Droplet Lifetime Research Articles

Experimental study on droplet dynamics behavior and combustion characteristics of high performance green propellant in electrical ignition mode

High performance green propellant represented by ammonium dinitramide-based liquid propellant and its new ignition method are the research hotspots of space propulsion in the 21st century. Exploring the complex multi-scale physical properties of multi-component ammonium dinitramide-based liquid propellant droplets in the electrical ignition mode has wide application significance for spray, propulsion system design and combustion control. The droplet dynamics behavior and combustion characteristics of propellant droplets at different ignition voltages were studied experimentally. The droplet dynamics behavior during the evaporation process, including violent volume oscillation, approximate steady-state expansion, contraction, secondary expansion, puffing and micro-explosion, have been determined by the generation, growth, and discharge of vapor bubbles. In the initial evaporation process, the heterogeneous nucleation is dominant. As the droplet is continuously heated, homogenization nucleation gradually dominates. The main physical and chemical mechanisms of bubble evolution driven by temperature involve methanol boiling, water overheating, ammonium dinitramide decomposition and combustion reaction between vapor molecules. Increasing the ignition voltage increases the droplet dynamics behavior and the combustion, but promotes the combustion instability. Increasing the ignition voltage increases the ignition delay time, puffing delay time, droplet lifetime, maximum temperature of droplet, and reduces the ignition critical diameter. It is proposed that the method of suppressing the droplet breakup dynamics at decomposition area and enhancing the droplet breakup dynamics at the combustion area are conducive to the combustion control of the thruster in electrical ignition mode. This research provides novel insight into the study of the electrical ignition mechanism of liquid fuels.

Characterizing Theta-Emitter Generation for Use in Microdroplet Reactions.

Theta emitters are useful for generating microdroplets for rapid-mixing reactions. Theta emitters are glass tips containing an internal septum that separates two channels. When used for mixing, the solutions from each channel are sprayed with mixing occurring during electrospray ionization (ESI) with reaction times on the order of microseconds to milliseconds. Theta emitters of increasing size cause the formation of ESI droplets of increasing size, which require longer times for desolvation and increase droplet lifetimes. Droplets with longer lifetimes provide more time for mixing and allow for increased reaction times prior to desolvation. Because theta emitters are typically produced in-house, there is a need to consistently pull tips with a variety of sizes. Herein, we characterize the effect of pull parameters on the generation of distinct-sized theta emitters using a P-1000 tip puller. Of the examined parameters, the velocity value had the largest impact on the channel diameter. This work also compares the effect of pulling parameters between single-channel and theta capillaries to examine how the internal septum in theta capillaries affects tip pulling. We demonstrate the utility of using theta emitters with different sizes for establishing distinct reaction times. Finally, we offer suggestions on producing theta emitters of various sizes while maintaining high repeatability. Through this work, we provide resources to establish a versatile and inexpensive rapid-mixing system for probing biologically relevant systems and performing rapid derivatizations.

Liquid fuel cloud detonation and droplet lifetime

The structures and mechanisms of aerosolized liquid-fuel cloud detonations are studied in a detonation facility using simultaneous high-speed optical diagnostics. The characteristic length scale of the droplet lifetime in liquid fuel detonations is not well predicted by established breakup and evaporation models, whereas it captured by calculations of the evaporation time of the droplet cloud. Novelty and significance statementDetonation research has mostly focused on gaseous fuels with limited investigations of purely liquid fueled detonations. This research explores the characteristic length scales of aerosolized liquid fuel droplets’ lifetime showing it does not scale with established breakup and evaporation models. It scales well with the evaporation time of child droplet clouds, highlighting the significance.

Technical Note: Lifetime of evaporating droplets in a closed volume

The numerical modeling of heat and mass transfer of droplets within a closed volume containing gas heated in relation to the droplets was conducted. The droplet lifetime at variation of initial values of gas and droplet temperatures and droplet mass fraction has been determined. It was found that droplet lifetime exhibits a weak dependence on the initial droplet temperature and a pronounced dependence on the initial gas temperature. Moreover, it was demonstrated that the droplet lifetime in a closed volume is longer than in infinite space. This is due to the fact that the cooling of the surrounding gas by droplets results in a decrease in the evaporation rate of the droplets. A parameter is proposed which allows for the consideration of the effect of evaporating droplets on the thermal regime of the gas-drop mixture, and which enables the generalization of the results of numerical simulation to obtain an expression for the droplet lifetime in a closed volume. The error in the use of the obtained expression is estimated. It was determined that the margin of error for the calculation results is less than ten percent.

Are Hydroxyl Radicals Spontaneously Generated in Unactivated Water Droplets?

Spontaneous ionization/breakup of water at the surface of aqueous droplets has been reported with evidence ranging from formation of hydrogen peroxide and hydroxyl radicals, indicated by ions at m/z 36 attributed to OH•‐H3O+ or (H2O‐OH2)+• as well as oxidation products of radical scavengers in mass spectra of water droplets formed by pneumatic nebulization. Here, aqueous droplets are formed both by nanoelectrospray, which produces highly charged nanodrops with initial diameters ~100 nm, and a vibrating mesh nebulizer, which produces 2 – 20 µm droplets that are less highly charged. The lifetimes of these droplets range from 10s of µs to 560 ms and the surface‐to‐volume ratios span ~100‐fold range. No ions at m/z 36 are detected with pure water, nor are significant oxidation products for the two radical scavengers that were previously reported to be formed in high abundance. These and other results indicate that prior conclusions about spontaneous hydroxyl radical formation in unactivated water droplets are not supported by the evidence and that water appears to be stable at droplet surfaces over a wide range of droplet size, charge and lifetime.

Intricate Evaporation Dynamics in Different Multidroplet Configurations.

We experimentally investigate the evaporation dynamics of an array of sessile droplets arranged in different configurations. Utilizing a customized goniometer, we capture side and top view profiles to monitor the evolution of height, spread, contact angle, and volume of the droplets. Our results reveal that the lifetime of a droplet array surpasses that of an isolated droplet, attributed to the shielding effect induced by neighboring droplets, which elevates the local vapor concentration, thereby reducing the evaporation rate. We found that lifetime increases as droplet separation distance decreases at a fixed configuration and substrate temperature. It is observed that the lifetimes increase with the number of droplets. We observe a decrease in lifetimes, following a power law trend, with increasing substrate temperature, with the shielding effect diminishing at higher substrate temperatures due to natural convective effects. We also observe a generalized behavior for the centrally placed droplet across various separation distances and substrate temperatures. This arises from different droplet configurations and substrate temperatures, which modify the local vapor concentration around the droplets without significantly impacting the contact line dynamics. Additionally, the experimental results are compared with a diffusion-based theoretical model that incorporates the evaporative cooling effect to predict the lifetime of the central droplet within the array. We observe that the theoretical model satisfactorily predicts the lifetime of the droplet at room temperature. However, for high-temperature cases, the model slightly overpredicts the evaporative lifetimes.

Planar Disk μ-Aptasensors by Monolayer Assembly in a Dissolving Microdroplet.

Electrochemical aptamer-based sensors provide a highly modular platform for real-time monitoring of small molecules. Their ability to selectively recognize target molecules in complex environments like biological fluids makes them an attractive technology for the analysis of micro- and nanoscale systems. The signal-to-noise of the measurement depends on the electroactive surface (i.e., how many aptamers one can place), which has previously precluded miniaturization of aptamer-based sensors to planar disk ultramicroelectrodes (r ∼ 5-10 μm). Here, we employ a concentration enrichment strategy based on the active dissolution of an aqueous, aptamer-containing microdroplet on an ultramicroelectrode submerged in an organic continuous phase (1,2-dichloroethane). We show consistent voltammetric signal increase as a function of droplet lifetime, indicating the successful immobalization of the thiol-terminated aminoglycoside aptamers to the electrode surface. We observe a diagnostic methylene blue peak and 10-fold increase in current magnitude as compared to bare microelectrodes. We report robust sensor behavior with a linear dynamic range extending from milli- to micromolar concentrations of kanamycin in buffer. This research offers a successful method for optimized electrochemical aptamer-based sensor fabrication and miniaturization on ultramicroelectrodes without the need for electrode surface area enhancement.

Droplet–droplet vapor-mediated interactions in confined environments

The evaporation of multiple droplets ensues ubiquitously in nature and industry. Vapor mediation caused by evaporating neighboring droplets is a demonstrated phenomenon that shows that droplets can interact with each other via the vapor in both open and confined configurations, i.e., the “shielding effect.” However, interactions between paired droplets in confined environments, more common in industrial processes, remain unexplored. In this Letter, we experimentally investigate the evaporation of water based paired sessile droplets on hydrophilic glass slides at different spacings in the absence and presence of an enclosed chamber. The results demonstrate that a confined environment significantly attenuates droplet evaporation, which intensifies with decreasing spacing between droplets. A 30%–82% increase in the droplet lifetime is found for the shortest distance studied in a confined environment, while results in an open environment are provided as a control. Both the local shielding effect and the global vapor accumulation due to confinement collaboratively induce such strong evaporation suppression. In addition, two well differentiated evaporation regimes ensue in a confined environment where the shielding effect initially dominates the evaporation suppression, whereas confinement governs the later evaporation stage. The later stage accounts for over 60% of the droplet lifetime. Such transition and further evaporation suppression, when compared to the classical shielding effect, highlights the importance of a confined environment in multiple droplet evaporation.

Suppressing the Leidenfrost effect by air discharge assisted electrowetting-on-dielectrics

The Leidenfrost effect for a droplet on an over-heated substrate always results in a superhydrophobic state, significantly hindering the water evaporation for heat dissipation. Here, we demonstrate a strategy of air discharge assisted electrowetting-on-dielectrics (ADA-EWOD), overcoming this challenge. This strategy increases the solid surface free energy by generating air discharge near the three-phase contact line of the droplet and combines it with the electromechanical force to decrease the contact angle, which makes ADA-EWOD have stronger wetting capabilities than traditional electrically control methods that only rely on electromechanical force. The water contact angle on an over-heated surface (above 350 °C) is decreased from nearly 180° down to less than 10°. This superhydrophilicity at high temperature reduces the droplet lifetime by at least 10 times, well inhabiting the Leidenfrost effect. Furthermore, we use ADA-EWOD in droplet evaporation for heat dissipation, where a heated silicon wafer at 600 °C is cooled down to less than 200 °C within 20 s. We believe that the present work provides a perspective on suppressing the Leidenfrost effect, which may have important potential applications in the field of heat dissipation.

Electrical ignition of ADN-based green liquid propellant in constant volume combustion chamber and continuous flow

Ammonium dinitramide (ADN) propellant for green space propulsion is perceived as a focal point in space propulsion research. The development of electrical ignition methods to realize the ignition for ADN-based liquid propellant can overcome the technical bottleneck of the catalytic ignition methods currently applied to ADN-based space thrusters. This work presents an experimental investigation of the electrical ignition and combustion characteristics of an ADN-based liquid propellant in a constant volume combustion chamber, considering ignition voltages and initial pressures. It is observed that increasing the ignition voltage and initial pressure reduces the flame retardation period, combustion duration, droplet lifetime, ignition energy, and total energy. Also, the combustion peak pressure decreases with an increase in the ignition voltage but increases with higher initial pressures. The energy consumption is mainly concentrated in the ignition stage, with the ignition energy accounting for approximately 76% to 96% of the total energy. The measurement of the concentrations of the keycombustion products are reported, highlighting that the mole fraction of CO decreases with an increase in ignition voltage and pressure. The ignition and combustion of the ADN-based liquid propellant in continuous flow were achieved for the first time through a combination of resistive ignition and high-temperature arc-assisted combustion. During the testing of the propellant ignition in continuous flow, there is a need for a continuous supply of electrical energy, and the combustion cannot be self-sustaining.

Experimental investigation on enhanced combustion of methanol/heavy fuel oil by droplet puffing at elevated temperatures

To achieve high-efficiency combustion of heavy fuel oil (HFO), this study investigated the combustion characteristics of methanol/HFO droplets with methanol content from 10 to 30% using the suspension method under ambient temperature from 923 to 1023 K. The combustion of methanol/HFO droplets was summarized as a two-phase process consisting of six typical stages, emphasizing liquid phase. Especially, the fluctuation evaporation stage, induced by frequent and intense puffing, was identified as prominent character. Both the ignition delay and lifetime of HFO and methanol/HFO droplets decreased with increasing ambient temperatures. For the methanol/HFO droplet, the ignition delay and droplet lifetime increased with the increasing methanol content. Prominently, compared to HFO, HM10 had the most significant reduction in droplet lifetime and TINL under the same operating conditions, which indicated that the addition of 10% methanol accelerated the combustion process and reduced soot generation. Additionally, the thermos-dynamic characteristics of methanol/HFO droplets were investigated. Puffing was primarily attributed to superheating of methanol and pyrolysis of heavy components in HFO, which resulted in active and passive rupture of bubbles. Similarity and maximum deformation were employed to qualitatively distinguish between them. The obtained findings aimed to develop a promising alternative fuel to reduce emissions and preserve energy.

A novel asymptotically consistent approximation for integral evaporation from a spherical cap droplet

The total evaporation rate due to a volatile capillarity-dominated droplet diffusively evaporating into the surrounding gas is a critically important quantity in industrial and engineering applications such as Q/OLED screen manufacturing. However, the analytical expression in terms of integrals in toroidal coordinates can be unwieldy in applications, as well as expensive to compute. Therefore, simple yet highly accurate approximate solutions are frequently used in practical settings. Herein we present a new approximate form that is both accurate and fast to compute, but also retains the correct asymptotic behaviour in the key physical regimes, namely hydrophilic and superhydrophobic substrates, and a hemispherical droplet. We illustrate this by comparison to several previous approximations and, in particular, illustrate its use in calculating droplet lifetimes, as well as approximating the local evaporative flux.

Enhancing the vaporization and secondary atomization of two-liquid droplets for fire suppression

Large-scale fires of various origins are most often contained and suppressed using fine water spray systems with a droplet size of 50–300 μm. Enhancing the extinguishing performance of these systems is an urgent task as this will reduce the amount of irreversible damage fires cause to forests, buildings, and compartments. Finer droplet size of the extinguishing liquid sprayed directly in the combustion zone prevents droplet entrainment, thus increasing the firefighting efficiency and reducing the extinguishing liquid consumption. However, too little is still known about alternative ways to intensify the vaporization and secondary atomization of fine water sprays in order to disrupt the chemical reactions responsible for combustion. Experimental findings on enhancing the secondary atomization and phase change of sprayed water with added vegetable oils for fire suppression are presented. Experiments on water droplets with sunflower, rapeseed, and tall oils were performed in the ethanol burner flame zone. The radii of two-liquid droplets were 0.85 ± 0.05 mm. The concentration of explosion-triggering additives to water was constant at 10 ± 2 vol.% in all the experiments. The addition of 10 vol.% rapeseed oil to water allows one to reduce the local gas temperature around the droplet up to 250 K in case of micro-explosion of an isolated droplet and up to 450 K in case of micro-explosion of a group of droplets. The experimental findings were processed to obtain approximations of the relationships established in the experiments: micro-explosion delay times, droplet lifetimes, and normalized evaporation surface area after breakup. A physical model is presented describing the processes involved in the experiments. An additional series of experiments involved suppressing a laboratory-scale wood crib fire using water and water/rapeseed oil emulsion to compare the fire suppression times and the extinguishing liquid consumption in both cases. The suppression times and the extinguishing liquid consumption were on average 15% lower when using a water/rapeseed oil emulsion compared with pure water. Practical recommendations are given on how to provide the intense disruption of extinguishing liquids with added vegetable oils to accelerate fire suppression. Two-liquid droplets exploding in the flame zone are established as a possible alternative to water capsules, extinguishing missiles, and cryogenic technologies for suppressing large-scale fires.

Evaluation of droplet evaporation characteristics and its influencing factors in high-temperature gaseous environment

Droplet evaporation is widely present in nature and industrial production, involving heat and mass transfer between the droplet and the surrounding gas environment. This study investigates the evaporation of individual spherical droplets in a high-temperature gas environment under both stationary and forced convection conditions, using theoretical and numerical approaches. The results show that the established evaporation model can effectively reflect the temperature change during droplet evaporation. When the droplet evaporates in a gaseous environment, the equilibrium temperature of the droplet is influenced by environmental parameters such as temperature and humidity. Compared to environmental temperature, humidity has a more significant impact on the equilibrium temperature of the droplet, making it the dominant factor affecting the evaporation temperature. The evaporation rate of the droplet is influenced by environmental humidity. When the air humidity is below 0.5, the variable-rate evaporation aligns closely with the numerical simulation results (droplet lifetime). When the air humidity is above 0.5, the simulation results are slightly higher but still have a small relative error. As the environmental humidity increases, the evaporation rate of the droplet is suppressed, leading to an increase in its lifetime. Establishing an appropriate droplet evaporation model not only deepens our understanding of the internal mechanisms of droplet evaporation but also provides practical guidance for enhancing heat and mass transfer in droplets.

Droplet evaporation on two-tier hierarchical micro-pillar array surface

As surface modification becomes a promising approach to thermal management, quantifying effects of surface structure geometrical dimension is of great significance to optimize heat transfer performance. In this paper, we emphatically report droplet evaporation on three types of two-tier hierarchical micro-pillar array (THM) surfaces. Utilizing the lattice Boltzmann model, evaporation characteristics including droplet morphology, triple-phase contact line (TPCL) behavior, liquid–vapor interface evolution and heat flux distribution are elucidated. In comparison to single-tier micro-pillar array (SM) surfaces, THM surfaces enable remarkable enhancement in heat transfer when secondary pillars are configured on the side of the primary pillar or configured parallelly to the primary pillar on the substrate, which is attributed to the synergistic effect of TPCL evaporation and liquid–vapor interface evaporation. The varying trend of droplet lifetime with the primary pillar geometrical dimension on all THM surfaces are also summarized, giving a guideline for optimization of heat dissipation by tailoring structures.

Investigation of the effect of curvature on the local mass flux of evaporating droplets using a phase field method

Droplet evaporation, a fundamental process with wide-ranging applications, involves complex physical mechanisms that significantly influence mass transfer rates and droplet lifetimes. Despite its significance, the influence of droplet deformation induced by capillary forces on local saturation conditions and its subsequent impact on phase change have received limited attention. This study delves into the impact of capillary-induced droplet deformation on local saturation conditions and its subsequent effect on phase change within a quiescent vapor environment. Contrary to most analytical and numerical studies that impose temperature and heat flux conditions at the vapor–liquid interface, our investigation employs direct numerical simulations based on the thermal Navier–Stokes–Korteweg model, capturing the dynamics of the droplet and its interface without relying on ad-hoc interface conditions. Our findings demonstrate that droplet deformation induces free oscillatory motion, which, although not significantly altering droplet lifetime due to viscous damping, leads to non-uniform mass fluxes along the droplet interface. We observe that areas of higher curvature are associated with increased local pressure and reduced temperatures, a discrepancy primarily attributed to evaporative cooling. This effect is further elucidated through an analysis of pressure and energy fluxes, revealing that regions of high curvature experience significant heat consumption, driving evaporative cooling and creating temperature gradients along the interface. Conversely, areas with negative curvature exhibit a reverse trend, indicating the complex interplay of capillarity, deformation, and phase change in droplet evaporation. Our results highlight the intricate relationship between droplet deformation, local pressure, mass flux, and temperature, underscoring the necessity for improved modeling of these factors in future studies on droplet evaporation dynamics.

Electrical effects on droplet behaviour

The effect of charge on water droplets modulates various aspects of their behaviour. These include the droplet stability, evaporation, and lifetime. Microphysical models have been developed such that a reasonably good understanding of these processes has been achieved. However, the specific effects of charge deserve further scrutiny as they are an intrinsic component of the factors controlling droplet characteristics. Describing the effects of these requires an understanding of the electrostatic pressure present in the droplet and its surface tension. One way to test these effects and assess droplet response to charge is to take an experimental approach to make observations directly. In this study, individual droplets are levitated in an acoustic wave to allow isolated measurements to be taken. The droplets are monitored using a CCD camera with a microscope objective lens. In some cases, with sufficient charge present, effects on droplet stability can be observed as Rayleigh explosions, where a sudden drop in mass is seen superimposed on the evaporation profile. These events also allow the charge on the droplet to be calculated, which is then compared with the droplet evaporation. Another factor that plays a part in droplet behaviour is droplet composition. Different substances have different surface tension, and this is explored by performing some experiments on sulphuric acid droplets. Theory predicts that the more highly charged a droplet is, the more resistant to evaporation it becomes. Experimental data collected during this study agrees with this, with more highly charged droplets observed to have slower evaporation rates. However, highly charged drops were also observed to periodically become unstable during evaporation and undergo Rayleigh explosions. Each instability of a highly charged drop removes mass, reducing the overall droplet lifetime regardless of the slower evaporation rate. The sulphuric acid droplets were observed to be much more resistant to evaporation and no Rayleigh instabilities were observed.

Evaporation characteristics of cis-1,1,1,4,4,4-hexafluoro-2-butene (R1336mzz(Z)) droplet in high pressure and temperature environments

Cis-1,1,1,4,4,4-hexafluoro-2-butene (R1336mzz(Z)) has emerged as an exceptionally promising low-global-warming-potential (GWP) refrigerant, ideal for spray cooling systems in the thermal management of electronic components. Research on the evaporation characteristics of an individual isolated cryogen droplet excludes uncertainties caused by droplet collisions and fusion, thereby laying the foundation for spray cooling. In this paper, a theoretical model for single R1336mzz (Z) droplet evaporation considering the effect of natural convection in a high pressure and temperature environment is proposed. The newly proposed model is validated by comparing the predicted results of the R1336mzz(Z) droplet evaporation with experimental data. Then, the effects of environmental temperature (323–523 K) and pressure (1–20 bar) on the R1336mzz(Z) droplet evaporation are investigated. The results reveal that the effect of increasing the ambient pressure on the droplet lifetime of R1336mzz(Z) undergoes a transition from deceleration to acceleration. Elevated temperature can promote droplet evaporation; however, the promoting effect of increasing the ambient temperature on droplet evaporation will be weakened in high-pressure cases. Increasing the ambient pressure and temperature both can enhance the heat transfer from the environment to the droplet through natural convection, while increasing the pressure greatly inhibits the molecular diffusion during droplet evaporation. Thus, the total evaporation rate depends on the competing effects of these two factors. In addition, the trend of the droplet temperature variation could differ based on droplet initial temperatures, ambient temperatures, and pressures. An increase in the ambient temperature or pressure corresponds to an increase in the droplet equilibrium temperature (Tequ). However, Tequ is almost independent of the droplet initial size and temperature.

Experimental study of the evaporation characteristics of unsymmetrical dimethylhydrazine in a sub-supercritical environment

In this study, the temperature–pressure separation technique was used to successfully obtain the process of the unsymmetrical dimethylhydrazine droplet diameter variation with time during evaporation in subcritical, supercritical, and far-supercritical environments at temperatures and pressures of 473–773 K and 1–9 MPa, respectively. The results show that unsymmetrical dimethylhydrazine droplet evaporation has two characteristic stages, transient heating and steady-state evaporation, under low-pressure and low-temperature environments; the steady-state evaporation stage influences droplet evaporation life more than the transient heating stage. Under high-pressure and high-temperature environments, droplet evaporation shows only a single feature of steady-state evaporation, and the droplet expansion process in the transient heating stage disappears. The normalized droplet lifetime decreased exponentially with temperature and decreased linearly and monotonically with pressure. The evaporation rate constant increases linearly and monotonically with temperature and exponentially with pressure.

Effect of non-ideal mixture on flame–spray interaction in counterflow n-heptane/ethanol flames

This work presents a numerical study of bi-component n-heptane/ethanol spray flames in a counterflow to improve the understanding of the effect of non-ideal mixtures on the flame–spray interaction. The simulations of this spray configuration allow for the detailed examination of the real fluid behavior and its influence on gas-phase combustion kinetics for a wide range of relevant parameters, i.e., droplet composition, size, and gas strain rate. The results reveal that due to the large real activity coefficient of n-heptane and the resulting selective evaporation of the heavier component, the burning characteristics of the spray with a high initial ethanol content do not differ if non-ideal mixture is considered. Conversely, the flame structure with a low ethanol content is substantially different, and displacement of the flame towards the spray side of the configuration is found. A multiple reaction zone, displaying both partially-premixed and diffusion flames, exists, which is overestimated spatially if the fluid is assumed ideal. In general, the ideal prediction of real fluid would misinterpret the composition evaporation order, prolonging the droplet lifetime and consequently, enhancing the flame–spray interaction locally, which results in a lower flame temperature. A parameter characterizing the multicomponent droplet penetration is introduced, which highlights the appreciable non-ideal effects that vary with droplet size and composition, and most importantly, further demonstrates the necessity of considering the real fluid properties in design of next-generation engines with blended fuels.