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Hot flocking Luneburg lens

Powder coating in layers to form a Luneburg lens
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In a recent idea I proposed using an array of Luneburg lenses as a 3D display. Luneburg lenses are not easy to make. It requires a gradient of refractive index from the centre to the surface of a sphere.

I have been pondering methods of forming uniform thickness spherical layers.

An added level of difficulty is making very small lenses (500µm diameter).

The best method I can think of is using a process called 'hot flocking' wherein the object to be coated is heated so that when the powder contacts the object the powder melts thus forming a layer. This process would be repeated to epitaxially form the lens.

To ensure the layer growth is uniform, the lens would need to be not contacting any support. This could be done in zero-gravity or using magnetic levitation, but the simplest way would be to drop the seed lens in a vacuum.

The seed lens is dropped and heated with radiation (microwave or IR laser). As the seed lens falls, a first blast of ultra-fine glass powder (diameter of about 2µm) is fired at the seed lens from all directions. The powder only attaches to the seed lens (and not the glass powder), thus forming a uniform layer. As the powder melts, the surface tension keeps the melted layer at a uniform depth.

The lens continues to fall and be heated, and then passes a second blast of ultra-fine glass powder of slightly lower refractive index.

The processes continues about 100 times; the lens having fallen a few hundred meters. The lens is then cooled and then gently decelarated in a low-density material.

xaviergisz, Jan 09 2020

Luneburg microlens array
[xaviergisz, Jan 09 2020]

Spaceframe Mountain Spaceframe_20Mountain
"mfd - redundant (see [Vernon]'s <link>) - kept only for the smartass remarks." [8th of 7, Jan 10 2020]

Wikipedia: Cold spraying https://en.wikipedi.../wiki/Cold_spraying
Mentioned in my anno. A thermal spraying process [notexactly, Jan 15 2020]

Wikipedia: Inkjet printing § Continuous inkjet https://en.wikipedi...g#Continuous_inkjet
Mentioned in my anno. Could be adapted to "continuous powder jet" [notexactly, Jan 15 2020]

Link 1 https://www.advance...-transparent-glass/
3D printed glass [bhumphrys, Jan 15 2020]

Link 2 https://3dprint.com...-lens-radar-system/
3D printed RF Luneburg lens [bhumphrys, Jan 15 2020]

[link]






       //fired at the seed lens from all directions//   

       Without compromising the vacuum?   

       Oh, right; you're doing this in space, aren't you?
pertinax, Jan 09 2020
  

       Why can't you just leave the hot glass on a flat surface and drop the glass powder onto it in a regular atmosphere?
Voice, Jan 09 2020
  

       //Without compromising the vacuum?    Oh, right; you're doing this in space, aren't you?//   

       This would be terrestrial; in a tall tower or deep mineshaft.   

       All the ultra-fine powder which didn't stick to the lens would fall to the bottom. Because it's a vacuum, the powder should fall. The tube might need to be cleaned regularly with a shuttle.
xaviergisz, Jan 09 2020
  

       //Why can't you just leave the hot glass on a flat surface and drop the glass powder onto it in a regular atmosphere?//   

       I don't think the hot glass would remain spherical, nor would it get coated uniformly.   

       Alternatively, you could try rolling the hot glass lens on a bed of glass powder, but again it's not going to be pretty.   

       Keeping it uniform at each stage is very important. Even small imperfections will grow bigger with each layer added.
xaviergisz, Jan 09 2020
  

       Neither technique will place a drop of near-molten glass directly upon each previously-deposited hardening drop.
Voice, Jan 09 2020
  

       In light of pertinax's comment I realise that I haven't explained the idea very well.   

       The seed lens would be dropped in a very high tube. Along the tube would be radially placed: a) powdered glass blasters, and b) heaters. The spacing (and size) of these would increase down the tube to account for the acceleration of the lens, so that each iteration would be the same length of time.   

       The bottom could have a series of quick-opening doors so that the lens could fall into a deceleration chamber without losing the vacuum in the tube.
xaviergisz, Jan 09 2020
  

       What does a radially-placed powdered glass blaster look like?   

       If I were inadvisedly riding the seed lens down the tube, would I see a series of rings of orifices out of which powdered glass would be blasted? In that case, the blasting would be asymmetrical in the vertical dimension. Or would I see a uniform coating of glass powder backed by an explosive substrate, whose detonation would somehow propagate down the tube at an accelerating rate? Or what?
pertinax, Jan 09 2020
  

       The blasters would be angled and timed so the powder would hit the seed lens at the same speed and approximately from an angle perpendicular to the surface of the seed lens. As the speed of the lens increases down the tube, the angles and spacing of the blasters would be more exaggerated.   

       The propulsion mechanism would be some kind of precisely controlled linear motor (piezoelectric or solenoid). The powder would be held in place on the end of the linear motor by a mesh with holes the same size as the powder diameter.   

       Alternatively the powder could be kept in thousands of tubes. An impact at one end sends a shock wave through the spherical powder particles so that the last one is ejected (like a Newton's cradle).   

       There are a few other ways I can envisage, e.g. similar to electrospraying.
xaviergisz, Jan 09 2020
  

       Instead of the seed lens falling past the powder blasters, the seed lens and blasters and heater could all be in self-contained shuttle. The seed lens would initially be held in the centre by a releasable mechanism. When the shuttle drops and everything in the shuttle experiences zero-g, the seed lens is released and then heated and then the powder is blastered at the lens iteratively. At the end of the process the lens is cooled and captured by the mechanism, and then the whole shuttle gently decelarated. The tower could be relatively small so the shuttle would do the first 10% of the layers in the first drop, then raised back up to do the next 10% and so on.
xaviergisz, Jan 10 2020
  

       hmmm Would a Prince Rupert drop qualify as a Luneburg lens with a fiber optic focal point or is the shape not spherical enough to lens properly?   

       It has been proposed to make a perfectly spherical version of a Prince Rupert's Drop. This could be done by rapidly cooling a blob of molten glass in a zero-g environment. Such a Prince Rupert's Drop would have a gradient of stress from the centre to the surface. It is a tantalizing possibility that (with a carefully chosen material) the gradient of stress could affect the gradient of refractive index such that it forms a Luneburg lens.
xaviergisz, Jan 10 2020
  

       What about molecular beam epitaxy ?   

       // The tube might need to be cleaned regularly with a shuttle. //   

       You might have a job borrowing one of the retired ones. Museums can be pretty reluctant to lend out their prize exhibits, no matter how politely they're asked. Even if you only want a small part as a souvenir, they can cut up really rough. It's not like they even paid for the damn thing, the Wrights donated it ... petty, we call it.
8th of 7, Jan 10 2020
  

       //It has been proposed to make a perfectly spherical version of a Prince Rupert's Drop. This could be done by rapidly cooling a blob of molten glass in a zero-g environment.//   

       I know...
: ]
  

       I proposed it in my Prince Rupert Spheres posting... that's what made me think of it. You might be able to do away with the enormous tower and a bunch of steps by making your lenses in microgravity.   

       // the enormous tower and a bunch of steps //   

       <Fondly recalls all-time-favourite anno, in "Spaceframe Mountain" <link>/>
8th of 7, Jan 10 2020
  

       This seems like a thermal spraying-based process whereby the glass powder is sprayed on and then melted by the heat that the nascent lens already has. What about the kinetic energy of the powder, though? Cold spraying [link], despite the name, is a thermal spraying process where the kinetic energy of the powder is what's used to melt it. It generally produces incomplete melting and therefore voids in the coating, but that could theoretically be controlled as a means of varying the refractive index, as long as the powder particles and voids are considerably smaller than the wavelengths the lens will be used with.   

       What about suspending the nascent lenses in an upward hot gas jet? You can shoot glass powder at them from the side, and the imbalance will cause rotation to allow even coverage. You'll have to shoot from multiple sides in a carefully planned sequence to ensure they rotate in such a way that all points on the surface can be blasted. Also, to compensate for recoil, the gas jet could be at an angle to vertical, or an additional gas jet can be added that provides horizontal wind (in pulses timed to coincide with glass powder blasts, if those are intermittent).   

       What about using the method used for manufacturing glass marbles? Those are made with a pair of screws that roll the marble along between them as it cools down. Instead of just dropping a full-size blob of glass on straight screws and using them to form it into a sphere, you could use tapered screws that grow from the size of the seed to the size of the finished lens, and set up blasters for each grade of glass powder along their length. The lens would grow as it goes along the screws, and the size of the screws would increase at the same rate. The two screws might need to be slightly different in size, and/or have different surface materials or textures (or maybe even different surfaces between the forward- and backward-facing sides of the thread), to ensure the lenses rotate in a way that results in even coverage. The screws would need to be very long in total, but you could easily break them up into multiple stages that you could stack. Also, either the screws need to be either made of or coated with something that the glass powder won't stick to, or the blasters need to produce very narrow streams. (Yes, that sentence has "either" twice. It's grammatically correct.)   

       // electrospraying //   

       What I thought of for producing very narrow streams of glass powder was something like continuous inkjet technology [link] (not to be confused with a drop-on-demand inkjet printer with a continuous ink supply system). Basically, a CIJ printer shoots a stream of electrically charged ink droplets and controls their trajectory similarly to how a CRT monitor controls its electron beam's trajectory. (This technology is one of the ones commonly used on packaging lines, for printing best before dates, lot numbers, etc., along with drop-on-demand inkjet and laser marking. That's why those markings often have letters made of dots of black ink, that can be scraped off if the substrate isn't porous, on a white background that's incorporated into the main label printing, which is done in advance of packaging.)   

       That would also enable easy control of when it's blasting glass powder and when it isn't, as well as fine aiming control. You could also, maybe, charge the nascent lenses to attract the glass powder, and the gas/screws to repel it, but it would be difficult to achieve that because glass is generally a dielectric, and the nascent lenses are in direct contact with the gas/screws. Maybe a beam of positive ions aimed at the top of each nascent lens as it goes along? Or a gas/surface on the screws that charges the lenses triboelectrically?
notexactly, Jan 15 2020
  

       There are many manufacturing possibilities indeed.   

       I'm attracted to the super precise methods. For example, imagine a geodesic arrangement of hundreds of thousands of needles and lasers. The molten seed lens is held in place in the centre (with zero-g or mag-lev). The needles simultaneously fire individual glass powder spheres; the first layer only requiring 1,000 powder glass spheres, progressing to the last layer requiring hundreds of thousands. Each needle would contain a series of progressively lower refractive index glass powder spheres, and ejected with a Newton' s Cradle impulse.   

       The glass powder spheres could be made with electrospray techniques.
xaviergisz, Jan 15 2020
  

       Simply 3D print a structure in glass (link 1) consisting of an appropriately spaced matrix of filaments in accordance with the eventual ly required spatial distribution of refractive indea required to form the Luneburg lens (link 2). Place the matrix thus formed inside a spherical mould. Fill the mould containing the matrix with powder of a different refractive index. Sinter it. Remove the sintered sphere and polish it: pronto.
bhumphrys, Jan 15 2020
  
      
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