Water flowing on a staircase

Liquid droplets can climb stairs with no external force, simulation shows

Image credit: Dreamstime

Using a computer simulation, researchers from the Indian Institute of Technology in Roorkee have demonstrated that liquid droplets can be made to “climb a staircase” on their own.

Other studies have succeeded in forcing liquid droplets to slide up slopes, although these complex approaches typically rely on the application of external forces. This new technique, described in Physics of Fluids, uses a passive approach; droplets are allowed to climb a microscopic surface “sustainably”, with no external forces required.

The researchers achieved this feat by creating a staircase with a surface that adheres to the droplets more readily with each step. A surface with greater wettability – the property of ‘stickiness’ of a surface to a liquid – causes the droplet to flatten, while a surface with lower wettability allows the droplet to remain rounded.

Previously, the same team of investigators were able to encourage a droplet to pull itself up a slope using a gradient of increasing wettability. At a microscopic scale however, surfaces are not perfectly smooth, and instead resemble a bumpy staircase.

“Most surfaces are textured, and mobility of a droplet over such surfaces requires climbing stairs,” said Professor Arup Kumar Das, a mechanical engineer at the Indian Institute of Technology in Roorkee. The Indian team worked alongside researchers from York University in Toronto, Canada.

The team set about testing whether a droplet could climb a bumpy surface – more relevant to real world applications – with a simulation of the physics of tiny droplets on staircases with increasing wettability as steps ascend.

As the droplets are wider than the length of a single step, the front of the droplet is in contact with a step of greater wettability. This causes the entire droplet to flatten towards the front, and – if the size of the droplet, steepness of the step and wettability gradient are suitable – spill onto the next step. This process allows the liquid to climb the staircase with no external forces.

Now, the researchers are working on practical experiments to confirm their computer simulations. They hope that their work could eventually have a range of applications, such as in water treatment, laboratory equipment, medical diagnostic devices and biochemical processing.

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