On the effective surface tension of powder-derived liquid marbles

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.

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

Liquid marbles (LMs) are nonsticking droplets whose surfaces are covered with low-wettability particles. Owing to their high mobility, shape reconfigurability, and widely accessible liquid/particle possibilities, the research on LMs has flourished since 2001. Their physical properties, fabrication mechanisms, and functionalisation capabilities indicate their potential for various applications. This review summarises the fundamental properties of LMs, the recent advances (mainly works published in 2020-2023) in the concept of LMs, physical properties, formation methods, LM-templated material design, and biochemical applications. Finally, the potential development and variations of LMs are discussed.

Exploring the Effects of Liquid Marbles' Deformation on Their Rolling Resistance

Liquid marbles (LMs) are nonwetting droplets manufactured by encapsulating droplets with micro- or nanoscale particles. These marbles are widely used as transport carriers for digital microfluidics due to their rapid displacement velocity and leak-free transport. An improved understanding of the resistance mechanism of rolling LMs is crucial for their transport and manipulation. In this study, we investigated the rolling resistance of LMs obtained with different powders and volumes using a high-speed camera. Our findings suggest that the deformation of liquid marbles would hinder their rolling by a resistance torque. To depict this resistance effect, we propose a theoretical model ( f ∼ λ ( ε - 1 2 Bo 1 / 2 ε 2 + 1 4 Bo ε 3 ) ) , where f is the rolling resistance of marbles, λ is the deflection coefficient, Bo is the Bond number, and (ε is the contact surface deformation) that accurately predicts the relationship between deformation and rolling resistance, which is supported by our experimental results. To further validate our theoretical model, we conducted three independent experiments: shape detection of prepared LMs, measuring the elastic force of LMs, and detecting the diffusive motion of the encapsulating particles. Furthermore, we discuss three factors that affect the rolling resistance: the volume of the marbles, the type and size of the encapsulating particles, and the substrate roughness. This comprehensive study not only generalizes the mechanism of deformation hindering the rolling of liquid marbles but also provides a theoretical framework to predict the relationship between the deformation and rolling resistance. These findings have practical implications for improving the manipulation efficiency and advancing the use of LMs as microfluidic carriers.

Open millifluidics based on powder-encased channels

Millifluidics, the manipulation of liquid flow in millimeter-sized channels, has been a revolutionary concept in chemical processing and engineering. The solid channels that contain the liquids, though, are not flexible in their design and modification, and prevent contact with the external environment. All-liquid constructs, on the other hand, while flexible and open, are imbedded in a liquid environment. Here, we provide a route to circumvent these limitations by encasing the liquids in a hydrophobic powder in air that jams on the surface, containing and isolating flowing fluids, offering flexibility and adaptability in design, as manifest in the ability to reconfigure, graft, and segment the constructs. Along with the open nature of these powder-contained channels that allow arbitrary connections/disconnections and substance addition/extraction, numerous applications can be opened in the biological, chemical, and material arenas.

Suppression of droplet pinch-off by early onset of interfacial instability

HYPOTHESIS

Interfacial instabilities cause undesirable droplet breakage during impact. Such breakage affects many applications, such as printing, spraying, etc. Particle coating over a droplet can significantly change the impact process and stabilize it against breakage. This work investigates the impact dynamics of particle-coated droplets, which mostly remains unexplored.

EXPERIMENTS

Particle-coated droplets of different mass loading were formed using volume addition. The prepared droplets were impacted on superhydrophobic surfaces, and their dynamics were recorded using a high-speed camera.

FINDINGS

We report an intriguing phenomenon where an interfacial fingering instability helps suppress pinch-off in particle-coated droplets. This island of breakage suppression, where the droplet maintains its intactness upon impact, appears in a regime of Weber numbers where bare droplet breakage is inevitable. The onset of fingering instability in particle-coated droplets is observed at much lower impact energy, around two times less than the bare droplet. The instability is characterized and explained using the rim Bond number. The instability suppresses pinch-off because of the higher losses associated with the formation of stable fingers. Such instability can also be seen in dust/pollen-covered surfaces, making it useful in many applications related to cooling, self-cleaning, anti-icing etc.

Preparation of Phase Change Melt Marbles with High Thermal Stability by Spontaneous Shrinkage and Encapsulation

Liquid marbles (LMs) are widely used in the fields of microfluids, gas sensitivity equipment, and microreactors. However, the thermal stability of the encapsulated liquid poses difficulty to the high-temperature stability of LMs. In this study, polar phase-change materials (PCMs) with high melting points were used as the encapsulated liquid of LMs. According to the required temperature, suitable PCMs were selected as the core and encapsulated by hydrophobic SiO₂ particles to form melt marbles (MMs). The types of PCMs used to prepare the MMs include erythritol, elemental sulfur, urea, and molten salts. Based on the premixed melting method, a series of MMs with high melting points and thermal stability were successfully developed. The highest acceptable temperature of the MMs exceeded 323 °C, and the evaporation rate of erythritol MMs was less than 1% at 140 °C in 8 h. Thus, the MMs maintained their excellent stability through multiple phase transitions. In the molten state, the MMs exhibited the properties of bounce ability, cuttability, and deformation resistance. The performance of the PCMs in energy storage and release during phase transition demonstrates their potential applications in the field of heat storage.

Effect of particle size on the stripping dynamics during impact of liquid marbles onto a liquid film

The robust attachment of particles at fluid interfaces is favorable for engineering new materials due to the large capillary energy, but it meets significant challenges when particle removal is a requirement. A previous study has shown that soap films can be utilized to achieve particle separation from liquid marbles. Here, we investigate the effects of particle size on the particle separation from liquid marbles using fast dynamics of drop impact on a soap film. Experimental observations disclose that the fast dynamics of the liquid marble involves coalescence, bouncing, stripping, or tunneling through the film by controlling the falling height and drop volume. More importantly, the active regime of the stripping mode can be selective-controlled by tuning the particle size, and the smaller stabilizing particles make a wider stripping regime. This is attributed to the smaller change of the surface energy resulting from the larger surface tension of LMs wrapped by smaller particles. Theoretical analysis reveals that the stripping thresholds are determined by the energy competition between kinetic energy, the increased surface energy and viscous dissipation, which offers important insights into particle separation by tuning the particle size. The present study provides guidelines for applications that involve phase separation.

Oscillation-Induced Mixing Advances the Functionality of Liquid Marble Microreactors

Droplet-based microreactors often uncover fascinating phenomena and exhibit diverse functionality, which make them applicable in various fields. Liquid marbles (LMs) are non-wetting droplets coated with particles, and these features highlight their potential as microreactors. However, sophisticated experimental designs are typically hindered because it is difficult to obtain sufficient substance mixing in these miniature, damage-prone, self-supporting liquid containers. Here, we demonstrate that subjecting LMs to vertical oscillations by audio signals represents a controllable approach that allows sufficient mixing with variable dynamic modes. The characteristics and key issues in LM oscillation are systematically explored. The effects of oscillation on application potential are examined. Under oscillation conditions, homogeneous mixing can be achieved within a few seconds in LMs consisting of either water or viscous liquids. Importantly, the structures of materials synthesized in LMs can be regulated by modulating the oscillation modes. The variable modes, flexible adjustability, high efficiency, and wide applicability of this oscillation method make it a verified manipulation strategy for advancing the functionality of LM microreactors.

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.

Measuring the effective surface tension of a floating liquid marble using X-ray imaging

A liquid marble (LM) is a droplet coated with microparticles that isolate the liquid interior from its surroundings, making it perfectly non-wetting. This attractive feature allows the LM to perform useful tasks such as coalescence, targeted delivery, and controlled release. The non-wetting characteristic also allows the LM to float on a carrier liquid. The growing number of applications in digital microfluidics requires further insights into the fundamental properties of a LM such as its effective surface tension. Although the coating provides the LM with various desirable characteristics, its random construction presents a major obstacle to accurate optical analysis. This paper presents a novel method to measure the effective surface tension of a floating LM using X-ray imaging and curve fitting procedures. X-ray imaging reveals the true LM liquid-air interface hidden by the coating particles. Analysis of this interface showed that the effective surface tension of a LM is not significantly different from that of its liquid content. This indicates that the particle coating might not have significantly altered the behaviour of the liquid interface. We also found that our method is sensitive enough to detect the variations across individual LMs.

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.

Mechanical robustness of monolayer nanoparticle-covered liquid marbles

Powder-derived liquid marbles (LMs) are versatile nonwetting systems but are confronted with many limitations in application, as their surface particles are usually large and agglomerated. Recently, sol-gel film-derived LMs have come on the scene that are termed monolayer nanoparticle-covered (mNPc) LMs based on their unique characteristics, revealing great application potential but also generating many questions. Here, mechanical robustness, a very important yet to be addressed property, of mNPc LMs was systematically studied. Rolling, pendant contact, and compression experiments were designed using bare and coated glasses with water contact angles (WCAs) ranging from 23° to 157°. With rupture as a quality criteria, the mechanical robustness of mNPc LMs enhanced with the hydrophobicity of solid surfaces that exerted pressure on them, but maintained much weaker than typical powder LMs until the solid surface was superhydrophobic. In particular, when contacting hydrophilic surfaces of WCAs ≤53°, an mNPc LM did not have the capacity for nonwetting and ruptured immediately, even if the pressure approached zero. This was distinct from powder LMs and indicated that a particle shell as thin as ∼20 nm could not prevent intermolecular attractions between the internal liquid and external solid surface. An interface scenario consisting of solid surface microroughness was proposed to address this issue. On the other hand, mNPc LMs remained unruptured on superhydrophobic surfaces but presented degraded elasticity under extreme compression. Uncovering these properties could be of much help for developments of mNPc LMs and their counterparts, the mNPc liquid plasticines.