It's a deroofed blister!
Then rubbing takes the roof off . And takes pain and infection risk to another level!
What causes blister deroofing?
Shear causes the blister, which is painful enough.
Then rubbing takes the roof off (that is, rubbing in the presence of a high friction level). And takes pain and infection risk to another level!
3 ways to stop your blister from deroofing
1) Add a protective layer (tape / dressing / bandage)
This will serve as protection from rubbing. I favour non-adherent island dressings (the dressing sticks to the surrounding skin to keep it in place, but not the blister roof itself because there's an island of non-adherent absorbing material in the middle eg: in its simplest form, a bandaid). Don't put tape over your blister. When you come to take that tape off, it's likely to take the blister roof off with it, leaving you with the very problem you're trying to avoid.
2) Eliminate pressure (scuffs / donut pads)
The best way to prevent deroofing is by eliminating all pressure. By wearing shoes that don't touch the blister (eg: thongs or scuffs for blisters on the toes or back of the heel) or total nonweightbearing (eg: crutches for blisters on the ball of the foot). This will allow the blister to resolve in its own time. In reality, this is not always possible, like if you are in a race situation or miles from support.
Alternatively, you can try and reduce pressure. For example ... donut pads. By cutting a hole in a piece of thick orthopedic felt / moleskin and adhering it over the area so the blister is in the cavity, pressure and movement against the skin can be avoided or at least minimised. This will keep the blister roof intact. Podiatrist Emily Smith shows you how in this video.
3) Lower the friction level (ENGO patch / lubricant)
Reduce the friction level between your shoe and sock. This is blister prevention (and deroofing prevention) GOLD. Why? Because you allow the sock and skin to move as a single unit so the sock actually protects the skin. Watch this video to see what I mean. Or you can reduce the friction level between your foot and the sock (lubricants, powders, moisture-wicking socks). It's not quite as effective but better than not addressing the friction level at all. Read this to find all the ways you can reduce friction levels.
Treating a deroofed blister
In a word ... Hydrocolloid! Refer to my guide to blister treatment.
If you have a blister, you've missed the boat for pain-free blister prevention. But you'll be wanting to keep the roof intact so it's less sore! It will also keep the area sterile so it doesn't get infected - pretty important! In fact it's your priority at this point. The best ways to achieve this are by reducing friction levels and reducing pressure.