David Quéré - Academia.edu (original) (raw)

Papers by David Quéré

Physical review fluids, Mar 16, 2020

Nature Communications, Mar 29, 2019

Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the pres... more Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the presence of entrapped air within the texture, it is much more challenging to repel hot water. As it contacts a colder material, hot water generates condensation within the cavities at the solid surface, which eventually builds bridges between the substrate and the water, and thus destroys repellency. Here we show that both "small" (~100 nm) and "large" (~10 µm) model features do reflect hot drops at any drop temperature and in the whole range of explored impact velocities. Hence, we can define two structural recipes for repelling hot water: drops on nanometric features hardly stick owing to the miniaturization of water bridges, whereas kinetics of condensation in large features is too slow to connect the liquid to the solid at impact.

Physical Review E, Feb 21, 2013

We propose a model for the propulsion of Leidenfrost solids on ratchets based on viscous drag due... more We propose a model for the propulsion of Leidenfrost solids on ratchets based on viscous drag due to the flow of evaporating vapor. The model assumes pressure-driven flow described by the Navier-Stokes equations and is mainly studied in lubrication approximation. A scaling expression is derived for the dependence of the propulsive force on geometric parameters of the ratchet surface and properties of the sublimating solid. We show that the model results as well as the scaling law compare favorably with experiments and are able to reproduce the experimentally observed scaling with the size of the solid.

arXiv (Cornell University), Dec 18, 2020

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investiga... more Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flows structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10 • C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the baseflows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius R, yields the most unstable, thus, dominant, azimuthal modes (of wavenumber m). Our theory predicts well the radii R for the mode transitions and cascade with decreasing wavenumbers from m = 3, m = 2, down to m = 1 (the eventual rolling mode that entails propulsion) as the drop shrinks in size. The effect of the escaping vapor is not taken into account here, which may further destabilize the inner flow and couple to the liquid/vapor interface to give rise to motion [8, 9].

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are

arXiv (Cornell University), Dec 18, 2020

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investiga... more Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flows structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10 • C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the baseflows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius R, yields the most unstable, thus, dominant, azimuthal modes (of wavenumber m). Our theory predicts well the radii R for the mode transitions and cascade with decreasing wavenumbers from m = 3, m = 2, down to m = 1 (the eventual rolling mode that entails propulsion) as the drop shrinks in size. The effect of the escaping vapor is not taken into account here, which may further destabilize the inner flow and couple to the liquid/vapor interface to give rise to motion [8, 9].

Flow

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investiga... more Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flow structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10 circ^{\circ }circ C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the base flows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and the effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius RRR , yields the most unstable, thus, dominant, azimuthal modes (of wavenumber mmm ). Our theory predicts well the radii $...

arXiv (Cornell University), Feb 9, 2016

Superantiwettability, including superhydrophobicity, is an enhanced effect of surface ruggedness ... more Superantiwettability, including superhydrophobicity, is an enhanced effect of surface ruggedness via the Cassie-Baxter wetting state 1 , and has many applications such as antifouling 2,3 , drop manipulation 4,5 , and self-cleaning 6-9 ,. However, superantiwettability is easily broken due to Cassie-Baxter to Wenzel wetting state transition caused by various environmental disturbances 10-17. Since all observed reverse transitions required energy inputs 18-20 , it was believed that the Cassie-Baxter state couldn't be monostable 21. Here we show that there is a regime in the phase space of the receding contact angle and ruggedness parameters in which a Wenzel state can automatically transit into the Cassie-Baxter one without an external energy input, namely the Cassie-Baxter state in this regime is monostable. We further find a simple criterion that predicts very well experimentally observed Wenzel to Cassie-Baxter transitions for different liquids placed on various pillar-structured substrates. These results can be used as a guide for designing and engineering durable superantiwetting surfaces. To find rugged substrates that may have monostable Cassie-Baxter states, we use various periodical pillar-structures fabricated from a flat silicon wafer. Figure 1a shows a typical pillarstructured substrate with the pillar side length = 20 m, height ℎ = 100 m and separation = 100 m. This rugged surface is treated with a commercial coating (Glaco Mirror Coat "Zero", Soft 99, Japan) 22,23 that contains hydrophobic nanoparticles (see Supplementary Section 1). The receding and advancing contact angles of water on a flat silicon surface with this coating are measured as 150 ± 3° and 164 ± 2°, respectively. Figure 1b shows a side view of a water drop placed on this substrate, appearing clearly in the Cassie-Baxter state (namely the liquid rests on the tops of the

Applied Physics Letters, 2021

Water is extremely mobile on non-wetting surfaces, on which it glides at high velocities. We disc... more Water is extremely mobile on non-wetting surfaces, on which it glides at high velocities. We discuss how a few indentations placed on the surface markedly slow down drops forced to hit and jump above these hurdles. The corresponding “friction” is characterized and shown to be inertial in nature, which we interpret as the result of the successive soft shocks of the drops against obstacles.

Science Advances, 2019

Leidenfrost drops, known to levitate on very hot solids, exhibit a “cold” regime on superhydropho... more Leidenfrost drops, known to levitate on very hot solids, exhibit a “cold” regime on superhydrophobic solids.

Physical Review Fluids, 2018

Atomization and spray generation naturally occur around us in a wide variety of situations rangin... more Atomization and spray generation naturally occur around us in a wide variety of situations ranging from drop impacts to bubble bursting. However, controlling this process is key in many applications such as internal combustion engines, gas turbines, and agricultural spraying. Here we show how a drop can be fragmented into thousands of smaller droplets by impacting it onto a mesh. We demonstrate the unexpected possibility to transfer liquid outside the projected impact area of the drop and the existence of a well-defined cone envelope for the resulting spray. Self-similarity of the flow studied at the primary repeating unit-the hole-allows us to predict the global nature of the atomization process: mass transfer and spray geometry. We explain how these elementary units capture the momentum of the flow atop them and how side wall interactions can lead to saturation effects. At the grid level, this translates into surface fraction and hole aspect ratio being governing parameters of the system that can be tuned to control and optimize spray characteristics. As a result of the fragmentation, the momentum exerted on the target is reduced-a major advantage in crop protection and pathogen dispersion prevention under rain. In addition, pesticide drift in agricultural sprays can be controlled by using initially large drops that are subsequently atomized and conically sprayed by a mesh atop the crop. Beyond droplet-substrate interaction, this inexpensive spraying method enhances surface exchange phenomena such as evaporation and has major implications in many applications such as cooling towers or multieffect desalination.

Nature Communications, 2019

Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the pres... more Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the presence of entrapped air within the texture, it is much more challenging to repel hot water. As it contacts a colder material, hot water generates condensation within the cavities at the solid surface, which eventually builds bridges between the substrate and the water, and thus destroys repellency. Here we show that both “small” (~100 nm) and “large” (~10 µm) model features do reflect hot drops at any drop temperature and in the whole range of explored impact velocities. Hence, we can define two structural recipes for repelling hot water: drops on nanometric features hardly stick owing to the miniaturization of water bridges, whereas kinetics of condensation in large features is too slow to connect the liquid to the solid at impact.

Soft matter, Jan 28, 2018

Texturing a flat superhydrophobic substrate with point-like superhydrophobic macrotextures of the... more Texturing a flat superhydrophobic substrate with point-like superhydrophobic macrotextures of the same repellency makes impacting water droplets take off as rings, which leads to shorter bouncing times than on a flat substrate. We investigate the contact time reduction on such elementary macrotextures through experiment and simulations. We understand the observations by decomposing the impacting drop reshaped by the defect into sub-units (or blobs) whose size is fixed by the liquid ring width. We test the blob picture by looking at the reduction of contact time for off-centered impacts and for impacts in grooves that produce liquid ribbons where the blob size is fixed by the width of the channel.

Nature Materials, 2017

Nanometre-scale features with special shapes impart a broad spectrum of unique properties to the ... more Nanometre-scale features with special shapes impart a broad spectrum of unique properties to the surface of insects. These properties are essential for the animal's survival, and include the low light reflectance of moth eyes, the oil repellency of springtail carapaces and the ultra-adhesive nature of palmtree bugs. Antireflective mosquito eyes and cicada wings are also known to exhibit some antifogging and self-cleaning properties. In all cases, the combination of small feature size and optimal shape provides exceptional surface properties. In this work, we investigate the underlying antifogging mechanism in model materials designed to mimic natural systems, and explain the importance of the texture's feature size and shape. While exposure to fog strongly compromises the water-repellency of hydrophobic structures, this failure can be minimized by scaling the texture down to nanosize. This undesired e ect even becomes non-measurable if the hydrophobic surface consists of nanocones, which generate antifogging e ciency close to unity and water departure of droplets smaller than 2 µm. A lthough textured hydrophobic materials show spectacular water-repellency, which causes millimetre-size drops to bounce off such surfaces 1,2 , they generally get wet when exposed to fogs or to humid atmosphere 3-9. Droplets of a size comparable to that of the surface features can nucleate and grow within-rather than atop-the texture, and this so-called Wenzel state destroys superhydrophobicity 6-9. Water strongly pinned in this way remains stuck as dew accumulates, resulting in hydrophiliclike behaviour. A large drop contacting this infused solid will also be pinned, owing to the multiple bridges provided by the subjacent wet patches. In humid air, water drops of any size between mist and rain get captured on the previously repellent material, even when it is tilted 3-5. These considerations suggest that more effective antifogging might result from scaling the texture sizes to the submicrometre range 10,11 , which has other practical benefits such as rendering the coating robust against pressure 12 , or enhancing optical transparency 13 and heat transfer 14,15. It has been reported that droplets of 10-100 µm condensing on nanotextures can remain mobile enough to allow an efficient transfer of surface energy gained in coalescence to kinetic energy, causing them to sometimes jump off the surface 15-18. This property has been observed on solids with two-tier roughness 19-21 or colloids 22 , and on cicadae wings covered with nanocones 11-thus potentially self-cleaned by fogs.

EPL (Europhysics Letters), 2015

The Editorial Board invites you to submit your letters to EPL EPL is a leading international jour... more The Editorial Board invites you to submit your letters to EPL EPL is a leading international journal publishing original, innovative Letters in all areas of physics, ranging from condensed matter topics and interdisciplinary research to astrophysics, geophysics, plasma and fusion sciences, including those with application potential. The high profile of the journal combined with the excellent scientific quality of the articles ensures that EPL is an essential resource for its worldwide audience. EPL offers authors global visibility and a great opportunity to share their work with others across the whole of the physics community.

Nature Communications, 2015

It has been recently shown that the presence of macrotextures on superhydrophobic materials can m... more It has been recently shown that the presence of macrotextures on superhydrophobic materials can markedly modify the dynamics of water impacting them, and in particular significantly reduce the contact time of bouncing drops, compared with what is observed on a flat surface. This finding constitutes a significant step in the maximization of water repellency, since it enables to minimize even further the contact between solid and liquid. It also opens a new axis of research on the design of super-structures to induce specific functions such as anti-freezing, liquid fragmentation and/or recomposition, guiding, trapping and so on. Here we show that the contact time of drops bouncing on a repellent macrotexture takes discrete values when varying the impact speed. This allows us to propose a quantitative analysis of the reduction of contact time and thus to understand how and why macrotextures can control the dynamical properties of bouncing drops.

Soft Matter, 2005

Water-repellency is a property of some materials, either natural or synthetic, which makes water ... more Water-repellency is a property of some materials, either natural or synthetic, which makes water hardly stick to them: drops roll very easily off these solids, and bounce back upon impacting them. Here we discuss recent advances in this field, which has been particularly lively in recent years. We first examine the physical causes for this effect. Then we discuss the loss of adherence of the drops in such a state, and stress their remarkable dynamic behaviour. We finally suggest several remaining challenges in the field.

Physics of Fluids, 2003

A Leidenfrost drop forms when a volatile liquid is brought in contact with a very hot solid. Then... more A Leidenfrost drop forms when a volatile liquid is brought in contact with a very hot solid. Then, a vapor film comes in between the solid and the drop, giving to the latter the appearance of a liquid pearl. After a brief description of the shape of a Leidenfrost drop, we show that its size cannot exceed a certain value. Then, we describe the characteristics of the vapor layer on which it floats. We show how it is related to the drop size, and how both vary with time, as evaporation takes place. We finally deduce scaling laws for the lifetime of these drops.

Physical Review Letters, 2011

Drops placed on very hot solids levitate on a cushion of their own vapor, as discovered by Leiden... more Drops placed on very hot solids levitate on a cushion of their own vapor, as discovered by Leidenfrost. This confers to these drops a remarkable mobility, which makes problematic their control and manipulation. Here we show how crenelated surfaces can be used to increase the friction of Leidenfrost drops by a factor on the order of 100, making them decelerate and be trapped on centimetric distances instead of the usual metric ones. We measure and characterize the friction force as a function of the design of the crenelations.

Physical Review E, 2012

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Physical review fluids, Mar 16, 2020

Nature Communications, Mar 29, 2019

Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the pres... more Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the presence of entrapped air within the texture, it is much more challenging to repel hot water. As it contacts a colder material, hot water generates condensation within the cavities at the solid surface, which eventually builds bridges between the substrate and the water, and thus destroys repellency. Here we show that both "small" (~100 nm) and "large" (~10 µm) model features do reflect hot drops at any drop temperature and in the whole range of explored impact velocities. Hence, we can define two structural recipes for repelling hot water: drops on nanometric features hardly stick owing to the miniaturization of water bridges, whereas kinetics of condensation in large features is too slow to connect the liquid to the solid at impact.

Physical Review E, Feb 21, 2013

We propose a model for the propulsion of Leidenfrost solids on ratchets based on viscous drag due... more We propose a model for the propulsion of Leidenfrost solids on ratchets based on viscous drag due to the flow of evaporating vapor. The model assumes pressure-driven flow described by the Navier-Stokes equations and is mainly studied in lubrication approximation. A scaling expression is derived for the dependence of the propulsive force on geometric parameters of the ratchet surface and properties of the sublimating solid. We show that the model results as well as the scaling law compare favorably with experiments and are able to reproduce the experimentally observed scaling with the size of the solid.

arXiv (Cornell University), Dec 18, 2020

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investiga... more Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flows structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10 • C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the baseflows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius R, yields the most unstable, thus, dominant, azimuthal modes (of wavenumber m). Our theory predicts well the radii R for the mode transitions and cascade with decreasing wavenumbers from m = 3, m = 2, down to m = 1 (the eventual rolling mode that entails propulsion) as the drop shrinks in size. The effect of the escaping vapor is not taken into account here, which may further destabilize the inner flow and couple to the liquid/vapor interface to give rise to motion [8, 9].

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are

arXiv (Cornell University), Dec 18, 2020

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investiga... more Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flows structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10 • C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the baseflows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius R, yields the most unstable, thus, dominant, azimuthal modes (of wavenumber m). Our theory predicts well the radii R for the mode transitions and cascade with decreasing wavenumbers from m = 3, m = 2, down to m = 1 (the eventual rolling mode that entails propulsion) as the drop shrinks in size. The effect of the escaping vapor is not taken into account here, which may further destabilize the inner flow and couple to the liquid/vapor interface to give rise to motion [8, 9].

Flow

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investiga... more Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flow structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10 circ^{\circ }circ C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the base flows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and the effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius RRR , yields the most unstable, thus, dominant, azimuthal modes (of wavenumber mmm ). Our theory predicts well the radii $...

arXiv (Cornell University), Feb 9, 2016

Superantiwettability, including superhydrophobicity, is an enhanced effect of surface ruggedness ... more Superantiwettability, including superhydrophobicity, is an enhanced effect of surface ruggedness via the Cassie-Baxter wetting state 1 , and has many applications such as antifouling 2,3 , drop manipulation 4,5 , and self-cleaning 6-9 ,. However, superantiwettability is easily broken due to Cassie-Baxter to Wenzel wetting state transition caused by various environmental disturbances 10-17. Since all observed reverse transitions required energy inputs 18-20 , it was believed that the Cassie-Baxter state couldn't be monostable 21. Here we show that there is a regime in the phase space of the receding contact angle and ruggedness parameters in which a Wenzel state can automatically transit into the Cassie-Baxter one without an external energy input, namely the Cassie-Baxter state in this regime is monostable. We further find a simple criterion that predicts very well experimentally observed Wenzel to Cassie-Baxter transitions for different liquids placed on various pillar-structured substrates. These results can be used as a guide for designing and engineering durable superantiwetting surfaces. To find rugged substrates that may have monostable Cassie-Baxter states, we use various periodical pillar-structures fabricated from a flat silicon wafer. Figure 1a shows a typical pillarstructured substrate with the pillar side length = 20 m, height ℎ = 100 m and separation = 100 m. This rugged surface is treated with a commercial coating (Glaco Mirror Coat "Zero", Soft 99, Japan) 22,23 that contains hydrophobic nanoparticles (see Supplementary Section 1). The receding and advancing contact angles of water on a flat silicon surface with this coating are measured as 150 ± 3° and 164 ± 2°, respectively. Figure 1b shows a side view of a water drop placed on this substrate, appearing clearly in the Cassie-Baxter state (namely the liquid rests on the tops of the

Applied Physics Letters, 2021

Water is extremely mobile on non-wetting surfaces, on which it glides at high velocities. We disc... more Water is extremely mobile on non-wetting surfaces, on which it glides at high velocities. We discuss how a few indentations placed on the surface markedly slow down drops forced to hit and jump above these hurdles. The corresponding “friction” is characterized and shown to be inertial in nature, which we interpret as the result of the successive soft shocks of the drops against obstacles.

Science Advances, 2019

Leidenfrost drops, known to levitate on very hot solids, exhibit a “cold” regime on superhydropho... more Leidenfrost drops, known to levitate on very hot solids, exhibit a “cold” regime on superhydrophobic solids.

Physical Review Fluids, 2018

Atomization and spray generation naturally occur around us in a wide variety of situations rangin... more Atomization and spray generation naturally occur around us in a wide variety of situations ranging from drop impacts to bubble bursting. However, controlling this process is key in many applications such as internal combustion engines, gas turbines, and agricultural spraying. Here we show how a drop can be fragmented into thousands of smaller droplets by impacting it onto a mesh. We demonstrate the unexpected possibility to transfer liquid outside the projected impact area of the drop and the existence of a well-defined cone envelope for the resulting spray. Self-similarity of the flow studied at the primary repeating unit-the hole-allows us to predict the global nature of the atomization process: mass transfer and spray geometry. We explain how these elementary units capture the momentum of the flow atop them and how side wall interactions can lead to saturation effects. At the grid level, this translates into surface fraction and hole aspect ratio being governing parameters of the system that can be tuned to control and optimize spray characteristics. As a result of the fragmentation, the momentum exerted on the target is reduced-a major advantage in crop protection and pathogen dispersion prevention under rain. In addition, pesticide drift in agricultural sprays can be controlled by using initially large drops that are subsequently atomized and conically sprayed by a mesh atop the crop. Beyond droplet-substrate interaction, this inexpensive spraying method enhances surface exchange phenomena such as evaporation and has major implications in many applications such as cooling towers or multieffect desalination.

Nature Communications, 2019

Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the pres... more Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the presence of entrapped air within the texture, it is much more challenging to repel hot water. As it contacts a colder material, hot water generates condensation within the cavities at the solid surface, which eventually builds bridges between the substrate and the water, and thus destroys repellency. Here we show that both “small” (~100 nm) and “large” (~10 µm) model features do reflect hot drops at any drop temperature and in the whole range of explored impact velocities. Hence, we can define two structural recipes for repelling hot water: drops on nanometric features hardly stick owing to the miniaturization of water bridges, whereas kinetics of condensation in large features is too slow to connect the liquid to the solid at impact.

Soft matter, Jan 28, 2018

Texturing a flat superhydrophobic substrate with point-like superhydrophobic macrotextures of the... more Texturing a flat superhydrophobic substrate with point-like superhydrophobic macrotextures of the same repellency makes impacting water droplets take off as rings, which leads to shorter bouncing times than on a flat substrate. We investigate the contact time reduction on such elementary macrotextures through experiment and simulations. We understand the observations by decomposing the impacting drop reshaped by the defect into sub-units (or blobs) whose size is fixed by the liquid ring width. We test the blob picture by looking at the reduction of contact time for off-centered impacts and for impacts in grooves that produce liquid ribbons where the blob size is fixed by the width of the channel.

Nature Materials, 2017

Nanometre-scale features with special shapes impart a broad spectrum of unique properties to the ... more Nanometre-scale features with special shapes impart a broad spectrum of unique properties to the surface of insects. These properties are essential for the animal's survival, and include the low light reflectance of moth eyes, the oil repellency of springtail carapaces and the ultra-adhesive nature of palmtree bugs. Antireflective mosquito eyes and cicada wings are also known to exhibit some antifogging and self-cleaning properties. In all cases, the combination of small feature size and optimal shape provides exceptional surface properties. In this work, we investigate the underlying antifogging mechanism in model materials designed to mimic natural systems, and explain the importance of the texture's feature size and shape. While exposure to fog strongly compromises the water-repellency of hydrophobic structures, this failure can be minimized by scaling the texture down to nanosize. This undesired e ect even becomes non-measurable if the hydrophobic surface consists of nanocones, which generate antifogging e ciency close to unity and water departure of droplets smaller than 2 µm. A lthough textured hydrophobic materials show spectacular water-repellency, which causes millimetre-size drops to bounce off such surfaces 1,2 , they generally get wet when exposed to fogs or to humid atmosphere 3-9. Droplets of a size comparable to that of the surface features can nucleate and grow within-rather than atop-the texture, and this so-called Wenzel state destroys superhydrophobicity 6-9. Water strongly pinned in this way remains stuck as dew accumulates, resulting in hydrophiliclike behaviour. A large drop contacting this infused solid will also be pinned, owing to the multiple bridges provided by the subjacent wet patches. In humid air, water drops of any size between mist and rain get captured on the previously repellent material, even when it is tilted 3-5. These considerations suggest that more effective antifogging might result from scaling the texture sizes to the submicrometre range 10,11 , which has other practical benefits such as rendering the coating robust against pressure 12 , or enhancing optical transparency 13 and heat transfer 14,15. It has been reported that droplets of 10-100 µm condensing on nanotextures can remain mobile enough to allow an efficient transfer of surface energy gained in coalescence to kinetic energy, causing them to sometimes jump off the surface 15-18. This property has been observed on solids with two-tier roughness 19-21 or colloids 22 , and on cicadae wings covered with nanocones 11-thus potentially self-cleaned by fogs.

EPL (Europhysics Letters), 2015

The Editorial Board invites you to submit your letters to EPL EPL is a leading international jour... more The Editorial Board invites you to submit your letters to EPL EPL is a leading international journal publishing original, innovative Letters in all areas of physics, ranging from condensed matter topics and interdisciplinary research to astrophysics, geophysics, plasma and fusion sciences, including those with application potential. The high profile of the journal combined with the excellent scientific quality of the articles ensures that EPL is an essential resource for its worldwide audience. EPL offers authors global visibility and a great opportunity to share their work with others across the whole of the physics community.

Nature Communications, 2015

It has been recently shown that the presence of macrotextures on superhydrophobic materials can m... more It has been recently shown that the presence of macrotextures on superhydrophobic materials can markedly modify the dynamics of water impacting them, and in particular significantly reduce the contact time of bouncing drops, compared with what is observed on a flat surface. This finding constitutes a significant step in the maximization of water repellency, since it enables to minimize even further the contact between solid and liquid. It also opens a new axis of research on the design of super-structures to induce specific functions such as anti-freezing, liquid fragmentation and/or recomposition, guiding, trapping and so on. Here we show that the contact time of drops bouncing on a repellent macrotexture takes discrete values when varying the impact speed. This allows us to propose a quantitative analysis of the reduction of contact time and thus to understand how and why macrotextures can control the dynamical properties of bouncing drops.

Soft Matter, 2005

Water-repellency is a property of some materials, either natural or synthetic, which makes water ... more Water-repellency is a property of some materials, either natural or synthetic, which makes water hardly stick to them: drops roll very easily off these solids, and bounce back upon impacting them. Here we discuss recent advances in this field, which has been particularly lively in recent years. We first examine the physical causes for this effect. Then we discuss the loss of adherence of the drops in such a state, and stress their remarkable dynamic behaviour. We finally suggest several remaining challenges in the field.

Physics of Fluids, 2003

A Leidenfrost drop forms when a volatile liquid is brought in contact with a very hot solid. Then... more A Leidenfrost drop forms when a volatile liquid is brought in contact with a very hot solid. Then, a vapor film comes in between the solid and the drop, giving to the latter the appearance of a liquid pearl. After a brief description of the shape of a Leidenfrost drop, we show that its size cannot exceed a certain value. Then, we describe the characteristics of the vapor layer on which it floats. We show how it is related to the drop size, and how both vary with time, as evaporation takes place. We finally deduce scaling laws for the lifetime of these drops.

Physical Review Letters, 2011

Drops placed on very hot solids levitate on a cushion of their own vapor, as discovered by Leiden... more Drops placed on very hot solids levitate on a cushion of their own vapor, as discovered by Leidenfrost. This confers to these drops a remarkable mobility, which makes problematic their control and manipulation. Here we show how crenelated surfaces can be used to increase the friction of Leidenfrost drops by a factor on the order of 100, making them decelerate and be trapped on centimetric distances instead of the usual metric ones. We measure and characterize the friction force as a function of the design of the crenelations.

Physical Review E, 2012

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.