Revisiting the surface tension of liquid marbles: Measurement of the effective surface tension of liquid marbles with the pendant marble method

Liquid Marbles and Drops on Superhydrophobic Surfaces: Interfacial Aspects and Dynamics of Formation: A Review

Liquid marbles (LMs) are droplets encapsulated with powders presenting varied roughness and wettability. These LMs have garnered a lot of attention due to their dual properties of leakage-free and quick transport on both solid and liquid surfaces. These droplets are in a Cassie-Baxter wetting state sitting on both roughness and air pockets existing between particles. They are also reminiscent of the state of a drop on a superhydrophobic (SH) surface. In this review, LMs and bare droplets on SH surfaces are comparatively investigated in terms of two aspects: interfacial and dynamical. LMs present a fascinating class of soft matter due to their superior interfacial activity and their remarkable stability. Inherently hydrophobic powders form stable LMs by simple rolling; however, particles with defined morphologies and chemistries contribute to the varied stability of LMs. The factors contributing to this interesting robustness with respect to bare droplets are then identified by tests of stability such as evaporation and compression. Next, the dynamics of the impact of a drop on a hydrophobic powder bed to form LMs is studied vis-à̀-vis that of drop impact on flat surfaces. The knowledge from drop impact phenomena on flat surfaces is used to build and complement insights to that of drop impact on powder surfaces. The maximum spread of the drop is empirically understood in terms of dimensionless numbers, and their drawbacks are highlighted. Various stages of drop impact-spreading, retraction and rebound, splashing, and final outcome-are systematically explored on both solid and hard surfaces. The implications of crater formation and energy dissipations are discussed in the case of granular beds. While the drop impact on solid surfaces is extensively reviewed, deep interpretation of the drop impact on granular surfaces needs to be improved. Additionally, the applications of each step in the sequence of drop impact phenomena on both substrates are also identified. Next, the criterion for the formation of peculiar jammed LMs was examined. Finally, the challenges and possible future perspectives are envisaged.

Liquid marbles: review of recent progress in physical properties, formation techniques, and lab-in-a-marble applications in microreactors and biosensors

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.

Universality of Scaling Laws Governing Contact and Spreading Time Spans of Bouncing Liquid Marbles and its Physical Origin

The impact of liquid marbles coated with a diversity of hydrophobic powders with various solid substrates, including hydrophobic, hydrophilic, and superhydrophobic ones, was investigated. The contact time of the bouncing marbles was studied. Universal scaling behavior of the contact time tc as a function of the Weber number (We) was established; the scaling law tc = tc(We) was independent of the kind of powder and the type of solid substrate. The total contact time consists of spreading time and retraction time. It is weakly dependent on We and this is true for all kinds of studied powders and substrates. This observation hints to the surface tension/inertia spring model governing the impact. By contrast, the spreading time ts scales as [Formula: see text], n = 0.28 - 0.30 ± 0.002. We relate the origin of this scaling law to the viscous dissipation occurring within the spreading marbles. The retraction time tr grows weakly with the Weber number. The scaling law was changed at threshold values of We ≅ 15-20. It is reasonable to explain this change with the breaking of the Leidenfrost regime of spreading under high values of We.

Liquid marbles, floating droplets: preparations, properties, operations and applications

Liquid marbles (LMs) are non-wettable droplets formed with a coating of hydrophobic particles. They can move easily across either solid or liquid surfaces since the hydrophobic particles protect the internal liquid from contacting the substrate. In recent years, mainly due to their simple preparation, abundant materials, non-wetting/non-adhesive properties, elasticities and stabilities, LMs have been applied in many fields such as microfluidics, sensors and biological incubators. In this review, the recent advances in the preparation, physical properties and applications of liquid marbles, especially operations and floating abilities, are summarized. Moreover, the challenges to achieve uniformity, slow volatilization and stronger stability are pointed out. Various applications generated by LMs’ structural characteristics are also expected.

The recent advances in the preparation, physical properties and applications of liquid marbles, especially operations and floating abilities, are summarized.

Measuring the effective surface tension of a floating liquid marble using X-ray imaging

A liquid marble (LM) is a droplet coated with microparticles that isolate the liquid interior from its surroundings, making it perfectly non-wetting. This attractive feature allows the LM to perform useful tasks such as coalescence, targeted delivery, and controlled release. The non-wetting characteristic also allows the LM to float on a carrier liquid. The growing number of applications in digital microfluidics requires further insights into the fundamental properties of a LM such as its effective surface tension. Although the coating provides the LM with various desirable characteristics, its random construction presents a major obstacle to accurate optical analysis. This paper presents a novel method to measure the effective surface tension of a floating LM using X-ray imaging and curve fitting procedures. X-ray imaging reveals the true LM liquid-air interface hidden by the coating particles. Analysis of this interface showed that the effective surface tension of a LM is not significantly different from that of its liquid content. This indicates that the particle coating might not have significantly altered the behaviour of the liquid interface. We also found that our method is sensitive enough to detect the variations across individual LMs.

Liquid marble-based digital microfluidics - fundamentals and applications

Liquid marbles are droplets with volume typically on the order of microliters coated with hydrophobic powder. Their versatility, ease of use and low cost make liquid marbles an attractive platform for digital microfluidics. This paper provides the state of the art of discoveries in the physics of liquid marbles and their practical applications. The paper first discusses the fundamental properties of liquid marbles, followed by the summary of different techniques for the synthesis of liquid marbles. Next, manipulation techniques for handling liquid marbles are discussed. Applications of liquid marbles are categorised according to their use as chemical and biological reactors. The paper concludes with perspectives on the future development of liquid marble-based digital microfluidics.

Manufacture and properties of composite liquid marbles

HYPOTHESIS

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.

EXPERIMENTS

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.

FINDINGS

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.

Liquid marbles from soot films

Soot films are the most easily available superhydrophobic surfaces. However, their cohesive forces are very weak such that they have been considered not suitable for direct use. Here we show that the seemingly undesirable mechanical weakness is actually an important property which allows a soot film to work as a superhydrophobic platform and tool, producing liquid marbles with fascinating properties and performances. A soot film is weak enough to lose component carbon nanoparticles (CNPs) on contact with water, but can adhere to a substrate stably on overturning or shaking the substrate. On this basis, we demonstrate that a liquid marble consisting of a liquid core and a CNP shell can be obtained by either rolling or an imprinting process. In addition, it is found that large-volume liquid puddles are easy to produce and manipulate with soot films by arbitrary shaking and pouring operations, without worrying about particles flying off that would occur in conventional powder-based liquid puddle production. The multifunctionality of CNPs endows soot liquid marbles/puddles with great potential in light shielding, electrical conduction, etc. This study reveals a direct application of soot films' superhydrophobicity, provides an alternative route for liquid marble production, and highlights the concept of disadvantage reversion.

Liquid marbles and liquid plasticines with nanoparticle monolayers

Liquid marbles, as particle-covered macroscopic liquid drops in an air environment, have exhibited great value as self-standing liquid containers in various areas, such as material synthesis, chemical analysis, and cell culture. However, conventional liquid marbles obtained by the rolling-on-powder-bed method usually feature micron-sized or larger particle agglomerates, which harm marble transparency and fine control of marble shape and thus results in considerable limitations for marble applications. Recently, monolayer nanoparticle (NP) coverage has been achieved using a sol-gel film instead of a powder as the particle source. The NP monolayer structure can not only result in highly transparent liquid marbles with very smooth and symmetrical profiles, but can also lead to liquid entities with arbitrarily designable shapes, as called liquid plasticines. Monolayer NP-covered (mNPc) liquid marbles and plasticines have generated important results in both fundamental and practical applications, as ideal physical models or advanced self-standing containers, showing great advantages in some conditions over conventional powder-derived liquid marbles. In this review, the preparations and current applications of the two mNPc systems are summarized and perspectives on their advantages, unclear issues, and application extension are provided.

Mapping outcomes of liquid marble collisions

Liquid marbles (LMs) have many promising roles in the ongoing development of microfluidics, microreactors, bioreactors, and unconventional computing. In many of these applications, the coalescence of two LMs is either required or actively discouraged, therefore it is important to study liquid marble collisions and establish parameters which enable the desired collision outcome. Recent reports on LM coalescence have focused on either two mobile LMs colliding, or an accelerating LM hitting a sessile LM with a backstop. A further possible scenario is the impact of a mobile LM against a non-supported static LM. This paper investigates such a collision, using high-speed videography for single-frame analysis. Multiple collisions were undertaken whilst varying the modified Weber number (We*) and offset ratios (X*). Parameter ranges of 1.0 < We* < 1.4 and 0.0 < X* < 0.1, resulted in a coalescence rate of approximately 50%. Whereas, parameter ranges X* > 0.25, and We* < 0.95 or We* > 1.55 resulted in 100% non-coalescence. Additionally, observations of LMs moving above a threshold velocity of 0.6 m s-1 have revealed a new and unusual deformation. Comparisons of the outcome of collisions whilst varying both the LM volume and the powder grain size have also been made, revealing a strong link. The results of this work provide a deeper understanding of LM coalescence, allowing improved control when designing future collision experiments.

Surfactant-Mediated Collapse of Liquid Marbles and Directed Assembly of Particles at the Liquid Surface

Extensive research is being devoted to both the fundamental and applied aspects of liquid marbles (LMs). However, influence of the surface tension of the liquid substrate on the stability of the LMs and LM-mediated capillary interaction remains unexplored. In this work, we unveil the role of the surface tension of the liquid substrate on the collapse of multilayered LMs and apply this knowledge for realizing a dense planar assembly of microparticles triggered by LM-mediated capillary interactions. Experiments and analysis show that the required surface tension for the collapse is dependent on the volume of the LMs. The larger LMs are less stable, and thus collapse at a higher surface tension than that required for smaller LMs. The results are explained on the basis of the balance between surface tension forces acting on the LM ( F_s) and its weight ( F_w). Force analysis reveals that the collapse of the LM on the liquid substrate occurs when the surface tension force approaches to its weight, that is, when F_s ≈ F_w. This has been verified for LMs having volume in the range 6-10 μL. The experiments with different surfactants (an anionic and a cationic) lead to similar results which indicate that the collapse condition of the LMs is mainly dependent on their weight and the surface tension of the liquid substrate. Further, we demonstrate the LM-mediated assembly of particles at the liquid surface, and interestingly, the LM can be collapsed once the assembly is completed, leading to a denser well-packed assembled structure. We believe that the presented results could provide new insights in the fields of microfluidics, particle patterning, and assembly.

A capillary rise method for studying the effective surface tension of monolayer nanoparticle-covered liquid marbles

The liquid marble covered with a xerogel-derived nanoparticle monolayer is emerging as a simple and ideal physical model for fundamental study. In this work, the effective surface tension, γeff, of this kind of water marble was studied by employing an optimized capillary rise method. This method is distinctive as it can reveal γeff at a specific point rather than the average γeff over the entire marble. γeff at the maximum lateral diameter position was investigated with particle coverage varying from 100% to 10%, showing a quite slow increasing trend from ∼64 to 72 mN m-1. The delay in the evolution of the γeff was attributed to the slow decline in the areal particle density on the side area of the marble. For comparative purposes, the maximum height method was also employed to determine γeff. The results of the two methods supported the position that a liquid marble's γeff depended on very specific conditions including the measurement method and all marble parameters.

Viscous effects on the interaction of granular particles with floating oils in water

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 10cSt to 1000cSt were used as the LHLs. Sand with two different particle sizes (40-100mesh and 20-30mesh) 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.

Liquid Marbles, Elastic Nonstick Droplets: From Minireactors to Self-Propulsion

Liquid marbles are nonstick droplets wrapped by micro- or nanometrically scaled colloidal particles, representing a platform for a variety of chemical, biological, and microfluidics applications. Liquid marbles demonstrate elastic properties and do not coalesce when bounced or pressed. The effective surface tension and Young modulus of liquid marbles are discussed. Physical sources of the elasticity of liquid marbles are considered. Liquids and powders used for the fabrication of liquid marbles are surveyed. This feature article reviews properties and applications of liquid marbles. Liquid marbles demonstrate potential as microreactors, microcontainers for growing micro-organisms and cells, and microfluidics devices. The Marangoni-flow-driven self-propulsion of marbles supported by liquids is addressed.

Evaporation of Ethanol-Water Binary Mixture Sessile Liquid Marbles

Liquid marble is a liquid droplet coated with particles. Recently, the evaporation process of a sessile liquid marble using geometric measurements has attracted great attention from the research community. However, the lack of gravimetric measurement limits further insights into the physical changes of a liquid marble during the evaporation process. Moreover, the evaporation process of a marble containing a liquid binary mixture has not been reported before. The present paper investigates the effective density and the effective surface tension of an evaporating liquid marble that contains aqueous ethanol at relatively low concentrations. The effective density of an evaporating liquid marble is determined from the concurrent measurement of instantaneous mass and volume. Density measurements combined with surface profile fitting provide the effective surface tension of the marble. We found that the density and surface tension of an evaporating marble are significantly affected by the particle coating.

Insertion and confinement of air bubbles inside a liquid marble

Nanoparticles at the air/liquid interface can serve as solid separating barriers to form stable foams or liquid marbles depending on the wettability of the nanoparticles. This paper presents an effect that enables the insertion and confinement of air bubbles inside a liquid marble, based on encapsulating an air bubble surrounded by surfactant molecules or hydrophilic particles. We have demonstrated that more than one bubble can be inserted and trapped inside one liquid marble so that liquid marbles can be divided into several separate compartments. The findings presented here may stimulate fundamental studies of this novel bubble-marble phenomenon, as well as developments of various practical applications.

Mechanical Compression to Characterize the Robustness of Liquid Marbles

In this work, we have devised a new approach to measure the critical pressure that a liquid marble can withstand. A liquid marble is gradually squeezed under a mechanical compression applied by two parallel plates. It ruptures at a sufficiently large applied pressure. Combining the force measurement and the high-speed imaging, we can determine the critical pressure that ruptures the liquid marble. This critical pressure, which reflects the mechanical robustness of liquid marbles, depends on the type and size of the stabilizing particles as well as the chemical nature of the liquid droplet. By investigating the surface of the liquid marble, we attribute its rupture under the critical pressure to the low surface coverage of particles when highly stretched. Moreover, the applied pressure can be reflected by the inner Laplace pressure of the liquid marble considering the squeezing test is a quasi-static process. By analyzing the Laplace pressure upon rupture of the liquid marble, we predict the dependence of the critical pressure on the size of the liquid marble, which agrees well with experimental results.

Deformation of a floating liquid marble

A rigid spherical particle floating on a liquid is a known problem with well-defined solutions. Under the combined effect of gravity and surface tension, the rigid particle deforms the liquid surface. However, in the case of a floating soft particle such as a liquid marble, not only the liquid surface but also the particle itself deforms. In this paper, we investigate the deformation of a floating liquid marble and characterise its height as well as aspect ratio. The experimental results show that theoretical models for a rigid spherical particle suit well for small liquid marbles. Larger marbles require an oblate liquid spheroid model. We will discuss the limitations of the two models and characterise the deformation of these marbles.

Electrohydrodynamic Deformation and Rotation of a Particle-Coated Drop

A dielectric drop suspended in conducting liquid and subjected to a uniform electric field deforms into an ellipsoid whose major axis is either perpendicular or tilted (because of the Quincke rotation effect) relative to the applied field. We experimentally study the effect of surface-adsorbed colloidal particles on these classic electrohydrodynamic phenomena. We observe that at a high surface coverage (90%), the electrohydrodynamic flow is suppressed, oblate drop deformation is enhanced, and the threshold for tilt is decreased compared to that of the particle-free drop. The deformation data are explained well by a capsule model, which assumes that the particle monolayer acts as an elastic interface. The reduction of the threshold field for rotation is likely related to drop asphericity.

Liquid marbles: topical context within soft matter and recent progress

The study of particle stabilized interfaces has a long history in terms of emulsions, foams and related dry powders. The same underlying interfacial energy principles also allow hydrophobic particles to encapsulate individual droplets into a stable form as individual macroscopic objects, which have recently been called "Liquid Marbles". Here we discuss conceptual similarities to superhydrophobic surfaces, capillary origami, slippery liquids-infused porous surfaces (SLIPS) and Leidenfrost droplets. We provide a review of recent progress on liquid marbles, since our earlier Emerging Area article (Soft Matter, 2011, 7, 5473-5481), and speculate on possible future directions from new liquid-infused liquid marbles to microarray applications. We highlight a range of properties of liquid marbles and describe applications including detecting changes in physical properties (e.g. pH, UV, NIR, temperature), use for gas sensing, synthesis of compounds/composites, blood typing and cell culture.

Liquid marbles for high-throughput biological screening of anchorage-dependent cells

Stable liquid marbles (LM) are produced by coating liquid droplets with a hydrophobic powder. The used hydrophobic powder is produced by fluorosi-lanization of diatomaceous earth, used before to produce superhydrophobic structures. Here, the use of LM is proposed for high-throughput drug screening on anchorage-dependent cells. To provide the required cell adhesion sites inside the liquid environment of LM, surface-modified poly(l-lactic acid) microparticles are used. A simple method that takes advantage from LM appealing features is presented, such as the ability to inject liquid on LM without disrupting (self-healing ability), and to monitor color changes inside of LM. After promoting cell adhesion, a cytotoxic screening test is performed as a proof of concept. Fe(3+) is used as a model cytotoxic agent and is injected on LM. After incubation, AlamarBlue reagent is injected and used to assess the presence of viable cells, by monitoring color change from blue to red. Color intensity is measured by image processing and the analysis of pictures takes using an ordinary digital camera. The proposed method is fully validated in counterpoint to an MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carbo​xymethoxyphenyl)-2-(4-sulfophenyl)-2H-te​trazolium) colorimetric assay, a well-known method used for the cytotoxicity assessment.

A floating self-propelling liquid marble containing aqueous ethanol solutions

We report the behaviour of a self-propelling liquid marble containing an aqueous ethanol solution.

Foams stabilised by mixtures of nanoparticles and oppositely charged surfactants: relationship between bubble shrinkage and foam coarsening

We have studied foams stabilised by surfactant-decorated nanoparticles adsorbed at the bubble surfaces. We show that the controlled compression of a single bubble allows one to understand the coarsening behavior of these foams. When bubbles are compressed, the particles become tightly packed in the surface layer. They lose their mobility, and the interface becomes solid-like when the jammed state is reached. Further compression leads to interfacial buckling characterised by crumpled surfaces. We find that the surface concentration of particles at which the jamming and the buckling transitions occur are independent of the surfactant concentration. This is a surprising feature. It suggests that the surfactants are mandatory to help the particles adsorb at the interface and that they change the equilibrium surface concentration of the decorated particles. But they do not affect the surface properties once the particles are adsorbed. We measured the compression elastic modulus of the surface in the jammed state and found it to be compatible with the Gibbs condition for which the spontaneous dissolution of bubbles is arrested. Due to this effect, the coarsening process of a foam composed of many close-packed bubbles occurs in two steps. In the first step, coarsening is slow and coalescence of the bigger bubbles is observed. In the second step, a number of very small bubbles remains, which exhibit crumpled surfaces and are stable over long times. This suggests that foam coarsening is arrested once the smallest bubbles become fully covered after the initial shrinking step.

Armoring a droplet: soft jamming of a dense granular interface

Droplets and bubbles protected by armors of particles have found vast applications in encapsulation, stabilization of emulsions and foams, or flotation processes. The liquid phase stores capillary energy, while concurrently the solid contacts of the granular network induce friction and energy dissipation, leading to hybrid interfaces of combined properties. By means of nonintrusive tensiometric methods and structural measurements, we distinguish three surface phases of increasing rigidity during the evaporation of armored droplets. The emergence of surface rigidity is reminiscent of jamming of granular matter, but it occurs differently since it is marked by a step by step hardening under surface compression. These results show that the concept of the effective surface tension remains useful only below the first jamming transition. Beyond this point, the surface stresses become anisotropic.

Liquid marbles supported by monodisperse poly(methylsilsesquioxane) particles

The preparation of model, well-controlled colloidal assemblies has been a central approach to understanding and optimizing the characteristics and functionality of complex colloidal dispersion systems. This approach, which has created a significant literature and rather deep understanding for emulsions and foams, has yet to be established for the liquid marble (water-in-air) motif. In this article we report the preparation of well-controlled liquid marbles using monodisperse micron-size particles of poly(methylsilsesquioxane) (PMSQ). The low cohesive nature of the stabilizing particles, their narrow size distribution, and their hydrophobicity permit the formation of liquid marbles containing a particulate monolayer with a hexagonally close-packed (HCP) structure. The "cleaning process" by rolling of liquid marbles under a flow of air on a hydrophobic substrate was useful to obtain the monolayer structure. Moreover, the monolayer structure was only obtained from liquids with high surface energy, whereas the others were not useful even though multilayered structure was formed from liquids that have intermediate surface energy.