Coalescence Phenomena Research Articles

Experimental investigation on effect of reservoir conditions on stability and rheology of carbon dioxide foams of nonionic surfactant and polymer: Implications of carbon geo-storage

Carbon dioxide (CO2) injection, in form of a viscous foam, is one of the effective techniques to control premature CO2 breakthrough in subsurface carbon storage and utilization (CSU). Polysaccharides are often used in oilfield and its use for developing viscous CO2 foam will make the process reservoir compatible and economic. However, their efficacy should be tested at real conditions to promote better CO2 utilization in oilfield projects. This study aims to quantify the role of subsurface conditions viz., pressure, temperature, and salinity on CO2-foams prepared by non-ionic polymer (guar gum, 4000 ppm) and surfactant (TX-100, 0.25 mM). Salts i.e. potassium chloride (KCl) and magnesium sulfate (MgSO4) of varying concentration (0–8 wt%) were used. Increasing salt concentration were found to significantly affect foam stability (maximum fall of 88.88%) and CO2 molality in surfactant-polymer (SP) solution (by 73.1% at 70 bar pressure). KCl showed a greater reduction (≈8–15%) than MgSO4 (6–12%) however, increasing surfactant amount had negligible impact on CO2 molality. Additionally, microscopic analysis was performed in which initial bubble size (i.e. 40-60 μm) due to coalescence phenomenon was observed to increase to (400–460 μm) after 1 h. CO2 foams exhibited non-Newtonian shear thinning behavior where foam viscosity and elasticity were positively influenced by increase in pressure, which suggested foam potential for enhanced oil recovery (EOR) and carbon storage in subsurface environment. Foam viscosity decreased with increasing salt concentration (fall of maximum 55% when compared to zero salt content) and temperature resulting at 90 °C, foam viscosity reduces to a value of 0.05 Pa s at pressure≈ 70 bar. Finally, dynamic rheological measurements were reported to visualize viscoelastic response of foams. The viscoelastic response of CO2 foams by strain sweep measurements reported a maximum fall of 60% while 40% reduction in case of frequency sweep measurements was observed. Also, foams exhibited both elastic and viscous effects with clear cross-over between G′ and G″ at each test pressure and temperature.

Growth of Phosphazene Film Onto Undoped n-InP

An anodic electrochemical treatment in liquid ammonia (-55°C, Patm) provides the formation of an ultrathin film onto p-InP (1017 atoms.cm-3) and n-InP (1018 atoms.cm-3). This overlayer exhibits a polyphosphazene like structure (H2N–P=NH) which presents a very specific spectral signature when analyzed by X-ray photoelectron spectroscopy (XPS[1]). Based on the atomic surface ratios measured, the film growth would require a nucleation step which would be followed by a phosphazene coalescence phenomenon in the two dimensions of the surface[2]. After this anodic treatment no evolution of the surface under air exposure is observed (> 1 year) [3]. This passivation layer, electrochemically obtained, appears promising for InP incorporation into (opto)electronic devices.Despite the reproducibility of the results, questions still arise, such as the anodic charge which widely varies according to the electrochemical process used. An anodic charge around 7 mC.cm-2 is required by cyclic voltametric1 whereas only 0,5 mC.cm-2 is enough by galvanometry[4] and a charge higher than 3 mC.cm-2 is observed by potentiometry[5]. Some disparities are also observed on XPS atomic surface ratios of the phosphazene like structure. For instance, the N to P133eV atomic ratio is expected to be 2 (see formula above). However, the value obtained on some of the passivated samples is slightly higher.In order to figure out these variations, the n-type is reinforced but with low doping carriers (1015atoms.cm-3) and under low photon flux. A very different behavior from n-InP (1018 atoms.cm-3) is observed by both cyclic voltametry and galvanometry. Under illumination, holes involved in the anodic process are directly proportional to the photon flux which explains the relevance of n type. n-InP (1015 atoms.cm-3) provides a photoanodic current at lower potential due to its larger space layer where an effective separation of photogenerated electron–hole pairs occurs. A larger range of potential is then available to explore the anodic treatment preventing holes by tunnel effect. By cyclic voltametry under low photon flux, onto n-InP (1015 atoms.cm-3) a constant photoanodic current is observed whereas an anodic wave was observed onto n-InP with higher doping level (1018 atoms.cm-3). As the cycles progress, the rising front of the photocurrent moves from -0.5 V to 1.2 V vs SRE while the intensity of the photocurrent remains constant.In the same illumination conditions, the galvanometry onto n-InP (1015 atoms.cm-3) required a current magnified by 200 (».500 mC.cm-2) compared to n-InP (1018 atoms.cm-3).Onto undopped n-InP the current efficiency gives then a poor performance in comparison with n-InP at higher doping level.Whatever the process used, the phosphazen film is detected by XPS. However, by galvanometry, the N to P133eV XPS atomic ratio is close to 3. A fundamental question is raised about the holes engaged during the anodic process by galvanometry. [1] D. Aureau, M. Fregnaux, A. M. Gonçalves, A. Etcheberry. Surface and Interface Analysis (2018) 1-5. [2] A.-M. Gonçalves C. Njel, D. Aureau, A. Etcheberry Thin Solid Films 538 (2013) 21–24 [3] A.-M. Goncalves, N. Mezailles, C. Mathieu, P. Le Floch, A. Etcheberry. ´Chem. Mat., 22, (2010) 3114-3120 [4] A.-M. Goncalves, C. Njel, D. Aureau, A. Etcheberry. Appl. Surf. Sci.391 (2017) 44–48 [5] C. Njel, A.-M. Goncalves, A. Etcheberry, Electrochim. Acta 139 (2014) 152–156

Relationship of rheological and thermal properties in organogel emulsions (W/O): Influence of temperature, time, and surfactant concentration on thermomechanical behavior

Emulsions are thermodynamically unstable systems that, by incorporating more components in their formulation, increase their complexity and make their study challenging. However, a good selection of compounds in the formulation, and suitable methods for their preparation, could result in systems with favorable thermomechanical and microstructural properties for multiple applications. The objective of this research was to evaluate the influence of temperature, time, and surfactant concentration on the microstructural, thermodynamic, and rheological properties of organogel emulsions, and to present in a simple way the relationship between enthalpy of fusion and rheological properties (i.e., viscosity) in organogel emulsions W/O. Canola oil, monoglycerides and polyglyceryl polyricinoleate (PGPR) were used as the oil phase. Microscopy, DSC, and Creep-recovery tests were performed. A molecular organization phenomenon was observed that resulted in systems with a more dispersed and ordered microstructure, delaying the phase separation for 28 days of evaluation. In addition, an increase in the internal viscosity of the organogel emulsions was reflected in higher enthalpy of fusion values at 10 °C. An inverse behavior was also presented in emulsions at 25 °C. The combined activity of stabilizing agents, as well as the three-dimensional network formed in continuous phase, avoided phenomena of coalescence and phase separation by keeping distant the drops of water and the oily liquid phase, while leading to a better organized microstructure over time and retaining good rheological properties.

Metastable States of Water Aerosols: Comparison by Experiment

Free energy of water aerosol plasma was calculated using the Debye–Hückel method. It was established that free energies of droplets, ions and simultaneously of all charged particles had local minima (metastable states) at certain concentrations and charges of particles. The calculation results were confirmed by experimental data taken from the literature on a droplet cluster in water vapor and droplet structures in water fog. The possible connection of metastable states with the phenomenon of drop coalescence and rain formation in real clouds, as well as with the generation of stable spatially arranged drop structures, has been indicated.

Shape transition and coalescence of Au islands on Ag (110) by molecular dynamics simulation.

We have used molecular dynamics simulations based on many body semi-empirical potentials described by the embedded atom method, to analyze and understand the diffusion and coalescence phenomena of Au-clusters during the heteroepitaxial growth on Ag (110) surface. Temperature ranging from 300 to 700K were considered. In this study, we examined the heterogeneous system Aun/Ag(110), where n is the number of atoms in each cluster/island (withn = 15, ….35). Our results show that the clusters diffuse on the Ag (110) surface via different diffusion processes, namely, the exchange mechanism and the simple jump, which generate a 2D to 3D transition. Formation and adsorption energies of clusters with different sizes have been computed using static simulations. The dynamic study of coalescence for two islands of system Au15; Au0 - 9/Ag(110) at different temperatures makes it possible to deduce the detail of cluster shape and the influence of its temperature on the stability of the system and its growth during this evolution.

Phase coalescence in a population of heterogeneous Kuramoto oscillators.

Phase coalescence (PC) is an emerging phenomenon in an ensemble of oscillators that manifests itself as a spontaneous rise in the order parameter. This increment in the order parameter is due to the overlaying of oscillator phases to a pre-existing system state. In the current work, we present a comprehensive analysis of the phenomenon of phase coalescence observed in a population of Kuramoto phase oscillators. The given population is divided into responsive and non-responsive oscillators depending on the position of the phases of the oscillators. The responsive set of oscillators is then reset by a pulse perturbation. This resetting leads to a temporary rise in a macroscopic observable, namely, order parameter. The provoked rise thus induced in the order parameter is followed by unprovoked increments separated by a constant time τPC. These unprovoked increments in the order parameter are caused due to a temporary gathering of the oscillator phases in a configuration similar to the initial system state, i.e., the state of the network immediately following the perturbation. A theoretical framework corroborating this phenomenon as well as the corresponding simulation results are presented. Dependence of τPC and the magnitude of spontaneous order parameter augmentation on various network parameters such as coupling strength, network size, degree of the network, and frequency distribution are then explored. The size of the phase resetting region would also affect the magnitude of the order parameter at τPC since it directly affects the number of oscillators reset by the perturbation. Therefore, the dependence of order parameter on the size of the phase resetting region is also analyzed.

Coalescence Phenomenon in the Three Photon Quantum Interferences

We present the theoretical study of three-photon interference in the six ports Mach-Zehnder Interferometer (6p-MZI). The 6p-MZI consist of two lossless symmetrically balanced tritters and three independently controllable phase modulators inserted in each photon line path between the tritters. Three photons (Fock state) are injected onto the first tritter input ports and measured in second tritter output ports. By independently varying the phases states of the three photons, one can control the detection probability of photon numbers at final tritter output ports. The calculation results demonstrated that, at the specific combination of phase states, the three photons interference exhibited a coalescence phenomenon, which is the three-photons detected at the same output port. Interestingly, the probability of detecting photons in all combination modes of |0, 0, 3), |0, 3, 0〉, and |3, 0, 0〉 are monotonously in the equal values.

Coalescence of Droplets in a Microwell Driven by Surface Acoustic Waves.

Microwell arrays are amongst the most commonly used platforms for biochemical assays. However, the coalescence of droplets that constitute the dispersed phase of suspensions housed within microwells has not received much attention to date. Herein, we study the coalescence of droplets in a two-phase system in a microwell driven by surface acoustic waves (SAWs). The microwell structure, together with symmetric exposure to SAW irradiation, coupled from beneath the microwell via a piezoelectric substrate, gives rise to the formation of a pair of counter-rotating vortices that enable droplet transport, trapping, and coalescence. We elucidate the physics of the coalescence phenomenon using a scaling analysis of the relevant forces, namely, the acoustic streaming-induced drag force, the capillary and viscous forces associated with the drainage of the thin continuous phase film between the droplets and the van der Waals attraction force. We confirm that droplet-droplet interface contact is established through the formation of a liquid bridge, whose neck radius grows linearly in time in the preceding viscous regime and proportionally with the square root of time in the subsequent inertial regime. Further, we investigate the influence of the input SAW power and droplet size on the film drainage time and demarcate the coalescence and non-coalescence regimes to derive a criterion for the onset of coalescence. The distinct deformation patterns observed for a pair of contacting droplets in both the regimes are elucidated and the possibility for driving concurrent coalescence of multiple droplets is demonstrated. We expect the study will find relevance in the demulsification of immiscible phases and the mixing of samples/reagents within microwells for a variety of biochemical applications.

Experimental and numerical study on crack propagation and coalescence in rock-like materials under compression

The discontinuous crack surface in a rock affects the stability of the whole rock system. The experiments in this paper were carried out by prefabricating rock-like specimens with different types of flaws, then the specimens were tested under uniaxial compression. Moreover, based on the theory of particle flow, PFC2D software was used for numerical simulation, and the comparative analysis of the experimental and simulative results was carried out to obtain the crack initiation sequence, propagation phenomenon, and failure mode of rock specimens with different flaw types. The results indicated that the wing crack started at the tip of flaw and the form of crack assumed split failure, followed by shear failure caused by the secondary crack. The tensile failure degree decreases and the influence of shear failure increases with the increase of flaw angle. The wing crack and secondary initiation stress value is 35%–55% and 85%–95% of the peak stress value. Crack coalescence appeared in adjacent flaws of rock-like specimens with multiple parallel single flaws, single-cross flaws and double-cross flaws, and the coalescence phenomenon always occurs when the stress peak value is reached. With the number of flaws increasing, the splitting failure of rock-like specimens became more and more serious, the splitting failure of double cross-flaw specimen is the most serious. As for the specimen with single-cross flaw, the wing crack would be produced at the tip of the flaw with larger obliquity. The results of this paper may offer certain reference value for the study on the mechanism of rock crack.

Stabilization of the emulsion of Alkenyl Succinic Anhydride (ASA) in water using cellulose nanofibrils

Alkenyl Succinic Anhydride (ASA) is sizing agent that is applied as an aqueous emulsion during the papermaking process to improve paper’s hydrophobicity. However, its application becomes difficult as the emulsion quickly destabilizes due to the coalescence phenomenon and the high reactivity of ASA with water. In this work, we used cellulose nanofibrils (CNFs) to increase the stability of the ASA emulsion over time. Small amounts of CNFs added to the emulsion helps to increase its storage life by almost 3 times, by means of stabilizing the average droplet size and reducing the amount of hydrolyzed ASA. The time extension allows higher flexibility of the sizing processes. In this work, we identify the negative impact that the temperature and the presence of metal ions in water have on the quality on the optimal ASA emulsion, resulting in a reduction in their effects for both factors with the presence of cellulose nanofibrils.

Porous scaffolds with the structure of an interpenetrating polymer network made by gelatin methacrylated nanoparticle-stabilized high internal phase emulsion polymerization targeted for tissue engineering.

The interlacing of biopolymers and synthetic polymers is a promising strategy to fabricate hydrogel-based tissue scaffolds to biomimic a natural extracellular matrix for cell growth. Herein, open-cellular macroporous 3D scaffolds with a semi-interpenetrating network were fabricated through high internal phase emulsion templating. The scaffolds are prepared by (I) the curing of PEG diacrylate (PEGDAC) and gelatin methacrylate (GelMA) in the continuous aquatic phase of a coconut oil-in-water emulsion stabilized by GelMA nanoparticles, and (II) the removal of the internal phase. The effect of the main contributing parameters such as pH, GelMA content, and GelMA/PEGDAC weight ratio on the emulsion features was investigated systematically. Due to the isoelectric point of GelMA at around pH 6, the GelMA particle (aggregation) size decreased at both sides of pH from 1000 to 100–140 nm because of the increased number of positive and negative charges on GelMA. These GelMA nanoparticles were able to produce stable emulsions with narrowly distributed small emulsion droplets. Moreover, the stability and emulsion droplet size were enhanced and increased, respectively, with GelMA content increasing and GelMA/PEGDAC weight ratio decreasing. These trends lie in the prevented coalescence phenomenon caused by the improved viscosity and likely partially formed network by GelMA chains in the continuous phase. Hence, the following formulation was selected for scaffold preparation: φoil = 74%, pH = 12, GeMA = 4 wt%, and GelMA/PEGDAC = 10/8. Then, PCL in different contents was infiltrated into the scaffold to balance hydrophilicity and hydrophobicity. The cell culture assay proved that the scaffold with a pore size of 60–180 μm and containing 51.2 wt% GelMA, 10.3 wt% PEG, and PCL 27.2 wt% provided a suitable microenvironment for mouse fibroblast cell (L929) adhesion, growth, and spreading. These results show that this strategy suggests promising culture for tissue engineering applications.

Numerical investigation of falling ferrofluid droplets under magnetic fields

A methodical analysis on the dynamic interaction behavior between a pair of uneven sized ferrofluid droplets freely falling under gravity and uniform magnetic fields is presented in this article. Here, a conservative level set method (LSM) is adopted to precisely calculate the free interface curvature of the droplet, which again couples both magnetic and flow fields. The results indicate that at a unity viscosity ratio (i.e., λ = 1) and a fixed Galilei number (i.e., Ga = 1600), in the absence of any external forces except gravity, a critical initial vertical separation distance between the droplets prevails (ΔYcr⋆ ≥ 6), which prohibits them from undergoing coalescence phenomenon before hitting the bottom wall of the computational domain. However, enacting a magnetic field along α = 0∘ hinders coalescence, while it is expedited by the implementation of the magnetic field along α = 90∘. Contrarily, at α = 45∘, the droplets exhibit downward lateral migration along the secondary diagonal of the domain, leading to a larger separation between them at higher magnetic Bond numbers Bom (i.e., Bom = 8). Additionally, the investigation on the effects of surface tension suggests an increase in the vertical separation between droplets at higher Eötvös numbers (i.e., Eo = 9.48). Moreover, a magnetic field along α = 0∘ results in the formation of round-bottom hull shaped droplets, whereas they transform into teardrop shaped droplets before coalescence at α = 90∘. Furthermore, the magnetic field along α = 45∘ greatly impedes the coalescence phenomenon, which eventually leads to migration of droplets along divergent lateral directions. Finally, if the droplets are dispersed in air, they do not exhibit any coalescence event before the impact under gravity or gravity and magnetic fields.

Experimental investigation on sliding bubble coalescence of subcooled flow boiling in rectangular narrow channel

The behavior of bubble coalescence in subcooled boiling has a significant influence on the void fraction and heat transfer capacity. In this paper, a visual flow boiling experiment is carried out to investigate the phenomenon of bubble coalescence and the effect of sliding bubbles on coalescence. The phenomenon of bubble coalescence, the velocity of bubble and interface evolution characteristics after coalescence are analyzed in detail. The annular waves caused by the rupture of the thick liquid film and the severe deformation caused by bubble coalescence are observed during the experiment. The propagation of annular waves on bubble surfaces makes bubbles unstable and then easily breaks down to produce secondary bubbles. Unlike the secondary bubbles produced by air bubble coalescence, the critical diameter of initial average bubbles that produce secondary bubbles after bubble coalescence is about 0.87 mm. This difference is because the viscosity of the fluid at high temperature in boiling two-phase flow is different from that in adiabatic water-air two-phase flow. Besides, it is found that there is an oscillation adjustment zone after bubble coalescence. This stage is divided into the violent oscillation region and moderation region, and their duration is basically the same. The relative velocity of sliding bubbles during collision is obtained according to the velocity difference between bubbles and the growth rate of the sliding bubble. Also, the effect of sliding bubbles on bubble coalescence is analyzed in this paper.

Surface and foam property of perfluoroalkyl polyoxyethylene ether phosphate salt in aqueous-ethanol system

Foam is used in many fields with excellent performance. Moreover, disinfectants with ethanol as the main ingredient have increasingly been used in people's lives. However, the foam research of an ethanol-water mixed system is still in the initial stage. In this work, a perfluoroalkyl polyoxyethylene ether phosphate salt surfactant (FEP-Na) was synthesized through a new molecular design. The surfactant has good interfacial adsorption behavior in 50% ethanol-water mixing system, and the approximately spherical aggregates are observed in the volume phase of the solution. Moreover, FEP-Na showed good foaming property in 50% ethanol-water mixed solution. Interestingly, high concentration solutions with high surface activity have lower foam stability. And under the microscope, we see that the bubble burst is dominated by the phenomenon of coalescence. In this regard, we explain the mechanism of foam stability by electrostatic double layer theory and interfacial dilation rheology. It provides a new choice for the synthesis of foaming agent in the ethanol-water mixed system at a concentration of 50%.

Intestinal release of biofilm-like microcolonies encased in calcium-pectinate beads increases probiotic properties of Lacticaseibacillus paracasei

In this study, we show that calcium pectinate beads (CPB) allow the formation of 20 µm spherical microcolonies of the probiotic bacteria Lacticaseibacillus paracasei (formerly designated as Lactobacillus paracasei) ATCC334 with a high cell density, reaching more than 10 log (CFU/g). The bacteria within these microcolonies are well structured and adhere to a three-dimensional network made of calcium-pectinate through the synthesis of extracellular polymeric substances (EPS) and thus display a biofilm-like phenotype, an attractive property for their use as probiotics. During bacterial development in the CPB, a coalescence phenomenon arises between neighboring microcolonies accompanied by their peripheral spatialization within the bead. Moreover, the cells of L. paracasei ATCC334 encased in these pectinate beads exhibit increased resistance to acidic stress (pH 1.5), osmotic stress (4.5 M NaCl), the freeze-drying process and combined stresses, simulating the harsh conditions encountered in the gastrointestinal (GI) tract. In vivo, the oral administration of CPB-formulated L. paracasei ATCC334 in mice demonstrated that biofilm-like microcolonies are successfully released from the CPB matrix in the colonic environment. In addition, these CPB-formulated probiotic bacteria display the ability to reduce the severity of a DSS-induced colitis mouse model, with a decrease in colonic mucosal injuries, less inflammation, and reduced weight loss compared to DSS control mice. To conclude, this work paves the way for a new form of probiotic administration in the form of biofilm-like microcolonies with enhanced functionalities.

The effect of fluctuating pressure gradient on the coalescence of Taylor bubble

The oscillatory coalescence phenomenon of the Taylor bubble flow subjected to the action of a fluctuating pressure gradient in a pulsating heat pipe was investigated using the front tracking method (FTM). The effects of amplitude and frequency of fluctuating pressure, bubble size, Reynolds number (Re), and Weber number (We) on the bubble coalescence process were studied. The results demonstrated that the lower the pulsation frequency, the longer the period of bubble oscillation, which could provide enough time for bubbles to drain and promoted the coalescence of the bubbles. The larger Euler number (Eu) was, the more easily bubbles coalesced. On the contrary, when Eu was small, the bubbles were slow to coalesce. The size of the top bubble and the bottom bubble had different effects on coalescence. An increase of the length of the top bubble (Lt) was beneficial to bubble coalescence while increasing the length of the bottom bubble (Lb) restrained bubble coalescence. Time required for bubble coalescence increased with the increase of Re and decreased with increasing We.

Microscopic characteristics of multiple droplets behaviors at the near-wall region during the quasi-steady state

It is well known that the liquid adhesion by impingement in the direct-injection spark-ignition (DISI) engine hinders the engine combustion efficiency and increases the particular matter (PM) emissions. Although numerous investigations were done on it, it is still blur for the scholars owing to the complicated droplets-wall dynamics. Due to the dense liquid near the wall, it is difficult to observe the impinging droplets in this region, let alone analyzing it on the micro level. Therefore, the “multiple droplets producer” was applied to cut the spray for clear observation in this study. Tslicer was finally determined at 40 μm to make the multiple droplets impacting on the wall at the near-wall region under various injection pressure from 10 to 30 MPa during the quasi-steady state. Four different locations of (15, 15), (17, 15), (20, 15), (22, 15) were selected along the jet development after impingement. Particle image analysis (PIA) technology was applied to capture the micro behaviors. The diameter and velocity of droplets were calculated and analyzed as well as the Weber number. Results show that droplet behaviors near the wall can be recorded at Tslicer = 40 μm during the injection. Droplet size becomes larger with spray propagation at the near-wall region. Observations demonstrate that the coalescence phenomenon of the secondary droplets as well as the splashing crown structures occurrence leads to the larger droplet and lower velocity. Moreover, non-dimensional parameter We was used to further prove the splashing transition occurring from (17, 15) to (20, 15). Additionally, the experimental results can provide strong evidence and verification basis for computational fluid dynamics (CFD) simulations.

Dynamics of respiratory droplets carrying SARS-CoV-2 virus in closed atmosphere

From the epidemiological point of view, the lifetime of cough and sneeze droplets in the ambient atmosphere plays a significant role in the transmission rate of Coronavirus. The lifetime of indoor respiratory droplets, per se, is a function of droplet size, ambient temperature, and humidity. In the attempt to explore the effective factors of droplet lifetime, sufficient knowledge of atomic-scale interactions and dynamics of the droplet with themselves, as well as the airflow molecules in the room space, is necessary. In this study, the vertical traveling of a wide range (100 nm–10 μm) of representative carrier droplets is studied in three ambient temperatures of 258, 298, and 318 K using all-atom molecular dynamics simulation. Our obtained results confirm that by increasing the room temperature, the suspending time of aerosol (suspended droplets carrying virus particles) increases due to the higher dynamics of air and evaporated water molecules in room space. In fact, by increasing the indoor temperature, the collision rate of aerosol and ambient atmosphere molecules increases significantly. Our result shows this higher rate of collision could have a dual effect on the lifetime of aerosol considering the fact of faster deposition of larger (heavier) droplet due to the gravitational force. On one hand, in higher temperatures, the higher collision can split the droplets to smaller ones with a semi-permanent suspension period. On the other hand, the higher dynamics of ambient molecules can lead to meet and coalesce of smaller cough/sneeze droplets making larger (heavier) droplets with faster sediment times. So, the role of indoor humidity to fuel the probability of coalescence phenomenon and lifetime of droplets becomes more determinant in the warmer spaces.

Numerical simulation of the collision behaviors of binary unequal-sized droplets at high Weber number

In the spray combustion process, the distribution of droplet size and velocity will affect the atomization performance of the fuel and the combustion effect. Compared with binary equal-sized droplet collisions, binary unequal-sized droplet collisions are more in line with the actual situation. In this paper, a numerical investigation of binary unequal-sized droplet collision has been performed under different high Weber numbers (from 210 to 810) and impact parameters (B ≈ 0.3–0.9) by a coupled level-set and volume of fluid method with adaptive mesh refinement technology. Unlike the coalescence and separation phenomena at low and medium Weber numbers, at high Weber numbers, due to the difference in velocity between large and small droplets, the rim expands radially outward in different ways and further breaks up. The splashing behavior of the droplets can also be observed. As the Weber number increases, the breakup moment of the droplets advances and the maximum radial deformation diameter increases first (We = 210–360) and then decreases (We = 360–810). By changing the impact parameters, it can be found that binary off-center collisions are associated with rotational motion. At larger impact parameters, the features of the capillary wave instability can be observed on the surface of the ligament.

Distribution, coalescence, and freezing characteristics of water droplets on surfaces with different wettabilities under subfreezing convective flow

This paper investigates the effects of surface wettability and surface initial conditions on early frost formation for hydrophobic, hydrophilic, and bare aluminum flat surfaces. The new data revealed that the average droplet sizes were larger and droplet size distributions were wider for the hydrophilic surface than for the hydrophobic and bare aluminum surfaces. This was due to a coalescence phenomenon occurring before the droplets froze into ice beads. Because of the relative high sub-freezing plate temperatures used in this work, the surface wettability of the plates had a smaller impact on the freezing time. Supersaturation degree and temperature difference between the air stream and the surface controlled freezing times for all different surface wettability types. When testing was conducted without cleaning the surfaces in between individual test runs, a film deposited on the surfaces which smoothed surface imperfections, inhibited surface wettability effects on droplet size and shape, and delayed freezing. A large variation in the frost nucleation behavior was measured in the present work due to intentionally different generated initial surface cleanness conditions. This finding could explain some of the significant deviations observed in data from literature studies when comparing freezing times and droplet diameters for similar coatings and similar tests conditions.