Microscopic Droplets Research Articles

Multi-scale flow atomization characteristics of Jatropha biodiesel swirl liquid film breakup

The characteristics of the spray and droplets produced by liquid film breakup are crucial for understanding the atomization process in industrial furnace atomizers. This study employed high-speed camera technology to capture images of Jatropha biodiesel (JB) spray at various pressures and Ohnesorge numbers (Oh). Image processing methods were employed to analyze the evolution morphology and proper orthogonal decomposition (POD) of fuel swirl spray, revealing the instability, breakup length, spray cone angle, fractal characteristics of the liquid film breakup zone, and droplet distribution characteristics. The results indicated that the JB jet evolution progressed through stages: cylindrical jet, torsion liquid film, open swirl, complete swirl, and fully developed mode. The evolution and disintegration of the liquid film exhibited a complex structure with ligaments, perforations, and droplet separation. Flow disturbances induced the Klystron effect, characterizing microscopic droplet motion. As spray pressure increased, the growth rate of the disturbance wave increased, the breakup length decreased, the atomization cone angle enlarged, and the fractal dimension (FD) of the liquid film breakup zone increased. The droplet size distribution followed a log-normal distribution, with a substantial increase in the number of small droplets as injection pressure increased. This study provides valuable insights into the multi-scale flow atomization of biodiesel in industrial furnace, and is of great significance for the design, operation, and optimization of spraying systems in various industrial applications.

A laminar spray group combustion experiment (LSGCE) with in situ image diagnostic methods and image analysis algorithms.

Spray combustion is important for different engines. The understanding of spray combustion should be further promoted especially in the non-dilute region, and there is lack of well-defined spray experiments. In this study, an experimental platform was developed. Using this platform, a cylindrical quasi-laminar spray can be formed and ignited by a thin and straight hot wire, making it a simple configuration. Two image diagnostic methods were also developed to capture in situ microscopic droplet images and macroscopic droplet-flame images synchronously. Different image analysis algorithms were developed to obtain droplet statistics (diameter, velocity, and number density) and flame information (size, location, and flame propagation speed) from the raw images. The design, diagnostic methods, and image analysis methods are detailedly presented. This experimental platform can cover a wide range of operating conditions, with Gig in a range of 0.01-0.06 and temperature in a range from room temperature to 1400K. In addition, this platform is small in size and is capable of further implanting into a ground-based microgravity facilitaty. The whole experimental system can be applied in spray ignition and combustion studies and can provide legitimate data for further model development.

Dynamic control of active droplets using light-responsive chiral liquid crystal environment

Microscopic active droplets are of interest since they can be used to transport matter from one point to another. In this work, we demonstrate an approach to control the direction of active droplet propulsion by a photoresponsive cholesteric liquid crystal environment. The active droplet represents a water dispersion of bacterial Bacillus subtilis microswimmers. When placed in a cholesteric, a surfactant-stabilized active droplet distorts the local director field, producing a point defect-hedgehog, with fore-aft asymmetry, and allows for the chaotic motion of the bacteria inside the droplet to be rectified into directional motion. When the pitch of the cholesteric confined in a sandwich-like cell is altered by light irradiation, the droplet trajectory realigns along a new direction. The strategy allows for a non-contact dynamic control of active droplets trajectories and demonstrates the advantage of orientationally ordered media in control of active matter over their isotropic counterparts.

Surfactant Partitioning Dynamics in Freshly Generated Aerosol Droplets.

Aerosol droplets are unique microcompartments with relevance to areas as diverse as materials and chemical synthesis, atmospheric chemistry, and cloud formation. Observations of highly accelerated and unusual chemistry taking place in such droplets have challenged our understanding of chemical kinetics in these microscopic systems. Due to their large surface-area-to-volume ratios, interfacial processes can play a dominant role in governing chemical reactivity and other processes in droplets. Quantitative knowledge about droplet surface properties is required to explain reaction mechanisms and product yields. However, our understanding of the compositions and properties of these dynamic, microscopic interfaces is poor compared to our understanding of bulk processes. Here, we measure the dynamic surface tensions of 14-25 μm radius (11-65 pL) droplets containing a strong surfactant (either sodium dodecyl sulfate or octyl-β-D-thioglucopyranoside) using a stroboscopic imaging approach, enabling observation of the dynamics of surfactant partitioning to the droplet-air interface on time scales of 10s to 100s of microseconds after droplet generation. The experimental results are interpreted with a state-of-the-art kinetic model accounting for the unique high surface-area-to-volume ratio inherent to aerosol droplets, providing insights into both the surfactant diffusion and adsorption kinetics as well as the time-dependence of the interfacial surfactant concentration. This study demonstrates that microscopic droplet interfaces can take up to many milliseconds to reach equilibrium. Such time scales should be considered when attempting to explain observations of accelerated chemistry in microcompartments.

Microscopic behavior of nano-water droplets on a silica glass surface

Recent advancements in computational science and interfacial measurements have sparked interest in microscopic water droplets and their diverse behaviors. A previous study using nonlinear spectroscopy revealed the heterogeneous wetting phenomenon of silica glass in response to humidity. Building on this premise, we employed high-resolution atomic force microscopy to investigate the wetting dynamics of silica glass surfaces at various humidity levels. Our observations revealed the spontaneous formation of nano-water droplets at a relative humidity of 50%. In contrast to the conventional model, which predicts the spreading of nanodroplets to form a uniform water film, our findings demonstrate the coexistence of nano-water droplets and the liquid film. Moreover, the mobility of the nano-water droplets suggests their potential in inducing the transport of adsorbates on solid surfaces. These results may contribute to the catalytic function of solid materials.

Optimizing air quality predictions: A discrete wavelet transform and long short‐term memory approach with wavelet‐type selection for hourly PM10 concentrations

AbstractThe rapid advancement of industrialization and urbanization has led to the global problem of air pollution. Air quality can decrease due to pollutants in the air, including types of gases and particles that are carcinogenic, causing adverse health effects. Therefore, estimating the concentration of air pollutants is of great interest as it can provide accurate information about air quality with proper planning of future activities. In this manner, this study considers Istanbul, a province with a high concentration of industry, population, and vehicle traffic. Particulate matter (PM), one of the most basic air pollutants, is stated to contain microscopic solids or liquid droplets that are small enough to be inhaled and cause serious health problems. Thus, it is recommended to apply discrete wavelet transform (DWT) and deep learning method long short‐term memory (LSTM) as a hybrid model to predict the concentration of PM10. Using the mentioned methods, they can predict air pollution to have been developed within the scope of this study. Furthermore, the hybrid approach with LSTM by selecting the most appropriate discrete wavelet type emphasizes the difference of this study from the existing literature. The ability of these developed methods to make successful future predictions helps institutions and organizations that can take precautions on the subject to take action at the right time; in addition, the deep learning methods used contribute to the development of sustainable smart environmental systems. In today's environment when air pollution is increasing and threatening human health, any precaution that can be taken would improve the quality of life for all living things, reduce health issues and deaths caused by air pollution, and thus raise the degree of well‐being. These findings might offer a reliable scientific evidence for Istanbul City's air pollution management, which can serve as an example for other regions.

Tuning properties of phase-separated magnetic fluid with temperature

Phase-separated magnetic fluids provide a very strong magnetic response (μ>25) in a liquid state material. Even small fields can cause a notable material response, but this depends on its properties, which are often difficult to control. Here, we investigate how temperature affects the properties of the system, where phase separation is induced by an increase in ionic strength. Following the deformations of individual microscopic droplets, we can extract surface tension, magnetic permeability, and viscosity. We find that the temperature increase does not affect the surface tension, while the magnetic permeability and viscosity increase. Applying this knowledge to induce a shape instability for a drop only using temperature shows the potential of applicability of our findings.

Directed droplet motion along thin fibers.

When microscopic droplets are placed between fibers held at a fixed angle, the droplets spontaneously move toward the apex of the fibers. The speed of the droplet motion increases both with the angle between the fibers and the distance the droplet spans across the fibers. The speed of these droplets can be described by a simple scaling relationship. Bending these fibers into a sawtooth geometry results in a droplet ratchet where cyclic motion in a fiber results in extended linear motion of the droplet, and can even be used to induce droplet mergers.

Coupling of droplet-on-demand microfluidcs with ESI/MS to study single-cell catalysis.

Droplet microfluidics provides an efficient method for analysing reactions within the range of nanoliters to picoliters. However, the sensitive, label-free and versatile detection with ESI/MS poses some difficulties. One challenge is the difficult association of droplets with the MS signal in high-throughput droplet analysis. Hence, a droplet-on-demand system for the generation of a few droplets can address this and other problems such as surfactant concentration or cross-contamination. Accordingly, the system has been further developed for online coupling with ESI/MS. To achieve this, we developed a setup enabling on-demand droplet generation by hydrodynamic gating, with downstream microscopic droplet detection and MS analysis. This facilitated the incorporation of 1-9 yeast cells into individual 1-5 nL droplets and the monitoring of yeast-catalysed transformation from ketoester to ethyl-3-hydroxybutyrate by MS. With our method a mean production rate of 0.035 ± 0.017 fmol per cell per h was observed with a detection limit of 0.30 μM. In conclusion, our droplet-on-demand method is a versatile and advantageous tool for cell encapsulation in droplets, droplet imaging and reaction detection using ESI/MS.

Metal Surface Engineering for Extreme Sustenance of Jumping Droplet Condensation.

Water vapor condensation on metallic surfaces is critical to a broad range of applications, ranging from power generation to the chemical and pharmaceutical industries. Enhancing simultaneously the heat transfer efficiency, scalability, and durability of a condenser surface remains a persistent challenge. Coalescence-induced condensing droplet jumping is a capillarity-driven mechanism of self-ejection of microscopic condensate droplets from a surface. This mechanism is highly desired due to the fact that it continuously frees up the surface for new condensate to form directly on the surface, enhancing heat transfer without requiring the presence of the gravitational field. However, this condensate ejection mechanism typically requires the fabrication of surface nanotextures coated by an ultrathin (<10 nm) conformal hydrophobic coating (hydrophobic self-assembled monolayers such as silanes), which results in poor durability. Here, we present a scalable approach for the fabrication of a hierarchically structured superhydrophobic surface on aluminum substrates, which is able to withstand adverse conditions characterized by condensation of superheated steam shear flow at pressure and temperature up to ≈1.42 bar and ≈111 °C, respectively, and velocities in the range ≈3-9 m/s. The synergetic function of micro- and nanotextures, combined with a chemically grafted, robust ultrathin (≈4.0 nm) poly-1H,1H,2H,2H-perfluorodecyl acrylate (pPFDA) coating, which is 1 order of magnitude thinner than the current state of the art, allows the sustenance of long-term coalescence-induced condensate jumping drop condensation for at least 72 h. This yields unprecedented, up to an order of magnitude higher heat transfer coefficients compared to filmwise condensation under the same conditions and significantly outperforms the current state of the art in terms of both durability and performance establishing a new milestone.

Dual biomimetic surfaces with anisotropic wettability for multi-scale droplets manipulation

Droplets manipulation on functional biomimetic surfaces have been highly anticipated for their wide range of applications. However, preparing a metal platform for manipulating multi-scale droplets without external energy input remains a challenge. Herein, dual biomimetic surfaces (DBSs) were fabricated by twice laser and thiol modification. The unique interfacial behavior of the droplets can be realized under the synergistic effect of the special “millimeter-micrometer-nanometer” multi-hierarchical composite structure and chemical composition. The as-prepared DBSs can achieve multi-scale droplets manipulation, directional self-cleaning in various media, and efficient directional fog collection. Further, the mechanism of specific interface behavior is analyzed in depth. Nanoparticles endow DBSs with more capture points and condensation cores and micrometer mastoids realized the transmission and aggregation of microscopic droplets through the action of Laplace force. Based on resistance, energy barriers and three-phase contact line, influence of millimeter grooves on anisotropic wettability and directional transportation of the droplets is analyzed. Besides, the multi-hierarchical structure also endows DBSs with good droplets manipulation ability and stability. This work not only provides an advanced biomimetic platform, but also provides an important reference for metal-based lab-on-a-chip for multi-scale droplets manipulation.

Comparison of six approaches to predicting droplet activation of surface active aerosol – Part 2: Strong surfactants

Abstract. Surfactants have been a focus of investigation in atmospheric sciences for decades due to their ability to modify the water uptake and cloud formation potential of aerosols. Surfactants adsorb at the surface and can decrease the surface tension of aqueous solutions. In microscopic aqueous droplets with finite amounts of solute, surface adsorption may simultaneously deplete the droplet bulk of the surfactant. While this mechanism is now broadly accepted, the representation in atmospheric and cloud droplet models is still not well constrained. We compare the predictions of five bulk–surface partitioning models documented in the literature to represent aerosol surface activity in Köhler calculations of cloud droplet activation. The models are applied to common aerosol systems, consisting of strong atmospheric surfactants (sodium myristate or myristic acid) and sodium chloride in a wide range of relative mixing ratios. For the same particles, the partitioning models predict similar critical droplet properties at small surfactant mass fractions, but differences between the model predictions increase significantly with the surfactant mass fraction in the particles. Furthermore, significantly different surface tensions are predicted for growing droplets at given ambient conditions along the Köhler curves. The inter-model variation for these strong surfactant particles is different than previously observed for moderately surface active atmospheric aerosol components. Our results highlight the importance of establishing bulk–surface partitioning effects in Köhler calculations for a wide range of conditions and aerosol types relevant to the atmosphere. In particular, conclusions made for a single type of surface active aerosol and surface activity model may not be immediately generalized.

Surface tension and evaporation behavior of liquid fuel droplets at transcritical conditions: Towards bridging the gap between molecular dynamics and continuum simulations

The phase transition from subcritical to supercritical conditions, referred to as transcritical behavior, significantly impacts the evaporation and fuel–air mixing in high-pressure liquid-fuel propulsion systems. Transcritical behavior is characterized as a transition from classical two-phase evaporation to a single-phase gas-like diffusion regime as surface tension and latent heat of vaporization reduce. However, the interfacial behavior represented by the surface tension coefficient and evaporation rate during this transition which are crucial inputs for Computational Fluid Dynamics (CFD) simulations of practical transcritical fuel spray is still missing. This study aims at developing new evaporation rate and surface tension models for transcritical n-dodecane droplets using molecular dynamics (MD) simulations irrespective of the droplet size. As MD simulations are primarily limited to the nanoscale, the new models can bridge the gap between MD and continuum simulations and enable the direct application of these findings to microscopic droplets. A new characteristic timescale, i.e., “undroplet time,” is defined which marks the transition from classical two-phase evaporation to single-phase gas-like diffusion behavior. The undroplet time indicates the onset of droplet core disintegration and penetration of nitrogen molecules into the droplet, which occurs after the vanishment of the surface tension. By normalizing the time with respect to the undroplet time, the rate of surface tension decay, evaporation rate, and the rate of droplet mass depletion become independent of the droplet size. Calculation of pairwise correlation coefficients for the entire MD results shows that both surface tension coefficient and evaporation rate are strongly correlated with the background temperature, while pressure and droplet size play a less significant role past the critical point. Therefore, new models for surface tension coefficient and evaporation rate spanning from sub- to supercritical conditions are developed as a function of background pressure and temperature, which can be used in continuum simulations. The identified phase change behavior based on the undroplet time shows a good agreement with the phase change regime maps obtained using microscale experiments and nanoscale MD predictions.

Axillary silicone lymphadenopathy caused by gel bleeding with intact silicone breast implants: a case report

Gel bleeding following breast augmentation using silicone breast implants (SBIs) occurs when microscopic silicone droplets diffuse through the implant surface, potentially resulting in complications such as capsular contracture and immune responses related to breast implant illness. Prompt and reliable diagnostic measures are crucial, as the presentation of gel bleeding can resemble cancer, making an accurate diagnosis challenging. This report discusses a rare case of axillary silicone lymphadenopathy caused by gel bleeding in a 48-year-old woman with intact SBIs. Silicone lymphadenopathy can be suspected based on mammography, ultrasonography, and magnetic resonance imaging in patients with a history of SBI insertion, and confirmation can be obtained through a pathological examination. Excisional biopsy is generally recommended for symptomatic patients, while treatment may not be necessary for asymptomatic patients; However, removal can be considered if the patient indicates a preference for it. Patients with silicone lymphadenopathy require replacement of SBIs to examine the breast capsule and verify the integrity of the implant. This case highlights the importance of considering gel bleeding as a potential cause of silicone lymphadenopathy, even in patients with intact SBIs.

Hydrodynamic Treadmill Reveals Reduced Rising Speeds of Oil Droplets Deformed by Marine Bacteria.

In marine environments, microscopic droplets of oil can be transported over large distances in the water column. Bacterial growth on the droplets' surface can deform the oil-water interface to generate complex shapes and significantly enlarge droplets. Understanding the fate of spilled oil droplets requires bridging these length scales and determining how microscale processes affect the large-scale transport of oil. Here, we describe an experimental setup, the hydrodynamic treadmill, developed to keep rising oil droplets stationary in the lab frame for continuous and direct observation. Oil droplets with radii 10 < R < 100 μm were colonized and deformed by bacteria over several days before their effective rising speeds were measured. The rising speeds of deformed droplets were significantly slower than those of droplets without bacteria. This decrease in rising speed is understood by an increase in drag force and a decrease in buoyancy as a result of bio-aggregate formation at the droplet surface. Additionally, we found sinking bio-aggregate particles of oil and bacterial biofilms and quantified their composition using fluorescence microscopy. Our experiments can be adapted to further study the interactions between oil droplets and marine organisms and could significantly improve our understanding of the transport of hydrocarbons and complex aggregates.

Single hole ammonia spray macroscopic and microscopic characteristics at flare and transition flash boiling regions

As a potential zero-carbon fuel for internal combustion engine to mitigate the greenhouse gas emission, ammonia’s unique flash boiling spray behaviors have not been well understood. In this study, the macroscopic and the microscopic characteristics of the liquid ammonia spray at the flare and transition flash boiling regions were experimentally investigated under different pressure ratios (RP, the ambient over the saturated pressure) and ambient temperatures. The spray macroscopic morphologies captured from the high-speed camera in the flare flash boiling region (RP ≤ 0.47) show that the spray expands significantly in radial direction while that in the transition flash boiling region (0.47 < RP ≤ 1.06) is more contracted in the penetration direction. Additionally, in flare flash boiling region, spray tip penetration and velocity increases generally with the increase of RP, while that in the transition flash boiling region in versus. Furthermore, the microscopic droplet statistics clearly demonstrate show that the most probable droplet diameter moves to a larger value with the increase of RP. The peak probability of droplet size and the droplet number density decreases at larger RP cases, resulting in a more uniformly distributed droplet sizes and an increased Sauter Mean Diameter. Finally, the ambient temperature show limited influence on the macroscopic spray penetration behaviors or the microscopic droplet size distribution, but evaporation is significantly enhanced since at highest ambient temperature, there are minimized droplet number density in some of the test locations.

Using Fume Hood to Reduce Nurses' Exposure to Particulate Matters Dispersed Into the Air During Pill Crushing.

Pill crushing is a common practice in patient care settings. Crushing pills can disperse particulate matter (PM) into indoor air. The PM is a widespread air pollutant composed of microscopic particles and droplets of various sizes and may carry active and/or inactive ingredients nurses can inhale. This study aimed to quantify PM sizes and concentration in indoor air when pills are crushed and examine the role of a fume hood in reducing particulate pollution. Two scenarios (with and without a fume hood) representing nurses' pill-crushing behaviors were set up in a positive-pressure cleanroom. Two acetaminophen tablets (325 mg/tablet) were crushed into powder and mixed with unsweetened applesauce. The PM sizes and concentrations were measured before and during crushing. Different sizes of PM, including inhalable, respirable, and thoracic particles, were emitted during medication crushing. The total count of all particle sizes and mass concentrations of particles were significantly lower during crushing when a fume hood was used (p = .00). Pill crushing increases PM and should be considered a workplace safety health hazard for nurses. Healthcare professionals should work under a fume hood when crushing pills and wear proper protective equipment. The findings of significant particulate pollution related to pill crushing suggest that further research is warranted.

Method to Measure Surface Tension of Microdroplets Using Standard AFM Cantilever Tips.

Surface tension is a physical property that is central to our understanding of wetting phenomena. One could easily measure liquid surface tension using commercially available tensiometers (e.g., Wilhelmy plate method) or by optical imaging (e.g., pendant drop method). However, such instruments are designed for bulk liquid volumes on the order of milliliters. In order to perform similar measurements on extremely small sample volumes in the range of femtoliters, atomic force microscope (AFM) is considered as a promising tool. It was previously reported that by fabricating a special "nanoneedle"-shaped cantilever probe, a Wilhelmy-like experiment can be performed with AFM. By measuring the capillary force between such special probes and a liquid surface, surface tension could be calculated. Here, we carried out measurements on microscopic droplets with AFM, but instead, using standard pyramidal cantilever tips. The cantilevers were coated with a hydrophilic polyethylene glycol-based polymer brush in a simple one-step process, which reduced its contact angle hysteresis for most liquids. Numerical simulations of a liquid drop interacting with a pyramidal or conical geometry were used to calculate surface tension from the experimentally measured force. The results on micrometer-sized drops agree well with bulk tensiometer measurement of three test liquids (mineral oil, ionic liquid, and glycerol), within a maximum error of 10%. Our method eliminates the need for specially fabricated "nanoneedle" tips, thus reducing the complexity and cost of measurement.

Effects of bendable P3CT polymers layer on the photovoltaic performance of perovskite solar cells

We report on the formation of bendable and edge-on poly[3-(4-carboxybutyl)thiophene-2,5-diyl] (P3CT) polymers thin layer used as a hole modification layer (HML) in the inverted perovskite solar cell. The aggregations of 2D layer-like P3CT polymers in dimethylformamide (DMF) solution can be formed via aromatic π–π stacking interactions and/or hydrogen-bonding interactions with the different concentration from 0.01 to 0.02 wt%, which highly influences the photovoltaic performance of the inverted perovskite solar cells. The atomic-force microscopic images and water droplet contact angle images show that the P3CT polymers modify the surface properties of the transparent conductive substrate and thereby dominating the formation of perovskite crystalline thin films, which play important roles in the highly efficient and stable perovskite solar cells. It is noted that the V OC (J SC) of the encapsulated solar cells values are maintained to be higher than 1.115 V (22 mA cm−2) after 104 d when an optimized π–π stacked and hydrogen-bonded P3CT polymer is used as the HML. On the other hand, the solar cell showed a high long-term stability by maintaining 85% of the initial power conversion efficiency in the ambient air for 103 d.

Experimental investigation on heat transfer performance during electrospray cooling with ethanol–R141b mixture

Electrospray (ES) cooling is promising to achieve energy-efficient liquid atomization and high-heat-flux thermal dissipation at relatively low operating temperatures (<75 °C) when using an ethanol–R141b mixture as a coolant, making it suitable for micro-sized electronics. Nevertheless, there has been little research on ES cooling using the ethanol–R141b mixture. This paper presents an experimental investigation on the cooling performance triggered by ES impingement. Results demonstrate that adding ethanol to R141b improves the electrohydrodynamic (EHD) atomization, namely reducing Sauter mean diameter and increasing spray angle. Meanwhile, it also causes the deposition of large-size isolated liquid films on the surface. As the mass concentration of ethanol increases from 10% to 60%, the cooling performance in terms of the temperature drop (Ts,d) and time-averaged heat transfer coefficient (hm), first increases and then decreases or almost remains unchanged. This is a complicated consequence of microscopic droplet behaviors, film deposition, and thermal properties, which are dependent on the mass concentration of ethanol. For volumetric flow rates of 100 and 150 mL/h, the ES sprays with mass fractions of 50% and 20% produce preferable cooling capabilities with respective hm equivalent to 1.6 and 1.9 kW/(m2·K) under the current tests. A Nusselt number correlation with the electric Weber, hydrodynamic Weber, and Prandtl numbers is derived. This correlation covers more than 97.7% of the experimental data within a ± 15% deviation. The derived correlation implies the increase in the flow rate has a superior contribution to the heat transfer in comparison with the applied voltage.