Liquid Marbles Research Articles

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.

Coalescence of liquid marbles on oil-infused surface

While droplets typically merge instantly in an air medium, alterations to the outer medium can complicate the coalescence process. This study investigates droplet coalescence dynamics when encapsulated by a uniform liquid–solid composite shell, aiming to understand its implications for mechanical stability and merging behavior. The uniform shell around a sessile droplet is produced by using liquid marble on oil-infused surfaces (LMOI). The coalescence dynamics was studied under two different conditions: droplet with LMOI and LMOI with LMOI. In contrast to merging of bare droplets, coalescence involving at least one LMOI reveals a three-step process, including spreading, depletion, and eventual merging phases. Higher oil viscosity influences the merging process, with increased viscosity leading to delayed merging with longer spreading and depletion phases. LMOI exhibits significant resistance to merging with another LMOI, necessitating external triggers like pressure or electric fields for coalescence. These findings provide insights into designing microreactor systems based on LMOI, contributing to the comprehension of their dynamics and functionalities.

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials.

The incorporation of superhydrophobic properties into metal organic framework (MOF) materials is highly desirable to enhance their hydrolytic stability, gas capture selectivity in the presence of humidity and efficiency in oil-water separations, among others. The existing strategies for inducing superhydrophobicity into MOFs have several weaknesses, such as increased cost, utilization of toxic reagents and solvents, applicability for limited MOFs, etc. Here, we report the simplest, most eco-friendly, and cost-effective process to impart superhydrophobicity to MOFs, involving a rapid (90 min) treatment of MOF materials with solutions of sodium oleate, a main component of soap. The method can be applied to both hydrolytically stable and unstable MOFs, with the porosity of modified MOFs approaching, in most cases, that of the pristine materials. Interestingly, this approach was used to isolate superhydrophobic magnetic MOF composites, and one of these materials formed stable liquid marbles, whose motion could be easily guided using an external magnetic field. We also successfully fabricated superhydrophobic MOF-coated cotton fabric and fiber composites. These composites exhibited exceptional oil sorption properties achieving rapid removal of floating crude oil from water, as well as efficient purification of oil-in-water emulsions. They are also regenerable and reusable for multiple sorption processes. Overall, the results described here pave the way for an unprecedented expansion of the family of MOF-based superhydrophobic materials, as virtually any MOF could be converted into a superhydrophobic compound by applying the new synthetic approach.

Vibration‐Enabled Mobility of Liquid Metal

AbstractDirected liquid metal (gallium‐based) manipulation and actuation are paramount for copious applications, including soft robotics, soft electronics, and targeted drug delivery. Although there are several strategies available to achieve mobility of liquid metals in a “wet” environment. Strategies to achieve and improve mobility of liquid metal droplets and puddles in a “dry” environment are scarce and rely on metallophobic surface design or liquid metal marbles. Here, high mobility of Galinstan is elucidated by combining metallophobic surface design and vertical vibrations. Vibration frequencies between 20 and 30 Hz are conducive to droplet movement and threshold inclination angles of 0.5° to 1° are observed upon actuation by these vibrations. The method itself is applicable for a wide range of droplet sizes (30 and 2000 µL) and very robust. The droplet movement typically comprises of periodic receding and advancing of the droplet and commences via a rolling mechanism rather than a gliding mechanism. Finally, it is shown that small (0.5 mm height) obstacles can be traversed by this method, indicating that it can be used in concert with other strategies, such as surface structuring strategies, which open up pathways for mobility and controlled actuation of liquid metal droplets in air.

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.

Impact of coating particles on liquid marble lifetime: reactor engineering approach to evaporation.

Liquid marbles are soft matter objects characterised by a liquid droplet enclosed within a hydrophobic particle coating, preventing wetting. This distinctive structure serves as active sites for solid-liquid-gas reactions. However, the impact the chosen coating material has on liquid marble stability, particularly regarding the number of coating layers and material wetting, remains uncertain. There is a need for a modelling approach to predict the overall lifetime considering these coating characteristics. This study reveals that for PTFE liquid marbles evaporating at ambient temperature, smaller coating particles (250 nm) extend their lifetime by forming a multilayered coating. Conversely, using larger particle sizes (200 μm) results in the formation of monolayer liquid marbles with shorter lifetimes than their equivalent naked droplets. Additionally, a higher number of particle layers and a larger contact angle generally enhance the liquid marble's lifetime. For multilayered liquid marbles comprised of smaller particles (250 nm), the particle contact angle is found to have a more significant impact than the number of layers on lifetime extension, whereas the opposite holds true for larger particle sizes (20 μm). A modelling approach using the reactor engineering method for liquid marble evaporation demonstrates excellent agreement with experimental results, yielding an R2 of 0.996. The implementation of this specific model, capable of assessing lifetime across various physical modifications, will enhance our understanding of liquid marble properties before their application in biomedical, microreactor, and green technologies.

Dynamic behavior of floating magnetic liquid marbles under steady and pulse-width-modulated magnetic fields.

Liquid marbles show promising potential for digital microfluidic devices due to their lower friction with the platform surface than non-covered droplets. In this study, the manipulation of a biocompatible magnetic liquid marble with a magnetic shell (LMMS) is experimentally studied. The movement of the floating LMMS on the water surface, which is actuated by DC and pulse width modulation (PWM) magnetic fields, is investigated under the influence of various parameters, including the LMMS volume, the initial distance of the LMMS from the magnetic coil tip, the magnetic coil current, the PWM frequency and its duty cycle. The LMMS has a shorter travel time to the magnetic coil tip under a DC magnetic field by increasing the magnetic coil current, decreasing the initial distance and its volume. In the PWM mode, these parameters show similar behavior; moreover, increasing the PWM duty cycle and decreasing the PWM frequency shorten the travel time. It is demonstrated that actuation by a PWM magnetic field with step-by-step movement provides better control over manipulation of the floating magnetic marble. The dynamic behavior of an LMMS is compared to a ferrofluid marble (FM), which is formed using a ferrofluid instead of water as its core. It is observed that the LMMS has a lower velocity than the FM.

Particulate gel liquid marble

Nonsticking water droplets stabilized by particulate polydimethylsiloxane (PDMS) organogel, namely "particulate gel liquid marbles (PGLMs)" are prepared via mechanochemistry. PGLM exhibited superior compression/impact stability to conventional particle-stabilized LM owing to...

PH-triggered adjustable bursting of liquid marbles in water pools

Readily functionalizable MOFs are introduced to precisely control the wettability transition from hydrophobicity to hydrophilicity via (i) triggering the structural instability of MOFs under an alkaline condition and (ii) protonating the available amine moiety under an acidic condition.

Biocompatible Hydrogel-Based Liquid Marbles with Magnetosomes.

Liquid marbles are widely known for their potential biomedical applications, especially due to their versatility and ease of preparation. In the present work, we prepared liquid marbles with various cores composed of water, agar-based hydrogels, magnetic fluids, or non-aqueous substances. As a coating material, we used biocompatible particles of plant origin, such as turmeric grains and Lycopodium pollen. Additionally, we provided marbles with magnetic properties by incorporating either magnetosomes or iron oxide nanoparticles as a powder or by injecting another magnetic fluid. Structures obtained in this way were stable and susceptible to manipulation by an external magnetic field. The properties of the magnetic components of our marbles were verified using electron paramagnetic resonance (EPR) spectroscopy and vibrating sample magnetometry (VSM). Our approach to encapsulation of active substances such as antibiotics within a protective hydrogel core opens up new perspectives for the delivery of hydrophobic payloads to the inherently hydrophilic biological environment. Additionally, hydrogel marbles enriched with magnetic materials showed promise as biocompatible heating agents under alternating magnetic fields. A significant innovation of our research was also the fabrication of composite structures in which the gel-like core was surrounded without mixing by a magnetic fluid covered on the outside by the particle shell. Our liquid marbles, especially those with a hydrogel core and magnetic content, due to the ease of preparation and favorable properties, have great potential for biomedical use. The fact that we were able to simultaneously produce, functionalize (by filling with predefined cargo), and manipulate (by means of an external magnetic field) several marbles also seems to be important from an application point of view.

Fluorine-free superhydrophobic PDMS-coated silica particles for oil–water separation and liquid marbles

In this paper, we report the fabrication of fluorine-free hydrophobic particles via grafting fluidic polydimethylsiloxane onto silica particles through a facile wet lab technique at room temperature. A cotton fabric coated with the prepared hydrophobic particles shows potential as a membrane to separate oil and water due to hydrophobic behavior (equilibrium water contact angle ~ 145°) and superoleophilicity. The hydrophobic nature of the fabric is varied only slightly after immersing the fabric in 1 M of acidic and basic solution for seven days and thus ensured the chemical stability of the fabric, whereas the physical stability examined via abrasion test elucidates minimal change in wettability even after 50 cycles of abrasion tests. The oil–water separation efficiency of the membrane made from particle-coated cotton fabric is found to be greater than 95% even after 50 cycles of usage for various oils. Further, these particles are used to cover aqueous droplets to create liquid marbles. The stability of the liquid marble is investigated against solution with different surface tension, pH values, and viscosity. The particles demonstrate the potential to create stable liquid marbles even when the surface tension of liquid is as low as ~ 32 mN m−1. In addition, the liquid marbles are created from aqueous solutions having pH values ranging from 1 to 14 and with liquids having viscosity over a wide range. The substrate temperature-dependent evaporation studies of the liquid marbles unambiguously elucidate the porous nature of the surface and manifest a temperature-dependent liquid marble height decrease in evaporation time scale.Graphical

Solar evaporation of liquid marbles with tunable nanowire array

Solar interfacial evaporation is a potential and sustainable desalination technology. The structural design of photothermal materials plays an important role in improving the evaporation performance. Here, we propose a new evaporating structure consisting of liquid marble with tunable nanowire array to enhance solar evaporation. Such magnetically responsive nanowire arrays are enabled by controllable assembly of nanoparticles over the surface of liquid marble. Different from the traditional structure, the nanowire array increases light absorption through multiple reflection and to promote steam generation. Besides, the narrow space and local aggregation of particles within the nanowire array hinder heat diffusion to enhance evaporation efficiency. A series of experiments are conducted to validate the concept. The liquid marble with upward nanowire array (ULM) shows outstanding performance with the highest evaporation rate of 8.7 μg/s. This rate is 1.21, 1.04 and 1.04 times larger than that of liquid marble with nanoparticles (LM), left-tilt nanowire array (LLM) and right-tilt nanowire array (RLM). Numerical analysis further illustrates that the nanowire array concentrates the electromagnetic field and heat over the evaporative interface. The induced thermal-boundary layer with high-temperature thus improves the evaporation. This work provides a new concept of solar interfacial evaporation with tunable nanowire array, which has far-reaching implications in seawater desalination or wastewater treatment.

Advancing Inorganic Microcapsule Fabrication through Frozen-Assisted Interfacial Reactions Utilizing Liquid Marbles.

Inorganic microcapsules (IMs) have gained significant attention as versatile platforms for delivering functional agents in various fields. Traditional template-dependent methods employing hard templates often involve complex and harsh template removal processes. Achieving IMs with diverse composition and structure remains challenging with current preparation strategies. Therefore, in this work, we have for the first time demonstrated an extremely facile and efficient liquid-marbles-based template approach for fabricating pure inorganic microcapsules via interfacial reaction in a mild aqueous solution. The water-water reaction interface is created by changing the wettability of the liquid marble (LM) surface through the icing-melting process. The composition and function of the inorganic shell could be easily adjusted by varying the inorganic reagent species of the interfacial reaction, the hydrophobic particle of the shell, and the reaction environment according to the specific requirements of the application field. Such an approach provides a flexible platform for material preparation.

The movement pattern of particles on the surface of liquid marble during rupture based on the DIC algorithm

The stability of liquid marbles is critical to their application in sensors, microfluidic control, microreactors, and other fields. A more comprehensive understanding of the rupture kinetics of liquid marble is instructive for studies in improving the stability of liquid marble. However, the mechanism of rupture of liquid marbles is not clear at present, therefore, this paper addressed this issue by calculating the displacement of particles on the surface of a liquid marble using the DIC (Digital Image Correlation) algorithm. The distribution characteristics of the particles on the surface of the liquid marble and their movement patterns were clarified, the causes of cracks on the marble surface were analyzed, the mechanism of liquid marble rupture was derived, and the effect of volume on rupture kinetics was analyzed. Based on the obtained data, a method was proposed to control the maximum compression that the liquid marble can withstand by spraying different sizes of superhydrophobic coatings. This paper contributes to the study of the stability of liquid marble, allowing its wider application in industrial production.

Tunable particle shells of thermo-responsive liquid marbles under alternating magnetic field

Liquid marbles, droplets entrapped in a solid particle shell at the air–liquid interface, can respond to stimuli such as alternating magnetic fields. In this study, we examined liquid marbles composed of the ferrofluid core covered by diverse particles, including thermo-responsive polymer microspheres. The temperature elevation induced by an alternating magnetic field provided the particle surface of the marbles with enhanced properties and broadened the functionality of the entire structure. The thermal response partially rigidified the particle shell, and despite temperature-induced evaporation of the liquid core, the residual shell could be easily refilled with an intended cargo, e.g., an additional portion of ferrofluid or an antibiotic suspension. Due to the presence of magnetic particles inside the droplets, liquid marbles were also responsive to static magnetic fields, enabling control over their position and spatial orientation using magnets. We also obtained patchy and Janus structures employing magneto-coalescence of two or more liquid marbles, underscoring the innovative use of alternating magnetic fields to modulate liquid marble properties potentially serving as delivery agents in various applications.

An investigation of drum granulation of post‐pyrolysis washed biochar

AbstractBiochar, unwashed and washed with a solution of Triton and hydrogen peroxide, was wet drum granulated using molasses binder solutions. Unwashed biochar was very hydrophobic and granulation proceeded through forming liquid marbles and layering. Washing reduced the hydrophobicity of the biochar. The effectiveness of the wash depended on the biochar source; it significantly reduced the hydrophobicity of biochar from woodchips and moderately reduced the hydrophobicity of biochar from flower digestate. Therefore, washed biochar from woodchips was granulated using a hydrophilic mechanism, while washed biochar from digestate was granulated according to a combination mechanism of liquid marbles collapsing and then coalescing. The change in granulation mechanism produced stronger and denser granules with higher yields of granules in the 1–4 mm optimal size range. Washing and then granulating biochar created a product that could be further tailored for optimal soil amendment.

Two modes of contact‐time reduction in the impact of particle‐coated droplets on superhydrophobic surfaces

AbstractReducing the contact time during droplet impact is essential for many scientific and industrial applications, such as self‐cleaning, anti‐icing, heat transfer, and condensation. This paper reports contact‐time reduction by coating droplets with micro–nano hydrophobic particles. Such particle‐coated droplets are known as liquid marbles (LM). LM impact on superhydrophobic surfaces reveals two different modes of contact‐time reduction. For lower impact energies, the reduced adhesion of LM with the surface is responsible for a reduction of up to 21%. Contact‐time reduction in this regime is found to be independent of particle size but dependent on the solid fraction of LM. However, a fragmentation‐based contact‐time reduction is observed for larger particle sizes and higher impact energies. Here, the reduction is as high as 65%. Such fragmentation occurs because the spreading LM lamella breaks when its thickness becomes similar to particle dimensions. Our findings reveal the potential of LM as a novel approach to reduce contact time during droplet impact, with implications for various scientific and industrial applications.

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

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

Ultraviolet‐Light–Triggered Coalescence of Liquid Marbles for Multistep Microreactions

AbstractLiquid marbles show promising potential for application in the microreaction field. The efficient and precise approach for the remote coalescence of liquid marbles is desirable. Herein, the ultraviolet‐light–induced wettability transition of TiO2 nanoparticles is exploited to develop an ingenious approach for efficient and controlled coalesce of contacting liquid marbles containing separate reagents. This approach is generic and provides ideas for the on‐demand initiation of multistep microreactions inside liquid marbles.

Controllable pore size of super-hydrophobic magnetic core-shell nanospheres with dendritic architecture and their pore-dependent performances in oil/water separation

The fundamental properties of porous materials strongly rely on their pore sizes. Pore-size-dependent performances play significant roles in their applications like catalysis, adsorption, delivery, and so forth. In this work, a feasible pore-expanding approach has been successfully explored to control the pore size of magnetic core–shell nanospheres with dendritic architecture (Fe3O4@DMSNs). The size can be adjusted from ca. 10 nm to 25 nm and further to 60 nm in an unprecedented wide range. Further, a concise modification route has been adopted to endow Fe3O4@DMSNs particles with super-hydrophobic abilities by the usage of a long-chain silane (Fe3O4@DMSNs-OTES). The as-prepared nanospheres with magnetic response function can not only form magnetic liquid marbles (MLMs) easily, but also separate n-hexane in oil–water mixtures or oil-in-water emulsions rapidly and efficiently. However, the above three particles display certain differences in their basic properties (like pore volume) and application performances (like oil removal efficiency). Most importantly, Fe3O4@DMSNs-OTES with 60 nm pores exhibit the best super-hydrophobicity in contact angles of H2O and MLMs, Fe3O4@DMSNs-OTES with 25 nm pores possess the maximum adsorption capacity, while the ones with 10 nm pores show the quickest adsorption rate (removal efficiency). The relationship between the performances and the pore size has been thoroughly revealed and well deciphered by wettability theory (e.g., Cassie-Baxter equation) and surface tension theory. In a word, all Fe3O4@DMSNs-OTES hold promising potentials as candidates for the practical wastewater purification on the basis of their characteristics.