I saw it just the other day in a mix of two miscible transparent liquids. There was a ripply, grainy quality to the liquids, a little like the effect you see in hot air rising above a heated object. It was perhaps like a millimeter or centimeter scale fluctuation in refractive index. But the cases I’m interested in are isothermal.
The first place I saw it vividly was in this very nifty toy that said it contained “liquid crystal”. It was a vinyl pouch the size of an unfolded wallet, and in the middle was a seam that went nearly the whole way across. By squeezing on the end, you could make the liquid squirt through the break in the seam into the other end. The flow patterns were very visible. I think the effects I’m talking about would arise from short range molecular order, which is what I vaguely understand liquid crystals to be about.
Where I see it most often is in concentrated soap solutions where, I think, the critical micelle concentration is exceeded.
And where I saw it yesterday was in a mix of solvents. They’re definitely miscible and there was no phase separation. It was all transparent with no internal boundaries. And yet it was… ripply.
Maybe something to do with the Schlieren effect?
If what you’re talking about is the same as what I’m thinking, I see it when adding water to saltwater. For example, when topping off my saltwater aquarium with (unsalted) water, it has a wavy look to it. It’s very similar to the wavy lines I can see above the heaters when they’re running. Both Veritasium and Smarter Every Day* have done Schlieren videos that likely explain it a whole lot better than I even understand it. But it seems, at least in part, it involves mixing liquids with different densities.
*The Smarter Every Day video where they use Schlieren photography to see a whip cracking is a good watch.
ETA, here’s the Veritasium video on Schlieren photography.
I was going to say that there’s more than one phenomenon being described here, and that’s sort of true, but Schlieren covers more than one kind of discontinuity - including I think refraction and polarisation. In the case of clear liquids being mixed, it’s just refractive index differentials at boundaries - if you were somehow able to pour a perfectly lens-shaped globule of clear liquid into another clear liquid with a lower refractive index, you would see it behave as a simple lens and you would observe distortions that are quite regular; when you pour irregular streams of liquid into another liquid, you’re sort of making a lens with a very irregular, wavy shape, so you see irregular wavy distortions - it’s not actually all that different from looking down into a rippling pool of water - there is a boundary between two fluids of different refractive index (air and water) - when that boundary is wavy, what you see through it will be wavy.
It’s hard to say without looking at the effect you see, but I think you’re making it too exotic.
You say that you were seeing the effect between two miscible liquids, but what you describe sounds more like to immiscible liquids, where you had “fingers” of one liquid penetrating into the bulk of the other liquid. You wouldn’t see a “boundary” effect, because the refractive indices were probabvly close enough that the fresnel reflection was minimal, but there’s enough of a refractive index difference to see the light bending . You yourself likened this to the rippling you see from hot air layers rising – that, too, is caused by refractive index differences. In fact, the index difference need not be large to see them.
It could be that your liquids are really miscible, but they aren’t yet well-mixed, and you’re seeing the effect of one liquid surrounded by the other, just as if they’re immiscible. Again, no temperature difference is needed if the liquids have different refractive indices at the same temperature.
It’s not really a “Schlieren effect”, since that requires s special optical setop ( see here – Schlieren photography - Wikipedia ). But you can see variatioins in refractive index due to hot air rising if you look at the shadows thrown in a collimated beam. This is how Jean-Paul Marat (more famous as a revolutionary, not as well known as an optical scientist() saw such index variations in rising hot air. In one famous experiment he showed member of the French academy the effect , with Benjamin Franklin’s head as the heat source.