Liquid Nanodroplets Research Articles

Deposition and splashing characteristics of ionic liquid nanodroplet impacting surfaces in electrospray

Nanodroplet collisions with solid surfaces can occur in various applications of electrospray technology. In electrospray propulsion, the deposition and splashing of droplets from the beam onto surfaces are critical to the performance and lifetime of the electrospray thrusters, but little is known about the underlying mechanisms. Therefore, this paper developed a detailed molecular dynamics model to simulate the collisions of an ionic liquid nanodroplet under varying electric fields and charges. The results show that under a 1000 V acceleration voltage, the droplet dissociates into cations and anions after a high-speed collision with the surface. The deposition characteristics are influenced by the oscillation between cations and anions and the applied electric field. When the electric field strength is low, the oscillation between ions leads to easier deposition of cations, causing neutral droplets to deposit a small amount of positive charge after the collision. Only negatively charged droplets may deposit a neutral or negative charge. When the electric field strength is high, it separates the cations and anions, resulting in significant charge deposition due to the imbalance of ion quantities on the surface. Furthermore, as the charge of the droplet increases, the deposition rate fluctuates, which is related to the momentum exchange caused by the oscillation between ions. This paper reveals the collision mechanism of ionic liquid nanodroplets in an electrospray environment, which may contribute to the further development of droplet–wall collision models in electrospray propulsion or deposition.

Precise synthesis of In–Bi alloy nanodroplets through controlled surface reactions on a Si(111) surface at ambient temperature

Precise synthesis of In–Bi alloy nanodroplets through controlled surface reactions on a Si(111) surface at ambient temperature

Liquid metal nanoparticles as photo-initiators for preparation of transparent hydrogel with adjustable mechanical properties

To achieve rapid preparation of hydrogels without using conventional chemical initiators, a stable suspension of eutectic gallium indium (EGaIn) liquid metal nanoparticles is explored by probe-sonicating the metal in an aqueous solution. Liquid metal suspension was sonicated to serve as a photo-initiator for acrylamide polymerization and produce hydrogels. The initiation effect comes from the fact that liquid metal suspension after sonication can produce a large number of free radicals when exposed to ultraviolet (UV) radiation, leading to initiation. The changes of liquid metal nanodroplets under UV light irradiation have been systematically investigated. Further, the liquid metal colloidal solutions were used to prepare hydrogels with the same transparency and adjustable mechanical properties as the samples initiated by commercial photo-initiators. This work shows the great application potential of liquid metal in the preparation of hydrogels and provides a new technical idea for the design of multifunctional hydrogels.

Size-dependent melting and freezing and thermal expansion of Pb nanoparticles in lead-borate glass

Size-dependent melting and freezing and thermal expansion of Pb nanoparticles in lead-borate glass

Effect of molecular branching and surface wettability on solid-liquid surface tension and line-tension of liquid alkane surface nanodroplets

Surface nanodroplets have important technological applications. Previous experiments and simulations have shown that their contact angle deviates from Young's equation. A modified version of Young's equation considering the three-phase line tension (τ) has been widely used in literature, and a wide range of values for τ are reported. We have recently shown that molecular branching affects the liquid-vapour surface tension γlv of liquid alkanes. Therefore, the wetting behaviour of surface nanodroplets should be affected by molecular branching. This study conducted molecular dynamics (MD) simulations to gain insight into the wetting behaviour of linear and branched alkane nanodroplets on oleophilic and oleophobic surfaces. We aim to examine the Young equation's validity and branching's effect on fundamental properties, including solid-liquid surface tension γsl and line tension τ. The simulations were performed on a linear alkane, triacontane (C30H62), as well as four of its branched isomers: 2,6,13,17-tetrapropyloctadecane,2,6,9,10,13,17-hexaethyloctadecane, 2,5,7,8,11,12,15-heptaethylhexadecane and 2,3,6,7,10,11-hexapropyldodecane. Nanodroplets with a diameter of approximately 15nm were released onto the surfaces, and their contact angles were measured. Additionally, using a novel approach, the solid-liquid surface tension (γsl), the validity of Young's equation and line tension for all alkane and surface combinations are determined. It was discovered that the calculated γsl, deviated from the theoretical γsl,Young predicted from Young's equation for all alkanes on oleophilic surfaces. However, this deviation was minimal for branched alkanes on the oleophobic surfaces but more significant for the linear alkane. The findings indicated that γsl<0 for oleophilic surfaces and γsl>0 for oleophobic surfaces. Moreover, it was observed that |γsl| was lower for branched molecules and decreased as branching increased. Line tension values were then determined through a novel method, showing τ was positive for oleophilic surfaces ranging from 1.30×10-10 to 6.27×10-11N. On an oleophobic surface, linear alkane shows a negative line tension of -1.15×10-10N and branched alkanes up to two orders of magnitude lower values ranging from -2.09×10-12 to 2.43×10-11N. Line tension values between -1.15×10-10 and+1.1×10-10N are calculated for various linear alkane and surface combinations. These findings show the dependence of line tension on the contact angle and branching, demonstrating that for linear alkanes, τ is significant, whereas, for branched alkanes, line tension is smaller or negligible for large contact angles.

Predicting the spontaneous vaporisation of superheated perfluorocarbon droplets

Superheated perfluorocarbon droplets have been widely explored as agents for ultrasound imaging and therapy, as well as for other applications such as radiation dosimetry. Submicrometre, or “nano” droplets offer a number of potential advantages over microbubbles, e.g., longer circulation half-lives, higher surface area-to-volume ratio and the ability to perfuse the microvasculature more easily. A key challenge in the use of nanodroplets, however, is the need to avoid spontaneous vaporisation whilst keeping the energy required for acoustic activation within the range of pressures that can be used safely in humans. This is especially important for imaging applications. Perfluoropropane (C;3F;8) microbubbles can be condensed to form liquid nanodroplets that offer a good trade-off between thermal stability and acoustic vaporisation threshold. Anecdotal reports, however, suggest that C;3F;8 droplets can spontaneously vaporise, and may therefore pose a potential safety risk, especially if bubble coalescence occurs. The aim of this study was to build on recent theoretical models of droplet vaporisation and investigate the probability of vaporisation as a function of temperature and interfacial tension. The results are compared with experimental measurements of vaporisation rates for different droplet formulations.

Laser manufactured-liquid metal nanodroplets intercalated Mxene as oil-based lubricant additives for reducing friction and wear

Laser manufactured-liquid metal nanodroplets intercalated Mxene as oil-based lubricant additives for reducing friction and wear

The fluidic molecular trajectory and the Nano-droplet production ability

This paper studies the liquid Nano-droplet production ability using molecular dynamics simulation methodology. The research parameter is performed at the temperature of 310 Kelvin (K), the pressing force of 10.0 × 10−10 Newton (N) and the ejective hole diameters of 25 and 40 Angstrom (Å). The research result shows that liquid Nano-droplets finally were not produced for the ejective diameter of 25 Å. The Nano-jets were not only non-destruction from nozzle’s surface to produce the droplets but also movement downward to come back the nozzle’s surface. The molecular trajectory is very zigzag and curved both inside and outside the ejective container. In the contrary, when increasing the ejective diameter to 40 Å, the liquid Nano-droplet was produced in the same the ejective time and compressible force magnitude. The molecular trajectory is quite straight after ejecting out the outside of the container. Meanwhile, for the nozzle diameter size of 40 Å, the Nano-droplet was not only production but also movement up to leave away the nozzle’s surface under same above conditions. That proves that the ejective diameter has the influences to the moveable direction and Nano-droplets formation ability in the whole ejective process.

Interface and surface segregation of germanium in the SiGe semiconductor

Interface and surface segregation of germanium in the SiGe semiconductor

Rapid self-assembly of self-healable and transferable liquid metal epidermis

Healable electronic skins, an essential component for future soft robotics, implantable bioelectronics, and smart wearable systems, necessitate self-healable and pliable materials that exhibit functionality at intricate interfaces. Although a plethora of self-healable materials have been developed, the fabrication of highly conformal biocompatible functional materials on complex biological surfaces remains a formidable challenge. Inspired by regenerative properties of skin, we present the self-assembled transfer-printable liquid metal epidermis (SALME), which possesses autonomous self-healing capabilities at the oil-water interface. SALME comprises a layer of surfactant-grafted liquid metal nanodroplets that spontaneously assemble at the oil-water interface within a few seconds. This unique self-assembly property facilitates rapid restoration (<10s) of SALME following mechanical damage. In addition to its self-healing ability, SALME exhibits excellent shear resistance and can be seamlessly transferred to arbitrary hydrophilic/hydrophobic curved surfaces. The transferred SALME effectively preserves submicron-scale surface textures on biological substrates, thus displaying tremendous potential for future epidermal bioelectronics.

Activating Tumor-Selective Liquid Metal Nanomedicine through Galvanic Replacement.

Advanced chemotherapeutic strategies including prodrug and nanocatalytic medicine have significantly advanced tumor-selective theranostics, but delicate prodrug screening, tedious synthesis, low degradability/biocompatibility of inorganic components, and unsatisfied reaction activity complicate treatment efficacies. Here, the intrinsic anticancer bioactivity of liquid metal nanodroplets (LMNDs) is explored through galvanic replacement. By utilizing a mechano-degradable ligand, the resultant size of the aqueous LMND is unexpectedly controlled as small as ≈20nm (LMND20). It is demonstrated that LMND20 presents excellent tumor penetration and biocompatibility and activates tumor-selective carrier-to-drug conversion, synchronously depleting Cu2+ ions and producing Ga3+ ions through galvanic replacement. Together with abundant generation of reactive oxygen species, multiple anticancer pathways lead to selective apoptosis and anti-angiogenesis of breast cancer cells. Compared to the preclinical/clinical anticancer drugs of tetrathiomolybdate and Ga(NO3 )3 , LMND20 administration significantly improves the therapeutic efficacy and survival in a BCap-37 xenograft mouse model, yet without obvious side effects.

Hydrophobicity of molecular-scale textured surfaces: The case of zeolitic imidazolate frameworks, an atomistic perspective.

Hydrophobicity has proven fundamental in an inexhaustible amount of everyday applications. Material hydrophobicity is determined by chemical composition and geometrical characteristics of its macroscopic surface. Surface roughness or texturing enhances intrinsic hydrophilic or hydrophobic characteristics of a material. Here we consider crystalline surfaces presenting molecular-scale texturing typical of crystalline porous materials, e.g., metal-organic frameworks. In particular, we investigate one such material with remarkable hydrophobic qualities, ZIF-8. We show that ZIF-8 hydrophobicity is driven not only by its chemical composition but also its sub-nanoscale surface corrugations, a physical enhancement rare amongst hydrophobes. Studying ZIF-8's hydrophobic properties is challenging as experimentally it is difficult to distinguish between the materials' and the macroscopic corrugations' contributions to the hydrophobicity. The computational contact angle determination is also difficult as the standard "geometric" technique of liquid nanodroplet deposition is prone to many artifacts. Here, we characterise ZIF-8 hydrophobicity via: (i) the "geometric" approach and (ii) the "energetic" method, utilising the Young-Dupré formula and computationally determining the liquid-solid adhesion energy. Both approaches reveal nanoscale Wenzel-like bathing of the corrugated surface. Moreover, we illustrate the importance of surface linker termination in ZIF-8 hydrophobicity, which reduces when varied from sp3 N to sp2 N termination. We also consider halogenated analogues of the methyl-imidazole linker, which promote the transition from nanoWenzel-like to nanoCassie-Baxter-like states, further enhancing surface hydrophobicity. Present results reveal the complex interface physics and chemistry between water and complex porous, molecular crystalline surfaces, providing a hint to tune their hydrophobicity.

Simple preparation of lignosulfonate stabilized eutectic gallium/indium liquid metal nanodroplets through ball milling process

The combination of biomass and liquid metal (LM) makes the preparation process "greener" and application of LM composite materials more sustainable. Here we reported the solvent free preparation of lignosulfonate (LS) stabilized eutectic gallium/indium (EGaIn) LM nanodroplets through ball milling (BM), which was recognized to be efficient and environmentally-friendly alternatives to solution-based methods. By regulating the BM frequency and milling time, uniform LM nanodroplets with a size <200nm can be achieved. Moreover, the surface of the EGaIn nanodroplets was covered by LS molecules, owing to the hydrogen bond formed between Ga2O3 and LS. Hydrophilic LS shell endowed the LS@EGaIn nanodroplets excellent colloidal stability in the aqueous media. The elongation at break and fracture strength of hydrogel with the addition of LS@EGaIn significantly improved with the addition of LS@EGaIn. Besides, the conductivity and excellent stress responsibility of the LS@EGaIn composite hydrogel illustrated its potential application as s a stress sensor, flexible wearable devices and other related applications. Moreover, it was predicted that LS can be replaced by other synthesized or biological macromolecules, and induced the formation of types of LM based composite materials through such a simple method.

Highly Stretchable and Conductive MXene‐Encapsulated Liquid Metal Hydrogels for Bioinspired Self‐Sensing Soft Actuators

AbstractAdvanced sensation and actuation abilities of various living organisms in nature have inspired researchers to design bioinspired self‐sensing soft actuators. However, the majority of conventional soft actuators primarily possess actuation capabilities while lacking a real‐time sensing signal feedback. Here, a promising strategy is reported to develop highly stretchable and conductive hydrogels for bioinspired self‐sensing soft actuators, which integrate actuation and strain‐sensing functions into a single materials system. The conductive hydrogels are designed and fabricated by in situ copolymerization of amino‐functionalized MXene‐encapsulated liquid metal nanodroplets (LM@A‐MXene) and poly(N‐isopropylacrylamide) hydrogels with controllable activated nanogels as nano‐cross‐linkers. The resulting hydrogel presents a compacted conducting network and highly porous microstructure, giving rise to robust integration of high conductivity, excellent strain sensitivity, broad stretchability, high stability, and fast response speed. Interestingly, the gradient network structure, formed by self‐precipitation of LM@A‐MXene, endows the hydrogel with shape‐programmable actuation, light‐driven remote control, and self‐sensing function. As a proof‐of‐concept application, the soft gripper based on the self‐sensing hydrogel actuators is developed, which can not only grasp, lift, and release objects, but also perceive every movement state by monitoring resistance changes. The proposed self‐sensing soft actuator can offer new insights for developing smart soft robotics and other artificial intelligent devices.

Uncovering the Properties of Dicationic Ionic Liquid Nanodroplets through Ab Initio Molecular Dynamics Simulations.

The behavior of nanodroplets of an imidazolium-based dicationic ionic liquid, i.e., [C1(mim)2][PF6]2, was investigated in this study using ab initio molecular dynamics simulations. The vibrational features as well as the structural, interfacial, and dynamical properties of different sized droplets were analyzed and compared to the bulk phase system. Structural properties of the droplets, such as π-π stacking, radial distribution functions, structure factors, combined distribution functions, and angular distribution functions were analyzed to understand the interactions and orientations of their ions. The vibrational features and hydrogen bonding strength of droplets were studied by calculating their infrared (IR) and power spectra, determining the contribution of different types of hydrogen bonding to each vibrational mode. The calculated spectra showed good overall agreement with the experimental results. The interfacial properties of the droplets and the orientation of their ions were analyzed using density profiles and an exposed surface. The results showed that, in all systems studied, cations and anions were equally likely to exist in both inner and outer layers, and the cations tended to be oriented toward the center of droplets with obtuse angles. Additionally, the droplet densities were extrapolated to predict the bulk phase density with less than 2% deviation. The dynamical properties of hydrogen bonds, mean square displacement, and van Hove correlations of cations and anions were also analyzed. The results indicated that there was no regular trend in the dynamic properties of droplets with an increasing system size.

Supramolecular gelator functionalized liquid metal nanodroplets as lubricant additive for friction reduction and anti-wear

In this work, dopamine n-butenylamide (DBA) modified GLM nanodroplets were prepared via directional ultrasound of bulk liquid metal in ethanol aqueous solution as well as DBA self-assembly, followed by grafting with urea-based gelators via radical polymerization to obtain GLM-based supramolecular gelators (Gelator@GLM). The grafting gelators can impart their good compatibility between the GLM nanodroplets and the base oil, so that the Gelator@GLM nanodroplets can be dispersed in the base oil uniformly and stably for more than 3weeks. Meanwhile, the tribological properties of Gelator@GLM nanodroplets was significantly enhanced, with a reduction of coefficient of friction (COF) and the wear volume of 41.18% and 92.13%, respectively, when compared with the base oil. Furthermore, Gelator@GLM additives exhibited stable lubrication performance even under variable temperature and frequency conditions. The synergistic effect of GLM nanodroplets and the gels generating a physical adsorption film and a chemically protective film (containing iron and chromium oxides, nitrides and carbides) can be credited with the improved tribological performance.

Synthesis, characterization, and photo‐responsiveness of liquid metal/azo polymer hybrid material

AbstractLiquid metals have garnered increasing attention as an emerging class of functional materials in recent years. In this work, an azo polymer coat with photo‐responsive properties is successfully applied onto the surface of liquid metal through reversible addition‐fragmentation chain transfer (RAFT) polymerization. The photo‐responsiveness of the resulting liquid metal/azo polymer hybrid material is thoroughly investigated. Liquid metal nanodroplets grafted with azo polymers exhibit exceptional ultraviolet and near infrared photo‐responsiveness due to photothermal effect of liquid metal and isomerization of azo polymer.

Single-Molecule Pycnometry and Shape Analysis of Ions in the Gas Phase.

The analysis of ions and clusters by mobility-classified mass spectrometry provides information on the mobility of analytes in the drift gas and the analyte mass. Mass equivalent and mobility equivalent diameters of globular analytes, such as ions, poly(ethylene glycol) (PEG), and ionic liquid nanodroplets, can be correlated with good accuracy by the Stokes-Millikan mobility model. A prerequisite to such an analysis is, however, the assumption of a globular analyte shape, which then allows determination of material density for globular ions. We show that the analyte density can be evaluated with high precision, independent of any assumptions on the analyte shape, by careful analysis of analyte-PEG-cluster ions following the concept of classical pycnometry. In particular, the analyte is entrapped in a globular PEG-analyte droplet. Based on the now independently derived mobility diameter and volume equivalent diameter, it is possible to attribute two parameters, size and shape, to the analyte molecule. We demonstrate the approach for lysozyme, cyano-cobalamin (vitamin B12), and glucose, which cover two orders of magnitude in analyte mass (180···14 300 Da). The derived densities for these analytes are highly accurate, i.e., they deviate less than 1% from literature values. Our method can be applied to newly synthesized molecules, supramolecular assemblies, isolated biomolecules, and molecular clusters, where only minor amounts of materials are available. The obtained shape parameters of lysozyme and cyano-cobalamin agree well with the expected molecular shapes. Data evaluation relies only on locations of the species in the mass-mobility plane and is in principle independent of any mobility theory. Our approach is thus robust with respect to experimental uncertainties and produces identical results irrespective of the type of mobility classification and drift gas.

Three-dimensional polymer-nanoparticle-liquid ternary composite for ultrahigh augmentation of piezoelectric nanogenerators

The development of flexible nanogenerators that can convert passively generated environmental energy into electricity is crucial for sustainable energy generation. Herein, we rationally designed a polymer-nanoparticle-liquid (PNL) ternary composite, comprising polydopamine-modified barium titanate nanoparticles (PDA-BTO NPs) doped polyvinylidene fluoride (PVDF), and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFOES) liquid nanodroplets, for application in piezoelectric nanogenerators (PENGs). The inter-discrete PFOES nanodroplets enabled the formation of a three-dimensional (3D) scaffold matrix in the ternary composite, which is highly beneficial for the stress transfer of PENGs. Finite-element analysis revealed that the ternary composite had a remarkably improved stress transfer ability due to the incorporation of highly deformable PFOES liquid nanodroplets, which increased the net stress exerted on PDA-BTO NPs and the PVDF matrix. The PNL composite-based PENG (PNL-PENG) device exhibited an enhanced piezoelectric performance, achieving an output voltage, current, and power density of 102 V, 10 µA, and 70 μW/cm2, respectively, which are record-high results compared to those achieved by binary composite-based PENGs. Moreover, the PNL-PENG also demonstrated tremendous potential to function as a highly sensitive tactile perception tool for shape recognition. The concept of PNL-PENG devices represents a milestone in the domain of human-machine interaction, taking a significant step toward the advancement of flexible and wearable electronics.

Ion emission from nanodroplets undergoing Coulomb explosions: a continuum numerical study

A droplet charged above the Rayleigh limit is unstable. In the resulting dynamical process, referred to as a Coulomb explosion, smaller droplets with higher charge-to-mass ratios are ejected, reducing the charge of the parent droplet below the Rayleigh limit. Furthermore, if the droplet is sufficiently small, the electric field on its surface can promote ion field emission. Ion emission can lower the charge of a spherical droplet below its Rayleigh limit, keeping it stable, or reduce the charge of a deforming droplet, changing its dynamics and potentially preventing the Coulomb explosion. This article develops a continuum phase field electrohydrodynamic model to study the interplay between Coulomb explosions and ion emission, using charged nanodroplets of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-Im) as a case study. In small droplets (diameter $D \lesssim 20$ nm for EMI-Im), the electric field is strong enough to emit ions in the early phase of the droplet's evolution, suppressing the Coulomb explosion. For $20 \lesssim D \lesssim 45$ nm, the electric field on the EMI-Im droplet may not promote significant ion emission; however, as the unstable droplet becomes ellipsoidal, ions are emitted from its vertices, ultimately suppressing the Coulomb explosion while shedding $20\unicode{x2013}40\,\%$ of the initial charge. For larger EMI-Im droplets, $45 \lesssim D \lesssim 100$ nm, the evolution typical of a Coulomb explosion is observed, accompanied by ion emission which is however insufficient to prevent the Coulomb explosion. Ion emission and the smaller progeny droplets account for $24\,\%$ and $16\,\%$ of the initial charge, respectively.