Effect of particle morphology on mechanical properties of liquid marbles

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.

Dry Water as a Promoter for Gas Hydrate Formation: A Review

Applications of clathrate hydrate require fast formation kinetics of it, which is the long-standing technological bottleneck due to mass transfer and heat transfer limitations. Although several methods, such as surfactants and mechanical stirring, have been employed to accelerate gas hydrate formation, the problems they bring are not negligible. Recently, a new water-in-air dispersion stabilized by hydrophobic nanosilica, dry water, has been used as an effective promoter for hydrate formation. In this review, we summarize the preparation procedure of dry water and factors affecting the physical properties of dry water dispersion. The effect of dry water dispersion on gas hydrate formation is discussed from the thermodynamic and kinetic points of view. Dry water dispersion shifts the gas hydrate phase boundary to milder conditions. Dry water increases the gas hydrate formation rate and improves gas storage capacity by enhancing water-guest gas contact. The performance comparison and synergy of dry water with other common hydrate promoters are also summarized. The self-preservation effect of dry water hydrate was investigated. Despite the prominent effect of dry water in promoting gas hydrate formation, its reusability problem still remains to be solved. We present and compare several methods to improve its reusability. Finally, we propose knowledge gaps in dry water hydrate research and future research directions.

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.

pH-Responsive Liquid Marbles Based on Dihydroxystearic Acid

Herein, we report pH-responsive liquid marbles stabilized by 9,10-dihydroxystearic acid (DHSA). The particle morphology and the pH-responsive behavior of the liquid marbles were investigated. The rolling time during the preparation of liquid marbles has a great influence on the thickness of powder adsorption and the stability of the marbles. Compared with the liquid marbles stabilized by other fatty acids (e.g., stearic acid and docosoic acid), the liquid marbles prepared by DHSA have a much higher mechanical robustness. The increase in the number of hydroxyl groups on the carbon chain of fatty acids improves the mechanical robustness of the liquid marbles. Such liquid marbles immediately disintegrated on the surface of an alkaline solution or after exposure to NH₃ gas, which extends their applications in the NH₃ sensor and chemical reactions.

How Liquid Marbles Break Down: Direct Evidence for Two Breakage Scenarios

Liquid marbles are nonsticking droplets wrapped with hydrophobic nano- to micrometer particles and are expected to be useful for various applications, especially in industrial and biomedical fields. However, the practical use of liquid marbles is limited by their fragility. In this study, the dynamics of particle monolayer-stabilized liquid marble breakage upon impacting a solid surface are monitored in situ by high-speed interfacial microscopy. The experiments show that the breakage of liquid marbles can be induced by either i) cracking or ii) water penetration depending on the impact energy. The applicable scenario is determined by whether a jamming transition of the wrapping particles occurs during impact. The breakage mechanisms provide insights on how to improve the robustness of liquid marbles in accordance with these scenarios.

Electric-field-induced deformation, yielding, and crumpling of jammed particle shells formed on non-spherical Pickering droplets

Droplets covered with densely packed solid particles, often called Pickering droplets, are used in a variety of fundamental studies and practical applications. For many applications, it is essential to understand the mechanics of such particle-laden droplets subjected to external stresses. Several research groups have studied theoretically and experimentally the deformation, relaxation, rotation, and stability of Pickering droplets. Most of the research concerns spherical Pickering droplets. However, little is known about non-spherical Pickering droplets with arrested particle shells subjected to compressive stress. The experimental results presented here contribute to filling this gap in research. We deform arrested non-spherical Pickering droplets by subjecting them to electric fields, and study the effect of droplet geometry and size, as well as particle size and electric field strength, on the deformation and yielding of arrested non-spherical Pickering droplets. We explain why a more aspherical droplet and/or a droplet covered with a shell made of larger particles required higher electric stress to deform and yield. We also show that an armored droplet can absorb the electric stress differently (i.e., through either in-plane or out-of-plane particle rearrangements) depending on the strength of the applied electric field. Furthermore, we demonstrate that particle shells may fail through various crumpling instabilities, including ridge formation, folding, and wrinkling, as well as inward indentation.

Particle Size Determines the Shape of Supraparticles in Self-Lubricating Ternary Droplets

Supraparticles are large clusters of much smaller colloidal particles. Controlling the shape and anisotropy of supraparticles can enhance their functionality, enabling applications in fields such as optics, magnetics, and medicine. The evaporation of self-lubricating colloidal ouzo droplets is an easy and efficient strategy to create supraparticles, overcoming the problem of the "coffee-stain effect" during drop evaporation. Yet, the parameters that control the shape of the supraparticles formed in such evaporating droplets are not fully understood. Here, we show that the size of the colloidal particles determines the shape of the supraparticle. We compared the shape of the supraparticles made of seven different sizes of spherical silica particles, namely from 20 to 1000 nm, and of the mixtures of small and large colloidal particles at different mixing ratios. Specifically, our in situ measurements revealed that the supraparticle formation proceeds via the formation of a flexible shell of colloidal particles at the rapidly moving interfaces of the evaporating droplet. The time t_c0 when the shell ceases to shrink and loses its flexibility is closely related to the size of particles. A lower t_c0, as observed for smaller colloidal particles, leads to a flat pancake-like supraparticle, in contrast to a more curved American football-like supraparticle from larger colloidal particles. Furthermore, using a mixture of large and small colloidal particles, we obtained supraparticles that display a spatial variation in particle distribution, with small colloids forming the outer surface of the supraparticle. Our findings provide a guideline for controlling the supraparticle shape and the spatial distribution of the colloidal particles in supraparticles by simply self-lubricating ternary drops filled with colloidal particles.

Elemental Sulfur-Stabilized Liquid Marbles: Properties and Applications

Sulfur-stabilized liquid marbles were readily prepared by rolling water droplets on a sulfur (S₈) powder bed. Because of the construction of a gel layer on the surface of liquid marbles, the resulting liquid marbles have shape-designable characteristics. The effects of rolling time and volume of droplets on the deformability of sulfur-stabilized liquid marbles were investigated along with their mechanical stability and lifetime. The capability of sulfur-stabilized liquid marbles to be deformed at different pH values enables these liquid marbles to act as microreservoirs with desired shapes for aqueous solutions. Immersing the sulfur-stabilized liquid marbles into organic liquids leads to an increase in the liquid marbles' lifetime, and thereby they can survive at the interface of aqueous-organic two-phased systems for a long time. Finally, the applications of sulfur-stabilized liquid marbles as photocatalytic microreactors, electrochemical microcells, and monodisperse Pickering-like emulsions were demonstrated.

Mechanical robustness of monolayer nanoparticle-covered liquid marbles

Powder-derived liquid marbles (LMs) are versatile nonwetting systems but are confronted with many limitations in application, as their surface particles are usually large and agglomerated. Recently, sol-gel film-derived LMs have come on the scene that are termed monolayer nanoparticle-covered (mNPc) LMs based on their unique characteristics, revealing great application potential but also generating many questions. Here, mechanical robustness, a very important yet to be addressed property, of mNPc LMs was systematically studied. Rolling, pendant contact, and compression experiments were designed using bare and coated glasses with water contact angles (WCAs) ranging from 23° to 157°. With rupture as a quality criteria, the mechanical robustness of mNPc LMs enhanced with the hydrophobicity of solid surfaces that exerted pressure on them, but maintained much weaker than typical powder LMs until the solid surface was superhydrophobic. In particular, when contacting hydrophilic surfaces of WCAs ≤53°, an mNPc LM did not have the capacity for nonwetting and ruptured immediately, even if the pressure approached zero. This was distinct from powder LMs and indicated that a particle shell as thin as ∼20 nm could not prevent intermolecular attractions between the internal liquid and external solid surface. An interface scenario consisting of solid surface microroughness was proposed to address this issue. On the other hand, mNPc LMs remained unruptured on superhydrophobic surfaces but presented degraded elasticity under extreme compression. Uncovering these properties could be of much help for developments of mNPc LMs and their counterparts, the mNPc liquid plasticines.

Particle Monolayer-Stabilized Light-Sensitive Liquid Marbles from Polypyrrole-Coated Microparticles

Liquid marbles are water droplets coated with solid particles that prevent coalescence and allow storage, transport, and handling of liquids in the form of a powder. Here, we report on the formation of liquid marbles that are stabilized entirely by a single monolayer of solid particles and thus minimize the amount of required solid material. As stabilizing particles, we synthesize relatively monodisperse, 80 μm-sized polystyrene (PS) particles coated with heptadecafluorooctanesulfonic acid-doped polypyrrole (PPy-C₈F) shell (PS/PPy-C₈F particles) by aqueous chemical oxidative seeded polymerization of pyrrole using FeCl₃ as an oxidant and heptadecafluorooctanesulfonic acid as a hydrophobic dopant. We characterize the physicochemical properties of the particles as a function of the PPy-C₈F loading. Laser diffraction particle size analyses of dilute aqueous suspensions indicate that the polymer particles disperse stably in water medium before and after coating with the PPy-C₈F shell. X-ray photoelectron spectroscopy studies indicate the formation of a PPy-C₈F shell around the PS seed particles, which was further supported by deflated morphologies observed by scanning electron microscopy after extraction of the PS component from the PS/PPy-C₈F particles. We investigate the performance of the dried PS/PPy-C₈F particles to stabilize liquid marbles. Stereo- and laser microscope observations, as well as gravimetric studies, confirm that the PS/PPy-C₈F particles adsorb to the water droplet surface in the form of a particle monolayer with the characteristic hexagonal close-packed structure expected for spherical (colloidal) particles. Mechanical integrity of the liquid marble increases with an increase of PPy-C₈F loading of the PS/PPy-C₈F particles. The water contact angle of the PS/PPy-C₈F particles at air-water interface increases from 82 ± 12° to 102 ± 17° with an increase of PPy-C₈F loading. This increase in water contact angle directly correlates with the shape of the LM, with higher contact angles giving more spherical LMs. Furthermore, the broadband light absorption properties of the PPy coating was used to control evaporation rate of the enclosed water phase on demand by irradiation with a near-infrared laser. The evaporation rate could be finely controlled by the thickness of the PPy-C₈F shell of the particles stabilizing the liquid marbles.

Tuning the Porosity of Supraparticles

Supraparticles consisting of nano- or microparticles have potential applications as, for example, photonic crystals, drug carriers, or heterogeneous catalysts. To avoid the use of solvent or processing liquid, one can make supraparticles by evaporating droplets of aqueous suspensions from super-liquid-repellent surfaces. Herein, a method to adjust the porosity of supraparticles is described; a high porosity is desired, for example, in catalysis. To prepare highly porous TiO₂ supraparticles, polymer nanoparticles are co-dispersed in the suspension. Supraparticles are formed through evaporation of aqueous suspension droplets on superamphiphobic surfaces followed by calcination of the sacrificial polymer particles. The increase of porosity of up to 92% resulted in enhanced photocatalytic activity while maintaining sufficient mechanical stability.

Shape-Designable Liquid Marbles Stabilized by Gel Layer

Shape-designable liquid marbles were simply prepared by rolling the water droplets over the stearic acid powder for several seconds to encapsulate them. The effects of droplet volume, pH, and rolling time on the deformability of liquid marbles from sphere were investigated. The stearic acid-stabilized liquid marbles can be deformed to any desirable and stable shapes including ellipsoids and letters, thanks to the gel layer formed at liquid marble surfaces during the preparation. The gel layer works as a flexible and plastic membrane, which makes the liquid marbles irreversibly deformable. Finally, the applications of the liquid marbles as a microreactor were demonstrated.

Liquid Marbles in Nature: Craft of Aphids for Survival

Some aphids that live in the leaf galls of the host plant are known to fabricate liquid marbles consisting of honeydew and wax particles as an inner liquid and a stabilizer, respectively. In this study, the liquid marbles fabricated by the galling aphids, Eriosoma moriokense, were extensively characterized with respect to size and size distribution, shape, nanomorphology, liquid/solid weight ratio, and chemical compositions. The stereo microscopy studies confirmed that the liquid marbles have a near-spherical morphology and that the number-average diameter was 368 ± 152 μm, which is 1 order of magnitude smaller than the capillary length of the honeydew. The field emission scanning electron microscopy studies indicated that micrometer-sized wax particles with fiber- and dumpling-like shapes coated the honeydew droplets, which rendered the liquid marbles hydrophobic and nonwetting. Furthermore, the highly magnified scanning electron microscopy images confirmed that the wax particles were formed with assemblage of submicrometer-sized daughter fibers. The contact angle measurements indicated that the wax was intrinsically hydrophobic and that the liquid marbles were stabilized by the wax particles in the Cassie-Baxter model. The weight ratio of the honeydew and the wax particles was determined to be 96/4, and the honeydew consisted of 19 wt % nonvolatile components and 81 wt % water. The ¹H nuclear magnetic resonance, Fourier transform infrared spectroscopy, and mass spectroscopy studies confirmed that the wax mainly consisted of triglycerides and that the honeydew mainly consisted of saccharides (glucose and fructose) and ribitol. The atomic force microscopy studies confirmed that honeydew is sticky in nature.