Coalescence Phenomena Research Articles

Study on the Coalescence-Induced Jumping of Droplets with Different Radii on Superhydrophobic Surface

The phenomenon of droplet coalescence and jumping has received increasing attention due to its potential applications in the fields of condensation heat transfer and surface self-cleaning. Basic research on the process and mechanism of coalescence-induced droplet jumping has been carried out, and some universal laws have been established. However, it is found that the focus of these studies is based on two identical droplets, and the coalescence-induced jumping with different radii is rarely investigated, which is commonly encountered in nature. Therefore, it is essential to proceed with the research of coalescence and jumping of droplets with unequal radii. In this paper, molecular dynamics (MD) simulations are performed to reveal the effects of radius ratio and radius of small droplets on jumping velocity. The results show that as the increasing of radius ratio with an unchanged small droplet radius of 8.1 nm, the jumping velocity increases then decreases, which indicates there is an optimal radius ratio to maximize the jumping velocity. Additionally, it is found that if the small droplet radius is changed, the critical radius ratio for characterizing whether the coalesced droplet jumping increases with increasing the small droplet radius. Furthermore, according to energy conservation, the conversion efficiency of energy is discussed. The results show that when the radius ratio is greater than 1.3 with three different small droplet radii, the energy conversion efficiency rapidly decreases to below 1.0%; and the critical radius ratios are consistent with the result obtained from the velocity analysis. This work broadens the understanding of the more general phenomenon of coalescence-induced droplet jumping and can better guide industrial applications.

Quantification of Surface Tension Effects and Nucleation-and-Growth Rates during Self-Assembly of Biological Condensates.

Liquid-solid and liquid-liquid phase separation (PS) drives the formation of functional and disease-associated biological assemblies. Principles of phase equilibrium are here employed to derive a general kinetic solution that predicts the evolution of the mass and size of biological assemblies. Thermodynamically, protein PS is determined by two measurable concentration limits: the saturation concentration and the critical solubility. Due to surface tension effects, the critical solubility can be higher than the saturation concentration for small, curved nuclei. Kinetically, PS is characterized by the primary nucleation rate constant and a combined rate constant accounting for growth and secondary nucleation. It is demonstrated that the formation of a limited number of large condensates is possible without active mechanisms of size control and in the absence of coalescence phenomena. The exact analytical solution can be used to interrogate how the elementary steps of PS are affected by candidate drugs.

Ion irradiation-induced nano pore/fiber formation and areal-vacancy distribution in semiconductor materials

In this study, the formation of nano-porous/fibrous structures was investigated by spot-ion irradiation on Ge and GaSb substrates to consider the formation range of nanostructures and point defects. Spot-ion irradiation was performed in a focused ion beam system with a beam diameter of 35.1 nm, which is determined by the system. The fluence and ion beam current ranged within 5 × 1019–1 × 1022 ions/m2 and 500–520 pA, respectively. All the irradiated ions were Ga+ with incident energy adjusted at 30 keV, and the angle of incidence to the surface normal was 0°. The interval of the irradiated spot ranged from 100 to 1000 nm. For line-patterned irradiations, these spots overlapped with one another at an overlap ratio of 50%, producing a 35.1 nm × 10 μm irradiated region. Result shows that nanostructured regions followed a spot interval. When an interval was shorter than 200 nm, each nanostructured spot coalesced with neighbors to form large and uniform structures. Furthermore, line-patterned irradiation indicated that the migration distance of vacancy was ∼300 nm at maximum, which is consistent with the coalescence phenomenon.

Study of a Glass-Ceramic Seal Porosity

Sealing is a major issue in Solid Oxide Cell (SOC) used for hydrogen production. Currently, a slurry composed of glass powder, organic solvents and adjuvants, is used as a seal. After the deposition of the slurry on the areas to be sealed, a heat treatment allows to obtain a glass-ceramic. This shaping process is very convenient but the obtained material presents porosity. This study aims to understand the evolution of porosity as a function of time and temperature in order to minimize it. A significant increase in porosity is observed when the temperature increases. The porosity evolution is characterized by an increase in pores size and a reduction of pores number, typical of a coalescence phenomenon. At high temperature, the viscosity is low enough to allow pores to rise in the sample. This weakens the upper part of the seal and may cause a loss of sealing.

Eulerian-lagrangian investigation of de-oiling hydrocyclones: The effects of droplet interaction and size distribution

The widespread use of de-oiling hydrocyclones to separate oil and water from wastewater has attracted the interest of many researchers. The close density of oil and water, and their breakup and coalescence, make research in this area quite challenging. In the present research, a numerical Eulerian-Lagrangian (E-L) framework was developed to investigate the effects of droplet interaction, inlet size distribution, and phase-coupling schemes on hydrocyclone efficiency. The results show that droplet interaction, including the breakup and coalescence phenomena and the inlet size distribution, should be considered to obtain accurate numerical results in the E-L framework. Using the E-L approach, the efficiency of de-oiling hydrocyclones was estimated with a minimum error of 7%. Also, the phase-coupling schemes affected the velocity field of the continuous phase (water) and the separation efficiency. The coalescence and breakup-dominated regions were investigated in different designs of de-oiling hydrocyclone using the E-L approach. Examining different designs shows that large changes in cone angle create a region of high turbulence and enhance droplet breakup. It can be concluded that the Sauter mean diameter of droplets and, consequently, the separation efficiency decrease in designs with the sharp cone angle.

Study of the coalescence phenomena in biogenic nano-hydroxyapatite produced by controlled calcination processes at low temperature

The coalescence of nanocrystals generated by thermal treatments is a phenomenon observed in many materials. In the case of hydroxyapatite (HAp) derived from a biological source such as bovine bone, this transition was observed at temperatures around 700 °C. However, whether this is also the case at lower temperatures has not yet been investigated. With this aim, this work examined HAp samples from cattle calcined at 400–720 °C in increments of 20 °C, to determine the temperature effect on HAp nanocrystals due to coalescence. Differential scanning calorimetry showed that the raw (as prepared) bovine hydroxyapatite exhibited four thermal transition events at which the coalescence of HAp nanocrystals occurred. Infrared analysis showed that the full width at half maximum (FWHM) of the phosphate band decreases with increasing calcination temperature. The FWHM of the X-ray diffraction peaks showed minima values just at the coalescence events, while the unit cell volumes exhibited an opposite trend. The crystallite size of the HAp increases with increasing calcination temperature during the coalescence events, and the Rietveld method shows that the unit cell volume follows the same behavior and shows an opposite behavior in terms of the micro-strain. Scanning electron microscopy (SEM) images of samples calcined at temperatures above 520 °C show the first signs of coalescence, as necks are formed by the union of adjacent HAp nanocrystals. N2 adsorption-desorption measurements have also shown that the average pore diameter is maintained during thermal processes. Finally, transmission electron microscopy (TEM) was used to observe the coalescence of the nanocrystals of the sample burned at 420 and 520 °C. These thermal events could be because the surface vibrational states of at least two adjacent HAp nanocrystals collapse and form new bulk vibrational states.

Electrocoalescence dynamics of two unequal-sized droplets

The present study investigates the electrocoalescence dynamics of two unequal-sized water droplets surrounded by a different fluid (typically considered oil). Finite element method-based level-set formalism has been adopted to solve the governing transport equations for a two-dimensional framework, and the accuracy of the solver is ascertained by validating with previously reported experimental results. The study extensively explores the characteristics of the droplet-to-droplet coalescence phenomenon, which is governed by key determinants like electric field strength , diameter ratio of the coalescing droplets, and viscosity of the fluids. It reveals that the electric field application shifts the coalescence regime from complete to partial beyond a limiting field strength for a corresponding diameter ratio. For a larger diameter ratio, the electric field strength required to get partial coalescence is relatively smaller than that needed for a smaller diameter ratio. In the context of partial coalescence, increasing electric field generates larger satellite droplets. With significantly higher electric field strength application, the satellite droplet goes under the second stage pinch-off phenomenon without coalescence. The partial coalescence gets suppressed with the increase in viscosity of the surrounding fluid; however, with the application of a larger voltage, partial coalescence can be achieved even for higher viscous fluid, and the same becomes more susceptible at a higher diameter ratio of the droplets. Finally, it is also established with an extensive comparison of how the present work is fundamentally remarkable and holds the utmost significance in the community of droplet dynamics research.

Electric-field-mediated morpho-dynamic evolution in drop–drop coalescence phenomena in the inertio-capillary regime

When two drops collide, they may either exhibit complete coalescence or selectively generate secondary drops, depending on their relative sizes and physical properties, as dictated by a decisive interplay of the viscous, capillary, inertia and gravity effects. Electric field, however, is known to induce distinctive alterations in the topological evolution of the interfaces post-collision, by influencing a two-way nonlinear coupling between electro-mechanics and fluid flow as mediated by a topologically intriguing interfacial deformation. While prior studies primarily focused on the viscous-dominated regime of the resulting electro-coalescence dynamics, several non-intuitive features of the underlying morpho-dynamic evolution over the intertio-capillary regime have thus far remained unaddressed. In this study, we computationally investigate electrically modulated coalescence dynamics along with secondary drop formation mechanisms in the inertio-capillary regime, probing the interactions of two unequal-sized drops subjected to a uniform electric field. Our results bring out an explicit mapping between the observed topological evolution as a function of the respective initial sizes of the parent drops as well as their pertinent electro-physical property ratios. These findings establish electric-field-mediated exclusive controllability of the observed topological features, as well as the critical conditions leading to the transition from partial to complete coalescence phenomena. In a coalescence cascade, an electric field is further shown to orchestrate the numbers of successive stages of coalescence before complete collapse. However, an increase of the numbers of cascade stages with the electric field strength and parent droplet size ratio is non-perpetual, and the same is demonstrated to continue until only a threshold number of cascade stages is reached. These illustrations offer significant insights into leveraging the interplay of electrical, inertial and capillary-driven interactions for controllable drop manipulation via multi-drop interactions for a variety of applications ranging from chemical processing to emulsion technology.

Nucleation of water vapor on nanodimpled surfaces: Effects of curvature radius and surface wettability

• The effects of dimple curvature radius and wettability on nucleation are studied. • Surface potential energy analysis confirms nucleation sites. • Increasing dimple curvature radius worsens nucleation. • More dimples increase vapor nucleation probability at lower surface subcooling. • The coupling effect of the surface structure affects the nucleation. The mechanisms of vapor nucleation on nanostructured surfaces, whose geometrical characteristic sizes are comparable to those of nuclei, remain unsatisfactorily explored. This study investigates the correlations among the wettability, dimple curvature radius, and nucleation characteristics of vapor molecules on nanodimpled surfaces through molecular dynamics simulations. The surface potential energy analysis demonstrates that nucleation sites are located in the interior of dimples. With an increase in surface hydrophobicity or dimple curvature radius, a higher nucleation energy barrier would render nucleation more difficult. The critical sizes of nuclei obtained from MD simulation are larger than the results estimated by the classical nucleation theory. The discrepancy is attributed to the fact that the formed nucleus in simulations deviates from the spherical cap assumption. The formation of nuclei is not observed at all dimples, indicating that the coupling effects of surface structure would affect nucleation. Additionally, the comparisons of coalescence phenomena noted for different dimple numbers reveal the benefits of increasing the dimple number, which can be attributed to the fact that more high-energy nucleation sites increase the nucleation probability of vapor molecules at reduced surface subcooling. Therefore, an enhanced condensation performance can be achieved by artificially controlling the dimple number.

Three-dimensional X-FEM modeling of crack coalescence phenomena in the Smart CutTM technology

A new 3D X-FEM approach is proposed to model crack coalescence phenomena in the Smart Cut TM technology. An adaptive mesh is developed based on a 3D fractal mesh allowing a significant gain in computing time compared to regular meshes. In order to calculate stress intensity factors (SIFs), the term corresponding to internal pressure, that is normal loading on crack faces has been determined, added to the interaction integral and the results of SIFs in mode I have been compared to their analytical values with mean error less than 1%. An implicit algorithm is implemented to predict the evolution of the internal pressure for growing cracks of any shape; a constitutive law in pressure correlated with the equation of state of ideal gases has therefore been used. Compared to the analytical solution, numerically evaluated pressure is accurate with maximum errors less than 1% for relevant convergence tolerance. Finally, the coalescence of two cracks is taken into account in the model. Qualitative local stress analysis makes it possible to explain some phenomena experimentally observed in the Smart Cut TM such as neighboring effects of growing cracks, the outgrowth of nearby cracks or influence of the presence of concave areas at a front on growth. • A 3D X-FEM model has been proposed to model crack coalescence phenomena under variable pressure (Smart Cut TM process). • The interaction integral has been adapted to take into account the internal pressure when computing 3D stress intensity factors. • Prediction of the internal pressure of propagating cracks whatever are their shapes. • Analysis of neighboring effects by stress analysis on a two penny-shape crack coalescence model.

Investigation of gas bubble coalescence suppression and its effect on the operation of a booster pump as part of a pump-ejector system when pumping water-gas mixtures

Article presents an overview of studies of the gas bubbles interaction processes in the composition of water-gas mixtures. Data are given on theories that indirectly indicate the nature of the phenomenon of coalescence, or its suppression. It has been established that there are rational ranges of parameter values at which coalescence suppression is observed, however, when some critical values of these parameters are exceeded, the coalescence suppression regime breaks down. The experimental studies of the gas flow rate effect, passed in the bubbles flow form through a column with liquid, at various pressures and salinity of the liquid showed that an increase in the maximum gas flow rate with suppressed coalescence is observed in a certain range of salinity (with increasing pressure). However, further with an increase in salinity, the maximum the gas flow rate remains constant, after which the regime breaks. The addition of foaming surfactants increases the maximum gas flow rate. At the same time, a study of multistage centrifugal pump characteristics showed that the addition of surfactants improves the performance of the pump up to a certain concentration, with an increase in this concentration, the performance improves slightly. A further direction of studying the technology of creating stable water-gas mixtures is also formulated. Keywords: carbon footprint reduction; SWAG technology; pump-ejector systems; water-gas mixture; coalescence of gas bubbles.

Chitosan-carbamoylcarboxylic acid grafted polymers for removal of metal ions in wastewater

Water contamination by heavy metals is reported all over the world and requires strategic solutions to minimize its environmental impact. In this work, new materials based on chitosan (Q) were grafted with new mono carbamoylcarboxylic acids (mCCA) 1a and 1b were evaluated in the simultaneous removal of six metal ions by coagulation-flocculation process. The physicochemical properties and the interaction of carbamoylcarboxylic acid (CCA)-chitosans Q-1a and Q-1b, chitosan, 1a and 1b with metal ions were analyzed by zeta potential (ζ). CCA-chitosan Q-1a-5 shows significant removal of Cu2+ (99.8 %), Ni2+ (73.1 %) and Zn2+ (67.0 %) and optimal removal dose (ORD) of 20.0 mg/g of M2+, as well as Q-1b-10 performs the removal of Pb2+ (98.8 %), Cu2+ (99.1 %) and Ni2+ (81.1 %) with an ORD of 23.0 mg/g of M2+. The global metal ions removal is 81.2 % with Q-1a-5 and 79.3 % for Q-1b-10, highly superior than chitosan (42.3 % and ORD 34.3 mg/g of M2+). CCA-chitosan Q-1a-5 and Q-1b-10 show an improved clarification kinetics (18.3 and 56.8 T%/h, respectively), sedimentation ratio (10.6 and 48.5 mm/h, respectively) and a Turbiscan Stability Index (TSI) value (40.2 and 80.2, respectively) and the metal ions removal is associate to a sedimentation, flocculation and coalescence phenomena, displaying the best characteristics for the wastewater treatment containing different metal ions.

A population balance model for the flow-induced preparation of Pickering emulsions

In this work we studied the flow-induced preparation of oil-in-water Pickering emulsions by means of a bivariate population balance model. We characterized oil droplets by their size and surface coverage by solid particles, and we took into account all phenomena responsible for their change, namely coalescence and breakup of the droplets, and particle-droplet collisions leading to surface coverage. We studied the population dynamics in a uniform shear flow and in a turbulent flow field, and stabilization was observed in both cases. Under shear, the population evolved by coalescence phenomena, which, together with particle adsorption, led to a prompt and full stabilization of the emulsion droplets. An inverse scaling between particle load and droplets Sauter diameter was found, and seen to well compare with previously reported experimental data for arrested coalescence. In turbulence, stabilization was also obtained. In this case, the droplet size evolved through a peculiar path, showing a size undershoot before equilibrium conditions were reached. We explained this as a consequence of the coalescence phenomena, that alongside breakup, affected the transient of the process. Results shed a new light onto the dynamics of preparation of Pickering emulsions and aim at constituting a reference for the set-up and interpretation of experiments.

Methodology to assess interlayer quality in the material extrusion process: A temperature and adhesion prediction on a high performance polymer

The material extrusion additive manufacturing process consists in extruding a molten polymer along a path with trajectories defined from a 3D definition of the part. One of the major drawbacks of this process is the reduction of the mechanical properties of the parts compared with those made by conventional manufacturing methods such as injection molding. On the one hand, this reduction of properties is linked to the presence of macroporosities in the part and, on the other hand, to the limited adhesion between the deposited layers. These two phenomena strongly depend on the thermal history of the polymer. This work proposes to determine the thermal conditions leading to sufficient adhesion, and to the manufacture of parts with sufficient final interfilament strength. An instrumented bench was developed from a 3D printer in order to investigate the cooling, at the filament scale, of parts printed in PEKK material. In parallel, heat transfers in the material extrusion process were simulated using a model, which was validated with the experimental data obtained using the bench. The adhesion was also modeled by describing the phenomena of coalescence and healing of the interface. The physical properties necessary for the models were characterized taking into consideration their thermodependency. A parametric study then quantifies the influence of each parameter on the temperature of the polymer, as well as on the interlayer adhesion, thus contributing to the definition of the process window.

Quantitative kinetic analysis of γ′ precipitate evolution in a Co–Al–W superalloy during aging heat treatment

In this research, the effect of aging-heat treatment on evolution of γ′-precipitates in the Co–Al–W-based superalloy was studied. For this purpose, solution heat treatment was applied followed by water-quenching and then, aging different cycles were carried out at four temperatures of 720, 780, 840 and 900 °C for different holding time (1–48 h). The electron microscope micrographs showed that increasing size of γ′ particles happens with more holding time; while volume fraction stays approximately constant at 0.72. In addition, the γ-γ′ lattice mismatch, as positive values, increased with holding time increase due to simultaneous γ′ shape adaptation. While the γ′ morphology variation was seen to change from spherical to semi-cubic and cubic; the L- and stretched-shapes of γ′-particles with concurrent split of the coarse particles was detected by coarsening, coalescence and splitting phenomena, respectively. The proposed mechanisms for such microstructural evolution were interpreted based on n value as the power term in kinetics equation. As such, the γ′-particles diffusion across the γ/γ′ interface (n∼2), volume-diffusion (n∼3) and coalescence/splitting (n∼7) are key controlled processes at different aging temperature ranges. The characteristics of γ′-precipitates was discussed in detail based on total energy concept in terms of surface (Esur.), elastic strain (Estr..) and interactions of elastic (Eint.) energies and hence, a phenomenological model was proposed for evolution of γ′-particles. Modeling based a physical model and analysis of microhardness experimental revealed that the desired radius of γ′ phase should be between 80 nm and 90 nm respectively; and the optimized aging condition is estimated to be done at 840 °C for 36 h.

A new approach to analyze the equilibrium and transient behaviors of particulate systems and the subsequent application to multiphase fluid systems

Fast and robust computation for population balance models is a crucial requirement for simulating particulate systems. Despite all the recent advances in developing relevant computational algorithms, either efficiency or robustness can only be achieved at the expense of the other. However, optimal compromise is possible by having prior estimates of the system's internal length and time scales. Thus, an approximation technique is introduced in this study to extract equilibrium and transient behaviors for a particulate system considering coalescence and breakage phenomena. The approximation method is developed by simplifying assumptions on the available analytical solution for a spatially homogeneous population balance equation with simple kernels. The derived equilibrium and transient equations suggest that the system is governed by a dimensionless group that can describe the equilibrium distribution of the system as well as the rate and the direction that the system is likely to evolve. The method is applied to two different sets of complex breakage and coalescence kernels used for liquid-liquid dispersed systems. The approximated time and length scales were validated with numerical results; thus, the approximation can be used to generate targeted element-based orthogonal collocation grids for fast and robust computation of transient and steady-state particulate systems. This approach can significantly decrease the computation time, typically by 40–70 % for steady-state conditions.

TPV Foaming by CO2 Extrusion: Processing and Modelling.

This work focuses on the extrusion foaming under CO2 of commercial TPV and how the process influences the final morphology of the foam. Moreover, numerical modelling of the cell growth of the extrusion foaming is developed. The results show how a precise control on the saturation pressure, die geometry, temperature and nucleation can provide a homogeneous foam having a low density (<500 kg/m3). This work demonstrates that an optimum of CO2 content must be determined to control the coalescence phenomenon that appears for high levels of CO2. This is explained by longer residence times in the die (time of growth under confinement) and an early nucleation (expansion on the die destabilizes the polymer flow). Finally, this work proposes a model to predict the influence of CO2 on the flow (plasticizing effect) and a global model to simulate the extrusion process and foaming inside and outside the die. For well-chosen nucleation parameters, the model predicts the final mean radius of the cell foam as well as final foam density.

Dual pulsed laser deposition of Ag nanoparticles on calcium phosphate coatings for biomedical applications

Pulsed laser deposition (PLD) represents a promising bottom-up methodology for the synthesis and transference of nanoparticles to the surface of a biomedical device. Silver (Ag) nanoparticles directly incorporated on the metallic implant emerge as an alternative strategy for local action against prosthetic joint-associated infections. In the present research, a dual sequential PLD process is proposed to obtain a bilayer coating with (1) a bio-derived calcium phosphate (CaP) layer, to provide osteointegrative properties and (2) the controlled growth of the Ag nanoparticles over it, ranging the number of laser pulses from 100 to 500. The characterization by SEM, EDS, TEM, XPS and AFM revealed the uniform deposit of Ag rounded nanoparticles, with a narrow mean size distribution, in the original non-oxidized metallic state. Moreover, given the evidences from XPS and AFM techniques, the occurrence of a coalescence phenomenon from 400 pulses onwards was proposed together with the expected positive linear relation between the number of pulses and Ag contribution with a deposition rate of 0.05 at. % of Ag per pulse. Conversely, the decrease in roughness as the Ag content increased was also verified. Finally, the expected bacteriostatic activity for these PLD deposited metallic state Ag nanoparticles against the bacterial strain Staphylococcus aureus was confirmed. Moreover, the evaluation of the osteoblast-like MG-63 cells viability on the Ag(100–500)-CaP coatings revealed a significant increased proliferation (p > 0.05) on the Ag100-CaP coating compared to the control (Ag0-CaP). When same coating was evaluated against S. aureus the effect was not significant. The possibility of modulating the amount of nanoparticles in the bilayer coating to obtain a greater or lesser effect in combination with CaP was revealed.

Numerical investigation of coalescence phenomena, affected by surface acoustic waves

Numerical investigation of coalescence phenomena, affected by surface acoustic waves

Thermal behavior of porcine biogenic hydroxyapatite: An in-situ impedance spectroscopy study

This paper studies the electrical properties of porcine bone through in-situ impedance spectroscopy (IS) from 50 Hz-5 MHz at temperatures from room temperature to 1273 K. Also, other in-situ techniques, such as Differential Scanning Calorimetry (DSC), Thermal Gravimetric Analysis (TGA), High-Temperature X-ray diffraction (HT-XRD), and ex-situ complementary techniques, such as Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM), were carried out to explain the physicochemical transformations of the system. The conduction phenomena in biogenic hydroxyapatite are governed by intergranular contact and crystal size. A thermal phase transition occurs at about 973 K that DSC, HT-XRD, and IS can detect. The correlation between these techniques indicates that this transition is a coalescence phenomenon that generates a nano to microcrystal size transition and out-diffusion of Mg to the surface. These processes improve the crystalline quality and change the activation energy for each thermal cycle, showing a strong correlation between structural and electrical properties. Electrical modulus, electrical permittivity, and electrical conductivity are sensible to this structural change.