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# Questions tagged [diffusion]

Diffusion is the net movement (spreading out) of molecules or atoms down a concentration gradient: from a region of high concentration to a region of low concentration.

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### Diffusion of perfume according to fick's law

Fick's first law relates the diffusive flux to the concentration under the assumption of steady state. It postulates that the flux goes from regions of high concentration to regions of low ...
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### Water equilibrium while dying of thirst in a confined space

I'm trying to understand what happens with the water while a person is dying from thirst in a deep, narrow mine shaft. Assuming he has dry food for a long time, and some liters of water. Dry air is ...
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### Diffusion equation with external forces

I am given the diffusion equation including the external forces as follows: $$\frac{\partial c}{\partial t} = D\frac{\partial^2 c}{\partial x^2} - \frac{F}{\gamma} \frac{\partial c}{\partial x}$$ ...
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### Expression for “rotational diffusivity”; orientation random-walk of thin rod-like particles?

From this answer and from the Stokes-Einstein equation the diffusivity of a particle of radius $R$ in a fluid of viscosity $\eta$ is $$D=\frac{k_B T}{6 \pi \eta R}$$ where $\xi=6 \pi \eta R$ is a ...
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### Difference between diffusion and interface mobility?

I have had this question when reading about the growth of ferrite (in steel) that is controlled by carbon diffusion and by interface mobility of Carbon. I would like to know the difference between ...
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### Pouring Soda At A Constant Angle And Minimizing Filltime While Accounting For $\mathrm{CO}_2$

I was pouring soda, when the act made me think of a question. If you have an unopended $2L$ bottle of soda, and you wish to fill up a generic cylindrical glass cup - if you hold the $2L$ bottle at a ...
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### Why doesn't the diffusivity of a particle in a fluid depend on the particle's density?

From this answer and from the Stokes-Einstein equation the diffusivity of a particle of radius $R$ in a fluid of viscosity $\eta$ is $$D=\frac{k_B T}{6 \pi \eta R}$$ where $\xi=6 \pi \eta R$ is a ...
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### what's the difference between viscous flow and diffusion

For a one-componet gas flow in a channel. There are a pressure i.e density difference between two ends of the channel. There will be a mass flux. My question is how to describe this flux? it's a ...
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### The influence of a temperature gradient on the growth of dendrites in a solid liquid interface

Not sure if this is the correct place to ask... My book states that, for a Solid Liquid interface in which the liquid has an increasing positive temperature gradient (increasing temperatures in ...
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### Why and how do holes in the valence band of the p-type material of a p-n junction diffuse through to the valence band of the n-type material

tl;dr -- See the question title I quote from this Wikipedia article on the depletion region: By definition, the N-type semiconductor has an excess of free electrons (in the conduction band) ...
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### Why don't we use Leibniz integral rule when solving Diffusion equation using the Fourier transform?

My question concerns the solution to the diffusion equation: $$\frac{\partial{p(x,t)}}{\partial{t}}=D\frac{\partial^2{p(x,t)}}{\partial{x}^2}~.\tag{1}\label{1}$$ I have a question about the solution ...
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### What formula can serve as an approximate estimate of the time taken for the smell of a perfume to reach somebody?

I am in an attempt to calculate the time required for the smell of a bottle of perfume to reach a person's nose $10$m away. Real life experience tells me that it takes several seconds. I tried to work ...
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### Classification of 2D time dependent diffusion equation

I was trying to classify the following PDE: $$\frac{\partial{u}}{\partial{t}}=\frac{\partial^2{u}}{\partial{x^2}}+\frac{\partial^2{u}}{\partial{y^2}}$$ where $u = u(x,y,t)$. I was originally using ...
Morse and Feshbach state without elaboration that the diffusion equation for temperature or concentration $\psi$ and its "conjugate" $\psi^*$ (quotation marks theirs) has Lagrangian density: L=-\...