If you’ve been wondering just how the water witch works, you’ve come to the right place. We’ve told you so much about how we’re better than the competition because we have a controlled and constant daily emission. Others either let water fall out on its own through a tiny hole, so it flows fast at first and then slowly and empties waaay too quick or they use a porous membrane (ceramic) that is again, uncontrolled and unreliable. So what did we do that made us better?
The Water-Witch™ uses a selectively permeable membrane to control its water emission. A selectively permeable membrane is a material that is picky about what kind of things it'll allow to pass through it. It only lets select molecules pass through it and blocks everything else. Some synthetic examples of this are cellophane, parchment paper and coffee filters. Each of our cells is surrounded by a phospholipid bilayer membrane that's selectively permeable and so is the membrane on eggs!
The membrane we use is made of expanded polytetrafluoroethylene (PTFE). This material was created accidentally by Bob Gore and his family perfected it to create Gore-Tex! The image attached is Gore-Tex under a microscope. The amazing thing about this material is that it lets water vapour pass through itself but not liquid water. This capability is why it's used to make sleeping bags, diving suits and so on. It creates waterproof yet breathable fabric because it doesn't let water in but lets air in!
How does this work in the Water-Witch™? It has a small opening in it covered by a small disk of expanded PTFE. This disk does not allow any of the water to fall out of the bottle. It does however, slowly (on account of the size of this aperture) allow water vapour in. This displaces a small amount of water from the bottle, leading to slow drips of water into the soil. Every factor from the size of the aperture to the thickness of the disk affects how fast or slow it will release water. So we found an optimal spot and introduced the pipe to create a range of emission rates for the consumer (that’s you!) to customise to the needs of their plant.
Now you know how the Water-Witch™ really works and it's not magic, just membranes.
Aahanaa Tibrewal
Horto-Logic Intern for Summer ‘23
Macalester College Class of 2025