What is the change in entropy for a colloid settling out of solution?

[Timtam]

If it occurs spontaneously then it must increase entropy but the possible micro states reduce so what else is occurring to increase entropy

 

[Dr_Nate]

Let's assume a closed system.

I think you have to ask yourself: why were the colloids originally suspended? I think because the fluid was agitated. The fluid then settles down, and the colloids then settle out of suspension. Where did that KE of the fluid go to? (...and the PE of the colloids.) It raised the temperature of your system, so now the water molecules have more accessible microstates.

 

[Lord Jestocost]

If it occurs spontaneously then it must increase entropy...

A closed system which isn‘t in an equilibrium state “evolves” in such a way to minimize – depending on the constraints – its Gibbs or Helmholtz free energy.

 

[Andy Resnick]

Summary:: If it occurs spontaneously then it must increase entropy but the possible micro states reduce so what else is occurring to increase entropy

If it occurs spontaneously then it must increase entropy but the possible micro states reduce so what else is occurring to increase entropy

Settling occurs because of gravity- an external force- the colloid system is not closed. Take away gravity and while sedimentation no longer occurs, phase separations can occur by other mechanisms- an interesting one is entropy-driven phase transitions and self-assembly:

https://arxiv.org/abs/cond-mat/0402585
https://www.physics.upenn.edu/yodhlab/docs/yodh_Phys295_Entropic_Forces.pdf

 

[Dr_Nate]

Settling occurs because of gravity- an external force- the colloid system is not closed.

Your comment made me realize that I should have said closed and insulated. I believe you are trying to say the system is not isolated.

 

[hilbert2]

When the colloid particles settle on the bottom of the container, their motion causes frictional heating in the liquid. This increases the total entropy.

 

[Andy Resnick]

Your comment made me realize that I should have said closed and insulated. I believe you are trying to say the system is not isolated.

Yes, that's correct. If sedimentation occurs, the system is not isolated from the environment.

(Edit: https://www.sciencedirect.com/science/article/pii/S0031891456900283)