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Issue 15, 2014
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Length scale of Leidenfrost ratchet switches droplet directionality

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Abstract

Arrays of tilted pillars with characteristic heights spanning from hundreds of nanometers to tens of micrometers were created using wafer level processing and used as Leidenfrost ratchets to control droplet directionality. Dynamic Leidenfrost droplets on the ratchets with nanoscale features were found to move in the direction of the pillar tilt while the opposite directionality was observed on the microscale ratchets. This remarkable switch in the droplet directionality can be explained by varying contributions from the two distinct mechanisms controlling droplet motion on Leidenfrost ratchets with nanoscale and microscale features. In particular, asymmetric wettability of dynamic Leidenfrost droplets upon initial impact appears to be the dominant mechanism determining their directionality on tilted nanoscale pillar arrays. By contrast, asymmetric wetting does not provide a strong enough driving force compared to the forces induced by asymmetric vapour flow on arrays of much taller tilted microscale pillars. Furthermore, asymmetric wetting plays a role only in the dynamic Leidenfrost regime, for instance when droplets repeatedly jump after their initial impact. The point of crossover between the two mechanisms coincides with the pillar heights comparable to the values of the thinnest vapor layers still capable of cushioning Leidenfrost droplets upon their initial impact. The proposed model of the length scale dependent interplay between the two mechanisms points to the previously unexplored ability to bias movement of dynamic Leidenfrost droplets and even switch their directionality.

Graphical abstract: Length scale of Leidenfrost ratchet switches droplet directionality

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Publication details

The article was received on 01 May 2014, accepted on 13 Jun 2014 and first published on 19 Jun 2014


Article type: Paper
DOI: 10.1039/C4NR02362E
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Citation: Nanoscale, 2014,6, 9293-9299
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    Length scale of Leidenfrost ratchet switches droplet directionality

    R. L. Agapov, J. B. Boreyko, D. P. Briggs, B. R. Srijanto, S. T. Retterer, C. P. Collier and N. V. Lavrik, Nanoscale, 2014, 6, 9293
    DOI: 10.1039/C4NR02362E

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