Freezing of a liquid marble

Biofabrication with microbial cellulose: from bioadaptive designs to living materials

Nanocellulose is not only a renewable material but also brings functions that are opening new technological opportunities. Here we discuss a special subset of this material, in its fibrillated form, which is produced by aerobic microorganisms, namely, bacterial nanocellulose (BNC). BNC offers distinct advantages over plant-derived counterparts, including high purity and high degree of polymerization as well as crystallinity, strength, and water-holding capacity, among others. More remarkably, beyond classical fermentative protocols, it is possible to grow BNC on non-planar interfaces, opening new possibilities in the assembly of advanced bottom-up structures. In this review, we discuss the recent advances in the area of BNC-based biofabrication of three-dimensional (3D) designs by following solid- and soft-material templating. These methods are shown as suitable platforms to achieve bioadaptive constructs comprising highly interlocked biofilms that can be tailored with precise control over nanoscale morphological features. BNC-based biofabrication opens applications that are not possible by using traditional manufacturing routes, including direct ink writing of hydrogels. This review emphasizes the critical contributions of microbiology, colloid and surface science, as well as additive manufacturing in achieving bioadaptive designs from living matter. The future impact of BNC biofabrication is expected to take advantage of material and energy integration, residue utilization, circularity and social latitudes. Leveraging existing infrastructure, the scaleup of biofabrication routes will contribute to a new generation of advanced materials rooted in exciting synergies that combine biology, chemistry, engineering and material sciences.

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.

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.

Preparation of Biocompatible Liquid Marbles Stabilized by Food-Grade Stearate Microparticle for Aerobic Bacteria Cultivation

Liquid marble (LM), a non-stick drop coated with micro- or nano-scale particles, has great potential in a wide range of applications. LMs have an advantageous feature in which gas or vapor can freely transport through their particle shell; therefore, it makes them an ideal candidate to be utilized as microbioreactor containing aerobic microorganisms. In this study, safer and more biocompatible LMs were successfully prepared using a food-grade calcium stearate microparticle as a stabilizer. As the volume of core liquid increased, the height of LM increased and reached a constant value, as a similar trend has been reported in conventional LMs. The drying rate curve of the LMs confirmed that the LMs have a similar pattern with the drying of typical wet powders. The drying rate depended on the salt species in the core solution and the environmental humidity. For instance, in the case of MgCl₂, by changing humidity from 40 to 80% RH, the lifetime of LMs (time in which the LM dried completely) was increased to about 900 min. This is nearly three times longer than those have no salt and at 40% RH. Model aerobic bacteria Bacillus subtilis has actively proliferated inside the LM during 24-h incubation. Comparing with the test tube cultivations under O₂-rich stationary or O₂ rich-shaken conditions, the cultivation in the LM system showed a higher proliferation than the test tube systems. As a conclusion, we demonstrated that the calcium stearate LM system would be an ideal candidate for safer and easily available microbioreactor containing aerobic bacteria.

Cryoprotectant-Free Freezing of Cells Using Liquid Marbles Filled with Hydrogel

Cryopreservation without cryoprotectant remains a significant challenge for the re-establishment of cell culture after freeze-thaw. Thus, finding an alternative and a simple cryopreservation method is necessary. Liquid marble (LM)-based digital microfluidics is a promising approach for cryoprotectant-free cryopreservation. However, the use of this platform to efficiently preserve samples with low cell density and well-controlled serum concentrations has not been investigated. We addressed this issue by embedding an agarose-containing fetal bovine serum (FBS) inside the LM. A low density of 500 cells/μL of murine 3T3 cells was selected for evaluating the postcryogenic survivability. The effects on the post-thaw cell viability of the concentration of agarose, the amount of FBS inside the agarose, and the volume of the LM were investigated systematically. This paper also presents an analysis on the changes in shape and crack size of post-thawed agarose. The results revealed that the embedded agarose gel serves as a controlled release mechanism of FBS and significantly improves cell viability. Post-thaw recovery sustains major cellular features, such as viability, cell adhesion, and morphology. The platform technology reported here opens up new possibilities to cryopreserve rare biological samples without the toxicity risk of cryoprotectants.

Liquid Marbles: From Industrial to Medical Applications

Liquid marbles are versatile structures demonstrating a pseudo-Leidenfrost wetting regime formed by encapsulating microscale volumes of liquid in a particle shell. The liquid core is completely separated from the exterior through air pockets. The external phase consists of hydrophobic particles, in most cases, or hydrophilic ones distributed as aggregates. Their interesting features arise from the double solid-fluid character. Thus, these interesting formations, also known as “dry waters”, have gained attention in surface science. This review paper summarizes a series of proposed formulations, fabrication techniques and properties, in correlation with already discovered and emerging applications. A short general review of the surface properties of powders (contact angle, superficial tension) is proposed, followed by a presentation of liquid marbles’ properties (superficial characteristics, elasticity, self-propulsion etc.). Finally, applications of liquid marbles are discussed, mainly as helpful and yet to be exploited structures in the pharmaceutical and medical field. Innovative pharmaceutical forms (Pickering emulsions) are also means of use taken into account as applications which need further investigation.

Rationally Turning the Interface Activity of Mesoporous Silicas for Preparing Pickering Foam and "Dry Water"

We develop a novel protocol to prepare smart, gas/water interface-active, mesoporous silica particles. This protocol involves modification of highly mesoporous silicas with a mixture of hydrophobic octyl organosilane and hydrophilic triamine organosilane. Their structure and compositions are characterized by transmission electron microscopy (TEM), N₂ sorption, solid state NMR, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TGA), and elemental analysis. It is demonstrated that our protocol enables the interface activity of mesoporous silica particles to be facilely tuned, so that the stable gas-water interfaces ranging from air bubbles dispersed in water (Pickering foam) and water droplets dispersed in air ("dry water") can be achieved, depending on the molar ratio of these two organosilanes. The "dry water" is not otherwise attainable for the analogous nonporous silica particles, indicting the uniqueness of the chosen mesoporous structures. Moreover, these particle-stabilized Pickering foams and "dry waters" can be disassembled in response to pH. Interestingly, it was found that aqueous potassium carbonate droplets stabilized by these interface-active mesoporous silica particles ("dry K₂CO₃-containing water") could automatically capture CO₂ from a simulated flue gas with enhanced adsorption rate and adsorption capacity when compared to the aqueous potassium carbonate bulk solution. This study not only supplies a novel type of efficient, smart, gas/water interface-active mesoporous silica particles but also demonstrates an innovative application of mesoporous materials in gas adsorption.

Colloidal Particles at a Range of Fluid-Fluid Interfaces

The study of solid particles residing at fluid-fluid interfaces has become an established area in surface and colloid science recently, experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes such as antifoam formulations, crude oil emulsions, aerated foodstuffs, and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also, and the area is now multidisciplinary, involving research in the fundamental science and potential applications. In this Feature Article, the flavor of some of this interest is given on the basis of recent work from our own group and includes the behavior of particles at oil-water, air-water, oil-oil, air-oil, and water-water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilized emulsions, particle-stabilized aqueous and oil foams, dry liquids, liquid marbles, and powdered emulsions.

Continuous Production of Janus and Composite Liquid Marbles with Tunable Coverage

We report a simple method for on-demand continuous processing of composite liquid marbles with the aid of a 3D printed slide platform, which offers the potential for engineering novel functional surfaces for the production of combination drug therapies, particle-based barcode biomarkers and smart membranes, among other applications. Unlike other attempts at producing such liquid marbles, this novel technique not only facilitates controllable and reproducible production of the liquid marbles but also allows the selection of different morphologies such as banded, patchy, and Janus structures by controlling the coalescence conditions, with the possibility for tunable symmetric and asymmetric patterns, the latter by varying the particle species partitioning ratio.

Oil-in-oil emulsions stabilised solely by solid particles

A brief review of the stabilisation of emulsions of two immiscible oils is given. We then describe the use of fumed silica particles coated with either hydrocarbon or fluorocarbon groups in acting as sole stabilisers of emulsions of various vegetable oils with linear silicone oils (PDMS) of different viscosity. Transitional phase inversion of emulsions, containing equal volumes of the two oils, from silicone-in-vegetable (S/V) to vegetable-in-silicone (V/S) occurs upon increasing the hydrophobicity of the particles. Close to inversion, emulsions are stable to coalescence and gravity-induced separation for at least one year. Increasing the viscosity of the silicone oil enables stable S/V emulsions to be prepared even with relatively hydrophilic particles. Predictions of emulsion type from calculated contact angles of a silica particle at the oil-oil interface are in agreement with experiment provided a small polar contribution to the surface energy of the oils is included. We also show that stable multiple emulsions of V/S/V can be prepared in a two-step procedure using two particle types of different hydrophobicity. At fixed particle concentration, catastrophic phase inversion of emulsions from V/S to S/V can be effected by increasing the volume fraction of vegetable oil. Finally, in the case of sunflower oil + 20 cS PDMS, the study is extended to particles other than silica which differ in chemical type, particle size and particle shape. Consistent with the above findings, we find that only sufficiently hydrophobic particles (clay, zinc oxide, silicone, calcium carbonate) can act as efficient V/S emulsion stabilisers.

Switchable Opening and Closing of a Liquid Marble via Ultrasonic Levitation

Liquid marbles have promising applications in the field of microreactors, where the opening and closing of their surfaces plays a central role. We have levitated liquid water marbles using an acoustic levitator and, thereby, achieved the manipulation of the particle shell in a controlled manner. Upon increasing the sound intensity, the stable levitated liquid marble changes from a quasi-sphere to a flattened ellipsoid. Interestingly, a cavity on the particle shell can be produced on the polar areas, which can be completely healed when decreasing the sound intensity, allowing it to serve as a microreactor. The integral of the acoustic radiation pressure on the part of the particle surface protruding into air is responsible for particle migration from the center of the liquid marble to the edge. Our results demonstrate that the opening and closing of the liquid marble particle shell can be conveniently achieved via acoustic levitation, opening up a new possibility to manipulate liquid marbles coated with non-ferromagnetic particles.

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.

Tunable shape transformation of freezing liquid water marbles

Liquid water marbles coated with fumed silica nanoparticles exhibit various shape transformations upon freezing which are dependent on the hydrophobicity of the nanoparticles. The shape can be recovered during re-melting. For marbles coated with the most hydrophobic particles, a vertically prolonged morphology with a pointed protrusion on the top is formed on freezing. For marbles coated with less hydrophobic particles, a lateral expanded flying saucer-shaped morphology is formed. The different responses to freezing result from the different heterogeneous nucleation sites owing to the different positions of the particles at the air-water interface. If the particles are more immersed in water, ice embryos tend to form in the concave cavities between the particles. The volume expansion of water caused by freezing and continuous nucleation lead to continuous lateral stretching of the particle network coating the droplet surface and ultimately to the horizontally inflated shape of the marble. If the particles are more exposed to air, nucleation occurs on the convex surface of the particles, similar to that of a bare water droplet on a hydrophobic substrate.

Activating the microscale edge effect in a hierarchical surface for frosting suppression and defrosting promotion

Despite extensive progress, current icephobic materials are limited by the breakdown of their icephobicity in the condensation frosting environment. In particular, the frost formation over the entire surface is inevitable as a result of undesired inter-droplet freezing wave propagation initiated by the sample edges. Moreover, the frost formation directly results in an increased frost adhesion, posing severe challenges for the subsequent defrosting process. Here, we report a hierarchical surface which allows for interdroplet freezing wave propagation suppression and efficient frost removal. The enhanced performances are mainly owing to the activation of the microscale edge effect in the hierarchical surface, which increases the energy barrier for ice bridging as well as engendering the liquid lubrication during the defrosting process. We believe the concept of harnessing the surface morphology to achieve superior performances in two opposite phase transition processes might shed new light on the development of novel materials for various applications.