Levitation : from drops to plastic cards

Manipulating drops is a major issue in microfluidics applications such as lab-on-a-chip [1]. They are usually conveyed in microfluidic channels and the surrounding media is commonly a liquid. However, it has lately been shown [2],[3] that there is an alternate technique for manipulating droplets based on the conjunction of two physical phenomena. The first one relies on the levitation of the drop over a cushion of its own vapour: the Leidenfrost effect. This insulating layer allows us to have frictionless objects, hence very mobiles [4].
The second one takes advantage of the viscous entrainment of the escaping air. As the drop is sitting on top of the vapour layer, it squeezes it thus making a lubrication flow. By rectifying this flow with microscopic textures and introducing asymmetry drops self-propel [5].
Here we propose to use a new texture (see Fig. 1) where we completely control the rectification of the flow, and thus can model the role of all the physical parameters. We subsequently show new macroscopic devices to move (see Fig. 2) or trap (see Fig. 3) drops. Finally, we extend this self-motion ability to rigid objects such as plastic cards or PDMS flying carpets. As we cannot exploit the Leidenfrost effect anymore, we use a porous media through which we compressed air is injected to generate an air cushion. This new set-up allows us to explore new parameters such as the weight or the geometry of the propelled object.

Fig 1 : New herringbone propelling texture
Fig 2 : Time-lapse of a drop spinning over two herringbone textures (in the middle) connected through two hemispherical sections (on both sides). The white arrow is positioned at the drop starting position and shows the motion direction. The movement lasts until there is no more liquid to evaporate