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A scientist and an engineer were looking at the picture above, entitled Tails from the Nozzle Bank. The scientist waxed lyrical about the quintessential beauty of science at the micro-scale. “It’s upside down,” said the engineer.

He was right of course: Newton’s law of gravity applies to ink drops as well as apples. But Steve Hoath, a researcher in ink-jet printing, had the last laugh as he turned what is an engineering failure into a picture that took first prize in the 2006 Epson Photography Competition within the Department of Engineering at the University of Cambridge. Here’s how.

Hoath’s picture shows ink drops emerging from a bank of nozzles. Each drop is a mere 50 micrometres across and the tails are less than 10 micrometres wide. The thin column of ink ejected by the jetting process travels at 6 metres per second, and surface tension is supposed to force the column to become a sphere while gravity pulls the sphere and the remains of the column downwards.

Ideally the engineer wants the complete column of ink to reform into a single sphere before it hits the paper. Under the stress of the conflicting forces, the tail should rapidly merge with the droplet. The drops in this photograph represent a design failure because the tails are not coalescing with their droplets, but are breaking up into separate mini-droplets trailing the main drop.

If the printhead and paper were both stationary during the jetting and free fall of the ink, this wouldn’t cause a problem – the satellite droplets would arrive in precisely the same spot as the main drop, just slightly later. But in the real world, time is money, and either the printhead or the paper will be moving. If the droplet splits, the main drop and its satellites will land at different points. This can cause a slight blurring of the printed pattern – barely perceptible in a colour image, but if you are printing an electronic circuit, that blurring can cause the circuit to fail.

So while tails and satellites may make good art, they are just not welcome in real life.