loading . . . The LeidenForce true Force LeidenForce "True Force" initiative brings together ten cutting-edge doctoral projects exploring the physics and applications of the Leidenfrost effect. F rom droplet dynamics and levitation on hot surfaces to vapor film modeling, particle manipulation, and industrial heat transfer control, each research tracks blends advanced experimentation, theoretical modeling and real-world engineering. Supported by prestigious partners such as Airbus and top European institutions, the program pushes the boundaries of fluid dynamics and thermal science. The 10 doctoral positions included in the LeidenForce programme are detailed below. DC1 : Maybe Your name JOB OFFER DC1 - Click here to apply WP1 Title: Stability of the vapor film PI: Pierre Colinet (ULB, BE) Secondment: ESPCI, FR Objective A: To explore and characterize the dynamical regimes of a single drop in Leidenfrost state based on existing and new experiments (in particular, those enabling measurement of vapour film thickness dynamics) Objective B: To build different theoretical models, from simple dimensional analysis and scaling laws, to full numerical simulations Objective C: To validate theory against experiments and to test the predictive capability of the models (and this, for different liquids) DC2 : Maxim De Wilt WP1 Title: Leidenfrost effect for binary droplets and the role of thermal and solutal Marangoni and buoyancy forces PI: Detlef Lohse (U. Twente, NL) Secondment: CRM (Ghent, BE) Objective A: Experimentally explore both the static (gentle deposition) and the dynamic (drop impact) Leidenfrost effect for the droplets consisting of mixtures. Do so for various mixtures with different volatilities of the components to understand the role of the temperature and concentration dependences of both surface tension and buoyancy. Objective B: Understand under what conditions drops can be in the Leidenfrost state and when they will touch the surface. In particular: What is the role of distortions, either by controlled roughness or by immersed particles or by some impact velocity? Objective C: Numerically model the Marangoni flows in binary Leidenfrost droplets, employing Finite Element Methods (FEM). With their sharp interface representations of the free surface they are ideally suited for the study of binary Leidenfrost droplets, including the mass-exchange with the surrounding gas and including the consideration of temperature difference due to latent heat effects. This is achieved with a co-moving mesh which is always conforming with the interface and thus adaptive. We will do this with a so-called arbitrary Lagrangian-Eulerian (ALE) algorithm. This part will be done in close collaboration with the student in DC4. DC3 : Maybe Your name JOB OFFER DC3: Click here to apply WP1 Title: Controlling Leidenfrost dynamics with soft solids PI: Scott Waitukaitis (ISTA, AT) Secondment: ULiège, BE Objective A: Our first set of experiments will involve soft vaporizable solids LeidenForceing due to the traditional Leidenfrost effect, where we probe the effect of the vapor layer on the equilibrium shape. We expect results similar to the liquid case, except with (1) the restoring force provided by elasticity rather than surface tension, and (2) the added complication that the shape of the droplet changes over time due to vaporization. By building a minimal model to explain our findings, we may be able to harness soft solids to control motion. Objective B: Our second set of experiments will investigate how the Young’s modulus affects the bouncing dynamics in the elastic regime. We will cast spheres of a fixed size but with different Young’s moduli to map out changes the equilibrium energy injection per bounce. The effect of stiffness is non-trivial. The naïve hypotheses would be that stiffer objects have a reduced Hertzian contact area and contact time, thus permitting less energy release through vaporization per bounce. However, previous results indicate that couplings to spheroidal oscillations also affect injection efficiency, and stiffness directly alters the resonant spheroidal modes. By gathering comprehensive experimental data, we aim to map out a wide parameter space concerning the role of elasticity. We then intend to develop a minimal model for the interplay of these two effects. DC4 : Maybe Your name JOB OFFER DC4 - CLICK HERE TO APPLY WP2 Title: Leidenfrost jet PI: Stéphane Dorbolo (ULiège, BE) & Alexis Duchesne (CNRS-ULille, FR) Secondment: AIRBUS, FR Changing the inclination of the jet, the liquid properties (volatility, surface tension...), the diameter of the jet, the flow rate and the temperature difference between the plate and the boiling point of the liquid we will: Objective A: Investigate and characterize the different physical objects (rebound, levitating liquid sheet, Leidenfrost puddle...) that may be observed in the impact of a continuous liquid jet on a hot smooth plate. Objective B: Characterize the heat transfer in this configuration. Objective C: (in partnership with DC#10): Observe the role of particles in the fragmentation of the liquid sheet and in the rebound process. Objective D: (in partnership with DC#8): Create textured surface to control the Leidenfrost temperature for a liquid jet. DC5 : Maybe Your name JOB OFFER DC5 - Click here to apply WP2 Title: Control of Leidenfrost temperature with surface cavities PI: Maria Fernandino (NTNU, NO) Secondment: AIRBUS, FR & A. Duchesne, FR Objective A: To characterize the impact outcome of impinging droplets on surfaces with cavities with different geometrical characteristics (diameter, pitch, depth). Objective B: To investigate how the properties of the surface affect the vapor generation process and control the shift of the Leidenfrost temperature for the above configurations Objective C: (in partnership with DC5): Design of textured surfaces for control of the Leidenfrost temperature during jet impact DC6 : Maybe Your name JOB OFFER DC6 - Click here to apply WP2 Title: Local cancellation of LF assisted by an electrical field PI: Anne-Laure Biance (UCBL, FR) Secondment: CRM (Liège, BE) Objective A: Determine the stationary structure of a Leidenfrost film with impedance characterization of the thin film. We will begin with a water droplet on a flat silicon wafer. Objective B: Investigate the effect of an external field on the dynamics and statics of the vapour film Objective C: Explore the effect of surface structure (roughness, tips, materials) on the response to the electric field. Use of a semiconductor nanowire or a metallic nanotip in solution will be achieved. Objective D: Explore additional gas sources using electrochemically active species and large voltage. In particular, hydrogen peroxide will be used as a liquid solvent. DC7 : Cheikh Tidiane Dioum JOB OFFER DC7 - Click here to apply WP2 Title: Particle trapping and releasing PI: Stéphane Dorbolo (ULiège, BE) Secondment: CSL (Liège, BE) Objective A: The project aims to experimentally determine the conditions to capture and to release particles on a hot plate assisted by the Leidenfrost effect. For this objective, the model particles are glass spheres. The hot plate can be inclined, and the droplet (made of water) can be gently released or can impact the plate. The image analysis of the plate before and after the interaction of the droplet with the substrate is the key method to determine the probability of captured, released or pushed particles. The initial configuration of the granular packing is a relevant parameter. Three cases can be considered: (i) the monolayer, (ii) the thin layer and (iii) the bulk. Objective B: Influence of the particle shape. Particles can be of various shape, size, distribution, material (rigid or flexible). . The geometrical arrangement of the particles with respect to their physical properties is to be investigated. The polydispersity can be easily investigated with different glass bead sizes. As for the shape, needles are envisaged to be compared to the behaviour of the sphere. Spherical particles of tungsten or of silicon carbide will be used to tune the thermal properties of the particles. In particular, the deposit of the particles after the complete evaporation will be analysed. Finally, the nature of the liquid will be considered using usual solvent. Objective C: Model development. In parallel to the development of objectives A and B, models will be built on the experimental facts in collaboration with Pierre Colinet (ULB) since we have already an experience about the interaction particles-droplet. Objective D: The discovery will be applied to the cleaning of optical devices. Indeed, the non-contact conditions are relevant for the soft cleaning of lens. The proposed method can be an innovative alternative to the cryo-cleaning methods that use dry ice (CO2) projected under pressure on the surface to clean. Using liquid nitrogen is an interesting alternative as the droplets are in the Leidenfrost state on the surface to clean. This objective is related to the internship at CSL. DC8 : Maybe Your name JOB OFFER DC8 - click here to apply WP3 Title: Leidenfrost drops on a liquid surface PI: Benjamin Sobac (UPPA, FR) and Stéphane Dorbolo (ULiège, BE) Secondment: ULiège (BE) Objective A: Explore and understand the influence of some crucial parameters of the problem, related either to the liquid substrate (viscosity and thickness) or to the drop (size and density) on the LF levitation, i.e. its threshold, shapes and dynamics. Objective B: Explore and understand how to impose a flow motion in the liquid layer to efficiently control the drop motion or the mixture within the drop. Objective C: Explore and understand the interaction of multiple drops of the same/different nature and of the same/different size levitating at the top of a liquid layer. DC9 : Maybe Your name JOB OFFER DC9: Click here to apply WP3 Title: Establishment of Leidenfrost transition on a plate with defects and curved substrates PI: David Quéré (ESPCI, FR) Secondment: ULiège (BE) We plan to consider and model different situations for which a vapour film is formed starting from a droplet, namely on a plate which surface is textured by a single obstacle or is curved. We aim to describe the conditions for observing a Leidenfrost transition and the dynamics of the vapor film at short times. The presence of defects will be approached systematically in the different configurations. Objective A: The early stage of the “contact” of the droplet and the plate is crucial for triggering the Leidenfrost transition. The experimental set-up will be specially designed in order to visualize the first instants. The role of the defects will be analyzed giving information on the destabilization of the vapour film. On top of it, the dual system will be envisaged: the immersion of a hot plate into the liquid; the plate being textured with controlled defects. This process, which is similar to the inverse problem of Landau-Levich, will allow to observe how a vapor coating can start during the quenching of a hot object in a bath. A collaboration with ULiege will extend the work to cryogenics liquids. Objective B: Droplets will be introduced in curved substrate. The aim is to study how the speed of the droplet can be affected by the presence of the wall while the droplet remains in LF state. The presence of defects in the curved surface could be a relevant way to control the heat transfer between the droplet and the substrate. DC10 : Melanie Bulois JOB OFFER DC10 - Click here to apply WP3 Title: Leidenfrost effect in multiphase microfluidics PI: Benoit Scheid (ULB, BE) & Benjamin Sobac (UPPA, FR) Secondment: UPPA (FR) Objective A: Explore the feasibility of using the Leidenfrost effect in confined milli/micro-channel systems and investigate under what conditions does this levitating regime establish (in both situations anti-Bretherton and μ-antibubble). Objective B: Understand the shape of these drops, as well as their dynamics in terms of formation, motion and evaporation. To do so, it is planned to study in detail features related to objectives A and B for several couples of fluids, size of the drops, degree of confinement and superheating. Objective C: Mathematically model the shapes and the dynamics of the Leidenfrost drops and their surrounding vapor films. In the anti- Bretherton situation, we will extend the Bretherton model to include evaporation, similarly to self-propelled Leidenfrost drops. In the μ- antibubble situation, the dynamics of a vapor lubricating film fed by the evaporation of the drop will then be modelled with an intermediate approach between the modelling of an antibubble and that of a Leidenfrost drop. The modelling will be improved according to experimental agreement by accounting for heat and mass transfers under flow conditions in the surrounding phase. https://www.leidenforce.eu/trueforce
