SeeThrough

Glass is a wonderful material - you can see through it, but it lets nothing material pass. It does let heat through, of course. But with coatings and double glazing that can be minimised. However, that very see-through-ness is also a shortcoming; sometimes you want your windows not to be transparent. That's why you have curtains or blinds. Othertimes you want to be able to see out, but to deny people the ability to see in. That's why you have net curtains. There have been schemes to make glass windows that are liquid crystals, so that you can switch them electrically between transparent and opaque. But they are horrendously expensive.

What's needed is a cheap simple mechanism to allow a window to be switched from transparent to shaded to opaque and back again that requires minimal electricity (such as that that could be generated by putting PV cells around the frame). Why not pump coloured liquids between the inside two of the three panes of a tripple-glazed window? You could have any colour you want. You could use milky liquids to achieve opacity. You could even have fancy effects like a liquid with a suspension of neutral-density glitter to scatter the incoming sunlight all round a room rather than just dropping a quadrilateral of it on the floor. The pump would only have to run when a change was needed, so the electrical requirements would be minimal. It would obviously be essential for the glass to drain dry when the liquid was pumped out to achieve total transparency (or before a colour change; you probably wouldn't want the alternative liquids to mix over time). To get that, you just give the glass a highly hydrophobic coating, like that achieved by lotus paint. In a big deserted office block at night, you could even make a coarse-pixeled display of a whole glass curtain wall just by changing the colours and turning the room lights on and off under computer control...