[kroko]

im a student of architecture and just faced a question, for which i simply cant find an answer in the net. maybe dont use the use right keywords, but last time i learned chemistry, and what i learned...
therefore while seeking a chemistry professor will ask for help also here:)
::
is there any chemical X/coating (doesn't matter if organic, inorganic) which changes its appearance (color, structure) when NOT applied to the sunrays, but goes back to original appearance when the sun is applied again.
the idea is simple: "BURN". i want to cover a surface (an exterior wall which faces sun) with this chemical (if such exists). when a person stands in front of it with his (sun-)shadow he can burn his figure on to the wall. it should remain there at least for that long, that the person when jumping aside can see it (1 sec. and up). then the sun "erases" it again.
the second possibility is reagent and catalysator (or how is it called..). it is possible that between the person and the wall there is a transparent surface (catalysator), which transmits (or disables) some specific range of wavelength, further affecting the wall coating. but here: if such possibility exists, will i be able to actually see the shadow through this "ransparent" surface if it transmits only some part of wavelength (not that i see:))
third possibility is some fluoroscent surface. well, is it a possibility? and if yes, what kind of fluoroscent? will it keep the shadow burned enough long?

im not talking about a exterior wall with catptal W, it will be only a small area of wall, which is a part in some art structure/installation. and this is only an idea competioton, so its cost or reachability right now doesnt matter thaaaat much. but the best ideas will be done in real life, therefore, it shouldnt be radioactive or unhealthy . i can further cover the wall with simple glass, which prevents it from direct contact to an induvidual if it does harm, but insulation for unhealty gas or radiation... baaah
waving sos flag and hoping that someone finds my question interesting or at least shows a direction to dig

reinis
riga, latvia

p.s. to moderator: if you find that my question should be posted in other subdivision of your forum (physical, analytical- i just dunno), feel free to move it. thanks.

[lemonoman]

I did a bit of digging, and didn't find much as well.  One idea I did have though:

They have these windows, which are mostly used for bathrooms, where you flip a switch and the window frosts.  There's definitely a reactino of some kind happening there, and I THINK it MIGHT be caused by a potential difference between the two sides of the window (which, by the way, is two panes of glass with some kind of gaseous mixture in between.

Anyways, so what I thought was, if you could manufacture millions of tiny photovoltaic (solar) cells, and the same number of transparent capsules containing this gas, then you could have each cell power a voltage difference between each capsule (kinda like each cell is a battery powering an LED).  Then when someone steps in front, it would gradually turn the capsule clear.
 
Only problem, is that once they move out of the way, this particular reaction goes instantaneously when sunlight hits it...and you want it to be instantaneous when sunlight DOESN'T hit it....but something to think about

On the same note, if someone knows of a gas or mixture of gases that changes colour (due to a chemical reaction) where the activation in the forward AND reverse direction has a low activation energy...and preferably a slow equilibrium so that its not an instantaneous reaction...then we could build on this millions-or-tiny-transparent-capsules-in-a-wall thing...

[lemonoman]

Just had an idea...if you had a capsule of NO2 or N2O4 or both...this could happen...this is the classic equilibrium demostration if it rings a bell for anyone.

Anyways, if the capsule is made of something that conserves heat when sunlight hits it, but is still transparent, then you could have these capsules...

When the light hits them, their hot, and the equilibrium shifts towards the NO2 side and the wall is brown.  If you stand in front of the wall and for a little while, you'll cool the shadowed capsules down, and the equilibrium will shift towards N2O4...and the capsule will appear transparent as it is (or at least, lighter brown)

And since this is a temperature-induced equilibrium, it won't be instantaneous...they need time to heat up and cool down.

[Dude]

I haven't heard of anything specifically like that, however, a variation might be to use photochromic glass or plastic, similar to the Transitions lenses for eyesight.  This would be opposite (clear image when sun is blocked and darker image when in direct sunlight).  I've left my eyeglasses in my car with an ID badge partially obstructing one lens and the color will "leave an impression" (not diffuse).  The use of commercially available material should also keep the cost reasonable.

[kroko]

baaaah, had to go through the answers more than one time. im not a chemist/physician therefore my comment could be...

@lemonoman:
first: thanks, thanks, thanks!!!
second: what do you think by "capsule"? i understood that the capsule would be the wall itself. wall made of two panes of  glass- like "selective gass", only here the fillament woud be NO2&N2O4? |.'...'.'.| and due to temperature changes the gas would circulate through the doble-glass. hence the shadow (cooler area) will be transparent due to N204 and the hot area will be brown due to NO2. if this is your point then BINGO!!!!

actually i have also thought of the solar cells. before. only my idea was that the front pane is the "sun battery" and the back side pane is the "negative". the sollar battery outcharges through the air(gas) space and lets the gas fillament shine. when shadow steps in front the reaction in that place stops. i just didnt know that the reaction starts instantly after stepping back and i still dont know if there is transparent sollar cells.

=>i like the idea of gas better!!! there are some pros for the sun cell idea, for example it could generate energy for some exterior lightning for this whole installation, but the technology seems to be too comlicated.

so is it bingo?

[Mitch]

N2O4 is ridiculously toxic.

[kroko]

@dide:
thaks!
your idea is also meeting the subject!! only concern: i do photography as amateur and havent seen the PROCESS of photochromic lens filters changing "tint": it seems instant.
http://library.kcc.hawaii.edu/external/chemistry/everyday_photo_oxid.html
"(..)causing the lenses to darken. This process occurs almost instantaneously." that seems ok. but maybe there is some way to slowen down the reverse process? that would be also the solution for my question!

[kroko]

eh..toxic.
maybe some other gases? or even fluids?

[Borek]

N₂O₄-NO₂ or something similar - it will be better to divide area between glasses into many smaller compartments. If not, the gases will mix too fast. But maybe not.

As for photochromic glass solution - speed of color changes can vary, depending on the material used. Some react pretty fast, some are ridiculously slow. You should be able to find out one with the speed of changes fitting your needs. You will also need to select correct sensitivity, as too sensitive material can get dark even in the shadow, especially on the sunny day.

Sensitivity can be the major problem, as there are huge differences in the intensity of light between summer and winter and I understand you want it to work all year long.

Technical difficulties aside, I like the idea

[Donaldson Tan]

Ever consider LDR?

LDR stands for light dependent resistor. It is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

I propose a combined chemical-electrical method. We can construct capsules consisting a LDR with a chemical that exhibits colour change to electrical or heat stimulant.

We construct a panel made of millions of these capsules. When a person stands in front of the panel, the capsules in the area where his shadow casts on contain LDR that has electrical resistance. The capsules in the area where the light casts on (and not shadow) will exhibit low electrical resistance. So much electrical current can pass through the resistor. The increased current flow can act as an electrical or heat stimulant to cause a colour change in the active chemical.

We still end up with the same fundemental problem: what chemical?

[billnotgatez]

I try to visit science museums as often as possible. In some cases I see exhibits which is interesting, but I only recollect a limited amount of information about them. Thus is the case with the following phenomenon that I observed at some location now forgotten.
I went into a room that was dimly lit. Every once in a while there was a bright flash from one side of the room. It left an image on the opposite side of the room that slowly faded. That image was shadows of the people in the room that were between the flash and the image wall. It is possible that the dimly lit room was somewhat blue color, which may indicate phosphorescence, and UV light was involved. I can not remember if photography was the topic of the room or/and phosphorescence. So your general idea is plausible.
Regards,
Bill

[kroko]

today was to the faculty of chemistry and physics.
1) NO2 & N2O4 fails: #too toxic #the temperature when the gas would start to circulate (the temperature the sun must heat the glass for color (brown) changes to appear) is 80C   no comments...
2) solar cells would work, but too difficult and expensive
3) a fluid (gel) would work. by changing temperature it would extend-shrink and (dunno how its called in english:) the angle of reflection would alter
4) photochromic glass is the solution! its very likely that in real life its going to work. the solution for slowening the "tinting" process is simple: either a thick ph.ch. glass or ph.ch. glass made of numerous layers. BINGOOOOO!!!