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Self-propulsion of a Leidenfrost drop on a substrate with a thermal gradient

par Sobac, Benjamin ;Rednikov, Alexei ;Dorbolo, Stéphane;Colinet, Pierre
Référence (Twente, The Netherlands), Droplet 2015
Publication Publié, 2015

Abstract de conférence

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Résumé :

As discovered by Leidenfrost, liquids released over very hot solids levitate on a thin cushion of their own vapor. Placed on an asymmetrically textured surface (such as ratchets), a drop not only levitates, but also self-propels in a preferential direction at a velocity of the order of tens of cm/s. This finding opened new doors for the control and manipulation of drops, useful for systems where liquids need to be quickly transported within a device. The origin of the drop displacement has been under debate. However, recently, several studies presented solid evidences to support the “viscous mechanism” as the main driver of the phenomenon. The idea is that the vapor flow below a levitated drop is rectified by the asymmetric teeth of the ratchet. The vapor flows in a preferential direction, along the less steep slope of each tooth, and drags the drop with it. In a self-propelling situation, asymmetry is generally the primary cause of motion. If the idea is to induce an asymmetry of the vapor flow, it might be possible for a Leidenfrost drop to self-propel due to the presence of a thermal gradient along the horizontal substrate surface. Accordingly, we here theoretically investigate the behavior of Leidenfrost drops in such a situation and highlight that they are indeed able to self-propel in a preferential direction. Namely, they are found to self-propel in the opposite direction of the thermal gradient, as if they were trying to maximize their lifetime. In particular, a centimetric water drop can reach velocities of the order of tens of mm/s for thermal gradients of the order of K/mm. In general, the model, based upon the lubrication approximation in the vapor cushion, permits to predict the values of these velocities as a function of the drop size, superheat, thermal gradient and material properties.