The smaller a nozzle is, the faster water at the same initial pressure will spray out — and the smaller the emerging droplets will be. It is a commonplace phenomenon that often surprises, and sometimes delights (see picture). But what happens when the nozzle is very small? And what kind of nozzle produces the smallest droplets?

P. McGuinness and colleagues applied themselves to these questions (J. Phys. D 38, 3382–3386; 2005). Their motivation was by no means a frivolous one: the answers are crucial to improving the resolution of inkjet printing, as well as being more generally applicable to industrial techniques requiring the manipulation of small liquid samples. In such cases, the high surface tension that develops at nozzles of micrometre diameters could limit the scope for reducing droplet size.

Credit: BETTMANN/CORBIS

So the authors tested different sizes and shapes of small nozzles, using numerical techniques based on the Young–Laplace equation, which relates the pressure difference at a gas–liquid interface to its geometry. For two-dimensional (planar) nozzles, a triangular opening with sides curved slightly inwards proved the best choice: compared with a conventional, circular opening at the same pressure, it provided a 16% reduction in droplet volume.

But the authors didn't stop there. By bending the corners of the curvilinear triangle up or down to form a non-planar nozzle tip, they were able to bring the reduction in volume to around 33%. As they point out, this adds another dimension to questions of small-droplet generation.