Droplets surfing waves of creation and of destruction

I am continuing to play around with transient phase separation, as you can see in the movie above. Droplets of a blue phase appear then disappear. In the experiments that I am trying to understand, the experimentalists mix salt, which diffuses fast and promotes protein phase separation, with protein, which diffuses slower. So I have developed the model so that the ‘protein’ in my model diffuses slowly and downwards, and there is also a faster diffusing component, the ‘salt’ in the model, that diffuses upwards. Then the blue ‘protein’ droplets start to form when the ‘salt’ has diffused into the top half of the system, but then as the ‘protein’ diffuses downward and out of the top half, this causes dissolution of the droplets.

I think the model works well. There is a clear advancing front of droplet formation, followed by a slower moving dissolution front. For this system, it is very important that the ‘salt’ and the ‘protein’ move at different rates. The salt must win the race for the droplets to form.

This difference in speeds also has consequences. If you watch carefully you will see that the droplets that form near the top live for longer, and so have time to grow bigger, than the ones that form just above halfway. In some experiments in which they mixed real salt with a real protein (see Fig. 8A), then at the end of the experiment they found not droplets but crystals. The crystals were largest farthest away from where the salt was, i.e., they found the same trend of increasing size with distance, as I do. This could be a coincidence, but the protein solutions may well have undergone liquid/liquid phase separation as part of the process that formed the crystals. So maybe there is a connection. But even if there isn’t, this is a fun system to play around with. The relative speeds of diffusion must be controlling how big the droplets grow, so it will be fun to work that out.

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