Liquid marbles (LMs) are nonsticking droplets whose surfaces are covered with low-wettability particles. Owing to their high mobility, shape reconfigurability, and widely accessible liquid/particle possibilities, the research on LMs has flourished since 2001. Their physical properties, fabrication mechanisms, and functionalisation capabilities indicate their potential for various applications. This review summarises the fundamental properties of LMs, the recent advances (mainly works published in 2020-2023) in the concept of LMs, physical properties, formation methods, LM-templated material design, and biochemical applications. Finally, the potential development and variations of LMs are discussed.

The spectacular nature of non-wetting drops mainly arises from their extreme mobility, and quick-silver, for instance, was named after this property. There are two ways to make water non-wetting, and they both rely on texture: either we can roughen a hydrophobic solid, which makes drops looking like pearls, or we can texture the liquid with a hydrophobic powder that “isolates” the resulting marble from its substrate. We observe, here, races between pearls and marbles, and report two effects: (1) the static adhesion of the two objects is different in nature, which we interpret as a consequence of the way they meet their substrates; (2) when they move, pearls are generally quicker than marbles, which might arise from the dissimilarity of the liquid/air interface between these two kinds of globules.

Non-wetting solids have been extensively studied for about 20 years due to the spectacular mobility they provide to water drops. However, they have other noteworthy properties: For instance, such materials immersed in water entrain a plastron of air, a phenomenon exploited by many creatures to hunt or breathe underwater. We measure the thickness of this plastron and model the results by modifying the Landau–Levich–Derjaguin theory established for a plate drawn out of a wetting liquid.

Fluid and ionic transport at the nanoscale has recently demonstrated a wealth of exotic behaviours1–14. However, artificial nanofluidic devices15–18 are still far from demonstrating the advanced functionalities existing in biological systems, such as electrically and mechanically activated transport19,20. Here, we focus on ionic transport through 2-nm-radius individual multiwalled carbon nanotubes under the combination of mechanical and electrical forcings. Our findings evidence mechanically activated ionic transport in the form of an ionic conductance that depends quadratically on the applied pressure. Our theoretical study relates this behaviour to the complex interplay between electrical and mechanical drivings, and shows that the superlubricity of the carbon nanotubes4–8,21 is a prerequisite to attaining mechanically activated transport. The pressure sensitivity shares similarities with the response of biological mechanosensitive ion channels19,20, but observed here in an artificial system. This paves the way to build new active nanofluidic functionalities inspired by complex biological machinery. Carbon nanotubes with 2 nm channel radius are shown to display pressure-driven ionic currents, which share some similarities to the response of biological mechanosensitive ion channels to tension.

Contrasting with its sluggish behavior on standard solids, water is extremely mobile on superhydrophobic materials, as shown, for instance, by the continuous acceleration of drops on tilted water-repellent leaves. For much longer substrates, however, drops reach a terminal velocity that results from a balance between weight and friction, allowing us to question the nature of this friction. We report that the relationship between force and terminal velocity is nonlinear. This is interpreted by showing that classical sources of friction are minimized, so that the aerodynamical resistance to motion becomes dominant, which eventually explains the matchless mobility of water. Our results are finally extended to viscous liquids, also known to be unusually quick on these materials.

Water-repellent materials ideally operate at very different liquid scales: from centimetersize for bugs living on ponds through millimeter-size for antirain functions to micrometersize for antifogging solids. In the last situation, it was recently evidenced that microdrops condensing on a highly nonadhesive substrate can take advantage from coalescence to jump off the material, even if the dynamical characteristics of the jump were not established at such microscales. We demonstrate in this paper that the jumping speed of drops is nonmonotonic with the drop size, showing a maximum around 5 mu m (a size commonly observed in dew), below and above which viscous and inertial effects, respectively, impede the takeoff. We quantitatively describe this optimum in antifogging. We also studied the ballistics of the jumping microdrops, from the height they reached to their behavior at landing; a situation where retakeoff is surprisingly found to be nearly unachievable despite the extreme nonwettability of the material.

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.

Over the past decade, the ability to reduce the dimensions of fluidic devices to the nanometre scale (by using nanotubes1-5 or nanopores6-11, for example) has led to the discovery of unexpected water- and ion-transport phenomena12-14. More recently, van der Waals assembly of two-dimensional materials15 has allowed the creation of artificial channels with ångström-scale precision16. Such channels push fluid confinement to the molecular scale, wherein the limits of continuum transport equations17 are challenged. Water films on this scale can rearrange into one or two layers with strongly suppressed dielectric permittivity18,19 or form a room-temperature ice phase20. Ionic motion in such confined channels21 is affected by direct interactions between the channel walls and the hydration shells of the ions, and water transport becomes strongly dependent on the channel wall material22. We explore how water and ionic transport are coupled in such confinement. Here we report measurements of ionic fluid transport through molecular-sized slit-like channels. The transport, driven by pressure and by an applied electric field, reveals a transistor-like electrohydrodynamic effect. An applied bias of a fraction of a volt increases the measured pressure-driven ionic transport (characterized by streaming mobilities) by up to 20 times. This gating effect is observed in both graphite and hexagonal boron nitride channels but exhibits marked material-dependent differences. We use a modified continuum framework accounting for the material-dependent frictional interaction of water molecules, ions and the confining surfaces to explain the differences observed between channels made of graphene and hexagonal boron nitride. This highly nonlinear gating of fluid transport under molecular-scale confinement may offer new routes to control molecular and ion transport, and to explore electromechanical couplings that may have a role in recently discovered mechanosensitive ionic channels23.

Drops of volatile liquid placed on sufficiently hot plates are spectacularly mobile and move away from their initial position, as reported in 1756 by Leidenfrost [1]. The elusive character of Leidenfrost drops arises from the presence of a cushion of vapor beneath the liquid, which generates a frictionless situation and makes these hovercrafts sensitive to any external force (gravity, airflows). This “passive” behavior is questioned here. Indeed, even if the Leidenfrost phenomenon has been described for about a quarter of a millennium, the existence and the effect of internal flows were hardly addressed. To investigate such motions, we look at the drops while they are pinned by a needle or immobilized by deepening the substrate. Figure 1 shows the presence of strong internal flows, as revealed by long-time exposure (about 200 ms) photos of water drops containing tracers placed on brass brought to 300 °C.

In the version of this Letter originally published, the Supplementary Videos were incorrectly labelled; the descriptions of 1–4 should have gone with the videos of 6–9, and the descriptions of 5–9 should have gone with the videos of 1–5. This has now been corrected.

In this paper, we explore the effect of a finite surface charge mobility on the interfacial transport: conductance, streaming currents, electro- and diffusio-osmotic flows. We first show that the surface charge mobility modifies the hydrodynamic boundary condition for the fluid, which introduces a supplementary term depending on the applied electric field. In particular, the resulting slip length is found to decrease inversely with the surface charge. We then derive expressions for the various transport mobilities, high-lighting that the surface charge mobility merely moderates the amplification effect of interfacial slippage, to the noticeable exception of diffusio-osmosis and surface conductance. Our calculations, obtained within Poisson-Boltzmann framework, highlight the importance of non-linear electrostatic contributions to predict the small concentration/large charge limiting regimes for the transport mobilities. We discuss these predictions in the context of recent electrokinetic experiments with carbon nanotubes.

As reported in 1756 by Johann Gottlob Leidenfrost, volatile liquids on hot solids form “gleaming drops resembling quicksilver”, a consequence of their levitation on a vapour cushion1,2. This makes the drops spectacularly mobile, moving away as soon as they are deposited—an observation commonly attributed to gravity or surrounding airflows. This mobility has been exploited to manipulate drops, because tiny forces such as those generated on asymmetric substrates can move them in well-defined directions3–5, a situation that also provides heat evacuation6. Here we report that Leidenfrost droplets initially at rest on horizontal substrates self-rotate and self-propel in the direction they are rolling, in the absence of any source of asymmetry or external force. Their rapid internal flow is found to be accompanied by a tilting of their base, which creates a permanent ratchet-like mechanism, entraining the rolling liquid despite the fact that it is not in contact with its substrate.Water drops placed at rest on flat, hot solids are found to rotate and spontaneously propel themselves in the direction of their rotation. The effect is due to symmetry breaking of the flow inside the drop, which couples rotation to translation.

We report a method to directly measure the jumping force during the coalescence of two water droplets on a superhydrophobic surface using a MEMS-based force sensor as shown in Figure 1(a). The sensor is designed to measure the jumping force during the coalescence of two droplets whose radii are several hundred μm. The measurement results show that the maximum jumping force could be more than 10 times larger than the total weight of the merged droplet. Moreover, the impulse calculated from the measured force was on the same order with the jumping momentum of the droplet. Therefore, our sensor is a useful tool to investigate the mechanism of coalescence-induced droplet jumping.

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 effect even becomes non-measurable if the hydrophobic surface consists of nanocones, which generate antifogging efficiency close to unity and water departure of droplets smaller than 2 μm. The antifogging properties of a structured surface can be considerably enhanced if the feature size is small enough and if the feature shapes are cones rather than cylinders.

L’eau sur une feuille de Lotus est connue pour être étonnamment mobile, cette propriété émergeant de la présence sur la feuille de rugosités hydrophobes micrométriques. À l’image d’un fakir qui ne touche que les pointes des clous de son tapis, une goutte sur une telle surface ne repose que sur les sommets des rugosités. Le liquide est ainsi sur coussin d’air d’où sa grande mobilité. Cette propriété est appelée superhydrophobie et permet de repousser efficacement l’eau. Cependant en situations humides, comme au contact d’un liquide chaud, la buée qui se condense dans les textures micrométriques de la surface altère ces propriétés anti-eau. D’autres surfaces naturelles sont superhydrophobes, parmi elles, les ailes des cigales, qui sont pourvues de cônes de l’ordre de 100 nm. Sur ces ailes, et contrairement au lotus, l’eau sous forme de buée semble garder sa mobilité : des gouttes qui coalescent sur cette surface peuvent s’éjecter de la surface par transfert d’énergie de surface en énergie cinétique.Dans cette thèse, nous avons étudié avec des surfaces modèles l’effet de la taille et de la forme de nano-rugosités sur les propriétés antibuée de surfaces superhydrophobes. Cette thèse se divise en deux parties.Dans une première partie, nous avons étudié la résistance des surfaces nanostructurées aux figures de souffle. Nous avons mis en évidence avec des surfaces modèles que la forme des rugosités jouait un rôle clé dans l’antibuée. Des piliers coniques permettent d’obtenir une éjection des gouttes de buée pour la quasi-totalité (95%) des coalescences, alors que des piliers cylindriques de même échelle ont une efficacité proche de zéro. Nous nous sommes alors naturellement intéressés au mécanisme d’éjection de la buée et avons d’abord montré que la vitesse de saut des gouttes est gouvernée par un transfert de quantité de mouvement de l’horizontale à la verticale. Nous avons ensuite observé que la dissipation visqueuse limitait la vitesse de saut des gouttes de rayon inférieur à 5 µm.Dans la seconde partie de cette thèse, nous avons testé l’adhésion de gouttes d’eau chaudes sur des surfaces dont la rugosité va de la cinquantaine de nanomètres au micromètre. Nous avons montré que plus une structure est compacte, plus elle apporte une résistance aux liquides chauds. C’est du compartimentage de la buée par les piliers qu’émerge cette propriété : si les rugosités sont trop espacées, la buée qui se condense sous la goutte remplace l’air à l’origine de la mobilité; à l’inverse, des piliers suffisamment rapprochés permettent de bloquer le liquide et ainsi de conserver air et mobilité. Ces résultats fondés sur une étude statique ne prennent pas en compte, par définition, la dynamique de la formation de buée. Nous avons donc pour compléter cette étude, à la situation dynamique des impacts de gouttes chaudes. Contrairement à toutes les conclusions précédentes, dans ce cas anti-pluie, des surfaces de rugosité micrométrique peuvent avoir un meilleur comportement antibuée que celles de rugosité sub-micrométrique. Cela est dû au temps nécessaire pour que les gouttes de condensation remplissent les textures sous la goutte : il augmente avec la hauteur des piliers, si bien que la buée n’a pas d’effet quand le temps de rebond est inférieur au temps de remplissage.Au total, cette thèse a permis de mettre en évidence la grande diversité des propriétés antibuée que l’on peut obtenir en fonction de l’échelle des rugosités.