Work Packages
Scientific part - General motivations
Essentially, the Leidenfrost state required three parties: the droplet for sure (subject to a phase change and rapid internal liquid flow), the gap (a thin gas layer squeezed between the droplet and the substrate), the substrate (rough or not, good thermal conductor or not, liquid or solid)
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WP1 : DROPLET
Just after the release of the droplet, the contrast of temperature between the droplet and the substrate is the highest. On the other hand, when the droplet is in the Leidenfrost state, the temperature of the droplet is very close to the saturating temperature of the liquid. During the transient state between both situations, the vapor film is generated and reaches a stable shape. The stabilisation of the film is still an open question. Recent papers suggest that the flow in the droplet and the heating of the droplet between the ambient and the boiling temperatures are key parameters. As reported by Detlef Lohse’s Twente group, a paradigmatic change of our view on the LF transition is suggested, namely, to see the transition from the LF state to the contact state as directed percolation transition. These findings and the fresh view necessitate to revisit the LF transition and to understand the implications for the heat transfer between the droplet and the substrate and for the flow motion inside the droplet. This flow is driven by the gradients which emerge through the (selective) evaporation, namely gradients in density and in surface tension (i.e., Marangoni flow, both of thermal and of solution origin).
WP1.1- stability
This WP concerns the LF transition and the relationship with the flow inside the droplet. The flow and the stability of the vapour film below the droplet is to be investigated by DC#1. During the approach between the droplet and the substrate, the interactions between the droplet and the substrate are critical to the establishment of a sustainable vapour film and a sustainable flow in the droplet. The difficulty resides in the entanglement between the flow in the droplet and thermal stability of the gap. The coupling is to be rationalised. Moreover, collaboration with WP3.1 will bring new insight on the onset of the Leidenfrost effect.
WP1.2-droplet composition
The nature of the liquid the droplet is made of is the natural extension of the pure liquid Leidenfrost droplet in order to increase the heat transfer to the substrate. Liquids presenting phase transition are particularly interesting to suppress LF effect (DC#2). The second idea is to suppress the flow inside the droplet by considering hydrogel and soft spheres (DC#3). The hydrogel allows to keep the evaporation, but the droplet is « solid » suppressing the inner flow. Both DC programmes allow to go beyond the pure liquid LF transition by evidencing effects like phase transition that could allow the control of the LF transition. The applications will be performed at CRM.
WP1.3-from droplet to jets
The heat transfer between the droplet and the substrate through the vapour film is determinant and ubiquitous in industrial processes (cooling, heating). A generic situation is used (or is accidentally observed) to cool down a system: instead of droplets, a thin jet or an assembly of jets impacts the system to cool down. The DC#4 project aim is to switch from a droplet to one (or several) jet(s) of liquid. In a jet, the flow in the liquid is imposed. The jet allows to play on the flow rate, the angle of impact and the size of the jet. On top of it, a mixture of liquid and particles are envisaged to break the vapour gap by the particles increasing the heat transfer (the complement of DC#7).
WP2: GAP
The vapour gap between the droplet and the substrate is rather thin (micrometric) and made of pure vapour. This gap is the main obstacle to the heat transfer between the droplet and the substrate. In this WP, the gap will be modified by texturing the surface of the substrate. The second axis is the exploitation of this stable gas film to apply external electrical field. The third is the exploitation of the hovering droplet to collect particles out of the surface. The interests in local surface modification (CRM) and in cleaning process (CSL) are approached in the secondments.
WP2.1- Vapour flow control
The vapour flow through the film was shown to be sensitive to the texturing of the surface since the droplet can be entrained by the vapour. Complex graved substrates allow the manipulation of the droplet without contact. The modification of the heat transfer was not studied. This work package proposes to approach the problem by studying the impact of a jet on a substrate which surface is carved with small cavities. The work of the DC#5 is complementary but decoupled from DC#4 work. Since the relevant relation with WP1.3. Moreover, the connection with the cooling process in metallurgy is only natural.
WP2.2- Fields in the gap
The application of a high voltage between the droplet and the substrate has been shown to locally suppress the gap. The liquid can then react locally and on demand. The conditions to observe the phenomenon and the consequence of the electrostatic collapse of the gap are to be studied intensively (DC#6). The local collapse of the gap is envisaged to modified the substrate and chemically texture the surface according to a novel method. These applicative developments will be made in collaboration with CRM.
WP2.3- Grabbing particles
As the gap allows a high mobility of the droplet, the droplet can move on a surface without contact. Moreover, if particles are present at the surface, the particles can either be blown or trapped according to their relative size to the gap height. On the other hand, the droplet can also release particles or embarked salt as reported in the literature. The mechanism of trapping and releasing is still not available. The global idea of this WP is to obtain alternative surface cleaning process using no detergent. The DC#7’s project is to investigate control and model experiments in order to establish the blowing, trapping and releasing laws. The method will be tested on surface of industrial interest at CSL.
WP3: SUBSTRATE
The substrate is the energy provider of the levitating state of the droplet. Numerous works investigated how to increase the Leidenfrost transition by changing the surface state reaching Leidenfrost temperature up to 1000°C ; in so doing, the heat transfer collapses is nearly suppressed. On the other hand, the physical state of the substrate can be liquid. In this case, the droplet in levitation is also influenced by the flow in the bath. This constitutes one axis of this WP.
The geometry of the substrate has been considered up to now essentially at the microscopic level, i.e. the roughness, the texturing and rarely the shape of the substrate, e.g. a conical shape. This question is not purely academic since Leidenfrost state could be interesting in closed geometry like in tubes.
WP3.1- Liquid substrate
On a liquid surface, the interaction between a LF droplet and its underneath substrate, as well as the interaction between multiple independent LF droplets, are envisaged (DC#8) for controlling the coalescence, but also mastering the motion of the LF objects and the mixing the inner drop liquid at low superheats. Indeed, in previous research 22 we noticed that the flow generated inside the liquid substrate depends on the nature of the involved liquid. Heterogeneous systems constitute therefore unstable system which could trig the mixing of droplets.
WP3.2- Particular geometry
Designed microscopic defect or the curvature of the substrate influence the vapour flow in the gap and consequently, the flow in the droplet. A singularity on a flat surface has been demonstrated to be a smart procedure to obtain information on the splash of a droplet. In an analogous way, information could be obtained on the onset of the Leidenfrost effect when a droplet hits a substrate on which a designed obstacle has been placed. This argument holds also concerning the curvature of the substrate which curvature radius is comparable to the size of the droplet or even smaller! The DC#9 task is to investigate the phase diagrams including the temperature the curvature and the radius of the droplet. An extension to cryogenics liquid is envisaged in collaboration with ULiège.
Concerning the milli/microfluidics applications, the systematic formation of LF droplets in microchannels is the main challenge of DC#10. Depending on the nature of the surrounding continuous phase (air or liquid), we expect the formation of anti-Breterthon drops or thermal antibubbles. In the latter case, we expect to separate two miscible liquids, and thus overcome some current problems associated with microfluidic encapsulation of miscible phases by generating them at unprecedented frequencies, benefiting from the high surface tension of the vapour film (augmented by the presence of two interfaces). Moreover, we expect that the lifetime of thermal antibubbles would be larger than the conventional ones, as the gas film drainage is compensated by the drop evaporation.
