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[Display omitted] • The synergistic effect between Fe 3 O 4 nanoparticles and CNT lies in the hybrid nanostructure improves the both light-harvesting and thermal insulation. • The localized heat concentration and radiation on the surface of Fe 3 O 4 /CNT structures enhance the solar evaporation. • Liquid Marbles with Fe 3 O 4 /CNT nanostructures show high evaporation rate. • The Fe 3 O 4 /CNT nanostructure enhances the light-induced electric field and heat concentration. Through the rational design of nanomaterial composites, broadband light harvesting and good thermal insulation can be achieved simultaneously to improve the efficiency of water evaporation. Solar evaporation experiments were carried out on liquid marbles (LMs) coated with Fe 3 O 4 nanoparticles, carbon nanotubes (CNTs) and hybrid nanomaterials (Fe 3 O 4 /CNTs) with different mass ratios of 2:1, 1:1 and 1:2. The results showed that the mixture of Fe 3 O 4 /CNTs enhances the light harvesting ability and solar interfacial evaporation performance. Fe 3 O 4 /CNT-LM at the mass ratio of 2:1 case provides the highest evaporation rate of 11.03 μg/s, which is about 1.22 and 1.34 times higher than that of Fe 3 O 4 and CNT, respectively. This high performance is mainly due to the synergistic effect between Fe 3 O 4 nanoparticles and CNTs, as the hybrid nanostructure significantly improves the both photothermal conversion and heat localization capability. Numerical simulation further supports that the composite can concentrate the electromagnetic field and heat at the phase-change interface. This leads to a rapid evaporation of the boundary region. This study provides a novel approach to a three-dimensional interface by assembling nanomaterials on the drop surface to enhance evaporation, which may have far-reaching implications for seawater desalination. [ABSTRACT FROM AUTHOR]

Liquid marbles (LMs) are versatile soft matter systems comprising a liquid core encapsulated by a shell of hydrophobic particles. LMs exhibit non-wetting properties and hence have enhanced mobility compared with droplets on a solid substrate. Recent advances have expanded their fabrication methods, material choices, and applications, particularly in microfluidics. This minireview highlights the progress in LM research over the past 2 years, focusing on novel fabrication techniques, unique materials, multi-physical control schemes, and emerging applications including solar energy harvesting, cell culture, controlled substance delivery, gas sensing, and pollutant decontamination. We provide opinions on the unaddressed needs in LM research and on the trend of LM developments, specifically for the applications of LMs in lab-on-a-chip applications. [ABSTRACT FROM AUTHOR]

Suspension of solid particles at interfaces is a common phenomenon, pivotal in natural, and industrial contexts. This study focuses on gas marbles which are single soap‐like air bubbles in air environment stabilized by colloidal particles. A unique stabilization method employing edible cinnamon particles is introduced. Unlike previous studies using spherical particles, the irregularly‐shaped micrometer‐sized cinnamon particles demonstrate remarkable long‐term stability through jamming and interlocking at the air‐liquid interface. Comprehensive characterization via optical and scanning electron microscopy, and contact angle experiments, elucidates the stabilization mechanism. These gas marbles exhibit exceptional resilience against drying, heating, freezing, and mechanical stress. They can be fabricated with various edible liquids offering extensive customization possibilities. This breakthrough introduces a promising method for creating stable gas marbles with edible particles, offering versatility for various fields and applications. The application scope includes surface catalysis and shock/vibration sensors, and also molecular gastronomy. [ABSTRACT FROM AUTHOR]

Liquid marbles (LMs) are liquid droplets in gaseous phase, which are generally millimeter size, coated by solid particles. In this study, how wide and high LMs can be fabricated is investigated. Polyhedral LMs are fabricated using polymer plates with millimeter to meter sizes as a stabilizer and water as an inner liquid. Rectangular LMs with widths exceeding 1 m, the largest width ever reported, can be successfully fabricated. The height of the LMs is found to be subject to restriction by the hydrostatic pressure and cubic LMs with heights of up to 5 mm, which is the maximum height limit for LMs stabilized with nano/micrometer‐sized particles, are fabricated. Reduction of the hydrostatic pressure by changing the LM shape from cube to pyramid and introduction of particle‐stabilized bubble into the LM enabled the increase of height of the LM up to 9.8 mm, the highest height ever reported. Investigation using non‐aqueous liquids as an inner liquid confirmed that the longer the capillary length, the higher the maximum possible height of LMs. Finally, the polyhedral LMs are demonstrated to function as a microreactor for silver mirror reaction and chemical oxidative polymerization, resulting in the formation of Janus polymer plates. [ABSTRACT FROM AUTHOR]

Liquid marbles (LMs) are versatile soft matter systems comprising a liquid core encapsulated by a shell of hydrophobic particles. LMs exhibit non-wetting properties and hence have enhanced mobility compared with droplets on a solid substrate. Recent advances have expanded their fabrication methods, material choices, and applications, particularly in microfluidics. This mini-review highlights the progress in LM research over the past 2 years, focusing on novel fabrication techniques, unique materials, multi-physical control schemes, and emerging applications including solar energy harvesting, cell culture, controlled substance delivery, gas sensing, and pollutant decontamination. We provide opinions on the unaddressed needs in LM research and on the trend of LM developments, specifically for the applications of LMs in lab-on-a-chip applications.

Abstract Liquid marbles (LMs) are liquid droplets in gaseous phase, which are generally millimeter size, coated by solid particles. In this study, how wide and high LMs can be fabricated is investigated. Polyhedral LMs are fabricated using polymer plates with millimeter to meter sizes as a stabilizer and water as an inner liquid. Rectangular LMs with widths exceeding 1 m, the largest width ever reported, can be successfully fabricated. The height of the LMs is found to be subject to restriction by the hydrostatic pressure and cubic LMs with heights of up to 5 mm, which is the maximum height limit for LMs stabilized with nano/micrometer‐sized particles, are fabricated. Reduction of the hydrostatic pressure by changing the LM shape from cube to pyramid and introduction of particle‐stabilized bubble into the LM enabled the increase of height of the LM up to 9.8 mm, the highest height ever reported. Investigation using non‐aqueous liquids as an inner liquid confirmed that the longer the capillary length, the higher the maximum possible height of LMs. Finally, the polyhedral LMs are demonstrated to function as a microreactor for silver mirror reaction and chemical oxidative polymerization, resulting in the formation of Janus polymer plates.

Liquid marbles are widely known for their potential biomedical applications, especially due to their versatility and ease of preparation. In the present work, we prepared liquid marbles with various cores composed of water, agar-based hydrogels, magnetic fluids, or non-aqueous substances. As a coating material, we used biocompatible particles of plant origin, such as turmeric grains and Lycopodium pollen. Additionally, we provided marbles with magnetic properties by incorporating either magnetosomes or iron oxide nanoparticles as a powder or by injecting another magnetic fluid. Structures obtained in this way were stable and susceptible to manipulation by an external magnetic field. The properties of the magnetic components of our marbles were verified using electron paramagnetic resonance (EPR) spectroscopy and vibrating sample magnetometry (VSM). Our approach to encapsulation of active substances such as antibiotics within a protective hydrogel core opens up new perspectives for the delivery of hydrophobic payloads to the inherently hydrophilic biological environment. Additionally, hydrogel marbles enriched with magnetic materials showed promise as biocompatible heating agents under alternating magnetic fields. A significant innovation of our research was also the fabrication of composite structures in which the gel-like core was surrounded without mixing by a magnetic fluid covered on the outside by the particle shell. Our liquid marbles, especially those with a hydrogel core and magnetic content, due to the ease of preparation and favorable properties, have great potential for biomedical use. The fact that we were able to simultaneously produce, functionalize (by filling with predefined cargo), and manipulate (by means of an external magnetic field) several marbles also seems to be important from an application point of view. [ABSTRACT FROM AUTHOR]

Open-channel droplet arrays have attracted much attention in the fields of biochemical analysis, biofluid monitoring, biomarker recognition and cell interactions, as they have advantages with regard to miniaturization, parallelization, high-throughput, simplicity and accessibility. Such droplet arrays not only improve the sensitivity and accuracy of a biosensor, but also do not require sophisticated equipment or tedious processes, showing great potential in next-generation miniaturized sensing platforms. This review summarizes typical examples of open-channel microdroplet arrays and focuses on diversified biosensing integrated with multiple signal-output approaches (fluorescence, colorimetric, surface-enhanced Raman scattering (SERS), electrochemical, etc.). The limitations and development prospects of open-channel droplet arrays in biosensing are also discussed with regard to the increasing demand for biosensors. [ABSTRACT FROM AUTHOR]

A novel soft actuator is designed, fabricated, and optimized for applied use in soft robotics and biomedical applications. The soft actuator is powered by the expansion and contraction of a graphene‐containing and encased liquid marble using the photothermal effect. Unfortunately, conventional liquid marbles are found to be too fragile and prone to cracking and failure for such applications. After experimentation, it is possible to remedy this problem by synthesizing liquid marbles encased with polymeric shells–polymerized in situ–for added mechanical strength and robustness. These marbles are shown to have intrinsic photothermal activity. They are then situated in bimorph‐type soft actuators where one side of the actuator has a dramatically different Young's modulus than the other, leading to directional actuation which is successfully demonstrated in multistep walking soft robots. The soft actuators are shown to successfully activate the mechanosensitive Piezo protein in a transfected human cell line with high effectiveness and no toxicity. Overall, the liquid marble‐powered soft actuators described here represent a new soft actuation methodology and a novel tool for mechanobiological studies, such as stem cell fate and organoid differentiation. [ABSTRACT FROM AUTHOR]

Non‐sticking droplets wrapped with fine hydrophobic particles, namely liquid marbles, can be transported both on solid and water pool without an undesired spill of the inner encapsulated liquid. While the stimuli‐responsive release of the inner liquid in the target area is proposed, the time‐programmed release is not yet achieved. Herein, the hydrophobicity of nanoclay is modulated via a catalyst‐free 1,4‐conjugate addition reaction to form liquid marbles. This nanoclay liquid marble is robust and stable in air but collapses on the liquid pool with a specific lifetime. The lifetime of the liquid marble can be modulated over seconds to hours scale depending on the selection of chemically modulated wettability of the nanoclay. The critical mechanism of lifetime modulation is responsible for controlling the coalescence kinetics between the water pool and inner liquid by nanoclays' high diffusion length and chemically varied water spreading potential. The NC liquid marble's programmable lifetime to 'time‐bomb' type drug release and cascade chemical reaction is applied—without requiring any external intervention. [ABSTRACT FROM AUTHOR]

The intricate interplay between droplet dynamics and particle characteristics has led to the extensive use of particle-laden droplets in diverse applications, spanning industrial engineering and scientific research. Accurate particle distribution analysis at multiphase interfaces is crucial for understanding the complex behaviors of these soft matters, for example, liquid marbles (LMs). However, traditional image analysis methods for particle-laden interfaces often rely on manual processing, which is both time-consuming and susceptible to human error. In this study, we present a pixel-level image recognition technique based on fully convolutional networks (FCNs) to automatically analyze the interfacial particle distribution of LMs exhibiting clear core-shell structures. The FCNs-based method enables efficient and accurate segmentation of target particles on sessile LMs, utilizing experimental data obtained from an in-situ optical imaging device. This approach significantly expedites the analysis of particle distribution, including proportion, concentration, and cluster centers. We assess the performance of the proposed method using image data of monolayer LMs encapsulated by dispersed polystyrene microspheres, demonstrating its superiority over traditional techniques in terms of accuracy and generalization. This investigation aims to offer novel insights into determining pertinent particulate characteristics within similar solid-liquid hybrid microsystems. [Display omitted] • An FCN-based image recognition method was proposed to analyze the interfacial particle distribution at the pixel level. • Efficient and precise segmentation of particles at target interfaces was achieved in in-situ optical imaging experiments. • Rapid analysis of particulate information on proportion, concentration, and cluste centers was further conducted. • A performance evaluation of the proposed method on image data collection of monolayer LMs was demonstrated. [ABSTRACT FROM AUTHOR]

• The synergistic effect between Fe 3 O 4 nanoparticles and CNTs optimizes the light-harvesting capability. • Nanowire arrays extend the light path to promote light absorption. • The composite structure enhances the electromagnetic field. • The confine spaces among nanowire arrays improves heat concentration. • Liquid marbles with Fe 3 O 4 /CNT nanowire arrays exhibit high evaporation rate. Solar interfacial evaporation is a sustainable technology to produce fresh water. The synergistic realization of broadband light absorption and good thermal insulation is crucial to improving the thermal efficiency. Here, we propose a new structure over the surface of liquid marbles, which via nanowire arrays assembled with composite nanomaterials (Fe 3 O 4 /CNT nanoparticles) to enhance the evaporation efficiency. The composite materials can effectively absorb light across the entire solar spectrum. Multiple reflections of light over nanowire array further improve light absorption. The narrow spaces and local particle aggregation of the nanowire array enhance thermal insulation to improve the evaporation efficiency. A series of experiments were conducted to verify the concept. The results showed that liquid marbles with upward Fe 3 O 4 /CNT nanowire arrays (Fe 3 O 4 /CNT-ULM) enhance the light harvesting ability and thermal insulation. Fe 3 O 4 /CNT-ULM at a volume ratio 1:2 provides the highest evaporation rate of 12.25 μg/s, which is about 1.16 and 1.53 times higher than that of Fe 3 O 4 and CNT, respectively. Numerical analysis further illustrates that the combination of composite materials and nanowire arrays enhances concentration of electromagnetic field and heat at the air/water interfaces. This promotes rapid evaporation at the thermal boundary region, which has important implications for the structure design of solar interfacial evaporator. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] Liquid marbles i.e. droplets coated by hydrophobic particles may be formed not only on the solid substrates but also on the floating layers of hydrophobic powders such as fluorinated fumed silica or polytetrafluoroethylene. Formation and growth of liquid marbles on fluorinated fumed silica or polytetrafluoroethylene powder floating on a heated water-vapor interface is reported. Marbles emerge from condensation of water droplets levitating above the powder layer. The kinetics of the growth of droplets is reported. Growth of droplets results from three main mechanisms: water condensation, absorption of small droplets and merging of droplets with neighboring ones. Growing droplets are coated with the hydrophobic powder, eventually giving rise to the formation of stable liquid marbles. Formation of hierarchical liquid marbles is reported. Growth of liquid marbles emerging from water condensation follows the linear temporal dependence. A phenomenological model of the liquid marble growth is suggested. [ABSTRACT FROM AUTHOR]

[Display omitted] Three-dimensional cell culture systems are increasingly used for biological and anticancer drug screening as they mimic the structure and microenvironment of tumors more closely than conventional two-dimensional cell models. In this study, the growth kinetics of colon adenocarcinoma-derived spheroids (HT-29 cell line) cultivated in liquid marble micro-bioreactors and nonadherent PDMS-coated well plates was investigated in detail and enabled precise control of the spheroid size by the seed cell density and cultivation time. The therapeutic effect of 5-fluorouracil and irinotecan hydrochloride in 2D monolayer cell culture and 3D tumor spheroids revealed an unexpected twist in their efficacy due to different ability to penetrate through 3D microtissue. For 5-fluorouracil, the inhibitory concentration IC 50 after 48 h exposure increased from 11.3 µM for a 2D cell culture to 707.7 µM for a 3D spheroid. In the case of irinotecan, IC 50 increased from 24.9 µM to 77.8 µM. Despite its higher molar weight, irinotecan appeared to penetrate the 3D spheroid structure more efficiently than 5-fluorouracil. While 5-fluorouracil mainly caused a suppression of spheroid growth from the outside, irinotecan affected the entire spheroid and caused its originally compact structure to disintegrate. The acquired results highlight the need to screen cancer chemotherapeutics on 3D tumor models, as contrasting results can be obtained compared to standard 2D cell cultures. [ABSTRACT FROM AUTHOR]

The present study investigates the physicochemical properties and stability of a novel lipid-based formulation—surfactant-enriched oil marbles containing abiraterone acetate. While the biopharmaceutical performance of this formulation has been reported recently, this study aims to fill the gap between a promising in vivo performance and industrial applicability. A series of techniques were employed to assess the solid-state characteristics of oil marble cores along with their physicochemical properties upon stability testing. The chemical stability of abiraterone acetate in the formulation was also investigated. The core of the formulation was found to be stable both physically and chemically over 12 months of storage. The in vitro performance of stressed samples was evaluated using a dissolution experiment. The formulation has successfully self-emulsified upon incubation in bio-relevant media, resulting in a fast and complete API release. An important issue connected with the excipient used as a covering material of oil marbles has been identified. The seemingly insignificant water sorption caused agglomeration of the oil marbles and consequently compromised the dissolution rate in some of the stressed samples. Replacing HPMC with lactose as a covering material resulted in more favorable properties upon storage. Overall, it has been shown that oil marbles are an industrially applicable concept of the solidified lipid-based formulation. [ABSTRACT FROM AUTHOR]

• DWLMs with sizes of 100 to 250 μm were prepared with SiO 2 /graphene. • The DWLMs can still remain stable at a bed height of 5–7 cm. • Temperature of DWLMs can be increased over 95 °C under 1 × 106 Lux illumination. • Absorption of DWLMs to CO 2 was approximately 0.17–0.19 mol CO 2 /mol*amine. • Regeneration of DWLMs was realized via photothermal methods. Liquid marbles(LMs) are formed by encapsulating liquid with hydrophobic particles showed potential application in capturing CO 2. However, conventional LMs are mostly millimeter scale and easily deformed by extrusion. This study aimed to enhance the mechanical stability by controlling the particle size to prepare "dry water" type LMs (DWLMs). The N-methyldiethanolamine (MDEA) solution was used as the core in constructing DWLMs, and SiO 2 /graphene(GPE) was used as the shell material. The incorporation of GPE can endow DWLMs with photothermal conversion properties, with a particle size distribution around 100 ∼ 250 μm occupying the largest volume fraction. These DWLMs exhibited good flow ability and mechanical stability. Moreover, the effects of MDEA concentration, and content of SiO 2 / GPE on the preparation of DWLMs were investigated. Under an illumination intensity of 1 × 106 Lux, the temperature of DWLMs reached above 95 °C. Subsequently, the absorption performance was examined by a fixed-bed absorption tower, and it was found that decreasing the droplet size can increase the gas–liquid mass transfer area to reduce the mass transfer resistance. The height of the fixed bed constructed by DWLMs was higher than 7 cm, and the bottom particles can still maintain stability without agglomeration and blocking the supporting mesh. The breakthrough point (t 0.5 and t 0.95) of large-DWLMs (L-DWLMs) were greater than that of tiny-DWLMs (T-DWLMs). The absorption kinetics have validated that chemisorption is the primary mode of CO 2 absorption. In desorption process, the results showed that the desorption of CO 2 was realized by photothermic desorption, and the desorbed DWLMs were regenerated and recycled, which could prevent the corrosion of the device as well as utilize the energy provided by the sunlight. [ABSTRACT FROM AUTHOR]

[Display omitted] Electrostatic extraction of particles from a bed to a pendent droplet to form liquid marbles has previously been investigated with respect to particle conductivity, size and shape, however, interparticle forces have not been specifically interrogated. If cohesion is the dominant force within the particle bed, then particles will be more readily extracted with reduced surface free energy. Glass particles were surface-modified using various alkyltrichlorosilanes. The surface free energy was measured for each sample using colloid probe atomic force microscopy (AFM) and sessile drop measurements on similarly modified glass slides. The ease of electrostatic particle extraction of each particle sample to a pendent droplet was compared by quantifying the electric field force required for successful extraction as a function of the measured surface free energy. Surface free energy calculated from sessile droplet measurements and AFM were not in agreement, as work of adhesion of a liquid droplet on a planar substrate is not representative of the contact between particles. Ease of electrostatic extraction of particles was observed to generally decrease as a function of AFM-derived surface free energy, confirming this is a critical factor in electrostatic delivery of particles to a pendent droplet. Roughness was also shown to inhibit particle extraction. [ABSTRACT FROM AUTHOR]

Three different kinds of liquid marbles were prepared using polytetrafluoroethylene nanoparticles (PTFE-NPs), polytetrafluoroethylene microparticles (PTFE-MPs), and polyvinylidene fluoride nanoparticles (PVDF-NPs) as the surface layers, respectively. The surface properties and stability of the different marbles were studied. The surface energy of PTFE-NPs, PTFE-MPs, and PVDF-NPs marbles are 0.061 N/m, 0.065 N/m, and 0.067 N/m respectively. It was revealed that the PTFE-NPs liquid marbles have the smallest surface energy and the highest stability. In addition, we prepared the phenolphthalein regent marbles with the PTFE-NPs, PTFE-MPs, PVDF-NPs. Among the same basicity gas, compared with the pure phenolphthalein liquid drop, the color of the phenolphthalein regent marbles changed more quickly. [ABSTRACT FROM AUTHOR]

Hydrophobic polymer plates with smooth and rough surfaces are used as a stabilizer for cubic liquid marbles (LMs) to study the effect of surface roughness on their formation. The smooth and rough polymer plates can stabilize LMs using liquids with surface tensions of 72.8-26.6 and 72.8-22.9 mN m -1 , respectively. It is clarified that the higher the surface roughness, the lower the surface tension of the liquids are stabilized to form the LMs. These results indicated that the introduction of surface roughness improves the hydrophobicity of the polymer plates and the rough polymer plates can stabilize LMs using liquids with a wider surface tension range. Electron microscopy studies and numerical analyses confirmed that the LMs can be formed, when the Cassie-Baxter wetting state, where θ Y >90° (θ Y : the contact angle on smooth surfaces) and θ R >90° (θ R : the contact angle on rough surfaces), and the metastable Cassie-Baxter wetting state, where θ Y <90° and θ R >90°, are realized. Finally, the synthesis of cubic polymer particles are succeeded by free radical polymerization of the cubic LMs containing a hydrophobic vinyl monomer (dodecyl acrylate) in a solvent-free manner., (© 2024 Wiley‐VCH GmbH.)

Cryopreservation is highly desired for long-term maintenance of the viability of living biosamples, while effective cell cryopreservation still relies heavily on the addition of dimethyl sulfoxide (DMSO) and fetal bovine serum (FBS). However, the intrinsic toxicity of DMSO is still a bottleneck, which could not only cause the clinical side effect but also induce cell genetic variants. In the meantime, the addition of FBS may bring potentially the risk of pathogenic microorganism contamination. The liquid marbles (LMs), a novel biotechnology tool for cell cryopreservation, which not only have a small volume system that facilitated recovery, but the hydrophobic shell also resisted the harm to cells caused by adverse environments. Previous LM-based cell cryopreservation relied heavily on the addition of FBS. In this work, we introduced acidic polyaspartic acid and polyglutamic acid as cryoprotectants to construct LM systems. LMs could burst in an instant to facilitate and achieve ultrarapid recovery process, and the hydrophilic carboxyl groups of the cryoprotectants could form hydrogen bonds with water molecules and further inhibit ice growth/formation to protect cells from cryoinjuries. The L929 cells could be well cryopreserved by acidic polyamino acid-based LMs. This new biotechnology platform is expected to be widely used for cell cryopreservation, which has the potential to propel LMs for the preservation of various functional cells in the future. • The LM cryopreservation system was constructed by using acidic polyamino acids. • The acidic polyamino acid displayed a superior biocompatibility. • The preparation of LMs was easy, and the LMs could be recovered quickly. • The acidic polyamino acid-based LMs showed excellent cryopreservation efficiency. [ABSTRACT FROM AUTHOR]

Liquid marbles allow for quantities of various liquids to be encapsulated by hydrophobic particles, thus ensuring isolation from the external environment. The unique properties provided by this soft solid has allowed for use in a wide array of different applications. Liquid marbles do however have certain drawbacks, with lifetime and robustness often being limited. Within this review, particle characteristics that impact liquid marble stability are critically discussed, in addition to other factors, such as internal and external environments, that can be engineered to achieve a robust long‐lived liquid marble. New emerging applications, which will benefit from this improvement, are explored such as unconventional computing, cell mimicry, and soft lithography. Incorporation of liquid marbles and liquid crystal technologies shows promise in utilizing structural color for optical display applications, and within green and environmental applications, liquid marble technology is increasingly adapted for use in energy conversion, heavy metal recovery, CO2 capture, and oil removal. [ABSTRACT FROM AUTHOR]

[Display omitted] • Composite liquid marbles coated filled with saline grew when floated on the distilled water. • Floating of the composite marbles filled with water floating on saline water resulted in decrease of their volume. • Composite liquid marbles filled with saline water and floating on saline water remained stable during 25 h. • The reported findings are attributed to the osmotic mass transfer. • Qualitative model of the mass transport across the osmotic membrane is suggested. We hypothesized that the reported evolution (growth) of composite water marbles filled with saline water and coated with lycopodium dispersed in a thin layer of silicone oil is due to the osmotic mass transfer. The hypothesis is supported by the semi-empirical model of osmotic growth of small liquid marbles floating on distilled water. Saline composite, silicone oil-coated marbles floating on distilled water grew with time; whereas, composite marbles filled with distilled water floating on aqueous solutions of NaCl lost mass with time and shrunk. However, composite liquid marbles filled with saline water and floating on aqueous solutions of NaCl remained stable during 25 h of the laboratory experiment. The reported findings are reasonably attributed to osmotic mass transport through the thin silicon layer filled with lycopodium particles coating the marbles, acting as an osmotic membrane. This is supported by the suggested model for the osmotic growth of marbles. [ABSTRACT FROM AUTHOR]

[Display omitted] • Liquid marbles production and actuation methods are reviewed. • New advances in scale-up are highlighted. • External fields and forces are harnessed for liquid marble motion and deployment. • Future research directions are discussed. Liquid marbles may be traditionally formed by rolling a droplet on a bed of non-wetting particles resulting in encapsulation and stabilisation. Particles used in this process may range from nanometre to millimetre if handled with sufficient care. This method, however, runs the risk of droplet coalescence and is limited to non-wetting particles. Currently there exist some alternative methods of formulation including using electrostatics to either deliver a particle bed to the droplet or pull the droplet to the particles. The former has shown some promise in potential batch processes but is hindered by interparticle forces. Additional production methods include a form of blender, but this has shown to be unable to produce marbles of a narrow size distribution. Once formed, liquid marbles have demonstrated value as potential blood typing devices, as micro-reaction vessels due to the inherent barrier between the internal phase and the substrate whilst maintaining gas permeability, and as contaminant sensors. Liquid marbles also demonstrate a remarkable level of elasticity under compressive force and reduced evaporation rates when compared to bare water droplets, a function of the size and composition of the stabilising particles. In addition to this, liquid marbles have been proposed as actuators. Locomotion may easily be induced in these structures, using electrostatics, sound, magnetism or light depending on the particle/liquid combinations used in formation, and the environment of deployment. This review seeks to present and summarise recent advances in the field of liquid marble manufacture and methods for actuation. We also aim to highlight potential future avenues of further study within this arena. [ABSTRACT FROM AUTHOR]

Liquid marble (LM) is a droplet that is wrapped by hydrophobic solid particles, which behave as a non‐wetting soft solid. Based on these properties, LM can be applied in fluidics and soft device applications. A wide variety of functional particles have been synthesized to form functional LMs. However, the formation of multifunctional LMs by integrating several types of functional particles is challenging. Here, a general strategy for the flexible patterning of functional particles on droplet surfaces in a patchwork‐like design is reported. It is shown that LMs can switch their macroscopic behavior between a stable and active state on super‐repellent surfaces in situ by jamming/unjamming the surface particles. Active LMs hydrostatically coalesce to form a self‐sorted particle pattern on the droplet surface. With the support of LM handling robotics, on‐demand cyclic activation–manipulation–coalescence–stabilization protocols by LMs with different sizes and particle types result in the reliable design of multi‐faced LMs. Based on this concept, a single bi‐functional LM is designed from two mono‐functional LMs as an advanced droplet carrier. [ABSTRACT FROM AUTHOR]

Abstract As an emerging droplet system, monolayer nanoparticle‐covered (mNPc) liquid marbles have shown fascinating properties and application values. Their formation process has been clear, but liquid availability for production has yet to be clarified. This study addresses this issue from the angle of surface tension, revealing critical surface tension and contact angle values for mNPc liquid marble production with both rolling and compression methods. Water‐ethanol and water‐surfactant systems are employed as testing liquids. With irreversible pinning as criterion, critical surface tension values that allow mNPc liquid marble formation are obtained with the two kinds of liquids, forming ranges from 38.6 to 31.8 mN m−1, and from 36.7 to 27.6 mN m−1, for the rolling and compression methods, respectively. Correspondingly, contact angles in the ranges from 132° to 126° and from 116–124° are regarded as the lower limits for mNPc liquid marble production with the rolling and compression methods, respectively. In addition, rupture and deformation behaviors under low surface tension are systematically investigated and a depinning approach that facilitates marble formation is proposed. This study could provide important guidance for various applications that require low surface tension liquids to produce mNPc liquid marbles.

Abstract Liquid marbles represented a significant advance in the manipulation of fluids as they used particle films to confine liquid drops, creating a robust and durable soft solid. We exploit this technology to engineering a bioactive hydrogel marble (BHM). Specifically, pristine bioactive glass nanoparticles were chemically tuned to produce biocompatible hydrophobic bioactive glass nanoparticles (H-BGNPs) that shielded a gelatin-based bead. The designed BHM shell promoted the growth of a bone-like apatite layer upon immersion in a physiological environment. The fabrication process allowed the efficient incorporation of drugs and cells into the engineered structure. The BHM provided a simultaneously controlled release of distinct encapsulated therapeutic model molecules. Moreover, the BHM sustained cell encapsulation in a 3D environment as demonstrated by an excellent in vitro stability and cytocompatibility. The engineered structures also showed potential to regulate a pre-osteoblastic cell line into osteogenic commitment. Overall, these hierarchical nanostructured and functional marbles revealed a high potential for future applications in bone tissue engineering.

Digital microfluidics based on liquid marble (LM) has recently emerged as a promising platform for liquid handling and cell-based assays. However, evaporation is a critical problem in such platforms, hindering their widerange applications in various fields. This study aims to develop a functional sessile LM system for long-term 3D cell culture. Previously, this study group and others demonstrated that floating LM-based bioreactors could reduce the evaporation rate, and were thus suitable for growing multicellular spheroids. However, floating LMs are not robust and easily collapse. Herein, an evaporation-reducing sessile LM by embedding LM with agarose gel is proposed. Through a series of comprehensive mathematical modeling, numerical simulations, and experimental investigations (both with and without biological cells), it is shown that such a platform acts as a moisture absorption system to control the evaporation and thus extends the life span of LMs. It is also found that unlike pure LMs, the LMs filled with agarose maintain their spherical shapes within 72 h inside a humidified incubator. Moreover, the presence of agarose significantly contributes to minimizing evaporation and improves the viability of the harvested multicellular spheroids. These results can open up a new avenue in using LMs in life sciences and chemistry. [ABSTRACT FROM AUTHOR]

Polyhedral liquid marbles/plasticines are prepared using (sub)millimeter‐sized polymer plates as a stabilizer and water as an inner liquid. Precise control of size and shape can be successfully performed by tuning the size ratio of the water droplet and the plate, number of plates adsorbed to the droplet, coalescence (jointing) of multiple polyhedral liquid marbles/plasticines, and application of external mechanical stress. Thanks to interfacial jamming of the plates, plastic deformation of the liquid marbles/plasticines is achieved. The authors are able to fabricate liquid marbles/plasticines with various shapes including A–Z letters of alphabet. Liquid marble/plasticine with an aspect ratio exceeding 800, the largest aspect ratio ever reported, is also successfully prepared; the length of the liquid marble/plasticine exceeded 1.5 m. The liquid marbles can be picked up and be piled up on top of each other using tweezers or fingers. Furthermore, Janus‐type liquid marbles/plasticines with different curvatures and different stabilizers in a single liquid marble/plasticine can be fabricated by coalescence (jointing) of near‐spherical and cuboid liquid marbles/plasticines stabilized by plates with different sizes. An internal liquid flow from the near‐spherical liquid marble to the cuboid liquid marble/plasticine immediately after jointing is observed, making this system act as a micropump. [ABSTRACT FROM AUTHOR]

As an emerging droplet system, monolayer nanoparticle‐covered (mNPc) liquid marbles have shown fascinating properties and application values. Their formation process has been clear, but liquid availability for production has yet to be clarified. This study addresses this issue from the angle of surface tension, revealing critical surface tension and contact angle values for mNPc liquid marble production with both rolling and compression methods. Water‐ethanol and water‐surfactant systems are employed as testing liquids. With irreversible pinning as criterion, critical surface tension values that allow mNPc liquid marble formation are obtained with the two kinds of liquids, forming ranges from 38.6 to 31.8 mN m−1, and from 36.7 to 27.6 mN m−1, for the rolling and compression methods, respectively. Correspondingly, contact angles in the ranges from 132° to 126° and from 116–124° are regarded as the lower limits for mNPc liquid marble production with the rolling and compression methods, respectively. In addition, rupture and deformation behaviors under low surface tension are systematically investigated and a depinning approach that facilitates marble formation is proposed. This study could provide important guidance for various applications that require low surface tension liquids to produce mNPc liquid marbles. [ABSTRACT FROM AUTHOR]

• Water marbles coated with a thin layer of silicone oil containing superhydrophobic particles are reported. • The composite marbles remained stable on solid and liquid supports. • The composite marbles remained stable when placed on a vibrating water bath. • The composite marbles withstand coalescence on collision with each other. • Bouncing of composite marbles with solid substrates was explored. Liquid marbles are non-stick droplets coated with colloidal usually hydrophobic particles. We suggest that "composite" liquid marbles, i.e. bi-liquid droplets, may be prepared with water droplets coated by a thin silicone oil layer containing hydrophobic, colloidal particles. The process enabling manufacturing water marbles coated with silicone oil containing fumed fluorosilica particles is reported. The marbles remained stable when placed on solid and liquid supports. Bouncing and coalescence of the composite marbles was explored. Non-coalescence prolonged (ca. 20 min) jumping of composite marbles above a vibrating water bath was observed. Composite marbles withstand coalescence better than colloidal particle-stabilized liquid marbles. The effective surface tension of the composite marbles is markedly lower than that of water marbles coated with fumed fluorosilica particles. The coefficient of restitution of the composite marbles bouncing on a hydrophobic solid substrate is lower than that established for water marbles. This observation is related to the viscous dissipation occurring within the silicone layer making up the composite marbles. [ABSTRACT FROM AUTHOR]

Effective surface tension (γ eff) is a fundamental property of liquid marbles, i.e., particle-covered drops in an air environment. Recently, studies of monolayer nanoparticle-covered (mNPc) liquid marbles produced with a sol-gel film have significantly updated previous understandings regarding γ eff. Examining the derived explanations with the most common powder-derived liquid marbles should be meaningful. Here, the maximum height and pendant drop methods were employed for γ eff measurements of liquid marbles produced with a polytetrafluoroethylene powder. Following the methodology proposed in mNPc liquid marble studies, the measurement processes were addressed in detail with the uncertainties in parameter identification as the main concerns, and the results were analyzed by highlighting the effective surface tension's equivalence nature and liquid marbles' surface solidity. By combining the present findings and conclusions related to both mNPc and powder liquid marbles, the key issues in measuring and understanding liquid marble γ eff were summarized. Unlabelled Image • How to understand and measure effective surface tension was systematically addressed. • The equivalence principle and surface solidity were demonstrated very important. • The maximum height and pendant drop methods were revisited. [ABSTRACT FROM AUTHOR]

Abstract Coalescence and splitting of liquid marbles (LMs) are critical for the mixture of precise amount precursors and removal of the wastes in the microliter range. Here, the coalescence and splitting of LMs are realized by a simple gravity‐driven impact method and the two processes are systematically investigated to obtain the optimal parameters. The formation, coalescence, and splitting of LMs can be realized on‐demand with a designed channel box. By selecting the functional channels on the device, gravity‐based fusion and splitting of LMs are performed to mix medium/drugs and remove spent culture medium in a precise manner, thus ensuring that the microenvironment of the cells is maintained under optimal conditions. The LM‐based 3D stem cell spheroids are demonstrated to possess an approximately threefold of cell viability compared with the conventional spheroid obtained from nonadhesive plates. Delivery of the cell spheroid to a hydrophilic surface results in the in situ respreading of cells and gradual formation of typical 2D cell morphology, which offers the possibility for such spheroid‐based stem cell delivery in regenerative medicine.

Reversing the organization of oil-in-water Pickering emulsion formed with halloysite nanoclay, liquid marbles were created with water droplets encapsulated by a layer of clay nanotubes. After incorporating microorganisms inside the spherical liquid marble of 0.5 mm in diameter, the simple clay capsule has potential as a storage vehicle for bacterial cultures. Halloysite nanotubes are naturally formed biocompatible and widely available aluminosilicate clay. The inherently hydrophilic halloysite is rendered hydrophobic (contact angle >90°) through silane grafting of long alkane groups onto the external surface. Because of the water repellency, the nanotubes are able to trap water inside a thin shell making a stable interface between inner liquid and air, thus forming marbles with reversed emulsion architecture. The halloysite with its alkane modifications encourages the growth of selected bacteria inside the marble; Alcanivorax borkumensis is one such example. The biofilm produced at the inner walls of the halloysite shell by such bacteria strengthens the marbles' structure and reduced evaporation, keeping the bacteria viable for a period of up to 4 days after drying. A symbiotic correlation between the halloysite external shell and bacteria creates stable liquid marble structures, paving the way for a strategy to encapsulate and store bacteria at room temperature. [ABSTRACT FROM AUTHOR]

Liquid marbles with polyacrylamide as skeleton and eosin Y as core and hydrophobic silica as shell have been prepared to study the sustained release of eosin Y. The influences of the amount of crosslinker, the sustained-release medium, the volume of liquid marbles, the temperature, the adhesive powder and the addition of sodium bicarbonate on the sustainedrelease properties of eosin Y have been studied in detail. The results showed that with the increase of volume, the amount of eosin Y release increased but the release rate decreased. The release amount of eosin Y decreases as the amount of cross linking agent increase. The prepared liquid marbles have been hardly released in acidic conditions, but the release amount of eosin Y has been improved with the increase of pH. The amount of eosin Y release increased with the increase of temperature. The use of liquid marbles as a medium to preparation sustained-release hydrogel for dye release can not only control the dye loading by accurately controlling the volume of liquid marbles, but also the release rate of dyes can be controlled by tuning the sustained-release medium and other conditions. This method provides a new method for dye release. [ABSTRACT FROM AUTHOR]

Solar interfacial evaporation is a potential and sustainable desalination technology. The structural design of photothermal materials plays an important role in improving the evaporation performance. Here, we propose a new evaporating structure consisting of liquid marble with tunable nanowire array to enhance solar evaporation. Such magnetically responsive nanowire arrays are enabled by controllable assembly of nanoparticles over the surface of liquid marble. Different from the traditional structure, the nanowire array increases light absorption through multiple reflection and to promote steam generation. Besides, the narrow space and local aggregation of particles within the nanowire array hinder heat diffusion to enhance evaporation efficiency. A series of experiments are conducted to validate the concept. The liquid marble with upward nanowire array (ULM) shows outstanding performance with the highest evaporation rate of 8.7 μg/s. This rate is 1.21, 1.04 and 1.04 times larger than that of liquid marble with nanoparticles (LM), left-tilt nanowire array (LLM) and right-tilt nanowire array (RLM). Numerical analysis further illustrates that the nanowire array concentrates the electromagnetic field and heat over the evaporative interface. The induced thermal-boundary layer with high-temperature thus improves the evaporation. This work provides a new concept of solar interfacial evaporation with tunable nanowire array, which has far-reaching implications in seawater desalination or wastewater treatment. [Display omitted] • Magnetically responsive nanowires are proposed for photothermal conversion. • Liquid marbles with the tunable nanowires show a high evaporation rate. • The accumulate nanoparticles enhance the electric field and heat localization. • The induced thermal-boundary layer improves the solar evaporation. [ABSTRACT FROM AUTHOR]

Liquid marbles which can be created by encapsulating water droplets using superhydrophobic powders have attracted extensive attention for their various promising application prospects. Cupric stearate coated liquid marbles was created by encapsulating water droplet using cupric stearate powders in a droplet rolling process. Hierarchies of robustness have been constructed which showed that the cupric stearate coated liquid marbles survived on surfaces with different wettabilities and could keep intact after impact, manipulation, transportation and mechanical compression. To evaluate robustness of the liquid marbles in manipulation processes, the mass of superhydrophobic powders per unit area on the surface of liquid marbles (powder surface density) was investigated. Powder surface density was decreased with the increase of water volume in the liquid marble. Moreover, water volume of the liquid marble was adjusted in situ, and the critical powder surface density was obtained via increasing or decreasing water volume of the liquid marble. By analyzing the critical surface density upon rupture or collapse of the liquid marble, the dependence of the critical surface density on the water volume of the liquid marble was predicted, which agrees well with experimental results. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The preparation of Janus liquid marbles containing both oil and water is reported. The marbles were prepared by coalescing an oil liquid marble with a water liquid marble mechanically on a powdered bed of omniphobic solid particles, followed by further rolling on the particle bed so as to increase the particle coating and prevent disintegration upon removal. The coalescence of the marbles involves the rarification of the particle layers around the interacting marbles followed by the fusion of their liquid cores. Upon further rolling, the marbles exhibited four characteristic shapes depending on the oil to water ratio and the stabilising solid particles. Janus marbles containing equal volumes of oil and water and stabilised by fluorinated clay particles were 'egg-like' in shape with the water portion remaining almost spherical compared with the oil portion. The marbles elongate and change to a 'tadpole-like' shape as the volume fraction of oil increases, leaving the shape of the water portion which forms the head of the tadpole intact (quasi-spherical). On the contrary, increasing the volume fraction of water did not affect the marble shape. When stabilised by fluorinated fumed silica particles, a quasi-spherical shape forms at equal oil and water volumes, but an increase in either oil or water fraction changes the shape to a puddle. The stability of the Janus marbles to evaporation depends on the vapour pressure of the encapsulated liquids, with those of higher vapour pressure evaporating faster as in the case of liquid marbles containing only one liquid. Lastly, by coalescing a cyclohexane marble containing sebacoyl chloride with a water marble containing hexamethylene diamine, resulting in the formation of Nylon 6–10 at the interface, the possibility of performing chemical reactions in the Janus oil-water liquid marbles was illustrated. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The effective surface tension of lycopodium-coated liquid marbles is calculated from the analysis of their free vibrations. Levitation of liquid marbles was achieved by the catapult mechanism. The effective surface tension was calculated with the Rayleigh formula for eigen-frequencies of oscillating spherical droplets. The calculated effective surface tension is close to the values, obtained by other experimental methods and supports the suggestion that the effective surface tension of lycopodium-coated marbles is markedly lower than that of the bare water/vapor interface. The analysis of damping of oscillations of levitating marbles enabled introduction of the notion of ≪effective viscosity≫ of marbles. A semi-quantitative model elucidating the energy dissipation in oscillating marbles is proposed. The scaling law describing the viscous dissipation in liquid marbles is derived and shown to be in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Liquid marble is a suitable candidate for transporting and targeted releasing of reagents and analytes. The moment of the disruption of liquid marble are controlled by using external stimuli nowadays. This work aims to achieve a disruption of liquid marbles by an internal stimulus. Liquid marbles containing triton X-100 were prepared by rolling method. The influence of the concentration of triton X-100 in droplets on the stability and the life time of the liquid marbles was investigated. The mass of the silica nanoparticles coating liquid marbles and the effective surface tension of liquid marbles were measured. The resulted showed that the life time of liquid marbles could be tuned on time scales of several minutes to hours by controlling the concentration of triton X-100 in the droplets. The number of the layers of the silica particles coating the liquid marble was about 168. The decrease of the amount of silica nanoparticles at the surface of the droplet did not affect the effective surface tension of the liquid marbles significantly. By controlling the concentration of triton X-100 in the droplets, the timed coalescing of two liquid marbles also could be achieved. A mechanism for the disintegrating of liquid marbles triggered by the adsorption of triton X-100 was proposed. [ABSTRACT FROM AUTHOR]

Micro total analysis systems (microTAS) provide the opportunity to create complete analytical microsystems by integrating various functional modules, such as sample preparation, separation and detection, into a single chip-sized microfabricated device. Microfluidics is the enabling technology for implementing the concept of microTAS. Liquid marble (LM), as a promising separate digital microfluidic platform, has the great potential to enhance the broad applications of microTAS. LMs are small liquid droplets encapsulated by multilayered hydrophobic particles and have attracted a great interest from the microfluidics research community due to their non-wetting property. A LM maintains its integrity and exhibits low friction on various carrier surfaces, enabling the LM to be actuated by external electric, magnetic, gravitational and acoustic fields or other manipulation schemes. LMs can thus serve effectively for the storage and transportation of small liquid volumes. In addition, they have been widely used for the quick detection of water pollution or gas emission and, most importantly, micromixing and microreactions for chemical and biomedical purposes. This paper reviews the recent developments in the manipulation techniques and emerging applications of LMs. The review aims to facilitate better understanding of their use as a unique digital microfluidic platform to promote further advancement of microTAS. The paper begins with different manipulation schemes of LMs according to the nature of actuation energy. Next, it summarizes the diverse applications of LMs for various chemical and biological assays. Finally, this paper concludes with future perspectives regarding the research on LMs in microTAS technologies. [ABSTRACT FROM AUTHOR]

Millimetric liquid droplets coated with particles, known as “liquid marbles”, exhibit some curious physical properties such as extended lifetime with regards to evaporation, low-friction when in contact with other surfaces and non-coalescence with other liquids. The formation dynamics have been well-characterized as well as thermal properties, however, their mechanical properties have not. Here, we investigate the response of liquid marbles to compressive deformation between two plates. Above a compressive strain of approximately 40–50%, cracks appear in the particle coating and the liquid can wet the surface of the plates, i.e. the marble ruptures. However, more strikingly, we find that even for relatively small compressive strain (without rupturing), the marbles often undergo an irreversible deformation - that is they do not regain their original shape. We quantify this shape ‘hysteresis’ across a range of particle sizes and liquids, showing that it correlates primarily with compressive strain, but also particle type and underlying fluid. Furthermore, we analyse the compressed marble shapes upon approach to rupture in the context of previous analytical approximations, showing that the recent theory of Whyman & Bormashenko (2015) provides a good description of crushed marble shapes. [ABSTRACT FROM AUTHOR]

Light hydrophobic liquids (LHLs) can be submerged in water with granular particles by forming particle encapsulated liquid sacks. Formation and submergence of granular encapsulated LHL sacks can be an effective method for capturing and controlling the fate of floating oils. However, formation characteristics of the LHL sacks and effect of LHL viscosity on their behavior are not well understood. In this study, we examined the encapsulation characteristics of LHL sacks depending on liquid viscosity. Silicone oils with viscosities ranging from 10 cSt to 1000 cSt were used as the LHLs. Sand with two different particle sizes (40–100 mesh and 20–30 mesh) were used as the granular particles. The submerged LHL sacks were stable and remained separate from each other without collapsing or aggregating over time. They could be moved in water by sliding while keeping their encapsulation. [ABSTRACT FROM AUTHOR]

Superhydrophobicity attracted researchers' attention since the discovery of surfaces with special architectures. An important exponent is represented by particle enwrapped droplets, called liquid marbles. These special formations, known as "soft objects" are made of a liquid core wrapped in a solid shell. Liquid marbles covered in anti-inflammatory powders were previously obtained using salicylic acid as a model hydrophobic powder. The aim of this study is to establish an investigation method regarding the wettability of the model powders in correlation with liquid marbles' formation and with their stability in an external phase. Besides salicylic acid, niflumic acid and ketoprofen are used as shells. The next step of the experiments is represented by immersing previously manufactured liquid marbles into an oily phase (paraffin and coconut oil), resulting in Pickering-like emulsions, which are proposed as models in topical emulsions design, aiming a local anti-inflammatory and cosmetic effect. [ABSTRACT FROM AUTHOR]