Magnetic Liquid Marbles Research Articles

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.

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.

Magnetically Driven Manipulation of Nonmagnetic Liquid Marbles: Billiards with Liquid Marbles.

Liquid marbles are droplets encapsulated by a layer of hydrophobic nanoparticles and have been extensively employed in digital microfluidics and lab-on-a-chip systems in recent years. In this study, magnetic liquid marbles were used to manipulate nonmagnetic liquid marbles. To achieve this purpose, a ferrofluid liquid marble (FLM) was employed and attracted toward an electromagnet, resulting in an impulse to a water liquid marble (WLM) on its way to the electromagnet. It was observed that the manipulation of the WLM by the FLM was similar to the collision of billiard balls except that the liquid marbles exhibited an inelastic collision. Taking the FLM as the projectile ball and the WLM as the other target balls, one can adjust the displacement and direction of the WLM precisely, similar to an expert billiard player. Firstly, the WLM displacement can be adjusted by altering the liquid marble volumes, the initial distances from the electromagnet, and the coil current. Secondly, the WLM direction can be adjusted by changing the position of the WLM relative to the connecting line between the FLM center and the electromagnet. Results show that when the FLM or WLM volume increases by five times, the WLM shooting distance approximately increases by 200% and decreases by 75%, respectively.

Fabrication of robust superhydrophobic magnetic multifunctional coatings and liquid marbles

To obtain durable and multi-function superhydrophobic surfaces, we reported a facial method to prepare a multifunctional suspension (γ-Fe2O3@SiO2@PDMS suspension) named as FSP suspension, in which γ-Fe2O3 was coated by the silica shell and PDMS was used as the outer layer. Superhydrophobic magnetic polyurethane (SMPU) sponge was prepared by immersing the polyurethane (PU) sponge into the FSP suspension, exhibiting the superior ability to absorb oil. In addition, it could also move directionally by the attraction of magnets and absorb the oil along the fixed path. The heated superhydrophobic magnetic stainless steel (H-SMSS) mesh was acquired by spraying FSP suspension onto the stainless steel mesh and then heating at 400°C, which demonstrated superior superhydrophobicity and resistance to abrasion and chemical corrosion. Besides, the H-SMSS mesh displayed excellent flux and efficiency to separate the oil/water mixture. Rolling droplets on FSP particles, the superhydrophobic magnetic liquid marbles (SMLMs) were fabricated, in which liquids with different volumes were encapsulated and transported directionally. Further, it was convenient to inject liquid into the SMLM and withdraw liquid from it. Thus, the prepared FSP suspension has promising applications in constructing large-area, robust, and multifunctional surfaces and microreactors.

Multifunctional liquid marbles to stabilize and transport reactive fluids.

The self-organized transport and delivery of reactive liquids without spillage or loss of activity have been among the most daunting challenges for a long time. In this direction, we employ the concept of forming "liquid marbles" (LMs) to encapsulate and transport reactive hydrogen peroxide (H2O2) coated with functional microparticles. For example, peroxide marbles coated with a toner ink display remote-controlled magnetotactic movement inside a fluidic medium, thus overcoming the weaknesses associated with use of the bare droplets. Interestingly, in such a scenario, the coating of the marbles could also be removed or reformed by bringing the magnet towards or away from the marble. In this way, this process could ensure an on-demand remotely guided coating on the peroxide droplet or its removal. The liquid marbles carrying peroxide solutions are found to preserve the activity of the peroxide and exhibit a low evaporation rate compared with the uncoated peroxide fuel. Interestingly, oil droplets floating on the water could be recovered by introducing the armoured LMs into water under magnetic guidance. Further, the functionalized marbles could be employed as suicide bags for the on-demand delivery of reactive materials in targeted locations. Preliminary research on the antibacterial activity of such liquid marbles has proven to be effective in bacterial killing, which may create new avenues for emerging antibacterial and antibiofilm applications. Finally, such functionalized LMs have been employed to investigate the effects of surface charge on attachment of recombinant Escherichia coli bacteria expressing green fluorescent protein and monitoring the real-time imaging of bacterial death attached to the marble surface.

Magnetic Field-Induced Recoverable Dynamic Morphological Change of Gallium-Based Liquid Metal

We report magnetic field-driven dynamic morphological change of a liquid metal droplet: deformation and recovery; separation and merge. Low melting, non-toxic gallium-based liquid metal has been widely investigated due to its material properties and thus utilized to various applications. Morphological change of the liquid metal in required to tune characteristics of application. In order to enable the liquid metal to be non-wettable and magnetically controllable, we coated the surface with ferromagnetic iron (Fe) particles using hydrochloric acid (HCl) solution resulting in a magnetic liquid metal marble. With and without magnetic field, Fe particles-coated magnetic liquid metal marble was deformed and recovered readily. Against $\sim 8~\mu \text{L}$ magnetic liquid metal marble, depending on the coated Fe particles mass, the shape deformation under magnetic field was changed: conventional (Fe $\geqq ~4$ mg) wetting shape. Accordingly, the deformability and recovery time of the magnetic liquid metal marble were varied. With an applied magnetic field, one marble was readily separated into 2, 3, and 4 marbles. Due to re-oxidation of the magnetic liquid metal marble surface, we treated the marble with HCl solution and vapor. Split and merge occurred depending on treatment time of HCl solution or vapor. Finally, we demonstrated magnetic field-based dynamic behavior which is reversible. [2020-0218]

One-Pot Preparation of Fluorine-Free Magnetic Superhydrophobic Particles for Controllable Liquid Marbles and Robust Multifunctional Coatings.

In this paper, magnetic superhydrophobic particles were prepared by simultaneously coating silica microspheres and modifying 1,1,1,3,3,3-hexamethyl disilazane (HMDS) around the ferric oxide nanoparticles via a one-pot sol-gel process. The effect of the molar ratio of tetraethyl orthosilicate (TEOS) to HMDS on the wettability of superhydrophobic particles (Fe3O4@SiO2/HMDS) was investigated. Various stable liquid marble encapsulated solvents with different surface tensions, pH values, volumes, and temperatures could be obtained by simply rolling them on superhydrophobic particles. The magnetic liquid marbles could be directional transported and fixed-point volatilized. Furthermore, superhydrophobic particles were sprayed onto different surfaces using polydimethylsiloxane (PDMS) as the binder to construct organic-inorganic composite multifunctional coatings by a one-step process. By optimizing the content of Fe3O4@SiO2/HMDS and PDMS in the spraying solution, the prepared coatings showed superior superhydrophobicity with contact angles of larger than 150° and sliding angles of smaller than 10°. The coated fluorine-free fabric possessed excellent air permeability, tensile strength, and hydrostatic pressure resistance, thus fulfilling the practical wearable requirements. Besides, the prepared fabrics maintained stable water repellency even after withstanding mechanical damages or long-term exposure to severe environments. Moreover, the coated superhydrophobic materials could be applied for the on-demand separation of various oil/water mixtures. In addition, the superhydrophobic fabric presented excellent photothermal conversion performances, showing outstanding anti-icing and accelerated deicing properties. Thus, the prepared nonfluorinated and stable magnetic particles offer potential in the areas of controlled encapsulation and directional delivery and, as building blocks, are promising for the construction of robust, large-area, and multifunctional self-cleaning surfaces.

NIR light-steered magnetic liquid marbles with switchable positive/negative phototaxis

This work has demonstrated a magnetic liquid marble of switchable positive/negative phototaxis and excellent mechanical stability by encapsulating water droplet with oleic acid modified Fe3O4 nanoparticles (Fe3O4@OA NPs). The as-constructed marble can reach a height up to 4.7 mm, indicating an effective surface tension of ∼54 mN m−1. The marble on water surface can be steered by NIR light as well as magnetic field due to the photothermal effect and superparamagnetic feature of the Fe3O4@OA NPs. Its light-steered motion stems from the photothermal effect-induced Marangoni flow with speed controlled by the incident angles and light intensities. More interestingly, the marble can alter the motion direction from toward (positive phototaxis) to away from (negative phototaxis) NIR light source by simply changing the light irradiation position from upper to lower semi-marbles due to the unique light absorbance/transmittance characteristics of the Fe3O4@OA NPs. This is dramatically different from previously reported light-driven active matters that only show single phototaxis. This finding provides an advanced strategy to remotely steer magnetic liquid marbles, which may stimulate the exploitation of their application in chemistry and biology fields at microscale, and enlightens a method for phototactic regulation of active matters.

Electromagnet polarity dependent reversible dynamic behavior of magnetic liquid metal marble

We report reversible dynamic behavior of a magnetic liquid metal marble by changing magnetic field polarity. We coated iron (Fe) particles on the surface of the oxidized liquid metal by HCl-based chemical reaction to make it to be magnetized and to be non-wettable. We utilized electromagnets and changed current direction on-demand to change the magnetic polarity of the electromagnet. In 1-dimension (1-D), with an applied magnetic field of reverse polarity from the bottom, the conventional wetting shape of the magnetic liquid metal marble was observed, whereas the split was occurred with the applied magnetic field of same polarity due to attractive or repulsive force. With an applied magnetic field of reverse polarity, the split marbles was merged. By removing the magnetic field, the deformed magnetic liquid metal marble was recovered. The applied current dependent deformability and recoverability, and split and merge time were investigated. In addition, in 2-dimension (2-D), we observed various deformations and split behavior of the magnetic liquid metal marble with various magnetic field polarity combinations by utilizing electromagnets, which is reversible.

Magnetic liquid marbles to facilitate rapid manipulation of the oil phase: Synergistic effect of semifluorinated ligand and catanionic surfactant mixtures

Stimuli-responsive liquid marbles can serve as reservoirs for encapsulating and carrying different materials, which could be used on demand for several applications. Here, enhanced spreading behavior of catanionic mixtures with magnetic liquid marbles were combined and a new approach to the phase separation, actuation and manipulation of liquids that could be important in many micro/macroscale operations is proposed. The magnetic liquid marbles containing catanionic surfactant mixture are prepared using superparamagnetic iron oxide nanoparticles (SPIONs) decorated by non-covalently-bonded semifluorinated alcohol, which is used for a specific purpose that has never been used before. Besides its ability to increase the hydrophobicity of NPs, the autooscillation behavior of the fluorinated alcohol maintains the fast rupture of the marble and provides the convective flow that supports the rapid transformation of the oil phase into lens form which contributes to obtain a better phase separation. Semifluorinated ligand helps the nanoparticles to integrate into oil phase, allowing the actuation of oil droplets via a magnetic field for a target-directed control of phases. With this system, an innovative model that stands at the intersection of several fundamental concepts of colloid and interface science as well as materials science and engineering is proposed.

Fabrication of magnetic liquid marbles using superhydrophobic atmospheric pressure plasma jet-formed fluorinated silica nanocomposites

In this study, the surface properties of iron microparticles were modified for the manipulation of liquid droplets using atmospheric pressure plasma jets. These modified hydrophobic iron microparticles were prepared by synthesizing fluorinated silica nanocomposites on the surfaces of iron microparticles under atmospheric pressure plasma. The compositions of the silica nanocomposites were controlled by the deposition of hexamethyldisiloxane and fluoroalkylsilane precursors. The fluorinated silica nanocomposites were then used with iron microparticles to prepare magnetic liquid marbles. The contact angles of the iron microparticles and the fluorinated silica nanoparticle coating on the glass surface were both 154°, which indicated that the surfaces of these particles were superhydrophobic. Higher hexamethyldisiloxane precursor flow rates produced more silica nanocomposites and resulted in greater roughness and larger contact angles. Changes in surface roughness were characterized by atomic force microscopy. X-ray photoelectron spectroscopy showed that C–F bonds were present on the modified glass surface. The presented approach allows rapid and highly efficient modification of uneven surfaces and can therefore be employed to render hydrophilic, superhydrophobic, and oleophilic surfaces. Moreover, the described hydrophobic iron microparticles can be used for the controlled magnetic manipulation of water droplets and oil–water separation.

A magnet-actuated biomimetic device for isolating biological entities in microwells

Microwell platforms show great promise in single-cell studies and protein measurements because of their low volume sampling, rapid analysis and high throughput screening ability. However, the existing actuation mechanisms to manipulate the target samples and fabrication procedures involved in the microwell-based microfluidic devices are complex, resource-intensive and require an external power source. In this work, we present proof of concept of a simple, power-free and low-cost closed magnet digital microfluidics device for isolating biological entities in femtoliter-sized microwells. The target biological entities were encapsulated in magnetic liquid marbles and shuttled back and forth between micropatterned top and bottom plates in the microdevice to obtain high loading efficiency and short processing time. The microdevice performance was studied through fluorescent detection of three different entities: microbeads, bovine serum albumin (BSA) and Escherichia coli, captured in the microwell array. Almost 80% of the microwells were loaded with single microbeads in five shuttling cycles, in less than a minute. Further, a low volume of BSA was compartmentalized in the microwell array over a two order range of concentration. The microdevice exhibits two unique features: lotus leaf stamps were used to fabricate micropatterns (microwells and micropillars) on top and bottom plates to impart functionality and cost-effectiveness, and the target samples were actuated by a permanent magnet to make the microdevice power-free and simple in operation. The developed biomimetic microdevice is therefore capable of capturing a multitude of biological entities in low-resource settings.

Rolling performance of liquid marbles enhanced by optimizing particles as digital microfluidics actuators

This study investigated the use of liquid marbles to transport small volumes of water as digital microfluidics actuators by rolling the marbles down an inclined substrate. Hydrophobic magnetic particles were chosen as the encapsulating material because they are readily separated from the inner liquid using a magnet. The rolling angles at which the marbles on an inclined surface began to move, as well as the marble rolling characteristics, were assessed for various particle sizes and materials. Magnetic liquid marbles from 0.6 (0.1 μl) to 4.5 mm in diameter started rolling at small incline angles from 1.5 to 15° on Si and glass surfaces. It was revealed that the rolling angles and the rolling accelerations were highly influenced by the size of particles and the fine magnetic particles, approximately 10 μm in diameter, exhibited excellent rolling performances. Friction on rolling angle, rolling resistance, and rolling condition with slip of liquid marbles were also investigated in terms of particles and a design guideline on optimizing a particle size and an inclination angle (using fine particles and a small inclination angle) was presented. The excellent performance of the fine magnetic particles was related to high mobility of the particles which lowered the rolling resistance. The high mobility was probably derived from high cohesiveness of the particles based on observations of the structure of particles.

A novel numerical model to predict the morphological behavior of magnetic liquid marbles using coarse grained molecular dynamics concepts

Liquid marbles are liquid droplets coated with superhydrophobic powders whose morphology is governed by the gravitational and surface tension forces. Small liquid marbles take spherical shapes, while larger liquid marbles exhibit puddle shapes due to the dominance of gravitational forces. Liquid marbles coated with hydrophobic magnetic powders respond to an external magnetic field. This unique feature of magnetic liquid marbles is very attractive for digital microfluidics and drug delivery systems. Several experimental studies have reported the behavior of the liquid marbles. However, the complete behavior of liquid marbles under various environmental conditions is yet to be understood. Modeling techniques can be used to predict the properties and the behavior of the liquid marbles effectively and efficiently. A robust liquid marble model will inspire new experiments and provide new insights. This paper presents a novel numerical modeling technique to predict the morphology of magnetic liquid marbles based on coarse grained molecular dynamics concepts. The proposed model is employed to predict the changes in height of a magnetic liquid marble against its width and compared with the experimental data. The model predictions agree well with the experimental findings. Subsequently, the relationship between the morphology of a liquid marble with the properties of the liquid is investigated. Furthermore, the developed model is capable of simulating the reversible process of opening and closing of the magnetic liquid marble under the action of a magnetic force. The scaling analysis shows that the model predictions are consistent with the scaling laws. Finally, the proposed model is used to assess the compressibility of the liquid marbles. The proposed modeling approach has the potential to be a powerful tool to predict the behavior of magnetic liquid marbles serving as bioreactors.

Magnetic digital microfluidics - a review.

A digital microfluidic platform manipulates droplets on an open surface. Magnetic digital microfluidics utilizes magnetic forces for actuation and offers unique advantages compared to other digital microfluidic platforms. First, the magnetic particles used in magnetic digital microfluidics have multiple functions. In addition to serving as actuators, they also provide a functional solid substrate for molecule binding, which enables a wide range of applications in molecular diagnostics and immunodiagnostics. Second, magnetic digital microfluidics can be manually operated in a "power-free" manner, which allows for operation in low-resource environments for point-of-care diagnostics where even batteries are considered a luxury item. This review covers research areas related to magnetic digital microfluidics. This paper first summarizes the current development of magnetic digital microfluidics. Various methods of droplet manipulation using magnetic forces are discussed, ranging from conventional magnetic particle-based actuation to the recent development of ferrofluids and magnetic liquid marbles. This paper also discusses several new approaches that use magnetically controlled flexible substrates for droplet manipulation. In addition, we emphasize applications of magnetic digital microfluidics in biosensing and medical diagnostics, and identify the current limitations of magnetic digital microfluidics. We provide a perspective on possible solutions to close these gaps. Finally, the paper discusses the future improvement of magnetic digital microfluidics to explore potential new research directions.

Liquid Marbles Based on Magnetic Upconversion Nanoparticles as Magnetically and Optically Responsive Miniature Reactors for Photocatalysis and Photodynamic Therapy.

Magnetic liquid marbles have recently attracted extensive attention for various potential applications. However, conventional liquid marbles based on iron oxide nanoparticles are opaque and inadequate for photo-related applications. Herein, we report the first development of liquid marbles coated with magnetic lanthanide-doped upconversion nanoparticles (UCNPs) that can convert near-infrared light into visible light. Apart from their excellent magnetic and mechanical properties, which are attractive for repeatable tip opening and magnetically directed movements, the resultant UCNP-based liquid marbles can act as ideal miniature reactors for photodynamic therapy of cancer cells. This work opens new ways for the development of liquid marbles, and shows great promise for liquid marbles based on UCNPs to be used in a large variety of potential applications, such as photodynamic therapy for accelerated drug screening, magnetically guided controlled drug delivery and release, and multifunctional actuation.

Palygorskite@Fe3O4@polyperfluoroalkylsilane nanocomposites for superoleophobic coatings and magnetic liquid marbles

Magnetic palygorskite@Fe3O4@polyperfluoroalkylsilane nanocomposites were prepared, which can be used for fabricating superoleophobic coatings and magnetic liquid marbles.

Nanoparticles: Bioinspired Superhydrophobic Fe3O4@Polydopamine@Ag Hybrid Nanoparticles for Liquid Marble and Oil Spill (Adv. Mater. Interfaces 13/2015)

J. H. Xin, L. Zhang, and co-workers demonstrate in article 1500234 that the combination of mussel-inspired and thiol surface chemistry can result in stable hierarchical nanoparticles with excellent superparamagnetic, superhydrophobic, and antibacterial properties. The as-prepared nanoparticles cannot only directly serve as oil removing materials, but also be used to form magnetic liquid marbles for liquid transportation and other derived purposes.

From polymer latexes to multifunctional liquid marbles.

A simple method to prepare multifunctional liquid marbles and dry water with magnetic, color, and fluorescent properties is presented. Multifunctional liquid marbles were prepared by encapsulation of water droplets using flocculated polymer latexes. First, the emulsion polymerization reaction of polystyrene and poly(benzyl methacrylate) was carried out using cheap and commercially available cationic surfactants. Subsequently, flocculation of the latex was provoked by an anion-exchange reaction of the cationic surfactant by the addition of lithium bis(trifluoromethanesulfonyl)imide salt. The flocculated polymer latex was filtered and dried, leading to very hydrophobic micronanoparticulated powders. These powders showed a great ability to stabilize the air/water interface. Stable liquid marbles were obtained by rolling water droplets onto the hydrophobic powders previously prepared. The use of very small polystyrene nanoparticles led us to the preparation of very stable and the biggest known liquid marbles up to 2.5 mL of water. Furthermore, the introduction of fluorescent comonomer dyes into the polymer powders allowed us to obtain new morphological images and new knowledge about the structure of liquid marbles by confocal microscopy. Furthermore, the introduction of magnetic nanoparticles into the polymer latex led to magnetic responsive liquid marbles, where the iron oxide nanoparticles are protected within a polymer. Altogether this method represents an accessible and general platform for the preparation of multifunctional liquid marbles and dry water, which may contribute to extending of their actual range of applications.

Surface Functionalization and Magnetic Motion of Hydrophobic Magnetic Nanoparticles with Different Sizes

Abstract A facile and general strategy was successfully developed for the surface modification of hydrophobic Fe3O4 magnetic nanoparticles with various sizes (4–17 nm). The results show that the magnetic and hydrophobic properties are sensitive to the nanoparticle size. For example, the contact angle (CA) of the sample increases as the particle size increases. Using these surface modification techniques allowed the coating of water droplets with highly hydrophobic Fe3O4 nanoparticles to form magnetic liquid marbles through a mechanical method. The behavior of these liquid marbles under the action of a magnetic field revealed that their potential value in electronic, biomedical, self-cleaning, and biochemical applications.