Liquid marbles: review of recent progress in physical properties, formation techniques, and lab-in-a-marble applications in microreactors and biosensors

Solar evaporation of liquid marbles with Fe3O4/CNT hybrid nanostructures

HYPOTHESIS

Through the rational design of nanomaterial composites, broadband light harvesting and good thermal insulation can be achieved simultaneously to improve the efficiency of water evaporation.

EXPERIMENT

Solar evaporation experiments were carried out on liquid marbles (LMs) coated with Fe3O4 nanoparticles, carbon nanotubes (CNTs) and hybrid nanomaterials (Fe3O4/CNTs) with different mass ratios of 2:1, 1:1 and 1:2.

FINDING

The results showed that the mixture of Fe3O4/CNTs enhances the light harvesting ability and solar interfacial evaporation performance. Fe3O4/CNT-LM at the mass ratio of 2:1 case provides the highest evaporation rate of 11.03 μg/s, which is about 1.22 and 1.34 times higher than that of Fe3O4 and CNT, respectively. This high performance is mainly due to the synergistic effect between Fe3O4 nanoparticles and CNTs, as the hybrid nanostructure significantly improves the both photothermal conversion and heat localization capability. Numerical simulation further supports that the composite can concentrate the electromagnetic field and heat at the phase-change interface. This leads to a rapid evaporation of the boundary region. This study provides a novel approach to a three-dimensional interface by assembling nanomaterials on the drop surface to enhance evaporation, which may have far-reaching implications for seawater desalination.

Using Hybrid Coating to Fabricate Highly Stable and Expandable Transparent Liquid Marbles

Liquid marbles (LMs) are microliter-scale droplets coated with hydrophobic solid particles. The particle size and hydrophobicity of the surface coating determine their properties, such as transparency, expandability, and resistance to evaporation and coalescence, one or more of which can be critical to their application as microreactors. This study reports the use of a mixture of two different hydrophobic powders for fabrication of LMs for colorimetric assays: trichloro(1H,1H,2H,2H-perfluorooctyl) silane-linked silica gel (modified silica gel (MSG), particle size: 40-75 μm) and hexamethyldisilazane-linked fumed silica (modified fumed silica (MFS), average aggregate length: 200-300 nm). The hybrid coating mixture (MIX) prepared by mixing these MSG and MFS powders in a ratio of 3:7 (w/w), respectively, contained particles of different sizes as well as different hydrophobicity as the silane linked to MSG is more hydrophobic than the one linked to MFS. LMs fabricated using MIX as the surface coating were characterized and compared to LMs coated with MSG or MFS alone. It was observed that MIX LMs were comparable to the MFS LMs in transparency (higher than the MSG LMs), expandability (more than 20 times their initial volume), and stability against evaporation (for more than 4 h at 78% relative humidity at 26 °C). However, in terms of resistance to coalescence, the MIX LMs showed a resistance comparable to that of MSG LMs, much higher than that of MFS LMs. Further experiments demonstrated that it is the presence of the particles of different sizes (MSG particles are ∼100 times larger than MFS) that improves the resistance to coalescence rather than the higher hydrophobicity of the MSG. Three different colorimetric assays were performed in the MIX LMs, and the results obtained were comparable in accuracy and precision to those obtained in a standard polystyrene microwell plate system. Low quantities of the analytes could be detected and quantified, as evidenced by the limit of detection (alkaline phosphatase (AP): 0.18 μg/mL; bovine serum albumin (BSA): 2.28 μg/mL; and chymotrypsin: 3.69 μM) and limit of quantification (AP: 0.59 μg/mL; BSA: 12.29 μg/mL; and chymotrypsin: 7.59 μM) values. Color intensities in LMs were quantified using a smartphone application, which provides the added benefit of an instrument-free approach. These findings highlight the potential of using LMs stabilized with mixtures of nano- and microparticles as robust, versatile microreactors for portable and sensitive colorimetric assays, paving the way for more accessible and efficient diagnostic tools.

Non-Aqueous Polyhedral Liquid Marbles Stabilized with Polymer Plates Having Surface Roughness

Hydrophobic polymer plates with smooth and rough surfaces are used as a stabilizer for cubic liquid marbles (LMs) to study the effect of surface roughness on their formation. The smooth and rough polymer plates can stabilize LMs using liquids with surface tensions of 72.8-26.6 and 72.8-22.9 mN m-1, respectively. It is clarified that the higher the surface roughness, the lower the surface tension of the liquids are stabilized to form the LMs. These results indicated that the introduction of surface roughness improves the hydrophobicity of the polymer plates and the rough polymer plates can stabilize LMs using liquids with a wider surface tension range. Electron microscopy studies and numerical analyses confirmed that the LMs can be formed, when the Cassie-Baxter wetting state, where θY>90° (θY: the contact angle on smooth surfaces) and θR>90° (θR: the contact angle on rough surfaces), and the metastable Cassie-Baxter wetting state, where θY<90° and θR>90°, are realized. Finally, the synthesis of cubic polymer particles are succeeded by free radical polymerization of the cubic LMs containing a hydrophobic vinyl monomer (dodecyl acrylate) in a solvent-free manner.

Stimulus-Responsive Gas Marbles as an Amphibious Carrier for Gaseous Materials

Gas marbles are a new family of particle-stabilized soft dispersed system with a soap bubble-like air-in-water-in-air structure. Herein, stimulus-responsive character is successfully introduced to a gas marble system for the first time using polymer particles carrying a poly(tertiary amine methacrylate) (pKa ≈7) steric stabilizer on their surfaces as a particulate stabilizer. The gas marbles exhibited long-term stability when transferred onto the planar surface of liquid water, provided that the solution pH of the subphase is basic and neutral. In contrast, the use of acidic solutions led to immediate disintegration of the gas marbles, resulting in release of the inner gas. The critical minimum solution pH required for long-term gas marble stability correlates closely with the known pKa value for the poly(tertiary amine methacrylate) stabilizer. It also demonstrates amphibious motions of the gas marbles.

pH- and Photoresponsive Liquid Plasticine

Liquid marbles (LMs) can be prepared by adsorption of hydrophobic particles at the air-liquid interface of a water droplet. LMs have been studied for their application as microreaction vessels. However, their opaqueness poses challenges for internal observation. Liquid plasticines (LPs), akin to LMs, can be prepared by the adsorption of hydrophobic particles with a diameter of 50 nm or less, at the air-liquid interface of a water droplet. Unlike LMs, LPs are transparent, allowing for internal observation, thus presenting promising applications as reactors and culture vessels on a microliter scale. In this study, the surface of silica particles, approximately 20 nm in diameter, was rendered hydrophobic to prepare hydrophobic silica particles (SD0). A small amount of poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) was then grafted onto the surface of SD0, yielding SD1. SD0 particles exhibited consistent hydrophobicity irrespective of the environmental pH atmosphere. Under acidic conditions, SD1 became hydrophilic due to the protonation of pendant tertiary amines in the grafted PDPA chains. However, SD1 alone was unsuitable for LP preparation due to its high surface wettability regardless of atmospheric pH, attributable to the presence of PDPA-grafted chains. Therefore, to prepare pH-responsive LP, SD1 and SD0 were mixed (SD1/SD0 = 3/7). Upon exposure to HCl gas, these LPs ruptured, with the leaked water from the LPs being absorbed by adjacent paper. Moreover, clear LPs, prepared using an aqueous solution containing a water-soluble photoacid generator (PAG), disintegrated upon exposure to light as PAG generated acid, leading to LP breakdown. In summary, pH-responsive LPs, capable of disintegration under acidic conditions and upon light irradiation, were successfully prepared in this study.

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.