Training a Brick Wall

After writing the last (rather hand-wavy) post on the idea of using fields to grow exotic conductor structures, I found a paper on someone doing this sort of thing for real, using silver nanowires:

• "A technique for controlling the alignment of silver nanowires with an electric field"
  Yang Cao et al 2006 Nanotechnology 17 2378-2380 doi: 10.1088/0957-4484/17/9/050
  

There's also an article (and a larger version of the above image) at www.physorg.com

• "Electric field can align silver nanowires"
  PhysOrg.com May 17th, 2006

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Brick walls can learn

Another (slightly random) example of materials "memorising" and recreating the effect of an applied potential is what happens to an external house-wall when your guttering is leaky.

Suppose that rainwater runs down the outside of your house and creates a bit of a damp patch on the inside wall. If the patch is bad, it's tempting to try to dry this out quickly from the inside by training a blow heater on the offending splodge. You evaporate the water at the surface, more water moves in from within the wall to take its place by capillary action, and you hope that at some point, the process will stop. You hope that you'll dry out the surface deeply enough that the rest of the water will stay put, somewhere deep within the wall, and perhaps dissipate. Some of it might work its way to the inside surface over a larger area and evaporate slowly and less intrusively, and some of it might eventually find its way back to the outside, the same way it got in. You hope.

But damp spots on walls are pesky creatures ... if you remove the surface water from one place too "aggressively", you're encouraging moisture directly below that section of surface to seep in to take its place faster than the usual seepage rate. In the worst-case scenario, instead of water slowly spreading through the wall in all directions, you're creating a faster directional flow within the wall, with water from a wider region of brickwork and plaster all converging on the extraction point where your heater is trained ... and as it moves, it dissolves salts and minerals from the wall and deposits them where the water is being driven off. If you see a crusty surface deposit forming, you're in trouble – it means that all that material has been dissolved out of the wall, possibly from around the points of greatest proportional resistance to the directional flow that you've created. You've etched a series of tiny micro-channels into the wall that converge on your extraction point, as a microscopic three-dimensional counterpart of how rivers carve channels into their landscape.
There might also be a concentration of those salts and minerals within the wall, towards the extraction point. If that happens, then the water-loving salts have created an absorbency gradient within the wall that represents the original flow, and once that piece of wall has dried out, if the dried region butts up against a damper region, the more "salty" part of the wall will draw water into it preferentially. The water will continue moving preferentially towards the most absorbent neighbouring region, following the original gradient ... straight back to your dried-out patch. So what you've done is (1) optimised the wall microstructure to collect water from a large area and direct it towards your extraction point, and (2) created an absorbency gradient in the wall to attract and hold the moisture and recreate the original damp patch. You've trained the wall to take water from the outside and carry it to the exact point on the inside where you didn't want it!

Materials that Learn

Suppose that we have a suspension of long electrically-conductive particles (such as metal filings or buckytubes) suspended in an insulating liquid resin. If we then try to force the liquid to conduct electricity in a particular direction, the particles will tend to self-organise to make that outcome achievable more efficiently.

A physicist will say that what actually happens is that when we apply a high voltage across the material in an attempt to force it to conduct, the particles become charge-polarised, and line up "lengthwise" in the electric field ... then the oppositely-charged "heads" and "tails" of adjacent particles tend to link up, and pretty soon you have lines of conducting threads running through the material linking the two electrical contact points. If your insulating resin's electrical resistance breaks down over small distances (above a given threshold voltage between a pair of particles), and if the sides of your particles can be persuaded to repel each other, to prevent the formation of additional conductive paths at right angles to your applied voltage, then, if you allow the resin to set, you should have a new type of material whose electrical conductivity depends on direction.

In itself, this doesn't sound particularly interesting: after all, we can already produce a solid directionally-conducting block by mechanically glueing or fusing a stack of insulated wires together and then machining the block to the desired shape. The advantage of using self-organising materials is that we can use them to build conduction patterns into films or coatings, or to build more exotic structures into solid blocks. You might want to tailor the electrical response of the paint on an aircraft or satellite to produce certain effects when it's hit by an incoming EM wave (say, to deflect radar or focus an incoming signal), or you might want to produce solid waveguides or field guides for electrical engineering, without laboriously building them from layers of laminated conductors, or winding them as coils.

The idea of self-organising materials isn't new. We use the idea dynamically with liquid crystal displays, and we've recently spent a lot of R&D money coming up with a "freezable" counterpart to LCDs, "electronic paper" (as used in the Amazon Kindle). But the idea of being able to "print" field structures into or onto materials, in a way that automatically self-corrects for any structural defects or variations in the material, is rather interesting. You could use superconducting grains to build exotic superconducting structures in two or three dimensions, or you could use a resin that's conductive when liquid, and freeze it from one end while the applied field is varied, to grow field structures that would be impossible to achieve by other means. You could even try coupling the process with 3D printer technology to produce independent conduction-alignment of each point within a structure to produce extremely ornate conductor structures. Then we have the interesting idea of field holography: if we create a complex external field around a device, and "freeze" the critical regions into superconducting blocks, then when those blocks are milled and reassembled, will Nature tend to recreate the original field by "joining the dots" between the separated blocks? What if we have a containment field with complex external field junctions that tend to destabilise under load – if we could freeze that field junction topology into a set of surrounding superconducting blocks, would they tend to stabilise the field?

We might be able to use expensive hardware to set up, say, a toroidal containment field, place a container of "smart resin" in the field, and "freeze" the external EM image of the device into the external block. If this was a useful component to have, we'd have a method of mass-producing them for use as as field guides or field stabilising devices.
With a number of interconnected connected and energised surrounding blocks, and the original device removed and replaced with a container of "smart resin", you might also be able to use the process in reverse, to recreate a rough electromagnetic approximation of the internal structure of the original device (crudely analogous to the old stereotype process originally used by printers to preserve and recreate the shape of blocks of moveable type).

Admittedly most of the potential applications for this sort of process don't exist yet. We're not mass-producing cage-confinement fusion reactors, and the LHC's magnets don't need miniaturisation. Fusion-powered vehicles are still some way off. But it's nice to know that there are still some fabrication tricks that we might be able to use that don't require laborious hand-tooling and impossible levels of molecular-level precision.