Leidenfrost droplet dynamics on structured surfaces

Parnell, Matthew Thomas (2024) Leidenfrost droplet dynamics on structured surfaces. Doctoral thesis, Northumbria University.

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Abstract

Discovered in 1756, the Leidenfrost effect allows droplets, deposited onto super-heated surfaces, to be suspended on a layer of its own vapour. The dynamics of this vapour layer under the droplet can induce uni-directional self-propulsion, oscillations, and sound production, which can be used for nano-particle production, green chemistry, pharmaceutical mixing, as well as heat engines for energy generation. As the droplet does not make contact with the substrate, the Leidenfrost effect can realise an ideal super-hydrophobic surface, whereby droplets can be manoeuvred without moving parts. Surface engineering droplets under the Leidenfrost effect can achieve speeds of up to 350mms−1. After over two centuries since its discovery, many new phenomena are still being found involving the Leidenfrost effect. For example, in 2018, droplets above the capillary length, deposited onto concave surfaces, were also shown to produce sound waves with a frequency up to 1750 Hz

This work investigates two aspects of Leidenfrost dynamics on structured surfaces. In the first study, a novel substrate geometry for controlling droplet motion is investigated. Bi-directional Leidenfrost self-propulsion has, up until now, only been reported once previously, requiring highly involved lithographic processes. As such, a much simpler, and novel substrate geometry is explored that uses CNC machining to combine a ratchet and herringbone pattern. This herringbone-ratchet geometry has a line of symmetry that, when combined with the ratchet cross-section, allows for a droplet’s self-propulsion and bi-directional control through the use of a negative feedback loop. This surface geometry allows for more robust applications where droplet position is paramount. A mathematical model is created to predict the motion and self-centring of a Leidenfrost droplet deposited onto a herringbone-ratchet. The model is shown to be suitable for predicting experimental results. However, in situations where the herringbone angle is large, an unforeseen premature deflection of the droplet is occasionally observed, prior to its interaction with the herringbone centre.

In the second study, a new phenomenon of sound production by Leidenfrost drops on structured surfaces is investigated. This work shows that sounds can be produced by Leidenfrost droplets deposited onto crenellated surfaces, achieving geometrically controlled Leidenfrost sounds. Effects of droplet volume and crenel geometry on the emitted sound frequency are investigated. The results show that the frequency of the sound produced does comply with the existing theory of Leidenfrost effect induced sound production, that the droplet acts much like a wind instrument, but with a different scaling parameter compared to previous studies.

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