Larger Stabilizing Particles Make Stronger Liquid Marble

Rod-Shaped Liquid Plasticine as Cuttable Minireactor for Photodynamic Therapy of Tumors

Photodynamic therapy (PDT) has emerged as a promising non-invasive approach for cancer treatment. Enhancing its efficacy and understanding its absorption-induced attenuation are significant while the solutions are limited, particularly for the latter. In this study, a rod-shaped liquid plasticine (LP), comprised of a tumor cell solution encased by a nanoparticle monolayer, is used to serve as a powerful minireactor for addressing these issues. The channel structure, openness, and cuttability of the LP reactor are exploited for providing benefits to PDT. The resulting PDT efficacy is several times higher than those from droplet reactors with common spherical shapes. The attenuation law, which is fundamental in PDT yet poorly understood due to the lack of experimental approaches, is preliminarily uncovered here from the perspective of in vitro experiments by using the LP's cuttability, affording quantitative understanding on this difficult subject. These findings provide insights into the widely-concerned topics in PDT, and highlight the great potential of an LP reactor in offering innovation power for the biochemical and biomedical arenas.

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.

Preparation of Oriented Superhydrophobic Surface to Reduce Agglomeration in Preparing Melt Marbles

Numerous innovative granulation techniques utilizing the concept of liquid marbles have been proposed before. However, these processes frequently encounter issues such as collisions, aggregation, and fragmentation of liquid/melt marble during the granulation process. In this study, the oriented superhydrophobic surface (OSS) was successfully prepared by utilizing copper wire to solve the above problem, facilitating efficient batch production and guided transportation of uniform marbles. The parameters and mechanisms of this process were thoroughly studied. The optimized structure is that the copper wire spacing (d) and height (h) are set as 1.0 and 0.1 mm, respectively. This resulted in a surface contact angle (CA) of 156° and anisotropic sliding (ΔSA) of 16.3 ± 1.34°. Using the prepared substrate, high-quality urea products were successfully obtained through the controlled transport of urea melt marbles. The mechanism of guided and directional drag reduction, based on the solid/solid contact on the surface, is proposed. These findings in this study have significant implications for improving granulation processes.

Droplet-Impact Driven Formation of Ultralow Volume Liquid Marbles with Enhanced Mechanical Stability and Sensing Ability

Liquid marbles (LMs), droplets encapsulated with micro/nanoparticles, have attracted significant attention owing to their potential applications in various fields, ranging from microbioreactors to sensors. The volume of the LMs is a key parameter determining their mechanical stability and gas sensing ability. It is ideal to work with small volumes because of their better mechanical stability and gas sensing power compared to the larger LMs. Though many methods exist for producing LMs in the volume range above 2 μL, no reliable method exists to prepare fully coated submicroliter LMs with tunable volume. The situation becomes even more difficult when one attempts to produce tiny Janus Liquid Marbles (JLMs). This paper presents a simple, single-step, and efficient strategy for obtaining both the pristine LMs and JLMs in the volume range 200 nL to 18 μL. The core idea relies on the impact of a liquid drop on a particle bed at a Weber number of ∼55 to produce two daughter droplets and to convert these droplets into LMs/JLMs. The whole process takes only a few tens of milliseconds (∼50 ms). We have rendered the experimental schemes so that both the JLMs and pristine LMs can be produced in a single step, with control over their volume. The mechanical stability analysis of the prepared marbles indicates that 200 nL is 5 times more stable than 10 μL of LMs. The 0.72 μL LMs prepared with a 0.5 v/v % phenolphthalein indicator solution showed 3 times faster response time to ammonia gas sensing than 10 μL of LMs. The results presented in this work open up a new route for the rapid and reliable production of both multilayered LMs and JLMs with tunable volume in a wide range (200 nL to 18 μL).

Liquid Marbles under Electric Fields: New Capabilities for Non-wetting Droplet Manipulation and Beyond

The study of liquid marbles (LMs) composed of stabilizing liquid droplets with solid particles in a gaseous environment has matured into an established area in surface and colloid science. The minimized "solid-liquid-air" triphase interface enables LMs to drastically reduce adhesion to a solid substrate, making them unique non-wetting droplets transportable with limited energy. The small volume, enclosed environment, and simple preparation render them suitable microreactors in industrial applications and processes such as cell culture, material synthesis, and blood coagulation. Extensive application contexts request precise and highly efficient manipulations of these non-wetting droplets. Many external fields, including magnetic, acoustic, photothermal, and pH, have emerged to prepare, deform, actuate, coalesce, mix, and disrupt these non-wetting droplets. Electric fields are rising among these external stimuli as an efficient source for manipulating the LMs with high controllability and a significant ability to contribute further to proposed applications. This Feature Article attempts to outline the recent developments related to LMs with the aid of electric fields. The effects of electric fields on the preparation and manipulation of LMs with intricate interfacial processes are discussed in detail. We highlight a wealth of novel electric field-involved LM-based applications and beyond while also envisaging the challenges, opportunities, and new directions for future development in this emerging research area.

Particle Separation from Liquid Marbles by the Viscous Folding of Liquid Films

Particle separation from fluid interfaces is one of the major challenges due to the large capillary energy associated with particle adsorption. Previous approaches rely on physicochemical modification or tuning the electrostatic action. Here, we show experimentally that particle separation can be achieved by fast dynamics of drop impact on soap films. When a droplet wrapped with particles (liquid marble) collides with a soap film, it undergoes bouncing and coalescence, stripping and viscous separation, or tunneling through the film. Despite the violence of splashing events, the process robustly yields the stripping in a tunable range. This viscous separation is supported by the transfer front of dynamic contact among the film, particle crust, and drop and can be well controlled in a deterministic manner by selectable impact parameters. By extensive experiments, together with thermodynamic analysis, we disclose that the separation thresholds depend on the energy competition between the kinetic energy, the increased surface energy, and the viscous dissipation. The mechanical cracking of the particle crust arises from the complex coupling between interfacial stress and viscous forces. This study is of potential benefit in soft matter research and also permits the study of a drop with colloid and surface chemistry.

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.

Stimulus Responsive Remotely Rupturable Adhesive Marbles Realized through a Hybrid Nanoparticle Concept

Adhesive marbles, an innovative concept derived from liquid marble technology that is "remotely breakable on demand" by external stimuli, offer diverse application prospects. Therefore, a chemically linked superomniphobic hybrid perfluorinated carbon black-silica nanoparticle (PCBSN) was realized by functionalizing surface groups and used for encapsulating adhesives. PCBSN successfully encapsulated liquids and adhesives to form water (WM, contact angle 158°), epoxy (EM, contact angle 145°), and silicone (SM, contact angle 135°) marbles, regardless of the surface tension and polarity. Studies on the interface characteristics revealed that the work performed for marble formation maintained an inverse relationship with the surface energy of particles and the surface tension of encapsulated liquids. The marble formation energy was determined to be higher for EM (1.071 × 10^-17 J) and lower for SM (0.946 × 10^-17 J). Upon exposure to laser, marbles showed a rapid photothermal response, and the heat transferability on the surface of marbles followed the order SM > EM > WM. The marbles were remotely rupturable by regulating the applied laser power, with breaking time being tunable from <10 to 500 s. The photothermal efficiency (%) of marbles can be graded as good and falls in the range of 88.6 × 10^-3 (EM) and 162.9 × 10^-3 (SM) at 1.5 W laser power. The marbles possessed high mechanical integrity and repeated cyclability before breaking on the rolling impact test. These adhesive marbles formed from PCBSNs may represent attractive candidates for such applications as "bonding from a distance" through remote means.

Interfacial viscoelasticity and jamming of colloidal particles at fluid-fluid interfaces: a review

Colloidal particles can be adsorbed at fluid-fluid interfaces, a phenomenon frequently observed in particle-stabilized foams, Pickering emulsions, and bijels. Particles adsorbed at interfaces exhibit unique physical and chemical behaviors, which affect the mechanical properties of the interface. Therefore, interfacial colloidal particles are of interest in terms of both fundamental and applied research. In this paper, we review studies on the adsorption of colloidal particles at fluid-fluid interfaces, from both thermodynamic and mechanical points of view, and discuss the differences as compared with surfactants and polymers. The unique particle interactions induced by the interfaces as well as the particle dynamics including lateral diffusion and contact line relaxation will be presented. We focus on the rearrangement of the particles and the resultant interfacial viscoelasticity. Particular emphasis will be given to the effects of particle shape, size, and surface hydrophobicity on the interfacial particle assembly and the mechanical properties of the obtained particle layer. We will also summarize recent advances in interfacial jamming behavior caused by adsorption of particles at interfaces. The buckling and cracking behavior of particle layers will be discussed from a mechanical perspective. Finally, we suggest several potential directions for future research in this area.

Effect of Stabilizing Particle Size on the Structure and Properties of Liquid Marbles

A liquid marble (LM) describes a liquid droplet that is wrapped by nonwetting micro- or nanoparticles and therefore obtains characteristics of a solid powder particle. Here, we investigate the effect of the stabilizing particle size on the resulting structure and properties of the LM. We synthesize a series of polystyrene particles with ultrathin coatings of heptadecafluorooctanesulfonic acid-doped polypyrrole with diameters ranging between 1 and 1000 μm by an aqueous chemical oxidative seeded polymerization of pyrrole. The methodology produced a set of hydrophobic particles with similar surface characteristics to allow the formation of LMs and to probe size effects in the LM formation and stabilization efficiency. We found that particles with a size above 20 μm adsorb as a particle monolayer to the surface of the LM, while smaller particles are adsorbed as ill-defined, multilayered aggregates. These results indicate that the balance between particle-particle interaction and gravity is an important parameter to control the surface structure of the LMs. The assembly behavior and size of the particles also correlated with the mechanical integrity of the LM against fall impact. The mechanical resistance was affected by the gap distance between the inner liquid of the LM and supporting substrate, the capillary forces acting between the particles at the LM surface, and the potential energy that depended on the particle size. Last, we demonstrate that the broadband light-absorbing properties of the polypyrrole shell also allow manipulating the evaporation rate of the inner liquid.

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.

Electrocoalescence of liquid marbles driven by embedded electrodes for triggering bioreactions

Liquid marbles need to be controlled precisely to benefit applications, for instance, as microreactors on digital microfluidic platforms for chemical and biological assays. In this work, a strategy is introduced to coalesce liquid marbles via electrostatics, where two liquid marbles in contact can coalesce when a sufficiently high voltage is applied to embedded electrodes. With the understanding of the mechanism of coalescence through relating the electric stress and the restoring capillary pressure at the contact interface, this method coalesces liquid marbles efficiently. When compared with the existing electrocoalescence method, our approach does not require immersion of electrodes to trigger coalescence. We demonstrate this to exchange the medium for the culture of cell spheroids and to measure the cell metabolic activity through a CCK-8 assay. The manipulation of liquid marbles driven by electrostatics creates new opportunities to conduct chemical reactions and biomedical assays in these novel microreactors.

Vapor-Driven Transport of Different Types of Objects at the Air-Liquid Interface

Transportation and position control of objects on the surface of liquids is an important part of automation. To drive an object on the surface of a liquid, many methods have been proposed. However, these methods mainly focus on the driving of the object itself, and it is still difficult to precisely control its position. In our study, we propose a new method that uses vapor released from a suspended drop to achieve precise position control and transport of different types of objects at the air-liquid interface. These objects can be a plastic plate, a liquid marble, or an oil drop. The mechanism for controlling objects is that vapor released from a suspended drop causes a surface tension gradient around the object. When the vapor dissolves on the surface of a liquid, the surface tension of the liquid increases. Due to the surface tension gradient, the object moves from the surrounding area to the area below the suspended drop and follows the motion of the suspended drop with the trajectory of a letter. To show that the position of the objects can be precisely controlled by our method, we control the object on the center of a circle, and the maximum offset distance from the center of the circle is less than 3 mm. In addition, we also use vapor released from a suspended drop to transport an oil drop close to an object. After the drop adhered with the object, the object is driven by the oil drop. Compared with other methods that drive the motion of objects by reducing the surface tension of a liquid, our method is easy and the position of objects can be precisely controlled.