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.

Reciprocating Oscillation of a Floating Ferrofluid Marble Triggered by Magnetic Fields

Liquid marbles have the potential for microfluidic transport, medical diagnostics, and chemical analysis due to their negligible stickiness, environmental independence, and excellent mobility. Here, we report a non-contact manipulation strategy to arouse a reciprocating oscillation of ferrofluid marbles floating on the water surface, which can be used as microreactors. We experimentally investigated the quantitative relationship between the oscillation behavior, the applied magnetic field parameters, and the field regulation mechanism. The variables, including the magnetic field strength, marble volume, and switching period, are vital in determining the final state. The oscillation can be separated into three stages: transitional movement, compressive deformation, and rebound, before entering the next cycle. Accordingly, we created a manipulation technique for improving the mixing of inner reactants inside this marble container by remote-controlled shaking after optimizing with an oscillation model.

Liquid Marble as an Amphibious Carrier for the Controlled Delivery and Release of Substances

The motion control of small objects has received significant interest in the research field of soft active matter. Controlling the release of substances from small objects has also attracted attention in other fields, such as the agrochemical and biomaterial fields. Until now, these two research objectives have been conducted independently in most cases but have the same ultimate goal: to transport small objects loaded with functional substances in a controlled manner and to release the substances at a desired time within the same system. This Perspective aims to highlight the challenges for realizing both the amphibious motion control of objects and the controlled release of substances by the application of external stimuli within the same system, using liquid marbles, which are particle-layer-coated small droplets, as a carrier.

Dynamic Liquid-Liquid Interface: Applying a Spinning Interfacial Microreactor to Actively Converge Biphasic Reactants for the Enhanced Interfacial Reaction

A liquid-liquid interfacial reaction combines reactants with large polarity disparity to achieve greener and more efficient chemistry that is otherwise challenging in traditional single-phase systems. However, current interfacial approaches suffer from the need for a large amount of solvent/reactant/emulsifier and poor reaction performance arising from intrinsic thermodynamic constraints. Herein, we achieve an efficient interfacial reaction by creating a magnetic-responsive, microscale liquid-liquid interface and exploit its dynamic spinning motion to generate vortex-like hydrodynamic flows that rapidly converge biphasic reactants to the point-of-reaction. Notably, the spinning of this functional interface at 800 rpm boosts the reaction efficiency and its apparent equilibrium constant by > 500-fold and 10⁵-fold, respectively, higher than conventional methods that utilize bulk and/or non-dynamic liquid interfaces, even with external mechanical stirring. By driving reaction equilibrium toward favorable product formation, our unique design offers enormous opportunities to realize efficient multiphasic reactions crucial for diverse applications in chemical synthesis, environmental remediation, and even molecular recycling.

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.

Controllable high-performance liquid marble micromixer

A liquid marble is a liquid droplet coated with a shell of microparticles. Liquid marbles have served as a unique microreactor for chemical reactions and cell culture. Mixing is an essential task for liquid marbles as a microreactor. However, the potential of liquid marble-based microreactors is significantly limited due to the lack of effective mixing strategies. Most mixing strategies used manual and contact-based actuation schemes. This paper reports the development of a manipulation scheme that induces fluid motion into a liquid marble, leading to enhanced mixing. By inducing rotation on a horizontal axis, we significantly increased the mixing rate by 27.6 times compared to a non-actuated liquid marble and reduced the reaction time by more than 10 times. The proposed method provides a simple, continuous, precise, and controllable high-performance mixing strategy on a liquid marble platform.

Modes of synchrony in self-propelled pentanol drops

We report various modes of synchrony observed for a population of two, three and four pentanol drops in a rectangular channel at the air-water interface. Initially, the autonomous oscillations of a single 1-pentanol drop were studied in a ferroin DI water solution pre-mixed with some volume of pentanol. A pentanol drop performs continuous motion on the air-water interface due to Marangoni forces. A linear channel was prepared to study the uniaxial movement of the drop(s). Thereafter, a systematic study of the self-propelled motion of a 1-pentanol drop was reported as a function of the drop volume. Subsequently, the coupled dynamics were studied for two, three and four drops, respectively. We observed anti-phase oscillations in a pair of pentanol drops. In the case of three drops, relay synchronization was observed, wherein consecutive pairs of drops were exhibiting out-of-phase oscillations and alternate drops were performing in-phase oscillations. Four pentanol drops showed two different modes of synchrony: one was relay synchrony and the other was out-of-phase oscillations between two pairs of drops (within a pair, the drops exhibit in-phase oscillations).

Multimotion of Marangoni Propulsion Ships Controlled by Two-Wavelength Near-Infrared Light

Multimode motion of Marangoni propulsion ships on a water surface as per a near-infrared, two-wavelength selective response is achieved for the first time. The ships are rhombus- or propeller-shaped polyacrylamide or siloxane resin-based gels in which Nd₂O₃ and Yb₂O₃ nanoparticles are incorporated separately for photothermal conversion at 808 and 980 nm, respectively. The rhombus geometry is for straight locomotion, and the propeller geometry is for rotation. On/off remote control of the forward and backward locomotion of a rhombus-shaped ship and of the clockwise and counterclockwise rotations of a propeller-shaped ship via irradiation with 808 or 980 nm near-infrared light is demonstrated. The nanoparticles are incorporated into the desired locations of the gels, enabling selective local heating of the gels without focusing the light. The temperature gradient of the ships by local heating, based on a photothermal conversion, generates a Marangoni propulsion force to move the ship in the desired direction.

Self-Propelled Aero-GaN Based Liquid Marbles Exhibiting Pulsed Rotation on the Water Surface

We report on self-propelled rotating liquid marbles fabricated using droplets of alcoholic solution encapsulated in hollow microtetrapods of GaN with hydrophilic free ends of their arms and hydrophobic lateral walls. Apart from stationary rotation, elongated-spheroid-like liquid marbles were found, for the first time, to exhibit pulsed rotation on water surfaces characterized by a threshold speed of rotation, which increased with the weight of the liquid marble while the frequency of pulses proved to decrease. To throw light upon the unusual behavior of the developed self-propelled liquid marbles, we propose a model which takes into account skimming of the liquid marbles over the water surface similar to that inherent to flying water lily beetle and the so-called helicopter effect, causing a liquid marble to rise above the level of the water surface when rotating.

Osmotic evolution of composite liquid marbles

HYPOTHESIS

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.

EXPERIMENTS

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.

FINDINGS

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.

Locomotion of a Nonaqueous Liquid Marble Induced by Near-Infrared-Light Irradiation

Micrometer-sized hydrophobic polyaniline (PANI) grains were synthesized via an aqueous chemical oxidative polymerization protocol in the presence of dopant carrying perfluoroalkyl or alkyl groups. The critical surface tensions of the PANIs synthesized in the presence of heptadecafluorooctanesulfonic acid and sodium dodecyl sulfate dopants were lower than that of PANI synthesized in the absence of dopant, indicating the presence of hydrophobic dopant on the grain surfaces. The PANI grains could adsorb to air-liquid interfaces, and aqueous and nonaqueous liquid marbles (LMs) were successfully fabricated using liquids with surface tensions ranging between 72.8 and 42.9 mN/m. Thermography studies confirmed that the surface temperature of the LMs increased by near-infrared light irradiation thanks to the photothermal property of the PANI, and the maximum temperatures measured for nonaqueous LMs were higher than that measured for aqueous LM. We demonstrated that transport of the LMs on a planar water surface can be achieved via Marangoni flow generated by the near-infrared light-induced temperature gradient. Numerical analyses indicated that the LMs containing liquids with lower specific heat and thermal conductivity and higher density showed longer path length per one light irradiation shot and longer decay time. This is because generated heat could efficiently transfer from the LMs to the water surface and larger inertial force could work on the LMs. The LMs could also move over the solid substrate thanks to their near-spherical shapes. Furthermore, it was also demonstrated that the inner liquids of the LMs could be released on site by an external stimulus.

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.

Spiral Thermal Waves Generated by Self-Propelled Camphor Boats

Spiral thermal surface waves arising from self-propulsion of the camphor-driven objects are reported. Spiral thermal waves were registered for dissolution and evaporation-guided self-propulsion. Soluto-capillarity is accompanied by thermo-capillarity under self-propulsion of camphor boats. The jump in the surface tension due to the soluto-capillarity is much larger than that due to the thermo-capillarity. The spiral patterns inherent for the surface thermal waves are imposed by the self-rotational motion of camphor grains. The observed thermal effect is related to the adsorption of camphor molecules at the water/vapor interface. The observed spirals are shaped as Archimedean ones.

Inverse leidenfrost drop manipulation using menisci

Drops deposited on an evaporating liquid bath can be maintained in an inverse Leidenfrost state by the vapor emanating from the bath, making them levitate and hover without effective friction. These perfectly non-wetting droplets create a depression in the liquid interface that sustains their weight, which generates repellent forces when they approach a meniscus rising against a wall. Here, we study this reflection in detail, and show that frictionless Leidenfrost drops are a simple and efficient tool to probe the shape of an unknown interface. We then use the menisci to control the motion of the otherwise elusive drops. We create waveguides to direct and accelerate them and use parabolic walls to reflect and focus them. This could be particularly beneficial in the scale up of droplet cryopreservation processes: capillary interactions can be used to transport, gather and collect vitrified biological samples in absence of contact and contamination.

Cherenkov-Like Surface Thermal Waves Emerging from Self-Propulsion of a Liquid Marble

We explore the thermal field related to the self-propulsion of floating liquid marbles filled with aqueous ethanol. Cherenkov-like thermal waves arising from self-propulsion are registered. The opening angle of the thermal field Cherenkov triangle is governed by the inter-relation between the velocity of self-propulsion and the phase velocity of the capillary waves. The self-propulsion is driven by soluto-capillarity accompanied by thermo-capillarity. A semiquantitative analysis of the effect is presented. The empirical selection rule for capillary waves responsible for the mass, momentum, and heat transfer is outlined. The soluto-capillarity leads to much stronger spatial variations of the surface tension than the thermo-capillarity.

A brief history of liquid computers

A substrate does not have to be solid to compute. It is possible to make a computer purely from a liquid. I demonstrate this using a variety of experimental prototypes where a liquid carries signals, actuates mechanical computing devices and hosts chemical reactions. We show hydraulic mathematical machines that compute functions based on mass transfer analogies. I discuss several prototypes of computing devices that employ fluid flows and jets. They are fluid mappers, where the fluid flow explores a geometrically constrained space to find an optimal way around, e.g. the shortest path in a maze, and fluid logic devices where fluid jet streams interact at the junctions of inlets and results of the computation are represented by fluid jets at selected outlets. Fluid mappers and fluidic logic devices compute continuously valued functions albeit discretized. There is also an opportunity to do discrete operation directly by representing information by droplets and liquid marbles (droplets coated by hydrophobic powder). There, computation is implemented at the sites, in time and space, where droplets collide one with another. The liquid computers mentioned above use liquid as signal carrier or actuator: the exact nature of the liquid is not that important. What is inside the liquid becomes crucial when reaction-diffusion liquid-phase computing devices come into play: there, the liquid hosts families of chemical species that interact with each other in a massive-parallel fashion. I shall illustrate a range of computational tasks, including computational geometry, implementable by excitation wave fronts in nonlinear active chemical medium. The overview will enable scientists and engineers to understand how vast is the variety of liquid computers and will inspire them to design their own experimental laboratory prototypes. This article is part of the theme issue 'Liquid brains, solid brains: How distributed cognitive architectures process information'.

Predictive Framework for the Spreading of Liquid Drops and the Formation of Liquid Marbles on Hydrophobic Particle Bed

In this work, we have developed a model to describe the behavior of liquid drops upon impaction on hydrophobic particle bed and verified it experimentally. Poly(tetrafluoroethylene) (PTFE) particles were used to coat drops of water, aqueous solutions of glycerol (20, 40, and 60% v/v), and ethanol (5 and 12% v/v). The experiments were conducted for Weber number ( We) ranging from 8 to 130 and Reynolds number ( Re) ranging from 370 to 4460. The bed porosity was varied from 0.8 to 0.6. The experimental values of β_max (ratio of the diameter at the maximum spreading condition to the initial drop diameter) were estimated from the time-lapsed images captured using a high-speed camera. The theoretical β_max was estimated by making energy balances on the liquid drop. The proposed model accounts for the energy losses due to viscous dissipation and crater formation along with a change in kinetic energy and surface energy. A good agreement was obtained between the experimental β_max and the estimated theoretical β_max. The proposed model yielded a least % absolute average relative deviation (% AARD) of 5.5 ± 4.3 compared to other models available in the literature. Further, it was found that the liquid drops impacting on particle bed are completely coated with PTFE particles with β_max values greater than 2.

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.

Manipulation of a floating liquid marble using dielectrophoresis

A liquid marble is a microliter-sized droplet coated with hydrophobic powder. The porous coating prevents the liquid content from being in direct physical contact with its surroundings, making the liquid marble perfectly non-wetting. On the one hand, the non-wetting ability allows the liquid marble to float and move across a liquid surface with little resistance. On the other hand, the porosity enables gas exchange between the liquid marble and its surroundings. These properties allow the liquid marble to serve as a bioreactor platform for important applications such as cell culture. Liquid marbles floating on a free liquid surface prevent evaporation due to the high humidity near the liquid surface. Moving a floating liquid marble allows for stirring and mixing inside the liquid marble. This paper reports a novel technique for manipulating a floating liquid marble using dielectrophoresis. A relatively simple setup can move liquid marbles of various sizes across the water surface at high speeds. We also present an analytical model to model and accurately predict the motion of the floating liquid marble. The technique reported here potentially allows for high-throughput and efficient handling of floating liquid marbles as a digital microfluidics platform.

Liquid Marble Actuator for Microfluidic Logic Systems

A mechanical flip-flop actuator has been developed that allows for the facile re-routing and distribution of liquid marbles (LMs) in digital microfluidic devices. Shaped loosely like a triangle, the actuating switch pivots from one bistable position to another, being actuated by the very low mass and momentum of a LM rolling under gravity (~4 × 10⁻⁶ kg ms⁻¹). The actuator was laser-cut from cast acrylic, held on a PTFE coated pivot, and used a PTFE washer. Due to the rocking motion of the switch, sequential LMs are distributed along different channels, allowing for sequential LMs to traverse parallel paths. This distributing effect can be easily cascaded, for example to evenly divide sequential LMs down four different paths. This lightweight, cheap and versatile actuator has been demonstrated in the design and construction of a LM-operated mechanical multiplication device — establishing its effectiveness. The actuator can be operated solely by gravity, giving it potential use in point-of-care devices in low resource areas.

Experimental Investigation of the Self-Propelled Motion of a Sodium Oleate Tablet and Boat at an Oil-Water Interface

The self-propelled behaviors of macroscopic inanimate objects at surfaces and interfaces are ubiquitous phenomena of fundamental interest in interface science. However, given the existence of a large variety of systems with their own inherent chemical properties, the kinematics of the self-propelled motion and the dynamics of the forces driving these systems often remain largely unknown. Here, we experimentally investigate the spontaneous motion of a sodium oleate tablet at a water-nitrobenzene interface, under nonequilibrium and global isothermal conditions, through measurements of the interfacial tension with the noninvasive, quasi-elastic laser scattering method. The sodium oleate tablet was self-propelled due to an imbalance in the interfacial tension induced by the inhomogeneous adsorption of oleate/oleic acid molecules. The kinetics of the self-propelled motion of a boat-shaped plastic sheet bearing sodium oleate tablets at a sodium oleate aqueous solution-nitrobenzene interface was also studied. The interfacial tension difference between the front and rear of the boat was quantitatively identified as the force pushing the boat forward, although the Marangoni flow due to the uneven distribution of the interfacial tension behind the boat tended to decelerate the motion.

Period of Oscillatory Motion of a Camphor Boat Determined by the Dissolution and Diffusion of Camphor Molecules

Here, we investigated the oscillatory motion of a camphor boat on water to clarify how the dynamics of camphor concentration profile determines the period of oscillation. The boat, which was made of a plastic plate and a camphor disk, was glued below the plate at a distance from the edge. The dependence of oscillation period on temperature and viscosity of the water phase was measured in experiments. We reproduced the experimental results by calculating the period of oscillatory motion by considering the experimental values of physicochemical parameters describing the time evolution of camphor concentration profile and the friction acting on a boat, such as diffusion and dissolution rates of camphor, viscosity of the water phase, and the threshold concentration of camphor necessary to accelerate the boat from the resting state. The increase in the period of oscillatory motion at low temperatures was explained by the reduced dissolution rate of camphor into the water phase.

Superposition of Translational and Rotational Motions under Self-Propulsion of Liquid Marbles Filled with Aqueous Solutions of Camphor

Self-locomotion of liquid marbles, coated with lycopodium or fumed fluorosilica powder, filled with a saturated aqueous solution of camphor and placed on a water/vapor interface is reported. Self-propelled marbles demonstrated a complicated motion, representing a superposition of translational and rotational motions. Oscillations of the velocity of the center of mass and the angular velocity of marbles, occurring in the antiphase, were registered and explained qualitatively. Self-propulsion occurs because of the Marangoni solutocapillary flow inspired by the adsorption of camphor (evaporated from the liquid marble) by the water surface. Scaling laws describing translational and rotational motions are proposed and checked. The rotational motion of marbles arises from the asymmetry of the field of the Marangoni stresses because of the adsorption of camphor evaporated from marbles.

Coalescence Processes of Droplets and Liquid Marbles

The coalescence process of droplets and, more recently, of liquid marbles, has become one of the most essential manipulation schemes in digital microfluidics. This process is indispensable for realising microfluidic functions such as mixing and reactions at microscale. This paper reviews previous studies on droplet coalescence, paying particular attention to the coalescence of liquid marbles. Four coalescence systems have been reviewed, namely, the coalescence of two droplets freely suspended in a fluid; the coalescence of two sessile droplets on a solid substrate; the coalescence of a falling droplet and a sessile droplet on a solid substrate; and liquid marble coalescence. The review is presented according to the dynamic behaviors, physical mechanisms and experimental parameters of the coalescence process. It also provides a systematic overview of how the coalescence process of droplets and liquid marbles could be induced and manipulated using external energy. In addition, the practical applications of liquid marble coalescence as a novel microreactor are highlighted. Finally, future perspectives on the investigation of the coalescence process of liquid marbles are proposed. This review aims to facilitate better understanding of the coalescence of droplets and of liquid marbles as well as to shed new insight on future studies.

Chlorine dioxide-induced and Congo red-inhibited Marangoni effect on the chlorite-trithionate reaction front

Hydrodynamic flows can exert multiple effects on an exothermal autocatalytic reaction, such as buoyancy and the Marangoni convection, which can change the structure and velocity of chemical waves. Here we report that in the chlorite-trithionate reaction, the production and consumption of chlorine dioxide can induce and inhibit Marangoni flow, respectively, leading to different chemo-hydrodynamic patterns. The horizontal propagation of a reaction-diffusion-convection front was investigated with the upper surface open to the air. The Marangoni convection, induced by gaseous chlorine dioxide on the surface, produced from chlorite disproportionation after the proton autocatalysis, has the same effect as the heat convection. When the Marangoni effect is removed by the reaction of chlorine dioxide with the Congo red (CR) indicator, an oscillatory propagation of the front tip is observed under suitable conditions. Replacing CR with bromophenol blue (BPB) distinctly enhanced the floating, resulting in multiple vortexes, owing to the coexistence between BPB and chlorine dioxide. Using the incompressible Navier-Stokes equations coupled with reaction-diffusion and heat conduction equations, we numerically obtain various experimental scenarios of front instability for the exothermic autocatalytic reaction coupled with buoyancy-driven convection and Marangoni convection.

The Potential of Liquid Marbles for Biomedical Applications: A Critical Review

Liquid marbles (LM) are freestanding droplets covered by micro/nanoparticles with hydrophobic/hydrophilic properties, which can be manipulated as a soft solid. The phenomenon that generates these soft structures is regarded as a different method to generate a superhydrophobic behavior in the liquid/solid interface without modifying the surface. Several applications for the LM have been reported in very different fields, however the developments for biomedical applications are very recent. At first, the LM properties are reviewed, namely shell structure, LM shape, evaporation, floatability and robustness. The different strategies for LM manipulation are also described, which make use of magnetic, electrostatic and gravitational forces, ultraviolet and infrared radiation, and approaches that induce LM self-propulsion. Then, very distinctive applications for LM in the biomedical field are presented, namely for diagnostic assays, cell culture, drug screening and cryopreservation of mammalian cells. Finally, a critical outlook about the unexplored potential of LM for biomedical applications is presented, suggesting possible advances on this emergent scientific area.

Transfer of Materials from Water to Solid Surfaces Using Liquid Marbles

Remotely controlling the movement of small objects is desirable, especially for the transportation and selection of materials. Transfer of objects between liquid and solid surfaces and triggering their release would allow for development of novel material transportation technology. Here, we describe the remote transport of a material from a water film surface to a solid surface using quasispherical liquid marbles (LMs). A light-induced Marangoni flow or an air stream is used to propel the LMs on water. As the LMs approach the rim of the water film, gravity forces them to slide down the water rim and roll onto the solid surface. Through this method, LMs can be efficiently moved on water and placed on a solid surface. The materials encapsulated within LMs can be released at a specific time by an external stimulus. We analyzed the velocity, acceleration, and force of the LMs on the liquid and solid surfaces. On water, the sliding friction due to the drag force resists the movement of the LMs. On a solid surface, the rolling distance is affected by the surface roughness of the LMs.

Camphor-Engine-Driven Micro-Boat Guides Evolution of Chemical Gardens

A micro-boat self-propelled by a camphor engine, carrying seed crystals of FeCl₃, promoted the evolution of chemical gardens when placed on the surface of aqueous solutions of potassium hexacyanoferrate. Inverse chemical gardens (growing from the top downward) were observed. The growth of the “inverse” chemical gardens was slowed down with an increase in the concentration of the potassium hexacyanoferrate. Heliciform precipitates were formed under the self-propulsion of the micro-boat. A phenomenological model, satisfactorily describing the self-locomotion of the camphor-driven micro-boat, is introduced and checked.

Liquid marbles as bioreactors for the study of three-dimensional cell interactions

Liquid marble as a bioreactor platform for cell-based studies has received significant attention, especially for developing 3D cell-based assays. This platform is particularly suitable for 3D in-vitro modeling of cell-cell interactions. For the first time, we demonstrated the interaction of olfactory ensheathing cells (OECs) with nerve debris and meningeal fibroblast using liquid marbles. As the transplantation of OECs can be used for repairing nerve injury, degenerating cell debris within the transplantation site can adversely affect the survival of transplanted OECs. In this paper, we used liquid marbles to mimic the hostile 3D environment to analyze the functional behavior of the cells and to form the basis for cell-based therapy. We show that OECs interact with debris and enhanced cellular aggregation to form a larger 3D spheroidal tissue. However, these spheroids indicated limitation in biological functions such as the inability of cells within the spheroids to migrate out and adherence to neighboring tissue by fusion. The coalescence of two liquid marbles allows for analyzing the interaction between two distinct cell types and their respective environment. We created a microenvironment consisting of 3D fibroblast spheroids and nerve debris and let it interact with OECs. We found that OECs initiate adherence with nerve debris in this 3D environment. The results suggest that liquid marbles are ideal for developing bioassays that could substantially contribute to therapeutic applications. Especially, insights for improving the survival and adherence of transplanted cells.

Recent Advances and Future Perspectives on Microfluidic Liquid Handling

The interdisciplinary research field of microfluidics has the potential to revolutionize current technologies that require the handling of a small amount of fluid, a fast response, low costs and automation. Microfluidic platforms that handle small amounts of liquid have been categorised as continuous-flow microfluidics and digital microfluidics. The first part of this paper discusses the recent advances of the two main and opposing applications of liquid handling in continuous-flow microfluidics: mixing and separation. Mixing and separation are essential steps in most lab-on-a-chip platforms, as sample preparation and detection are required for a variety of biological and chemical assays. The second part discusses the various digital microfluidic strategies, based on droplets and liquid marbles, for the manipulation of discrete microdroplets. More advanced digital microfluidic devices combining electrowetting with other techniques are also introduced. The applications of the emerging field of liquid-marble-based digital microfluidics are also highlighted. Finally, future perspectives on microfluidic liquid handling are discussed.

Microfluidic Technology for the Generation of Cell Spheroids and Their Applications

A three-dimensional (3D) tissue model has significant advantages over the conventional two-dimensional (2D) model. A 3D model mimics the relevant in-vivo physiological conditions, allowing a cell culture to serve as an effective tool for drug discovery, tissue engineering, and the investigation of disease pathology. The present reviews highlight the recent advances and the development of microfluidics based methods for the generation of cell spheroids. The paper emphasizes on the application of microfluidic technology for tissue engineering including the formation of multicellular spheroids (MCS). Further, the paper discusses the recent technical advances in the integration of microfluidic devices for MCS-based high-throughput drug screening. The review compares the various microfluidic techniques and finally provides a perspective for the future opportunities in this research area.

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.

Measuring the Coefficient of Friction of a Small Floating Liquid Marble

This paper investigates the friction coefficient of a moving liquid marble, a small liquid droplet coated with hydrophobic powder and floating on another liquid surface. A floating marble can easily move across water surface due to the low friction, allowing for the transport of aqueous solutions with minimal energy input. However, the motion of a floating marble has yet to be systematically characterised due to the lack of insight into key parameters such as the coefficient of friction between the floating marble and the carrier liquid. We measured the coefficient of friction of a small floating marble using a novel experimental setup that exploits the non-wetting properties of a liquid marble. A floating liquid marble pair containing a minute amount magnetite particles were immobilised and then released in a controlled manner using permanent magnets. The capillarity-driven motion was analysed to determine the coefficient of friction of the liquid marbles. The "capillary charge" model was used to fit the experimental results. We varied the marble content and carrier liquid to establish a relationship between the friction correction factor and the meniscus angle.

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.

Digital microfluidics with a magnetically actuated floating liquid marble

Controlled actuation of a floating liquid marble, a liquid droplet coated with hydrophobic particles floating on another liquid surface, is a potential digital microfluidics platform for the transport of aqueous solution with minimal volume loss. This paper reports our recent investigation on the magnetic actuation of floating liquid marbles filled with magnetic particles. The magnetic force and frictional force acting on the floating liquid marble determine the horizontal movement of the marble. We varied the magnetic flux density, flux density gradient, concentration of magnetic particles and speed of the marble to elucidate the relationship between the acting forces. We subsequently determined the suitable operating conditions for the actuation and derived the scaling laws for the actuation parameters.