A pair of miscible liquids, some nice clean glass and a Sharpie: that's all it really takes to build your own microfluidic machines. Nate Cira and co-workers have devised a number of tiny lo-fi games for droplets (http://youtu.be/K8Wx2PHIYGI) — and they can even tell you the physics behind them (Nature http://doi.org/2zv; 2015)

Getting droplets to move autonomously on glass isn't new in itself. Surface-energy gradients can be engineered to induce droplet motion, but the techniques are tricky, the gradients required large. Cira et al. found a way to create motile droplets without resorting to complicated surface-preparation protocols. They noticed that droplets made from a mixture of water and propylene glycol didn't spread out like their pure counterparts. The water, being more volatile, evaporated faster. And as evaporation occurred more readily at the edge of the droplet, the concentration of propylene glycol was higher there. This set up a surface-tension gradient, which pulled the liquid towards the centre along the top of the droplet and slowed down spreading.

But the constant contact angle was only one piece of the puzzle. The droplets exhibited characteristics of both wetting and non-wetting liquids. They didn't spread readily, but the authors also noticed that they were sitting on a thin fluid film. This meant that they were effectively shielded from the surface — as well as any inhomogeneities that might cause pinning.

Credit: NPG

Without pinning, the droplets were free to roam. And roam they did — frequently towards one another. Cira et al. observed long-range interactions that were induced by gradients in the water vapour produced by neighbouring droplets. The gradient increased the local relative humidity, slowing evaporation in the thin film and boosting the energy of the liquid–vapour interface, effecting a net movement. Once the droplets were close enough, they coalesced.

And that's when the fun started: Cira et al. used a Sharpie marker to make the glass superhydrophobic — confining the droplets to certain configurations. By playing with the short- and long-range forces, they were able to engineer sustained droplet chasing (left), sorting (middle) and self-organization (right).

Games aside, the technique may provide a physical analogue to help us understand the migration of keratocytes and other motile cells.