Figure 1: Self-assembly and wire formation from a colloidal dispersion.

(a) The schematic shows the formation of a ‘coffee-ring stain’ as the solvent starts to evaporate (the fine dashed line corresponds to initial solvent front, that is, the outer edge of the ring; the coarse dashed line corresponds to current solvent front, that is, the interior edge of the ring). The nanoparticles begin coalescing once the drop begins receding. Through van der Waals forces, the nanoparticles stick and attempt to pack into the lowest free-energy hexagonally close packed or body-centred cubic lattices. The optical image shows a section of such a drop where the coffee-ring effect is present. As the evaporating drop recedes towards the centre, radial compressive stresses inwards are established. (b) Here we illustrate how within a larger drop diameter (φ~20 cm), where the taper angle towards the centre is significantly less, the stresses involved with the nanoparticle packing can equal or exceed the radial stress so that secondary bifurcating fractures are formed, which propagate in parallel with each other, allowing long and uniform waveguides to form. (c) For small drop sizes this leads to fracturing towards the drop centre, forming tapers with an angle dependent on drop diameter. (d) An optical microscope image of a section of a typical microwire with width w=6.81 μm at both ends of section of length l=0.5 mm. (e) Digital photograph of a batch of uniform, approximately l>5 cm long microwires fabricated on a glass substrate. All scale bars in a–d correspond to 200 μm; the scale bar in e corresponds to 1 cm.