It's a deroofed blister!
That's a blister with its top (roof) rubbed off. First the skin of the blister roof tears, then it dislodges (deroofs) and leaves a red raw sore.
What causes blister deroofing?
Shear causes the blister, which is painful enough.
Then rubbing takes the roof off and takes pain and infection risk to another level!
4 ways to stop your blister deroofing
1) Protective layer (a dressing)
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) No pressure (nonweightbearing)
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.
3) Reduce pressure (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.
4) Low friction level (ENGO Patch)
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.
Treating a deroofed blister
Hint ... 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.