# Cause of the Soret Effect/Thermophoresis?

• Declan
In summary, the conversation is about a person trying to simulate a gas mixture of He and Ar and observing thermophoresis, with the Ar going to the hot side and He going to the cold side instead of the expected opposite. They are looking for an explanation for this effect and have found some resources to further understand it. There is also a discussion about the Soret coefficient and the Dufour effect, and the question of whether or not there is convection involved in thermophoresis.
Declan
Hi everyone! I'm simulating a gas mixture of He and Ar, and I'm trying to get them to separate via Thermophoresis. For clarification, http://en.wikipedia.org/wiki/Thermophoresis

When I naively try it with what I have so far in my program, it exhibits Thermophoresis...but with the Ar going to the hot side and He going to the cold side! The wiki says it should generally be the opposite (lighter molecule, He, going to the hot side), and I was told by my professor to expect this as well.

Generally, when something doesn't work, I try to understand the physics behind what should be happening, and go from there. But I've got to admit...I have no idea why this effect happens! I tried a bit of googlin', but came up with nothing. I can't really think of why the gas would separate.

Right now, I'm putting a 50/50 He/Ar mix between one plate at 500K and another plate at 5K. The distance between the plates is about a micron and a half. This does seem like a really high temperature gradient, so maybe that's messing something up. But then again, I don't really know why this works or why what I'm doing could be bad.

Does anyone know? Can anyone help me?

Thanks!

Interesting...

The Soret coefficient comes out of the Onsager reciprocal relations, and couples diffusion and thermal conduction. The inverse effect (concentration gradient leading to the temperature difference) is called the Dufour effect.

I don't have much material on it, but there's a section in DeGroot's "Thermodynamics of irreversible processes"

I also found this:

and this (measurements at lower temps, but maybe applicable):

http://pof.aip.org/resource/1/pfldas/v4/i10/p1216_s1

Last edited by a moderator:
Andy Resnick said:
Interesting...

The Soret coefficient comes out of the Onsager reciprocal relations, and couples diffusion and thermal conduction. The inverse effect (concentration gradient leading to the temperature difference) is called the Dufour effect.

I don't have much material on it, but there's a section in DeGroot's "Thermodynamics of irreversible processes"

I also found this:

and this (measurements at lower temps, but maybe applicable):

http://pof.aip.org/resource/1/pfldas/v4/i10/p1216_s1

Cool, I'll take a look at those. Thanks!

Looking at the working examples my professor showed me, he the gas in between a plate at 300K and one at 2000K...with the plates being 1m apart. So, relatively, that's a waaay smaller temperature gradient. It's possible it breaks or something when you're doing such extreme conditions as mine.

Well, let me go read those.

Last edited by a moderator:
Hmmm. Read a bit, can't say I'm clearer on it.

A question about thermophoresis, if you know the answer: once the gas has separated, it should roughly stay in place, right? As in, there isn't much convection going on, right?

No clue, sorry...

## What is the Soret Effect/Thermophoresis?

The Soret Effect/Thermophoresis is a phenomenon in which particles in a fluid move due to a temperature gradient. This movement is caused by the difference in the thermophoretic mobility of the particles, which is a measure of their ability to move in response to a temperature gradient.

## What is the cause of the Soret Effect/Thermophoresis?

The cause of the Soret Effect/Thermophoresis is the difference in the thermodiffusion coefficients of the particles in a fluid. This difference leads to a net movement of particles from regions of high temperature to regions of low temperature, creating a thermal gradient.

## What factors affect the strength of the Soret Effect/Thermophoresis?

The strength of the Soret Effect/Thermophoresis is affected by several factors, including the size and shape of the particles, the properties of the fluid, the temperature gradient, and the strength of the intermolecular interactions between the particles and the fluid.

## What applications does the Soret Effect/Thermophoresis have?

The Soret Effect/Thermophoresis has several applications, including separation and purification techniques, drug delivery systems, and microfluidic devices. It can also be used to study the properties of fluids and particles at a microscopic level.

## How is the Soret Effect/Thermophoresis studied and measured?

The Soret Effect/Thermophoresis can be studied and measured using various experimental techniques, such as optical microscopy, particle tracking, and thermophoresis velocimetry. Mathematical models and computer simulations are also used to understand and predict the behavior of particles in a fluid under the influence of a temperature gradient.

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