this.

Mixing of a drop of ink in a glass of water is diffusion. Mixing of a drop of ink (chemicals) in a flowing river is both diffusion and advection. Think about tornadoes, anything that falls in the vortex of tornado tears down (diffusion), mixes up in the circular motion (advection) and carries away with the tornado (movement of the vortex) (convection). Hope it helps.

• Hi @MotaMota, welcome to Physics.SE. This is a very late response to a question that already has an accepted answer. The answer you have given here is also fairly unclear and imprecise. In general it is better to answer more recent (and)or unanswered questions, with more concrete answers. Dec 21, 2015 at 1:42

Diffusion: particules under random motion, go from high concetration to low concentration zone( cause they collide together), so they are'nt external force (pression gradient, electric field). Advection: particules moves under EXTERNAL FORCE. So that it the major difference, it's the origin of the force(collision or external force). I will add that these 2 mechanism can be added, or compete (like in the cells).

Let me try to give you a visual picture of these terms in the advection -diffusion equation(ADE): Coffee cream mixing Advection (1st term in ADE): It is the mechanical motion of the fluid. For example, stirring a coffee cream by spoon.

Diffusion(2nd term in ADE): After this mechanical/spoon action, cream will stretch and fold and form thin filaments. At this time we see a blur cream. This is called molecular diffusion. So, advection increases the concentration gradients in the fluid whereas diffusion wipe out these gradients.

Notice that advection is crucial in order to achive mixing (homogenisation of cream/sugar). In the absence of advection, diffusion will take months to mix your coffee (that will be boring).