Levitated Liquid Research Articles

Micro container made of levitated liquid bead

Liquid bead, an emerging digital microfluidic platform offers a wide range of applications in research and industry. Making liquid beads using acoustic levitation technology allows for the fabrication of micro container. Extensive insight into levitated liquid beads is essential for determining their performance. Firstly, this paper unfolds the unique interface of the levitated liquid bead, which has thin membrane of shell material on top and thus serves as a micro container. Next, we characterised the puncture toughness of liquid beads to assess the capability of releasing the liquid content. Liquid bead with larger corresponding shell volume and lower core volume recorded the highest resistance against needle penetration. Finally, we observed the thermal behaviour of liquid beads at elevated temperature. The liquid beads showed no morphological change over 10-minutes period at 50 °C. At 65 °C, liquid bead exceeded the critical temperature threshold, which resulted in breaking the thin membrane and releasing the liquid from the core.

The structure of CaO–MgO–Al2O3–SiO2 melts and glasses doped with FeOX–NiO

AbstractNeutron and x‐ray diffraction measurements have been performed on CaO–MgO–Al2O3–SiO2 (CMAS) glasses doped with NiO–FeXO at room temperature, along with x‐ray measurements on aerodynamically levitated liquids at ≥2000 K. The disordered structures have been modeled using empirical potential structure refinement to investigate the relation between the aluminosilicate network and the modifying cations. The SiO4 and AlO4 tetrahedra are found to have wider Si–O and Al–O bond distance distributions in the glass, and the first Ca–On coordination shell is highly distorted, redistributing different populations of long and short bonds between the liquid and the glass. The addition of Fe and Ni at low aluminosilicate content increases the number of free oxygens not bonded to AlO4 or SiO4. Mg–O and Fe–O are both found to be predominantly fourfold and fivefold in the liquid and glassy states. Despite these low coordination numbers, their bond angle distributions indicate that they are predominantly in nontetrahedral‐type geometries, with ferrous and ferric iron possessing similar coordination environments. The Ca–O and Mg–O average coordination numbers and enthalpies of solution are consistent with their higher reactivity within relatively acidic aluminosilicate melts.

Iron coordination in liquid FeAl2O4.

The structure of aerodynamically levitated liquid [Formula: see text] was measured by neutron diffraction with isotope substitution (NDIS). Classical and ab initio molecular dynamics simulations were performed and their results were found to be in close agreement with each other and the NDIS data. The results reveal that molten [Formula: see text] may be considered as an ionic liquid without any preference for particular short-range structural motifs. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

A hyperbaric aerodynamic levitator for containerless materials research.

A hyperbaric aerodynamic levitator has been developed for containerless materials research at specimen temperatures exceeding 2000 °C and pressures up to 10.3 MPa (1500 psi). This report describes the prototype instrument design and observations of the influence of specimen size, density, pressure, and flow rate on levitation behavior. The effect of pressure on heat transfer was also assessed by studying the heating and cooling behavior of levitated Al2O3 liquids. A threefold increase in the convective heat transfer coefficient was estimated as pressure increased to 10.3 MPa. The results demonstrate that hyperbaric aerodynamic levitation is a promising technique for containerless materials research at high gas pressures.

Subpatterns of Thin-Sheet Splash of a Droplet Impact on a Heated Surface

A thin-sheet splash of a droplet impact on a solid surface typically appears as secondary droplets are ejected from a levitated liquid lamella. A recent work (Qin, M.; Tang, C.; Guo, Y.; Zhang, P.; Huang, Z., Langmuir 2020, 36 (18), 4917-4922) identified three subpatterns of a thin-sheet splash on a smooth wall at room temperature (T0). In the present work concerning the high-temperature (TW) surface, we show that subpatterns of the thin-sheet splash can be unified in the three-dimensional phase diagram of Oh-We-TW, where Oh is the Ohnesorge number and We is the Weber number. As TW is sufficiently high, the Leidenfrost effect becomes so prominent that both deposition and thin-sheet splash make a transition to Leidenfrost breakup. For the transition surface temperature TW,cr from thin-sheet splash to deposition, a scaling correlation of TW,cr/T0 ∼ We3/2 is derived based on the analysis of the temperature-dependent destabilizing force on the levitated lamella and agrees well with our experimental data.

The Discovery of Floating Boats on the Oscillating Levitated Liquid Layer

When air bubbles are injected into a still bath with liquid, it’s known that they will rise to the top surface because of their small mass. However, oscillations, which can provide a dynamical stabilizing effect, can make air bubbles sink when created below a certain depth that changes along with the forcing amplitude. Three experiments are needed to show the defy-gravity behavior caused by vibration on liquid. Using silicon oil, a syringe, a needle, and a vertically oscillating shaker with amplitude, an air layer formed by sinking bubbles, which defy the well-known Archimedes’ principle, is being trapped under the levitating liquid layer acts as a spring-mass. With the further experiment using two light foam boats, it was observed that an upside-down buoyant force was acting on the liquid provided by vibration, supporting the boats to flow on both interfaces of the liquid layer. A symmetric Archimedes’ principle is discovered to be reflected on the lower interface created by the dynamic stabilization as well. This discovery about the effect vibration can have on liquid, and alternately the forces acting on the object floating on this liquid brings lots of future development to light and is new progress in the area of fluid mechanics.

Benchmarking surface tension measurement method using two oscillation modes in levitated liquid metals

The Faraday forcing method in levitated liquid droplets has recently been introduced as a method for measuring surface tension using resonance. By subjecting an electrostatically levitated liquid metal droplet to a continuous, oscillatory, electric field, at a frequency nearing that of the droplet’s first principal mode of oscillation (known as mode 2), the method was previously shown to determine surface tension of materials that would be particularly difficult to process by other means, e.g., liquid metals and alloys. It also offers distinct advantages in future work involving high viscosity samples because of the continuous forcing approach. This work presents (1) a benchmarking experimental method to measure surface tension by excitation of the second principal mode of oscillation (known as mode 3) in a levitated liquid droplet and (2) a more rigorous quantification of droplet excitation using a projection method. Surface tension measurements compare favorably to literature values for Zirconium, Inconel 625, and Rhodium, using both modes 2 and 3. Thus, this new method serves as a credible, self-consistent benchmarking technique for the measurement of surface tension.

Floating under a levitating liquid.

When placed over a less dense medium, a liquid layer will typically collapse downwards if it exceeds a certain size, as gravity acting on the lower liquid interface triggers a destabilizing effect called a Rayleigh-Taylor instability1,2. Of the many methods that have been developed to prevent the liquid from falling3-6, vertical shaking has proved to be efficient and has therefore been studied in detail7-13. Stabilization is the result of the dynamical averaging effect of the oscillating effective gravity. Vibrations of liquids also induce other paradoxical phenomena such as the sinking of air bubbles14-19 or the stabilization of heavy objects in columns of fluid at unexpected heights20. Here we take advantage of the excitation resonance of the supporting air layer to perform experiments with large levitating liquid layers of up to half a litre in volume and up to 20 centimetres in width. Moreover, we predict theoretically and show experimentally that vertical shaking also creates stable buoyancy positions on the lower interface of the liquid, which behave as though the gravitational force were inverted. Bodies can thus float upside down on the lower interface of levitating liquid layers. We use our model to predict the minimum excitation needed to withstand falling of such an inverted floater, which depends on its mass. Experimental observations confirm the possibility of selective falling of heavy bodies. Our findings invite us to rethink all interfacial phenomena in this exotic and counter-intuitive stable configuration.

Resistivity Saturation in Metallic Liquids Above a Dynamical Crossover Temperature Observed in Measurements Aboard the International Space Station.

Although a resistivity saturation (minimum conductivity) is often observed in disordered metallic solids, such phenomena in the corresponding liquids are not known. Here we report a saturation of the electrical resistivity in Zr_{64}Ni_{36} and Cu_{50}Zr_{50} liquids above a dynamical crossover temperature for the viscosity (T_{A}). The measurements were made for the levitated liquids under the microgravity conditions of the International Space Station. Based on recent molecular dynamics simulations, the saturation is likely due to the ineffectiveness of electron-phonon scattering above T_{A} when the phonon lifetime becomes too short compared to the electron relaxation time. This is different from the conventional resistivity saturation mechanisms in solids.

Numerical and experimental investigation of the stability of a drop in a single-axis acoustic levitator

Acoustic levitation can be employed to hold liquid drops in midair, enabling novel applications in X-ray scattering of proteins, amorphous crystallization of solutions, or contactless mixing. Multiple studies have characterized the physical behavior of a levitated drop inside an acoustic field. Here, we present a numerical and experimental study on the acoustic levitation of water drops in a single-axis acoustic levitator consisting of an ultrasonic transducer and an opposing reflector. Instead of modeling an abstract incident acoustic field, our model considers the shape of the drop as well as the real geometry of the levitator. We also use a high-speed camera to observe the disintegration and the undesired oscillations of the drops. Our results show that the insertion of a drop in the levitator provokes a shift in its resonant frequency that depends on the shape of the drop. Second, the levitation behavior depends on whether the levitator operates slightly below or above the resonance. Third, if the levitator is driven above the resonant frequency, it is possible to levitate with more strength and avoid disintegration of the drop. This research provides an insight on how to achieve more stable experiments that avoid the bursting and undesired oscillations of the levitated sample. We hope that it will facilitate numerous experiments involving acoustically levitated liquid drops.

Shape evolution and bubble formation of acoustically levitated drops

In this study, we investigated the shape evolution and bubble formation of acoustically levitated drops upon increasing the sound intensity. Here, a levitated liquid drop evolves progressively from an oblate spheroidal shape to a flattened film to a thin bowl-shaped film, eventually forming a closed bubble. Through systematic experiments, numerical simulation, and scaling analysis, we demonstrate that the buckled geometry of the liquid film can drastically enhance the suction effect of acoustic radiation pressure at its rim, forming a significant pressure gradient inside the film which causes an abrupt area expansion and bubble formation. Our results provide the mechanical origin responsible for the shape evolution and bubble formation of acoustically levitated drops, and highlight the role of buckled geometry in the levitation and manipulation of liquid films in an ultrasound field.

Spectroscopic Investigation of the Primary Reaction Intermediates in the Oxidation of Levitated Droplets of Energetic Ionic Liquids.

The production of the next generation of hypergolic, ionic-liquid-based fuels requires an understanding of the reaction mechanisms between the ionic liquid and oxidizer. We probed reactions between a levitated droplet of 1-methyl-4-amino-1,2,4-triazolium dicyanamide ([MAT][DCA]), with and without hydrogen-capped boron nanoparticles, and the nitrogen dioxide (NO2) oxidizer. The apparatus exploits an ultrasonic levitator enclosed within a pressure-compatible process chamber equipped with complementary Raman, ultraviolet-visible, and Fourier-transform infrared (FTIR) spectroscopic probes. Vibrational modes were first assigned to the FTIR and Raman spectra of droplets levitated in argon. Spectra were subsequently collected for pure and boron-doped [MAT][DCA] exposed to nitrogen dioxide. By comparison with electronic structure calculations, some of the newly formed modes suggest that the N atom of the NO2 molecule bonds to a terminal N on the dicyanamide anion yielding [O2N-NCNCN]-. This represents the first spectroscopic evidence of a key reaction intermediate in the oxidation of levitated ionic liquid droplets.

Mechanical Behavior of Liquid Route Processed SiCf/Ti Composites Under Longitudinal and Transverse Loadings

Due to the high melting point and strong chemical reactivity of titanium alloys, titanium matrix composites (TMCs) are usually processed through solid-state routes such as the foil-fiber-foil technique. An alternative method consists in the deposition of the matrix on the fibers. However, techniques such as physical vapor deposition lead to a very low deposition rate, contrary to liquid route processing using a levitating liquid alloy sphere held in a cold crucible. In order to investigate the effects of the resulting thermal shock on carbon-coated SiC fibers, and select an appropriate fiber, fibers are subjected to a pure thermal shock using a laser bench facility. These fibers are then tensile tested to failure in order to evaluate the resulting fiber strength degradation and, thus, the maximum acceptable temperature. Mechanical characterization of the liquid route processed TMC is then investigated through longitudinal and transverse tensile and creep tests at temperatures representative of aeronautical applications. The specimens, unbroken after long-duration creep tests, are then subjected to tensile loading to failure: conditions representative of service, i.e., short-time overspeeding of a gas turbine. Finally, interpretation of the mechanical tests through micrographical and microfractographical examinations is focused on the identification of the deformation and failure mechanisms specific to the liquid route processed composite, e.g., nucleation, under either longitudinal or transverse loadings, of internal cracks in the α-phase of the titanium-based matrix, explained through a physical model involving a high shear stress and normal stress combination, leading to cleavage.

Measurement of Surface Tension of Cu–5Sn by an Oscillating Drop Technique

The surface tension of liquid Cu–5Sn (copper with \({5}\,{\hbox {wt} \%}\) tin) in the temperature range from \({1290}\,\hbox {K}\) to \({1560}\,\hbox {K}\) was measured by an oscillating drop technique combined with electromagnetic levitation. The levitation device uses an inhomogeneous radiofrequency electromagnetic field inside a levitation coil to position and to heat metallic material. Eddy currents are induced in a specimen to heat it to the liquid phase and to exert a Lorentz force, pushing it against gravity towards regions of lower field strength. The levitating liquid specimen takes the shape of a sphere, which is rotating and oscillating. The oscillations are recorded by a high-speed camera at \(600\,\hbox {fps}\); the temperature of the specimen is measured by a fast near-infrared pyrometer. A linear fit to the measured surface tension \(\gamma \) of Cu–5Sn as a function of temperature T in Kelvin is given by: $$\gamma (T)({\hbox {mN}\cdot \mathrm{m}^{-1}})=1195-0.052\cdot (T-1318).$$

Switchable Opening and Closing of a Liquid Marble via Ultrasonic Levitation

Liquid marbles have promising applications in the field of microreactors, where the opening and closing of their surfaces plays a central role. We have levitated liquid water marbles using an acoustic levitator and, thereby, achieved the manipulation of the particle shell in a controlled manner. Upon increasing the sound intensity, the stable levitated liquid marble changes from a quasi-sphere to a flattened ellipsoid. Interestingly, a cavity on the particle shell can be produced on the polar areas, which can be completely healed when decreasing the sound intensity, allowing it to serve as a microreactor. The integral of the acoustic radiation pressure on the part of the particle surface protruding into air is responsible for particle migration from the center of the liquid marble to the edge. Our results demonstrate that the opening and closing of the liquid marble particle shell can be conveniently achieved via acoustic levitation, opening up a new possibility to manipulate liquid marbles coated with non-ferromagnetic particles.

Surface wave patterns on acoustically levitated viscous liquid alloys

We demonstrate two different kinds of surface wave patterns on viscous liquid alloys, which are melted and solidified under acoustic levitation condition. These patterns are consistent with the morphologies of standing capillary waves and ensembles of oscillons, respectively. The rapid solidification of two-dimensional liquid alloy surfaces may hold them down.

Leidenfrost Dynamics

This review discusses how drops can levitate on a cushion of vapor when brought in contact with a hot solid. This is the so-called Leidenfrost phenomenon, a dynamical and transient effect, as vapor is injected below the liquid and pressed by the drop weight. The absence of solid/liquid contact provides unique mobility for the levitating liquid, contrasting with the usual situations in which contact lines induce adhesion and enhanced friction: hence a frictionless motion, and the possibility of bouncing after impact. All these characteristics can be combined to create devices in which self-propulsion is obtained, using asymmetric textures on the hot solid surface.

Structural Transformations on Vitrification in the Fragile Glass-Forming SystemCaAl2O4

The structure of the fragile glass-forming material CaAl(2)O(4) was measured by applying the method of neutron diffraction with Ca isotope substitution to the laser-heated aerodynamically levitated liquid at 1973(30) K and to the glass at 300(1) K. The results, interpreted with the aid of molecular dynamics simulations, reveal key structural modifications on multiple length scales. Specifically, there is a reorganization on quenching that leads to an almost complete breakdown of the AlO(5) polyhedra and threefold coordinated oxygen atoms present in the liquid, and to their replacement by a predominantly corner-sharing network of AlO(4) tetrahedra in the glass. This process is accompanied by the formation of branched chains of edge and face-sharing Ca-centered polyhedra that give cationic ordering on an intermediate length scale, where the measured coordination number for O around Ca is 6.0(2) for the liquid and 6.4(2) for the glass.

Interaction of Levitated Ionic Liquid Droplets with Water

The influence of a humid or dry atmosphere on acoustically levitated ionic liquid droplets was studied by volumetric analysis and vibrational spectroscopy. Imidazolium-based ionic liquids with two types of anions, fluorinated (BF(4) and PF(6)) and alkylsulfate anions, were investigated. The amount of absorbed water was correlated with structural differences of the ionic liquids and analyzed in terms of equilibrium mole fraction as well as absorption rate. The type of anion had the most significant influence on the amount of adsorbed water from the atmosphere. Furthermore, spectral changes in the in situ Raman spectra due to absorbed water were studied for all investigated ionic liquids. For 1-ethyl-3-methylimidazolium ethylsulfate, an exemplary detailed analysis of the intermolecular interactions between cations, anions and water was carried out based on the spectroscopic data. The observed band shifts were explained with a hydrogen bond between the anion and water.

Composition and polyamorphism in supercooled yttria–alumina melts

By extending recent work on liquid–liquid transitions in supercooled yttria–alumina AYx liquids we draw attention to the compositional dependence of the structure factor of the high density liquid, arguing that this is sufficiently sensitive to discriminate between liquids at the level of a few %. Comparing structure factor differences between liquids of different compositions and in the same liquid AY20 between high and low temperatures straddling the transition at 1788 K between a high density liquid (HDL) and a low density liquid (LDL) enables compositional phase separation to be ruled out. It points instead to kinetic changes in polyhedral configurational order being the drivers for this polyamorphic transformation. Rotor behaviour observed in levitated liquid drops used in the high temperature experiments enables the reversibility of the LLT transition (LLT) and the associated changes in entropy and density to be identified. Evidence for critical-like behaviour in the structural relaxation time and in the fluctuation correlation length is presented. By re-examining recent work which failed to find the structural and thermal signatures for the LLT in liquid AY20 at 1788 K we present evidence for the LLT occurring instead in liquid AY15 at 1940 K, suggesting that the liquid-liquid transition temperature in AYx liquids decreases with increasing yttria content.