Levitated Droplets Research Articles

A videographic and numerical study of nonisothermal oscillation process for electrostatically levitated liquid Nb–Si alloy

The heat transfer and oscillation dynamics of the liquid Nb-17.3 at.% Si eutectic alloy at high temperatures of approximately 2200 K were explored with electrostatic levitation (ESL) technique. Different second-order oscillation modes of levitated alloy droplets were successfully induced by both excited oscillation with smaller amplitude and spontaneous oscillation with larger amplitude. Importantly, an oscillation mode transition from second to third order was observed as the alloy droplet size increased to 3.96 mm. Meanwhile, a mathematical model coupling heat transfer and droplet oscillation was developed to simulate this nonisothermal oscillation process. The entire thermal history was numerically predicted and proved consistent with experimental measurements. The shape evolution dynamics of second- and third-order oscillations together with the corresponding liquid flow patterns inside alloy droplet was revealed in detail, which helped to clarify the mutual influence mechanism between heat transfer and droplet oscillation.

Bubbling of Ferrofluid Droplets via Coupled Magnetic and Sound Fields

AbstractBubbles play essential roles in many natural and industrial processes because of the unique effects of their films on heat–mass transfer and their soft confined geometry for interfacial assembly or phase transitions. Here, a technique for noncontact bubbling via coupling of sound and magnetic fields in acoustic levitation is reported. An acoustically levitated ferrofluid droplet can be transformed into a closed bubble in a controlled manner. It has been found that the magnetic field pulls the flattened ferrofluid film downward, and making it deviate from its initial levitation position determined by the balance between acoustic radiation force and gravity. This results in variation in acoustic radiation pressure on the film surface and generates an upwards torque on the levitated film, leading to buckling and subsequent bubble transition via acoustic resonance. Moreover, the levitated ferrofluid bubble can remain unburst in the sound field for minutes to enable its complete evaporation; thus, solid hollow shells form. This work provides insight for understanding the coupled effect of sound and magnetic fields on ferrofluid drops and sheds light on evaporation‐driven self‐assembly of ferrofluid particles into shell structures

Characterization of oscillation modes in levitated droplets using image and non-image based techniques

The dynamics of levitated liquid droplets can be used to measure their thermophysical properties by correlating the frequencies at which normal modes of oscillation most strongly resonate when subject to an external oscillatory force. In two preliminary works, it was shown via electrostatic levitation and processing of various metals and alloys that (1) the resonance of the first principal mode of oscillation (mode n = 2) can be used to accurately measure surface tension and (2) that so-called “higher-order resonance” of n = 3 is observable at a predictable frequency. It was also shown, in the context of future space-based experimentation on the Electrostatic Levitation Furnace (ELF), a setup on the International Space Station (ISS) operated by Japan Aerospace Exploration Agency (JAXA), that while the shadow array method in which droplet behavior is visualized would be challenging to identify the n = 3 resonance, the normal mode n = 4 was predicted to be more easily identifiable. In this short communication, experimental evidence of the first three principal modes of oscillation is provided using molten samples of Tin and Indium and it is subsequently shown that, as predicted, an “image-less" approach can be used to identify both n = 2 and n = 4 resonances in levitated liquid droplets. This suggests that the shadow array method may be satisfactorily used to obtain a self-consistent benchmark of thermophysical properties by comparing results from two successive even-mode natural frequencies.

Sonomaglev: Combining acoustic and diamagnetic levitation

Acoustic levitation and diamagnetic levitation are experimental methods that enable the contact-free study of both liquid droplets and solid particles. Here, we combine both the techniques into a single system that takes advantage of the strengths of each, allowing for the manipulation of levitated spherical water droplets (30 nl–14 μl) under conditions akin to weightlessness, in the laboratory, using a superconducting magnet fitted with two low-power ultrasonic transducers. We show that multiple droplets, arranged horizontally along a line, can be stably levitated with this system and demonstrate controlled contactless coalescence of two droplets. Numerical simulation of the magnetogravitational and acoustic potential reproduces the multiple stable equilibrium points observed in our experiments.

Suspended in sound: New constraints on isotopic fractionation of falling hydrometeors using acoustically levitated water droplets

AbstractThe isotopic composition of precipitation (δ18O, δ2H, and δ17O) is affected by evaporation and exchange as hydrometeors descend. These processes can significantly alter the isotopic ratio of precipitation relative to its initial condensation state in the cloud yet are exceedingly difficult to study in situ. The most widely utilized model for droplet‐atmosphere exchange was derived from controlled experiments where droplets were suspended by forced air in a narrow glass tube‐ a design that manipulated the structure of the boundary layer around the droplet. Here, we provide a novel experimental test of atmosphere‐hydrometeor isotopic exchange using the mechanism of acoustic levitation, where sound waves are projected vertically to levitate droplets in free‐flowing ambient air. We present results from a series of droplet levitation experiments where the droplets' surface temperatures were measured by a thermal camera and the background atmospheric isotope concentration was measured via cavity‐ringdown spectroscopy, providing a high degree of constraint on the fractionation conditions. We show that isotope enrichment of the suspended droplets met first order expectations based on existing models. However, to account for the slope of δ18O versus δ2H (i.e., the meteoric water line) and deuterium excess of the droplets as they evolved, we had to modify the existing model for droplet‐atmosphere exchange to account for the fact that some portion of the evaporative flux from the droplet remained present in the boundary layer around the droplet leading to an evolving feedback between droplet and the atmosphere‐that is, a quasi closed‐system effect. The isotopic enrichment of the boundary layer surrounding the droplet as a consequence of the closed‐system dynamics, drives more rapid δ18O isotope enrichment relative to δ2H of the droplet compared to what is predicted using an open system model. Although these were controlled experiments, they illustrate important dynamics regarding the isotopic signature of feedbacks between droplet evaporation and atmospheric humidity.

Evaporation kinetics during containerless chemical synthesis of ZIF-8 in levitated droplets.

Zeolitic imidazolate framework-8 was synthesized in a containerless state via acoustic levitation. The cavitation effect of ultrasound affected the coordination connection of organic ligands in acoustically levitated droplets and they exhibited a conspicuous difference in the particle size distribution as compared with those under normal conditions. Herein, methanol was chosen as the solvent to investigate the influence of droplet evaporation on acoustic levitation synthesis. The kinetic parameters of droplet evaporation such as geometrical morphology transformation, concentration change and temperature evolution were measured for the levitation state. Surface evaporation resulted in the drastic deformation of the droplet during ZIF-8 synthesis and caused its vertical vibration and shape oscillation. The abrupt change of the levitation state aggravated the sound field effect on the containerless synthesis and caused a decrease of particle size distribution. A two-dimensional axis-symmetry model was used to visually simulate the sound field distribution during acoustic levitation synthesis based on the finite element method. The fabricated ZIF-8 was able to remove phthalic acid in wastewater through adsorption, and its kinetic features followed a pseudo second-order rate model.

Experimental study on levitation control of particles and liquid droplets by vortex acoustic field enhanced by subwavelength pipe

The nonlinear propagation of acoustic waves in a medium generates acoustic radiation force. Using acoustic radiation force, particles and liquid droplets in gases can be levitated and manipulated. Acoustic levitation techniques can manipulate larger objects in the medium without contact, and therefore have been widely used in chemical analysis, droplet dynamics, and bioreactors. The acoustic levitation researches mainly focus on manipulating particles and droplets in an open environment, which provides flexibility in its use. However, this approach has limitations in terms of its efficiency in utilizing acoustic field energy. In this work we propose a concept of subwavelength pipe-enhanced acoustic tweezers, in which the acoustic field is used to manipulate expanded polystyrene particles (EPS) and droplets inside an acoustic pipe with an inner diameter smaller than the wavelength. In this work, we use four small transducers to excite a single low-order mode of a circular waveguide and its simplex state, and we also use the vortex sound field generated inside the waveguide to levitate and manipulate expanded polystyrene particle and droplet in the air. Compared with previous work in an open environment, we significantly enhance the acoustic radiation force by means of the acoustic resonance effect of the subwavelength duct, with both radial and axial suspension force magnitude increasing considerably. Similar concepts of subwavelength optical waveguides and resonant cavities and their effectiveness were already well known and widely used in the field of optics. In this work we first explain theoretically the basis for the design of subwavelength pipe-enhanced acoustic tweezer dimensions. Then, we point out in simulation that the pipe-enhanced acoustic tweezers, compared with the open environment acoustic tweezers, have strong sound field gradient distribution and acoustic radiation force distribution in the pipe. This conclusion is demonstrated experimentally. Finally, the manipulation of droplet and particle levitation and rotation in subwavelength-pipe-enhanced acoustic tweezers is systematically carried out. In this work we introduce the concept of subwavelength acoustic pipe for acoustic manipulation, which is expected to deepen the physical understanding of the interaction between acoustic fields and matter, and to develop new miniaturized acoustic manipulation devices for levitating particles and droplets.

Fluorescence profiles of water droplets in stable levitating droplet clusters.

Clusters of nearly identical water microdroplets levitating over a locally heated water layer are considered. The high-resolution and high-speed fluorescence microscopy showed that there is a universal brightness profile of single droplets, and this profile does not depend on the droplet temperature and size. We explain this universal profile using the theory of light scattering and propose a new method for determining the parameters of possible optical inhomogeneity of a droplet from its fluorescent image. In particular, we report for the first time and explain the anomalous fluorescence of some large droplets with initially high brightness at the periphery of the droplet. The disappearance of this effect after a few seconds is related to the diffusion of the fluorescent substance in water. Understanding the fluorescence profiles paves the way for the application of droplet clusters to the laboratory study of biochemical processes in individual microdroplets.

An investigative study into an oscillatory reaction in acoustically levitated droplets.

For the first time we have studied an oscillatory chemical reaction (the well-known Belousov-Zhabotinsky (BZ) reaction) in acoustically levitated droplets. Acoustically levitated droplets allow wall-less reaction studies, reduce consumption of sample/reagents, offer high throughput measurements, and enable environmentally friendly chemistry by significantly reducing plastic waste. In this work, microdroplets of the BZ reactants were mixed at the central axis of a low-cost acoustic levitator. The chemical reaction observed in acoustically levitated droplets proceeded in the same way as that in both stirred and unstirred vials where the volume of droplets was 750-fold lower than the solutions in vials. The observed oscillation frequency in droplets was lower than that observed in vials, possibly as a result of evaporative cooling of the droplets. This work has shown that oscillatory reactions can be successfully carried out in acoustically levitated droplets, which allows the application of this technique to areas such as analysis, synthesis and actuation of smart materials and studies of the origins of life.

Evaporation Caused Invaginations of Acoustically Levitated Colloidal Droplets.

Controlled buckling of colloidal droplets via acoustic levitation plays an important role in pharmaceutical, coating, and material self-assembly. In this study, the evaporation process of PTFE colloidal droplets with two particle concentrations (60 wt% and 20 wt%) was investigated under acoustic levitation. We report the occurrence of surface invagination caused by evaporation. For the high particle concentration droplet, the upper surface was invaginated, eventually forming a bowl-shaped structure. While for the low particle concentration droplet, both the upper and lower surfaces of the droplet were invaginated, resulting in a doughnut-like structure. For the acoustically levitated oblate spherical droplet, the dispersant loss at the equatorial area of the droplet is greater than that at the two poles. Therefore, the thickness of the solid shell on the surface of the droplet was not uniform, resulting in invagination at the weaker pole area. Moreover, once the droplet surface was buckling, the hollow cavity on the droplet surface would absorb the sound energy and results in strong positive acoustic radiation pressure at bottom of the invagination, thus further prompting the invagination process.

Laser Ablation Secondary Electrospray Ionization for In Situ Mass Spectrometric Interrogation of Acoustically-Levitated Droplets.

The composition of acoustically levitated droplets was probed by a novel combination of mid-IR laser evaporation and subsequent postionization via secondary electrospray ionization. The combination of microliter samples and subnanoliter sampling provided time-resolved interrogation of droplets and allowed for a kinetic investigation of the laser-induced release of the analyte, which was found to strongly depend on the analytes. The observed substance-specific delayed release of the analytes permitted baseline-separated discrimination of the analytes, ideal for the study of complex samples. The additionally applied postionization scheme was found to enable efficient detection of small volatile compounds as well as peptides. The detection of small molecules and peptides occurred under very different sampling geometries, pointing to two distinct underlying ionization mechanisms. Overall, our results suggest that the experimental setup presented in this study can serve as a widely applicable platform to study chemical reactions in acoustically levitated droplets as model reactors.

Extraordinary Solidification Mechanism of Liquid Alloys Under Acoustic Levitation State.

The acoustic levitation of various materials can be realized by highly intensive ultrasound, which provides a free surface and containerless state for materials processing under space simulation conditions. The nonlinear effects such as acoustic radiation pressure, acoustic streaming, and ultrasonic cavitation open up special access to modulate the fluid dynamics and solidification mechanisms of liquid materials. Here, the physical characteristics of liquid flow, undercooling capability, phase separation, and crystal nucleation and growth within acoustically levitated droplets are explored comprehensively to reveal the extraordinary solidification kinetics of liquid alloys. The sectorial shape oscillations of the 2nd to 10th order modes accompanying internal potential flow are observed for water droplets with modulated ultrasound amplitudes, while the enhanced ultrasound intensity promotes ice nucleation and thus reduces water undercooling. The migration of Sn-rich globules during phase separation of immiscible Al-Cu-Sn alloy is dominated by the droplet deformation and rotation related to acoustic levitation. The high undercooling states of liquid Ag-Cu-Ge and Ni-Sn alloys during acoustic levitation result in the refinement of (Ag) dendrites and the formation of anomalous (Ni+Ni3 Sn) eutectics. The ultrasound-liquid interaction also induces surface waves during the containerless solidification of Ag-Cu and Ni-Sn eutectic alloys.

Combined effect of acoustic field and gas absorption on evaporation of slurry droplet

In this study, we developed a model of convective heat and mass transfer of an acoustically levitated slurry droplet that evaporates in an atmosphere of air, water vapor, and soluble gas. The advanced model considered the effects of acoustic streaming, forced convection, and non-isothermal gas absorption on the evaporation rate of a liquid droplet containing small solid particles. It is shown that the time of porous shell formation decreases with the increase of sound pressure level (SPL) and increases with the increase in frequency. Adding soluble gas to the gaseous phase shortens the time required for porous shell formation during slurry droplet drying. As shown by numerical calculations, in a gas mixture containing air with an ammonia content of 30%, at a temperature of 293 K and a humidity of 50%, with an acoustic field frequency of 2π·45 kHz and an SPL of 140 dB, the time of porous shell formation of a silica-water slurry droplet with the initial radius of 250 µm is about half shorter than that in a gas mixture not containing an active gas. The prediction of the developed model was in good agreement with the experimental results available in literature.

Oscillation Dynamics of Multiple Water Droplets Levitated in an Acoustic Field.

This study aimed to improve and investigate the oscillation dynamics and levitation stability of acoustically levitated water droplets. Contactless sample manipulation technology in mid-air has attracted significant attention in the fields of biochemistry and pharmaceutical science. Although one promising method is acoustic levitation, most studies have focused on a single sample. Therefore, it is important to determine the stability of multiple samples during acoustic levitation. Here, we aim to understand the effect of multiple-sample levitation on levitation stability in acoustic fields. We visualized the oscillatory motion of multiple levitated droplets using a high-speed video camera. To characterize the dynamics of multiple levitating droplets, the oscillation frequency and restoring force coefficients of the levitated samples, which were obtained from the experimental data, were analyzed to quantify the droplet–droplet interaction. The oscillation model of the spring-mass system was compared with the experimental results, and we found that the number of levitating droplets and their position played an important role in the levitation stability of the droplets. Our insights could help us understand the oscillatory behavior of levitated droplets to achieve more stable levitation.

Precise density measurement and its uncertainty evaluation for refractory liquid metals over 3000 K using electrostatic levitation

Accurate density measurement of molten refractory metals over 3000 K is very challenging, and difficult to achieve with conventional methods. Although containerless techniques have been the most effective and well-established methods to measure the density of molten metals at such high temperatures, a large discrepancy in the containerlessly measured density values has been reported. Here, we identify the uncertainty factors of the density measurement and their influence on the measured density of molten refractory metals over 3000 K using an electrostatic levitator (ESL). We find that intensely focused laser beams can cause rotation-induced deformation of a levitated droplet and thus the large uncertainty in the measured density. Moreover, the combination of sample rotation and precession seriously affects the measurements of density and temperature dependence of density (i.e., volume thermal expansion). By minimizing such rotation and precession, we successfully measure the density and volume expansion coefficient of refractory liquids (tantalum, molybdenum, and niobium) with significantly improved reproducibility and accuracy, and evaluate the uncertainties associated with the density measurement using ESL.

Floating synthesis with enhanced catalytic performance via acoustic levitation processing

Acoustic levitation supplies a containerless state to eliminate natural convection and heterogeneous crystal nucleation and thus provides a highly uniform and ultra clean condition in the confined levitating area. Herein, we attempt to make full use of these advantages to fabricate well dispersed metal nanoparticles. The gold nanoparticles, synthesized in an acoustically levitated droplet, exhibited a smaller size and improved catalytic performance in 4-nitrophenol reduction were synthesized in an acoustically levitated droplet. The sound field was simulated to understand the impact of acoustic levitation on gold nanoparticle growth with the aid of crystal growth theory. Chemical reducing reactions in the acoustic levitated space trend to occur in a better dispersed state because the sound field supplies continuous vibration energy. The bubble movement and the cavitation effect accelerate the nucleation, decrease the size, and the internal flow inside levitated droplet probably inhibit the particle fusion in the growth stage. These factors lead to a reduction in particle size compared with the normal wet chemical synthetic condition. The resultant higher surface area and more numerous active catalytic sites contribute to the improvement of the catalytic performance.

Three-Dimensional Droplet Manipulation with Electrostatic Levitation.

An active and precise method for three-dimensional (3D) droplet manipulation is introduced. By modulating the local electrostatic force acting on droplets in carrier oil between needle plate electrodes, the vertical motion of droplets can be controlled, including the droplet levitation at the interface between the carrier oil and the air. Levitated droplets can be translated horizontally with high efficiency by the motion of the needle electrode. With controllable local deformation on the flexible plate electrode, selective manipulation can be realized for multiple droplets. Applying the manipulation method proposed, a platform is built and various droplet handling, such as transport, merging, and mixing, is performed effectively. Complex droplet transport trajectories are achieved by moving the needle electrode. The droplet transport velocity can reach up to 37 mm/s. The introduced method has fundamental advantages of avoiding cross-contamination between droplets, enhancing the flexibility, eliminating the transport track constraint, and lowering costs with straightforward and precise droplet manipulation.

Light Droplet Levitation in Relation to Interface Morphology and Liquid Property.

Light droplet levitation is an elegant technique allowing for contact-less manipulation in a wall-free environment. However, direct generation of light levitated droplets remains limited by small-curvature interface and underlying mechanism remains unclear. Here we report that small-curvature interface limitation encountered in liquid water is overcome by using liquids with extremely small saturated vapor pressure, which allows for direct generation of light levitated droplets above large-curvature interface. It is demonstrated that the interface morphology and extremely small saturated vapor pressure of liquids together contribute to creation of the gravity-lift and evaporation-condensation balances, enabling droplet levitation even above large-curvature interface. We also propose a levitation number Lv to judge whether droplets can be directly levitated above a curved interface or not, which successfully predicts the occurrence of light droplet levitation. When Lv falls in the range of 2.25 × 10-4 ∼ 6 × 10-3, tiny condensed droplets can be stably levitated above the gas-liquid interface no matter interface morphology and liquid type. The study deepens the understanding of the underlying mechanism for generating light levitated droplets.

Evaporation dynamics of a surrogate respiratory droplet in a vortical environment

HypothesisVortex droplet interaction is crucial for understanding the route of disease transmission through expiratory jet where several such embedded droplets continuously interact with vortical structures of different strengths and sizes. ExperimentsA train of vortex rings with different vortex strength, quantified with vortex Reynolds number (Re′=0,53,221,297) are made to interact with an isolated levitated droplet, and the evolution dynamics is captured using shadowgraphy, particle image velocimetry (PIV), and backlight imaging technique. NaCl-DI water solution of 0, 1, 10 and 20 wt% concentrations are used as test fluids for the droplet. FindingsThe results show the dependence of evaporation characteristics on vortex strength, while the crystallization dynamics was found to be independent of it. A reduction of 12.23% and 14.6% in evaporation time was seen in case of de-ionized (DI) water and 1% wt NaCl solution respectively in presence of vortex ring train at Re′=221. In contrast to this, a minimal reduction in evaporation time (0.6% and 0.9% for DI water and 1% wt NaCl solution, respectively) is observed when Re′ is increased from 221 to 297. The mechanisms for evaporation time reduction due to enhancement of convective heat and mass transfer from the droplet and shearing away of vapor layer by vortex ring interaction are discussed in this work.

Heat and mass transfer during levitation of a liquid nitrogen Leidenfrost droplet on a water pool

The understanding of cryogenic droplets evaporating on a liquid surface is of great interest in many industrial applications such as the Dearman engine, or food and beverage processing. However, due to the complexity of the physics involved, the entire process is far from being fully understood. Therefore, in the present study, the dynamics of a liquid nitrogen droplet deposited on a water surface is experimentally studied and theoretically modeled. During continuous cooling through the floating nitrogen droplet, the liquid at the pool surface may eventually solidify, which may significantly alters the situation for the evaporating droplet compared to the situation in which the pool remains liquid for the entire time. A crude theoretical model for the evaporation process of a floating nitrogen droplet is developed to calculate the vaporization rate of the droplet and the vapor layer thickness as a function of the radius of the droplet. The predictions from the model are compared to the experimental results, showing the lifetime predicted by the model is in good agreement with the experimental results.