Spheres can form squares

Everybody who has tried to stack oranges in a box knows that a regular packing of spheres in a flat layer naturally leads to a hexagonal pattern, where each sphere is surrounded by six neighbours in a honeycomb-like fashion. In an article just published on-line in PNAS, researchers from Wageningen University report an exception to this rule: when small, micrometer-sized particles are placed on a curved oil-water surface, they arrange in a square pattern, as on a chessboard.

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