Aerial roots of phalaenopsis.
 

Why do aerial roots of phalaenopsis die when you put them inside the substrate?

My speculation is that it's the same scenario as repotting losses:

When roots grow, the cellular structure is "tailored" to the environment in which it's growing, and then is fixed. If you change that environment, those roots are no longer appropriately designed, so they fade and die, hopefully replaced by new roots.

If those aerial roots reach the medium by themselves, or are repositioned (without breaking) to come into contact with it, they will often successfully grow down into it.

Thanks, Ray, is that I have about 8 minis that since last year are in moss, full of aerial roots but few in the substrate and I want to pass them to bark when they finish this flowering. My experience placing the aerial roots inside the pot has always been bad.

Once the plant is in media that it likes (not soggy and airless) new roots will grow into that media. I'd suggest leaving the aerial roots to do what comes naturally - (they clearly are happy in the air, that is the environment to which they have adapted) and the new roots that grow into the media will be adapted to THAT environment.

You don't need to put them into the medium.

My take on aerial roots
 

Aerial roots are just roots, like those that grow down into the medium, but being that they emerge from higher up in the "trunk" of the plant, they offer a mechanical advantage for stabilizing the plant - something akin to guy wires on an antenna tower.

Their presence is an indication to me that the plant is in too small of a pot, and/or simply isn't stable in it.

I have some Phals that make a lot of aerial roots, and I try to get a few of them in the media. I start early and train them down into the pot as they grow. Since they are growing into the pot rather than being stuffed into it, the roots are adapted to that environment and stay healthy.

I note you live in Florida. It may be the roots just like the environment outside the pot (high humidity and more air movement) moreso than inside pot.

I always let roots go willy-nilly wherever they want. If they're not going where you "want" them to go, it's likely a sign you need to switch your media or method. I would think FL would be a great place to have mounts, if you have the space and time.

I think that they do not like the substrate or that the pot is very small, that is why I want to transplant them when they finish blooming. Does anyone know where to get small pots for minis?

Just something to keep in mind... Phals in nature grow on trees, with roots exposed to rain and wind. (Of course, they also experience very high humidity, and warm day and night temperatures) We put them in pots for our convenience so that we can bring them into homes and greenhouses to get more warmth, and to provide the "humidity" that they need. The closer our own environment is to what they experience in nature, the less we have to concern ourselves with pots and media to keep them happy. So it's a lot easier to let them "go natural" in Florida than in Vermont.