Droplet Diffusion Research Articles

Wind velocity and dispersion/advection–diffusion of artificial droplets and droplet nuclei in a domed all-weather multi-purpose stadium

To evaluate the COVID-19 infection risk and the effectiveness of countermeasures at mass-gathering events, we measured the dispersion and advective diffusion of artificial droplets and artificial droplet nuclei at the Tokyo Dome, Japan (capacity 55,000 people). We also measured and evaluated the effectiveness of wearing masks and increasing the space between seating areas. If people were seated facing forward, artificial droplets did not reach the mouths of surrounding people, suggesting low risk of droplet transmission. For an artificially generated cough or sneeze, the volume of droplets deposited on the hair, back of the neck, and back of the human in front, and the backs of the seats in front, decreased by two to three orders of magnitude when a mask was worn, regardless of the type of mask. However, when the mask was worn with the nose out, the amount deposited on the back of the seat in front was reduced by only 17%. Even in seats with the highest particle concentration in the vicinity of the source, only 0.097%–0.24% of the generated droplet nuclei (1.0–3.0 μm) from the source were inhaled. Our results suggest that the infection risk at the Tokyo Dome via droplet and airborne transmission was low.

Interface properties and applications in laundry of the BS12-SAS mixed systems

In the BS12/SAS mixed systems, the solution exhibits acidity. BS12 can act as a cationic surfactant to form anionic-cationic complexes with SAS. In this study, the surface properties and application characteristics of the BS12/SAS mixed systems were investigated. The static surface tension, dynamic surface tension, and contact angle of BS12, SAS, and their different mixed molar ratios were measured at 25 °C to evaluate interaction parameters, activity coefficient, molar fraction, and thermodynamic variables of surfactants in the mixed micelles. The results demonstrated that the BS12/SAS mixed systems deviated from the ideal state due to the strong molecular attraction between the two components, resulting in a synergistic effect. The mixed systems exhibited higher surface activity and lower critical micelle concentration. Thermodynamic analysis indicated that the spontaneous micellization of binary mixtures of BS12 and SAS occurred through both enthalpy-driven and entropy-driven processes. The adsorption kinetics data revealed that the mixed systems followed a diffusion-adsorption process. Contact angle measurements demonstrated that BS12/SAS possesses superior ability to reduce surface tension, which facilitated rapid droplet diffusion and efficient wetting of the matrix within a short time period. Furthermore, the aggregation behavior of individual surfactants and the two composite systems in aqueous solutions was analyzed using DLS and TEM to investigate their effects on the wetting, emulsification stability, foaming stability, and decontamination capability of liquid paraffin.

Microscopic Mechanism for Gradient Diffusion of Salt-Containing Droplets Induced by Electromagnetic Synergy: A Molecular Dynamics Study.

The electromagnetic synergy has been proven to be highly effective in separating oil-water emulsions. However, the dynamic impact mechanism of electromagnetic fields on the internal structure of salt droplets remains unclear. In this study, the molecular dynamics (MD) simulation was used to investigate the molecular diffusion of salt ions and water molecules, as well as the dynamic behavior of droplets under the combined influence of electromagnetic fields. The results indicate that ions accumulate in the electromagnetic synergistic field, causing the deformation amplitude of droplets to be smaller than that in a single electric field. The magnetic field affects the energy of the system, when the magnetic field strength is between 1 and 5T, the nonbonded energy significantly increases nonlinearly; when the magnetic field strength is greater than 5T, the total energy of the system significantly changes. In addition, the viscosity of the medium is significantly lower when the intensity of the magnetic and electric fields is controlled within a specific range, providing a new way to regulate the fluidity of fluids.

Effect of an accelerating metro cabin on the diffusion of cough droplets

Coronaviruses being capable of spreading through droplet contamination have raised significant concerns regarding high-capacity public rail transport, such as the metro. Within a rapidly moving railcar cabin, the internal airflow lags behind the bulkhead, generating internally induced airflow that accelerates droplet dispersion within a non-inertial reference system. This study investigates the impact of acceleration on the diffusion of cough droplets of varying sizes using computational fluid dynamics. The modified k–ε equation in ANSYS® Fluent was utilized to simulate droplet diffusion under different body orientations by adjusting the inertial force correction source term. Results indicate that droplets in the middle size range (50–175 μm) are primarily influenced by inertial forces, whereas smaller droplets (3.5–20 μm) are predominantly controlled by air drag forces. Regardless of facial orientation, the outlet of high-capacity public rail transport poses the highest risk of infection.

Constructing myosin/high-density lipoprotein composite emulsions: Roles of pH on emulsification stability, rheological and structural properties

The emulsification activity of myosin plays a significant role in affecting quality of emulsified meat products. High-density lipoprotein (HDL) possesses strong emulsification activity and stability due to its structural characteristics, suggesting potential for its utilization in developing functional emulsified meat products. In order to explore the effect of HDL addition on emulsification stability, rheological properties and structural features of myosin (MS) emulsions, HDL-MS emulsion was prepared by mixing soybean oil with isolated HDL and MS, with pH adjustments ranging from 3.0 to 11.0. The results found that emulsification activity and stability in two emulsion groups consistently improved as pH increased. Under identical pH, HDL-MS emulsion exhibited superior emulsification behavior as compared to MS emulsion. The HDL-MS emulsion under pH of 7.0–11.0 formed a viscoelastic protein layer at the interface, adsorbing more proteins and retarding oil droplet diffusion, leading to enhanced oxidative stability, compared to the MS emulsion. Raman spectroscopy analysis showed more flexible conformational changes in the HDL-MS emulsion. Microstructural observations corroborated these findings, showing a more uniform distribution of droplet sizes in the HDL-MS emulsion with smaller particle sizes. Overall, these determinations suggested that the addition of HDL enhanced the emulsification behavior of MS emulsions, and the composite emulsions demonstrated heightened responsiveness under alkaline conditions. This establishes a theoretical basis for the practical utilization of HDL in emulsified meat products.

Numerical study of the seepage behavior of droplets in porous materials

This article analyzes the spread and infiltration dynamic process of liquid droplets on the surface of porous materials and establishes a mathematical model of dynamic changes in droplets. To accurately describe the dynamic effects of droplet flow, a mathematical model of droplet dynamics was developed by applying the level-set method, and the seepage process of the droplets was also analyzed numerically. The effects of a series of control parameters on the droplet percolation process are analyzed, and the pore degree of porous materials performed a numerical study of the droplet deformation process. We observe the spread and infiltration process of liquid droplets in porous materials models. It was found that there is a competitive relationship between diffusion and penetration of droplets. The depth of penetration of droplets decreases with increasing viscosity and increases with increasing porosity. The results of the study help to understand the seepage behavior of the droplet on the surface of the porous material.

Understanding the Droplet Diffusion in Ionic Liquid Microemulsions.

Ionic liquids (ILs) as polar components in nonaqueous microemulsions are complex formulations that have interesting transport and structural properties, and offer broader applicability of ILs in areas such as drug delivery and cleaning technology. The phase behavior, electrical conductivity, and nanostructures of these formulations have been investigated for quite some time, but the characteristics of the diffusion of nanodroplets were rarely explored─ and hence little understood. This work investigates the droplet diffusion processes in a series of IL-microemulsions containing 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4) by means of viscosity and depolarized dynamic light scattering (DDLS) measurements. The intensity correlation functions are strikingly similar to polymeric solutions in nonaqueous media and aqueous microemulsions containing block copolymers─having bimodal relaxations that are separated by three to four decades of correlation delay times. The "faster" diffusion process is likely a collective process characterizing the correlated motions of droplets in droplet clusters. The collective diffusion coefficient Dcol values are quite comparable to aqueous microemulsions. The "slower" diffusion is likely due to the "caging" effect caused by nearby clusters and/or bulk solvent─this mode may be linked to the microemulsion bulk viscosity. Interestingly, the Dcol variations on increasing [C4mim]BF4 concentration are strongly correlated to the microemulsion viscosity changes as well as locations of these compositions on the microemulsion phase diagram.

Reducing the risk of droplet inhalation for taxi drivers by regulating the air supply position according to the exhalation source

The environment inside taxis is relatively closed, making it easy for the drivers to contact with droplets exhaled by the passengers and creating a high risk of respiratory diseases. To study the diffusion of droplets in taxis, the numerical simulation was performed based on gas–solid two–phase flow. The renormalization-group k–epsilon (RNG k–ε) turbulence model and the discrete phase model were used to study the transmission characteristics of droplets with different exhalation sources, and the air-supply positions were chosen to reduce the risk of driver infection from droplet inhalation. Based on the likelihood–exposure–consequence (LEC) method, the cumulative inhalation ratio of droplets was used to assess drivers' infection risk during working. The results show that, the driver's infection risk is least when the passenger sits in the front seat, reducing the risk by 32%–87%. Furthermore, the risk is reduced by 38%–85% when the air supply is changed from two-sided to central. Therefore, the combination that minimizes the risk of inhalation infection for taxi drivers is the passenger sitting in the front seat and the air being supplied centrally in the taxi, which can provide a reference for improving the car layout and ventilation system of the taxi.

Development and Characterization of a Contact-Charging Electrostatic Spray UAV System

Utilizing agricultural UAVs for pesticide and insecticide spraying is an effective measure for plant protection. However, achieving effective coverage on the back side of target is often challenging. To address this issue, this study combined a contact-charging spraying system with a UAV to develop an electrostatic plant protection UAV system. Upon activating the electrostatic component, strong electrostatic effects were observed at the nozzle, altering the distribution of the liquid flow; the distribution within the liquid flow became more homogeneous, while the edge regions experienced electrostatic repulsion, leading to changes in droplet size and an increase in droplet density. In the central area, droplet size reduced from 159 μm to 135 μm, while in the edge area, it changed from no value to 120 μm. During field tests using the UAV, the results showed an increase of 1.0 m in effective spray width (at a flight height of 4.0 m), indicating that the charges and propellor wind field contributed to the diffusion of droplets towards the edges. Additionally, the droplet density increased by an average of 19.7 droplets/cm2, and the overall deposition increased by 0.12 μL/cm2, resulting in an approximate three-fold increase compared to conventional spray, which aids in insect control and reduces pesticide usage.

Airborne pathogens diffusion: A comparison between tracer gas and pigmented aerosols for indoor environment analysis

The evaluation of airborne pathogens diffusion is a crucial practice in preventing airborne diseases like COVID-19, especially in indoor environments. Through this transmission route, pathogens can be carried by droplets, droplet nuclei and aerosols and be conveyed over long distances. Therefore, understanding their diffusion is vital for prevention and curbing disease transmission. There are different techniques used for this purpose, and one of the most common is the utilization of tracer gas, however, it has limitations such as the difference in size between the gas molecules and the respiratory droplets, as well as its incapability to take into account evaporation. For this reason, a new method for evaluating the diffusion of respiratory droplets has been developed. This approach involves the use of an ultrasonic emitter to release and disperse pigmented aerosols, and a colorimeter for the following quantitative evaluation. A comparison with the tracer gas technique has been carried out, showing for the pigmented aerosols methodology a response that is dependent on different relative humidity conditions, while there is no clear difference in the dispersion of tracer gas at high or low humidity.

Numerical Simulation of Droplet Dispersion and Passenger Infection Risk Assessment in a Subway Carriage

Droplet transmission is a critical pathway for the spread of respiratory infectious viruses. A thorough understanding of the mechanisms of droplet dispersion within subway carriages is crucial to curb the widespread transmission of the virus. This study utilizes computational fluid dynamics (CFD) to establish a full-scale numerical model of a subway carriage. The numerical model and droplet evaporation behavior are validated using experimental data and literature. The impact of primary parameters such as the initial droplet size, release velocity, release position, relative humidity, and passenger density on the droplet diffusion and probability of infection for passengers is investigated. The results indicate that large droplets (100 μm) are deposited on the carriage floor before complete evaporation, while tiny droplets (10 μm) evaporate rapidly, leading to a longer suspension time in the air within the carriage. The infected passenger’s position influences the ventilation system’s efficiency in removing the droplets; removal takes significantly longer when the infected passenger is closer to the carriage end. Additionally, a low relative humidity (35%) and high passenger density (4 p/m2) result in more droplets being trapped by passengers’ bodies. The infection probability for passengers depends on the initial size and quantity of droplets trapped by their bodies. Maintaining higher relative humidity levels and limiting the passenger numbers within the subway carriage can reduce the number of droplets captured by passengers’ bodies, thus helping to reduce the infection probability of fellow passengers.

A COMBINED ANALYTICAL/NUMERICAL APPROACH TO THE MODELING OF THE PROCESSES LEADING TO PUFFING AND MICRO-EXPLOSION IN A COMPOSITE MULTI-COMPONENT FUEL/WATER DROPLET

The previously developed analytical/numerical model for predicting heat transfer and component diffusion in composite multi-component droplets is adjusted for use in practical engineering applications related to the analysis of droplet heating and evaporation and the onset of puffing and micro-explosions in those droplets. This adjustment allowed us to gain new insights into the previously developed models of these processes. The focus of the analysis is on kerosene/water droplets. It is demonstrated that the number of terms in the series in the analytical solution to the heat transfer equation can be reduced to just one or two to ensure that the maximal error of the model prediction does not exceed 1%, unless we are interested in the processes at the very start of heating. At the same time, the minimal number of terms in the series in the analytical solution to the component diffusion equation should be at least seven to ensure that the errors of the prediction of the numerical code do not exceed 3%. It is shown that, to ensure that the analytical/numerical code predicts physically consistent results, the maximal absolute error of calculation of the eigenvalues based on the bisection method cannot exceed 10<sup>-7</sup>. It is shown that using these limiting values for each of these input parameters leads to about 50%-75% reduction in the CPU time required to obtain results close to those which were obtained using the nonoptimized version of the numerical code. The overall reduction in CPU time can be up to about 95%. The predictions of the adjusted analytical/numerical code are validated against in-house experimental data and data available in the literature.

Oil density and viscosity affect emulsion stability and destabilization mechanism

The effects of the oils density and viscosity (DV) on the emulsions kinetic stability and destabilization mechanisms were revealed from the perspective of droplets movement and emulsions viscosity. With the extension of storage time, severe creaming (20.20 ± 0.73 × 10−4 mm/s), flocculation and coalescence (7.26 ± 0.82 μm−2 min−1) occurred in emulsions prepared with oils of lowest DV; the extent of creaming (15.90 ± 0.68 × 10−4 mm/s) and coalescence (1.36 ± 0.10 μm−2 min−1) were alleviated upon increasing oils DV, accompanied by the occurrence of acute Ostwald ripening (17.03 ± 1.72 μm3 min−1); and only slight creaming (2.26 ± 0.24 × 10−4 mm/s), flocculation, coalescence (0.70 ± 0.09 μm−2 min−1) and Ostwald ripening (6.14 ± 0.42 μm3 min−1) occurred in emulsions prepared with oils of highest DV. This was because oils with higher DV (especially density) restricted the droplets fluidity due to an increased motion friction, leading to a more homogeneous internal network and a higher apparent/macroscopic viscosity (0.31 ± 0.04 Pa·sn and 0.1 nm−2 s) of emulsions. These impeded the Brownian motion and diffusion of droplets to decrease the collision frequency/efficiency and mass transfer respectively, which in turn conduced to retarding droplets size enlargement and stabilizing the emulsions against different destabilization mechanisms.

Enhanced lipophilicity and wear resistance of 20CrMnTi induced by laser surface texturing

Gear transmission is crucial in mechanical engineering, extensively used in automobiles, ships, machine tools, and airplanes. However, as gear systems advance towards higher speeds, heavier loads, and smaller sizes, the tooth surface becomes more prone to pitting, wear, and other failures. The use of laser surface texturing technology has shown great promise in enhancing the hydrophobic and lipophilic properties of material surfaces. Thus, this paper presents a novel approach for machining equally spaced parallel and perpendicular microgrooves on the surface of gear steels (20CrMnTi) to facilitate lubricant storage and enhance the material’s surface lipophilicity. Laser power and scanning speed are optimized as parameters for laser processing to investigate the lipophilic properties, including the contact angle, diffusion velocity, and contact behavior of oil droplets. Comparative analysis of the processed and unprocessed specimens revealed that both static and dynamic contact behaviors are significantly changed. Compared with the unprocessed specimens, the static contact angle of the processed specimens decreases from 47° to 0° and is super lipophilic at different temperatures; the oil droplet diffusion distance increases by a maximum of 1.27 times; the vertical diffusion height increases by a maximum of 2.8 mm; and the wetting hysteresis coefficient increases by a maximum of 2.55 times. Additionally, the friction coefficient decreases during the stable wear phase, and the wear depth is reduced by 33.53 %. The research has demonstrated that the oil-attracting properties of 20CrMnTi can be greatly enhanced using laser micro-nano texture technology. This improves lubrication conditions and wear resistance, providing a novel approach to mitigating failure risks such as pitting and wear in various gear mechanisms. Furthermore, this method effectively extends the service life of gears, making it a valuable tool for gear engineering applications.

Compensated printing and characterization of the droplet on the binder migration pattern during casting sand mold 3D printing

With its unique advantages, binder jetting additive manufacturing technology has shown significant development potential in the additive manufacturing of complex casting sand mold. After droplet deposition, the permeation and migration of the liquid binder in the porous powder bed have a critical impact on the final performance of the printed sand mold. Taking the piezoelectric printhead as the research object, using Furan resin as the liquid binder and silica particles for the powder bed, the effect of binder content on the dimensional accuracy and mechanical properties of the formed sand mold parts was discussed. Several samples were printed with different volumes of droplets, and the droplet diffusion mode, dimensional accuracy, mechanical properties, and formability were measured, respectively. These indicators were quantified based on the different binder saturation, by comprehensively considering the correlation between the indicators. The methods of grayscale printing and layer thickness compensation are given to improve the product quality. The results show that a more extensive liquid saturation produces better strength and formability with the tradeoff of a significant increase in dimension error. However, by independently adjusting the droplet amount of the liquid binder at each printhead, the desired dimension inaccuracy, strength, and formability can be achieved at each precise place. The future application can be implemented in the binder jetting of precision casting sand mold 3D printing to accelerate its industrial development.

Understanding the role of droplet clusters in a reactive mixing layer

Turbulent reactive flows laden with droplets appear in various energy systems but are difficult to understand and parametrize. Such flows involve interactions of turbulent fluctuations, phase changes, and chemical reactions that give rise to complex phenomena. To improve our knowledge, we performed direct numerical simulations of a canonical shear flow. It is composed of a hot, quiescent outer layer and a cold, turbulent inner layer that is laden with droplets. Due to the turbulent fluctuations, the droplets form clusters. Due to the high temperatures, the droplets evaporate quickly and flames emerge spontaneously at the interface of the two layers. We observed premixed flames that enclose droplet clusters and diffusion flames that enclose vapor pockets or single droplets. To examine these flame structures in more detail, we varied the droplet size, droplet loading, and shear rate. We found that the droplet size and droplet loading have significant effects, whereas the shear rate has only subtle effects.

The Rheological Behavior of Polymer Solution Threads

Theoretical results of the authors in the field of the capillary thinning of polymer solution threads are reviewed. The dynamics of threads of both concentrated solutions without entanglements and dilute solutions, where hydrodynamic interactions play an important role, is considered. A molecular approach, in which macromolecules are simulated by a semiflexible chain, is used as a basis. This makes it possible to describe, from common positions, the nonlinear elasticity of solution and interactions taking into account the orientation of the chains. Particular attention is given to thread thinning in the region of the elastic behavior of solution where macromolecules unfold along the axis of stretching. The results of analysis of the capillary stability of a thread and conditions for solvent droplets emergence on its surface are presented, and the dynamics of formation of the hierarchical structure of beads-on-string droplets is viewed. Mechanisms behind the subsequent merging of droplets related to solvent overflow and droplets diffusion along the polymer string are discussed. The polymer string breakup occurs at time scales higher than the Rouse relaxation time of the polymer chain. String hardening and fiber formation may be an alternative.

Comparing Design Schemes and Infection Risk Assessment of Negative Pressure Isolation Cabin

At present, various public health emergencies have forced a deeper study of measures to prevent infectious diseases. To prevent the spread of infectious diseases on large cruise ships, the use of negative pressure isolation cabins is an effective method. However, existing cruise ships rarely use negative pressure isolation cabins or use them with shortcomings such as unreasonable layout of air inlets, which do not effectively reduce the risk of infection for medical staff while isolating patients. To solve this problem, first, the method of active air interference and the principle of proximity of the fresh air outlet were examined in this paper, and four groups of optimization schemes were designed. Second, by analyzing the diffusion of droplets in three breathing modes (coughing, sneezing, and talking while wearing or not wearing a mask), the direction of droplets and the efficiency of pollutant discharge under the condition of coughing were compared. Finally, in this paper, the infection risk of contact transmission and aerosol transmission was optimized by using the linear quantitative evaluation method and MSDR method, respectively. The results showed that the auxiliary air intake layout of optimal scheme 2 can effectively reduce the risk of infection for medical personnel in negative pressure isolation cabins. This study provides a useful reference for the design and optimization of negative pressure isolation cabins in future cruise ships.

Complex treatment of oily polluted waters by modified melamine foams: from colloidal emulsions to a free oil removal

This study deals with the efficient, low-cost, and scalable treatment of oily polluted waters including colloidal emulsions, oil-in-water mixtures, and free oil removal using melamine foams (MFs) modified by ferric chloride (FeCl3). Modified foams have superhydrophobic character due to the coordination of Fe3+ with free electron pairs on nitrogen and oxygen atoms within the melamine structure. The water contact angles (WCA) were 146° ± 2°, 148° ± 4°, 153° ± 2°, and 150° ± 4° for foams modified by the solutions with concentrations of 0.001 M, 0.005 M, 0.01 M, and 0.02 M, respectively. This modification enables the efficient treatment of various oil/water systems, including oil/water colloidal emulsions (99 vol% of the droplets have dimensions below 500 nm), oil-in-water mixtures up to 40 weight % of the oil component, and “free” oil removal as it was demonstrated in this study for the first time. The emulsions containing 100 ppm diesel oil (DO) were separated with 91.4% efficiency, and the mixtures containing 20 and 40 weight % DO were separated with 99.9% efficiency. Modified foams also quickly remove free DO from the water surface, absorbing 95 g/g DO, whereas water sorption was negligible. The separation of colloidal oil in water emulsions represents the key finding of this study as it indicates the applicability of the treated MFs for the treatment of emulsified industrial wastewater. The demulsification mechanism is based on multiple diffusion processes running at different time scales, including diffusion of the emulsion into the foam and diffusion of oil droplets within the foam, combined with parallel adsorption of oil droplets onto the solid skeleton of the foam. A multiplied usage of these foams for all these niche operations was also proven. The application of our current study with previous studies on modified MFs and polyurethane for water oil separation utilization is summarized in Table S1 ESI.Graphical

Coughed droplets dispersion pattern in two-bed ward under different heating strategies

This study aims to compare the dispersion of coughed droplets with different diameters under two heating strategies of hospital ward in winter. An experimentally verified computational fluid dynamics (CFD) method has been used to simulate the evaporation and diffusion of droplets with five different diameters in a two-bed ward with air conditioning strategy (MV) and floor radiant heating strategy (FH). Results show that in the boundary of the human body, indoor air flow, human expiratory airflow and thermal plume play an important role in the dispersion of coughed droplets. Small and medium-sized droplets suspension in the ward for a longer time, while large droplets deposit rapidly. The removal ratio of 50 μm droplets under MV strategies is the highest. The 100 μm droplets in the two heating strategies are completely removed within 30 s. There is an obvious difference of droplets deposition ratio on each surface in the ward.