Behavior Of Droplets Research Articles

In Situ Monitoring of Droplet Behavior in Inverse Microemulsions.

Thermoresponsive polymer assemblies are of growing interest in fields ranging from photonics to drug delivery, with their phase transitions often attributed to upper- or lower-critical solution temperatures and cloud-point behaviors. However, the direct imaging of these nanoscale transitions remains underexplored. This study addresses that gap by developing a temperature-sensitive inverse microemulsion system and elucidating its dynamic structural transitions under heating. We present a temperature-sensitive inverse microemulsion system composed of the nonionic surfactants Brij 010 and Span 80. Upon heating within a stable microemulsion temperature range, the decrease in hydrogen bonding between the hydrophilic surfactant head and the dispersed phase results in an initial droplet contraction. Above a critical destabilization temperature, the droplets expand and destabilize as the affinity of the surfactant for the continuous phase increases. This intriguing behavior was observed via dynamic light scattering and liquid-phase transmission electron microscopy, which revealed a rapid and reversible droplet transformation during heating cycles. This versatile inverse microemulsion system also serves as a modular nanoreactor for polymerizations, demonstrated through both conventional radical and photoiniferter polymerization. Our research contributes to the understanding of inverse microemulsions, which offer a platform for precise nanoparticle synthesis.

Droplet Interactions with Hot Surfaces: Boiling Modes, Leidenfrost Temperature, Dynamics, and Applications.

The interaction of droplets with high-temperature solid surfaces is critical in processes like machining cooling and internal combustion engine operations. As surface temperature rises, droplets transition through distinct boiling regimes: film evaporation, contact boiling, transition boiling, and film boiling. In the film boiling regime, droplets are suspended on a vapor layer formed by their evaporation, known as the Leidenfrost effect, which occurs above the Leidenfrost point-the minimum temperature for this phenomenon. While the vapors layer impairs heat transfer by acting as an insulator, it also facilitates droplet mobility, enabling applications in fluid motion control and driving research interest in this area. This review provides a comprehensive overview of droplet interactions with heated surfaces. It begins with a classification of boiling regimes and the criteria defining them, followed by an analysis of factors influencing the Leidenfrost point, including surface properties, liquid characteristics, and external conditions. The motion behaviors of droplets on high-temperature structured surfaces such as horizontal transport, vertical detachment, and rotation are then explored. Finally, potential applications for controlling droplet behavior on hot surfaces are discussed, including enhanced heat transfer, self-cleaning, drag reduction, and energy conversion, while highlighting emerging directions for future research.

Spreading behavior of liquid droplets upon impact on wetted cylindrical surfaces

Abstract This study experimentally investigated the spreading behavior of droplets impacting wetted cylindrical surfaces. The temporal evolution of spreading dimensions along the circumferential and axial directions of the cylinder was measured. The effects of fluid type, impact velocity, cylinder diameter, and impact position on the spreading dimensions were analyzed. The findings show that the droplet velocity minimally influences the spreading dimensions. The spreading dimensions increase with cylinder diameter but remain nearly unchanged when the cylinder diameter exceeds four times the droplet diameter. The spreading characteristics of droplets under eccentric impact were also examined, revealing that forward spreading is significantly greater than backward spreading. Additionally, the phenomenon of one-sided spreading during eccentric impacts was investigated, and it was found that the critical condition for one-sided spreading is largely independent of the impact velocity.

Wettability-driven coalescence behavior of compound droplets over a horizontal surface

Wettability-driven coalescence behavior of compound droplets over a horizontal surface

Experimental study on the evaporation characteristics of sessile droplets under different substrate materials and relative humidity

Abstract Based on the demand to enhance static phase change evaporation heat transfer, this research examines the evaporation behavior of sessile droplets exposed to shear flow within a compact wind tunnel. The results indicate that various factors, including substrate materials, substrate structures, relative humidity (RH), and droplet types, influence the evaporation of droplets affected by shear flow. The evaporation of droplets on copper substrates predominantly aligns with the Constant Contact Radius (CCR) model, exhibiting a higher evaporation rate. Elevated base structures can mitigate the effects of the boundary layer, facilitating faster evaporation of droplets at lower shear flow rates. Increasing relative humidity raises the vapor concentration in the air, thereby inhibiting droplet evaporation. Ethanol droplets under shear flow demonstrate a distinct behavior, with evaporation rates initially increasing before slightly decreasing. At low wind velocities, evaporation is predominantly driven by vapor concentration, while at high wind velocities, it is primarily limited by energy transfer. This research may provide valuable insights for utilizing shear flow to enhance liquid evaporation rates and improve heat transfer efficiency.

Study on the coalesce behaviors of dual droplets adhering to inclined surfaces under airflow conditions

Study on the coalesce behaviors of dual droplets adhering to inclined surfaces under airflow conditions

Distinct evaporation and combustion behaviors of suspended and unsuspended nanodiesel droplets

Distinct evaporation and combustion behaviors of suspended and unsuspended nanodiesel droplets

Atomic-Scale Modeling of Water and Ice Behavior on Vibrating Surfaces: Toward the Design of Surface Acoustic Wave Anti-icing and Deicing Systems.

Within these studies, atomic-scale molecular dynamics simulations have been performed to analyze the behavior of water droplets and ice clusters on hydrophilic and hydrophobic surfaces subjected to high-frequency vibrations. The methodology applied herewith aimed at understanding the phenomena governing the anti-icing and deicing process enabled by surface acoustic waves (SAWs). The complex wave propagation was simplified by in-plane and out-of-plane substrate vibrations, which are relevant to the individual longitudinal and transverse components of SAWs. Since the efficiency of such an active system depends on the energy transfer from the vibrating substrate to water or ice, the agents influencing such transfer as well as the accompanying phenomena were studied in detail. Apart from the polarization of the substrate vibrations (in-plane/out-of-plane), the amplitude and frequency of these vibrations were analyzed through atomic-scale modeling. Further, the surface wettability effect was introduced as a critical factor within the simulation of water or ice sitting on the vibrating substrate. The results of these studies allow identification of the different phenomena responsible for water and ice removal from vibrating surfaces depending on the wave amplitude and frequency. The importance of substrate wetting for anti-icing and deicing has also been analyzed and discussed concerning the future design and optimization of SAW-based systems.

Theoretical model for droplet self-motion in hydrophilic and hydrophobic microchannels with wettability gradient surfaces

Spontaneous liquid transport in microchannels driven by wettability gradient surfaces has received considerable interest for microfluidic applications. In this paper, a theoretical model was developed to investigate the self-motion behaviors of droplet in square microchannels featured by wettability gradient surfaces using 3D (three dimensional) morphological construction and virtual work principle. The effects of wettability gradient and contact angle range on droplet movement were compared and evaluated. A larger wettability gradient contributes to a higher droplet velocity, and the droplet velocity could reach several centimeters per second. In a wettability-gradient microchannel, the droplet velocity increased and decreased along the channel length in the hydrophobic and hydrophilic channels, respectively. For a droplet moving in a microchannel with its surface wettability continuously changing from hydrophobic to hydrophilic zones, the maximum droplet velocity appears at the contact angle of 90°, and it could be about 2.7 cm/s in the 100 μm width channel under a wettability gradient of 4°/mm. At a same wettability gradient, a lower initial contact angle is associated with higher droplet velocity for hydrophobic channels, while it is opposite in hydrophilic channels. Dimensionless correlations were developed to predict the self-motion velocities in hydrophobic and hydrophilic microchannels, and droplet self-motion characteristics at different wettability gradient conditions were elucidated. The droplet velocities obtained from the theoretical model are in good agreement with that from a 3D numerical simulation, but featured by tens of times smaller in time consumption of computation, showing the reliability and high computation efficiency of the theoretical model.

Dynamic behavior of droplets impacting porous particles in oil–water separation

Dynamic behavior of droplets impacting porous particles in oil–water separation

On the direct initiation in n-heptane mists considering droplet fragmentation

In this paper, direct initiated detonation with various initial energy depositions and droplet diameters in a one-dimensional n-heptane/oxygen/nitrogen heterogeneous system is simulated. Detonation evolution, frontal structure, and fuel droplet behavior are investigated. For a stoichiometric mixture with d0 = 5 μm, the critical initiation energy is around 1.6 × 105 J/m2 with no evidence of non-uniqueness. Initiation energy is crucial to activate the reaction O2 + H = OH + O, significantly contributing to OH production and sustaining the detonation. For a self-sustained detonation, initiation energy has a marginal influence on the variations of induction zone length, which is primarily determined by the initial droplet diameter. As droplet size increases, extended induction zone lengths with pronounced fluctuations are seen because of the increased breakup and evaporation time. Elevated Weber number is found both in the vicinity of the leading shock and reaction front, caused by large gas–droplet velocity differences and the low surface tension at the boiling point, respectively. Critical droplet in post-detonation region is found, which leads to the flash vaporization despite the droplet size is still large and yields localized high equivalence ratio and elevated heat release rate, accompanied by amplified shock.

Catalytically Active Coacervates Sustained Out‐of‐Equilibrium

AbstractMetabolically active membraneless organelles of extant biology have the capability to maintain their structure under nonequilibrium conditions by leveraging chemical reactions. Herein, we report active coacervates accessed via a mixture of minimal building blocks that featured π‐electron rich short peptide, positively charged aldehyde, and a cyclic ketone under nonequilibrium conditions. Peptide bound with the aldehyde by a dynamic covalent bond and demixed to form coacervates through hydrophobic interactions. Importantly, the short‐peptide could utilize its free amine (β‐alanine) to catalyze C─C bond formation which eventually led to the depletion of one of the building blocks (aldehyde) via aldol reaction; an intrinsic catalytic role that helped the coacervate to suppress coalescence. Notably, under continuous additions (open system) of the depleting precursors, the active coacervates were able to demonstrate spatial stability for longer duration. This out‐of‐equilibrium behavior of phase separated droplets in presence of flux of building blocks is reminiscent of the active membraneless organelles seen in contemporary biochemistry.

Catalytically Active Coacervates Sustained Out-of-Equilibrium.

Metabolically active membraneless organelles of extant biology have the capability to maintain their structure under nonequilibrium conditions by leveraging chemical reactions. Herein, we report active coacervates accessed via a mixture of minimal building blocks that featured π-electron rich short peptide, positively charged aldehyde, and a cyclic ketone under nonequilibrium conditions. Peptide bound with the aldehyde by a dynamic covalent bond and demixed to form coacervates through hydrophobic interactions. Importantly, the short-peptide could utilize its free amine (β-alanine) to catalyze C─C bond formation which eventually led to the depletion of one of the building blocks (aldehyde) via aldol reaction; an intrinsic catalytic role that helped the coacervate to suppress coalescence. Notably, under continuous additions (open system) of the depleting precursors, the active coacervates were able to demonstrate spatial stability for longer duration. This out-of-equilibrium behavior of phase separated droplets in presence of flux of building blocks is reminiscent of the active membraneless organelles seen in contemporary biochemistry.

Self-Swimming Droplets Driven by the Synergistic Effects of the Fast Interfacial Polymerization and Hydrolysis Reactions.

Self-swimming droplets, as an intriguing phenomenon in the microscopic realm, have traditionally been achieved by harnessing the interfacial properties of small-molecule emulsifiers. Herein, we successfully prepared self-swimming droplets by employing polymers. Leveraging the synergistic effects of fast interfacial polymerization and hydrolysis reactions, a polymer layer is rapidly formed at the oil-water interface, followed by the generation of weakly amphiphilic substances that establish an interfacial tension gradient. Thus, strong Marangoni circulation flows were induced to propel the droplets to move spontaneously. The interfacial behavior of the self-swimming droplets was investigated, revealing regular oscillations and the spontaneous formation of microdroplets on the larger droplet surface. Comparative experiments confirmed the synergistic effects of the two types of reactions in forming self-swimming droplets, as the absence of either reaction hindered autonomous movement. The movement time reaches 758 s, and the maximum speed of droplet movement reaches 5.08 mm/s. This work not only enriches the variety of self-swimming droplets but also provides theoretical guidance for designing and preparing innovative self-swimming systems.

Predator-Prey Behavior of Droplets Propelling Through Self-Generated Channels in Crystalline Surfactant Layers.

Motile droplets provide an attractive platform for liquid matter-based applications and protocell analogues displaying life-like features. The functionality of collectively operating droplets increases by the advance of well-designed (physico)chemical systems directing droplet-droplet interactions. Here, we report a strategy based on crystalline surfactant layers at air/water interfaces, which sustain the propulsion of floating droplets and at the same time shape the paths for other droplets attracted by them. First, we show how decylamine forms a closed, crystalline layer that remains at the air/water interface. Second, we demonstrate how aldehyde-based oil droplets react to decylamine in the crystalline layer by forming an imine, causing the droplets to move through the layer while leaving behind an open channel (comparable to "Pac-Man"). Third, we introduce tri(ethylene glycol) monododecylether (C12E3) droplets in the crystalline layer. The crystalline layer suppresses the motion of the C12E3 droplets, however, the aldehyde droplets create surface tension gradients upon depletion of surfactants from the air/water interface, thereby driving Marangoni flows that attract the C12E3 droplets as well as the myelin filaments they grow: Causing the C12E3 droplets to chase, and ultimately catch, the aldehyde droplets along the channels they have created, featuring a predator-prey analogy established at an air/water interface.

Modeling a Continuously Operating Electrospray Ionization Emitter Using Molecular Dynamics Simulations: From Bulk Solution to Gaseous Ions.

Numerous analytical workflows involve electrospray ionization (ESI), a process that converts solution species into gaseous ions for detection by mass spectrometry (MS). Upon exposure of a solution-filled emitter capillary to an electric field, ESI proceeds via a liquid jet that forms at the apex of a Taylor cone. Breakup of this jet generates charged droplets that ultimately release analyte ions into the gas phase. Many facets of these events remain incompletely understood. Molecular dynamics (MD) simulations of ESI droplets have become an important tool for ESI mechanistic investigations. Here, we extend these modeling efforts to entire ESI emitter capillaries. Such simulations face significant challenges. Chief among these is the fact that experimental ESI emitters operate in steady-state mode, where ejected solution is constantly replenished from an upstream reservoir. Previous MD simulations employed capillaries with a finite amount of solution, resulting in only short bursts of sprayer operation. The current work develops an MD algorithm that combines trajectory stitching with "solution recycling". This algorithm periodically removes ejected droplets, while simultaneously replenishing the emitter inlet with fresh solution. For the first time, this approach provides continuous MD-based ESI operation, while keeping the instantaneous number of atoms (and hence computational cost) manageable. For a 50 ns simulation, we illustrate a reduction of wall clock time from an estimated 5000 days to 20 days. The ESI emitter MD simulations performed here are the most detailed to date, providing insights into the properties of the Taylor cone and the behavior of nascent ESI droplets. Focusing on aqueous NaCl solution, this work provides the first comprehensive perspective of the entire ESI process, from analyte solution in the emitter to MS-detectable gaseous ions.

Predator‐Prey Behavior of Droplets Propelling through Self‐Generated Channels in Crystalline Surfactant Layers.

Motile droplets provide an attractive platform for liquid matter‐based applications and protocell analogues displaying life‐like features. The functionality of collectively operating droplets increases by the advance of well‐designed (physico)chemical systems directing droplet‐droplet interactions. Here, we report a strategy based on crystalline surfactant layers at air/water interfaces, which sustain the propulsion of floating droplets and at the same time shape the paths for other droplets attracted by them. First, we show how decylamine forms a closed, crystalline layer that remains at the air/water interface. Second, we demonstrate how aldehyde‐based oil droplets react to decylamine in the crystalline layer by forming an imine, causing the droplets to move through the layer while leaving behind an open channel (comparable to “Pacman”). Third, we introduce tri(ethylene glycol) monododecylether (C12E3) droplets in the crystalline layer. Whereas the crystalline layer suppresses the motion of the C12E3 droplets, the aldehyde droplets create surface tension gradients upon depletion of surfactants from the air/water interface, thereby driving Marangoni flows that attract the C12E3 droplets as well as the myelin filaments they grow: Causing the C12E3 droplets to chase, and ultimately catch, the aldehyde droplets along the channels they have created, featuring a predator‐prey analogy established at an air/water interface.

Attentive ink MLP droplet detection algorithm based on the satellite droplets threshold domain

Satellite droplet are trailing droplets caused by improper voltage waveform control in the nozzles, which directly affect the quality of inkjet printing. Traditional empirical tuning methods struggle to effectively control and predict satellite droplet. In response, this paper establishes a simulation model of a ring-shaped piezoelectric ceramic device. establishes a simulation model of a ring-shaped piezoelectric ceramic nozzle through numerical analysis, collecting droplet behavior parameters for 13,650 different waveforms. It was found that the relative differential of the negative pressure peak at the nozzle is the primary cause of satellite droplet formation. Furthermore, the critical values of various behavior parameters leading to satellite droplet formation were investigated, resulting in the construction of a “Satellite droplet formation”. resulting in the construction of a “Satellite droplet threshold domain.” Based on this domain, an attention-based MLP algorithm, the Attentive Ink MLP Based on this domain, an attention-based MLP algorithm, the Attentive Ink MLP, was proposed to automatically predict whether a waveform will generate satellite droplet. algorithm reduces prediction time from 40 min to 5 min, with a validation accuracy of 0.988. The numerical analysis and algorithm were further validated through experiments with a droplet. The numerical analysis and algorithm were further validated through experiments with a droplet observation system.

Behaviors of a Droplet Impact on a Cylinder

Abstract The hydrodynamic behaviors of a droplet impacting a cylindrical surface were experimentally investigated, examining the effects of cylinder-to-droplet diameter ratio (d*), impact velocity (v0), contact angle (θ), and relative eccentric distance (e*). Temporal evolutions of droplet behavior in the circumferential and axial directions were captured using a high-speed camera. Results indicate that the spreading process can be categorized into four stages based on contact line movements: impact, spreading, oscillation, and stabilization. The rebound height after impact decreases progressively with decreasing d* and increasing Weber number (We). The maximum spreading length increases with droplet diameter and Weber number, while a lower contact angle also contributes to a greater maximum spreading length. For eccentric impacts, the effects of circumferential asymmetry and surface hydrophilicity on spreading become more pronounced with larger e*. Additionally, a novel correlation was developed to predict the maximum spreading lengths of the droplet in the circumferential and axial directions for central impacts.

Large Eddy Simulation of Kerosene Spray Characteristics in Lean Direction Injection Combustor

ABSTRACT The coupled vortex-droplet interaction process in an axial Lean Direction Injection (LDI) swirler was quantitatively investigated by Improved Delayed Detached Eddy Simulation (IDDES) together with Dynamic Zone Flamelet Model (DZFM). Reasonable agreements with the measurements were achieved for the frozen and combustion cases with mesh containing up to 60 million cells. The effect of recirculation zones, the combustion characteristics, and the evolution of kerosene droplets were analyzed. Statistical analysis of the droplet behaviors shows that 2.044% of the droplets are immediately “evaporated” at the critical temperature, 43.57% are quickly boiled at the boiling point, and 53.37% are evaporated, while 1.01% of them experience no phase change. Highly heterogeneous evaporation of liquid compositions was observed during the slow evaporation process in the low-temperature LDI combustor. Precise control of temperature distribution within the combustor by aids of recirculation zones can effectively mitigate NO emissions, which values the importance of high-fidelity modeling in the development of efficient and environmentally friendly combustion systems.