[ebgfsbvfvdvd]

So I have this project I am working on and was wondering if there was a compound, when mixed with paint, illuminates when water hits it. I know there is this foto luminizing powder but I do not want it to glow in the dark, only illuminate when it is wet. I did not know if you all knew of anything like that or what type of compound would be needed in case I decide to try a synthesis or two.

Thanks in advance!

[Arkcon]

Hmmm ... tricky.  You're going to have to define your application better.  Everyone has to, in the first place, remember that light generating reactions are rare.  Generating the quasi-stable excited state in a molecule, and having it drop back by light emission, instead of thermally is difficult.  Light emission is also quenched by water, most light emitting reactions happen in solvents.  As to you direct application, you want this compound suspended in paint, to react with water?  So not suspended in water based paint?  You want it to dry in a film of reagents, then react with water?  How much?  Just a few drops?  How will the molecules move about to react?  Flooded with water?  Dissolved in water?   What do you mean?

[ebgfsbvfvdvd]

The application would be for aesthetic purposes, something that could be mixed into the paint so it would light up when rain hit it. If I wanted to paint something in the yard and have it illuminate when the rain came and hit it, so it would be for large amounts of water. The paint would have to be oil based because I only want it to illuminate when it rains. I suppose a film could be useful but it would have to withstand wear and tear and not degrade much over a short period of time. I know this is tough due to the chemical restrictions of illuminating compounds, I just wanted to see if there was anything that could give me this effect.

[Enthalpy]

And then it should work again at the next rain?

I'd switch to electronics instead.  Either the complicated way, with impact or water sensors, signal processing, a panel of Led... or more simply, electrodes of different metals that produce current for a local Led when water provides the electrolyte. It takes several cells in series, and some area (mesh of one metal over sheet of the other metal) to get 20mA for one Led. That one would work for years before one metal is fully used. It will look corroded.

Or turbine the drainpipe to supply some Led.

[Intanjir]

You need an energy source each time to emit light. Water is not easily going to provide you with chemical energy. You could potentially have a one time reaction where you provided the energy initially in the chemicals chosen and this was released as light when combined with water, but having it repeat instead is unrealistic chemically. What would regenerate the chemical to the active form?

[Corribus]

Not to mention: Even if such a molecule existed, if it was dispersed in a hydrophobic phase, water from rain wouldn't contact it in appreciable amounts anyway.

[Enthalpy]

Trying to put figures.

One raindrop falls at 5m/s (reaching equilibrium in 1.3m) and weighs 4*10^-6kg. Flashes from individual kinetic energy would be extremely weak: 170nJ=10mW*17µs. Visible in a dark night at best.

Then you could abandon individual flashes and light continuously if it rains. A strong rainfall, say 10mm in 1h, provides 35mW/m², enough for one Led per m². Harvesting the kinetic energy is difficult; ETFE is ferroelectric. An expensive and perfectly dry plate might convert some kinetic energy to high-voltage electricity.

The drain pipe looks easier. With 3m drop, a 3m*5m roof provides 1.2W, which a watermill can convert to low voltage while resisting dirt and moisture. Enough for a strong Led flashlight. Not very aesthetic: I too would prefer a flash at each waterdrop impact.

My general conclusion is that an external energy source is needed - consumable chemicals or something else.

Maybe a mesh of zinc wires hold just above a roof of brass sheet, but individual drops won't suffice. At (very variable) 2mS/m, one single 4mm²*1mm drop resists 100kohm, too much to supply a Led. As one dm² is fully soaked as more drops arrive and take time to flow away, its 50ohm suffice to light continuously one Led. Maybe the zinc and copper can be printed side-by-side in a pattern on an insulator. With only one Cu-Zn cell, it would take an electronic circuit to provide the voltage for a Led.

My preferred solution for aesthetic is to have a transparent roof sheet (glass, Pmma, polycarbonate) where you inject light at one edge and absorb it at the others. With proper design, light stays completely trapped in the dry transparent sheet, but every individual drop permits light to escape the sheet to the drop and they the air. That would be the best visual effect. You need the sheet to dry and clean spontaneously (maybe FEP, but it costs). Led are better to save electricity, choose the colour and inject light at a good angle, that is, near to escape the sheet. Injecting light from several edges may keep a uniform effect in strong rain, and ambiant light detection can adapt the intensity (PWM) to day and night.

At proper angle, the edges too can reflect light to recycle it and save electricity. Less nice under heavy rain.

I wouldn't supply the light by the drainpipe watermill, because I like to see the very first drops on the roof.

Marc Schaefer, aka Enthalpy