Kinetics Of Coalescence Research Articles

Formation of a single quasicrystal upon collision of multiple grains

Quasicrystals exhibit long-range order but lack translational symmetry. When grown as single crystals, they possess distinctive and unusual properties owing to the absence of grain boundaries. Unfortunately, conventional methods such as bulk crystal growth or thin film deposition only allow us to synthesize either polycrystalline quasicrystals or quasicrystals that are at most a few centimeters in size. Here, we reveal through real-time and 3D imaging the formation of a single decagonal quasicrystal arising from a hard collision between multiple growing quasicrystals in an Al-Co-Ni liquid. Through corresponding molecular dynamics simulations, we examine the underlying kinetics of quasicrystal coalescence and investigate the effects of initial misorientation between the growing quasicrystalline grains on the formation of grain boundaries. At small misorientation, coalescence occurs following rigid rotation that is facilitated by phasons. Our joint experimental-computational discovery paves the way toward fabrication of single, large-scale quasicrystals for novel applications.

Two-step droplet formation in monodisperse nanodroplet generation in quenched hydrothermal solution as revealed by spontaneous transformation of nanodroplets to swollen micelles in octane‑in‑water nanoemulsions

HypothesisMonodisperse nanodroplet generation in quenched hydrothermal solution (MAGIQ) is a newly developed bottom-up process for preparing nanoemulsions. In this process, homogeneous solutions of oil in supercritical water are quenched by adding cold water containing a surfactant to induce rapid phase-separation, during which oil molecules self-assemble to form nano-sized oil droplets. The droplet size in MAGIQ is known to be influenced by the interplay of the phase‐separation dynamics, coalescence kinetics of the droplets, and adsorption kinetics of the surfactant on the droplet surface; however, the primary stages of the droplet formation are still elusive. ExperimentsOctane‑in‑water nanoemulsions containing 0.5, 1, and 3 vol% octane were prepared by the MAGIQ method. Their ripening was studied by dynamic light scattering, and the phase diagram was established. FindingsThe nanoemulsions containing 0.5 and 1 vol% octane transformed to thermodynamically stable microemulsions containing swollen micelles, whereas the nanoemulsion containing 3 vol% octane underwent Ostwald ripening. The initial formation of the nano-sized droplets in the former was ascribed to a unique mechanism of droplet formation in MAGIQ—the droplets are first formed by the phase separation of homogeneous binary solutions of oil in supercritical water and then stabilized upon surfactant adsorption.

Revealing Au13 as Elementary Clusters During the Early Formation of Au Nanocrystals.

Understanding the formation mechanism of nanocrystals in solution is fundamental to the development of materials science. For a metal nanocrystal, the cluster-mediated formation mechanism is still poorly understood. In particular, identifying what types of clusters are dominant and how they evolve into a nanocrystal in the early nucleation stage remains a great challenge. Here, using liquid-cell transmission electron microscopy, we directly observe the formation of ultrasmall Au clusters (∼0.84 nm) in the presence of PAA-Na. These clusters, which correspond to the size of the Au13 cluster, coalesce to form nanocrystals. Our molecular dynamics simulations suggest that Au13 in an aqueous environment has greater stability when compared to other cluster sizes and provide atomistic details of growth by cluster coalescence. Collectively, our demonstration of Au13 as the dominant species with an elaboration of their coalescence kinetics sheds light on nonclassical nanocrystal formation mechanisms and offers useful guidelines for designing innovative pathways for the synthesis of nanomaterials.

Omnidirectional droplet propulsion on surfaces with a Pac-Man coalescence mechanism

Electrohydrodynamic (EHD) nanoprinting is used to demonstrate a novel open-atmosphere microfluidic platform, where femtoliter sessile droplets can be formed sustainably despite high volatility, manipulated, and made mobile in any planar direction on-demand, by a sequence of controlled coalescence events. By studying sessile droplet coalescence kinetics in a partially wetting regime, we found that, even for minute droplet sizes, contact line motion still dominates. Coalescing moving droplets were tasked to perform typical microfluidic operations like collecting, transporting, and merging solid materials on otherwise unpatterned substrates.

Preparation of stable multiple emulsions using food-grade emulsifiers: evaluating the effects of emulsifier concentration, W/O phase ratio, and emulsification process

In this study, the effects of sodium caseinate (NaCN; emulsifier) concentration, a type of hydrophilic emulsifier, as well as concentration of primary W/O emulsion on the stability of water–oil-water (W/O/W) emulsions were investigated. Emulsions were made using two different emulsification techniques including ultrasonic liquid processing and high pressure homogenization (HPH). Microscopy images of W/O/W emulsions in combination with droplet size analysis and viscosity measurements showed that the sample with a higher percentage of primary W/O emulsion (50 wt% vs. 40 wt% and 25 wt%) was more resistant to coalescence (narrower droplet size distribution and lower creaming index). The higher stability of this emulsion at 50 wt% is due to the enhancement in the solution viscosity which slows down the coalescence and destabilization kinetics. The increase in the NaCN concentration from 0.3 wt% to 0.9 wt% (based on total weight of emulsion) led to formation of larger droplets possibly due to the destabilization of primary W/O emulsion through the disruption of polyglycerol polyricinoleate (PGPR) layer. Regarding the effect of emulsifier type, incorporation of Cremophor EL and Tween 60 in comparison with NaCN resulted in formation of smaller droplet size due to their enhanced surface activity at the interface. Finally, we found that using high pressure homogenizer (HPH) instead of ultrasonic processor was detrimental to the emulsion stability and size distribution. These findings further provide new pathways to design emulsion systems for food and drug delivery applications.

Ligand-Dependent Coalescence Behaviors of Gold Nanoparticles Studied by Multichamber Graphene Liquid Cell Transmission Electron Microscopy.

The formation mechanism of colloidal nanoparticles is complex because significant nonclassical pathways coexist with the conventional nucleation and growth processes. Particularly, the coalescence of the growing clusters determines the final morphology and crystallinity of the synthesized nanoparticles. However, the experimental investigation of the coalescence mechanism is a challenge because the process is highly kinetic and correlates with surface ligands that dynamically modify the surface energy and the interparticle interactions of nanoparticles. Here, we employ quantitative in situ TEM with multichamber graphene liquid cell to observe the coalescence processes occurring in the synthesis of gold nanoparticles in different ligand systems, thus affording us an insight into their ligand-dependent coalescence kinetics. The analyses of numerous liquid-phase TEM trajectories of the coalescence and MD simulations of the ligand shells demonstrate that enhanced ligand mobility, employing a heterogeneous ligand mixture, results in the rapid nanoparticle pairing approach and a fast post-merging structural relaxation.

Thermal kinetic and mechanical behaviors of pressure-assisted Cu nanoparticles sintering: A molecular dynamics study

A molecular dynamics (MD) simulation was performed on the coalescence kinetics and mechanical behavior of the pressure-assisted Cu nanoparticles (NPs) sintering at low temperature. To investigate the effects of sintering pressure and temperature on the coalescence of the nanoparticles, sintering simulations of two halve Cu NPs were conducted at the pressure of 0–300 MPa and the temperature of 300–500 K. A transition of the dominant coalescence kinetics from slight surface diffusion to intensive grain boundary diffusion and dislocation driven plastic flows were found when pressure was applied. Furthermore, atomic trajectories showed the effect of temperature on sintering was strongly dependent on the microstructures of Cu NPs. The atomic diffusion around defects can be significantly promoted by the elevated temperature. Additionally, based on the sintered structures, uniaxial tension simulation was implemented with a constant strain rate. Stress–strain curves and evolution of dislocation activities were derived. Improved mechanical behaviors, including larger elastic modulus and larger tensile strength, were obtained in the structure sintered under higher pressure and temperature. Among this study, sintering temperature and pressure consistently exhibited the same relative impact on affecting both coalescence and the mechanical properties of the sintered structure.

Recrystallization and coalescence kinetics of TiO2 and ZnO nano-catalysts towards enhanced photocatalytic activity and colloidal stability within slurry reactors

Advanced oxidation processes rely on the development of stable photocatalytic materials, offering specific band-gap attained upon reaching appropriate crystalline phases. A key research gap is related to the recrystallization of nano-catalysts and their impact on performance. This paper describes the systematic preparation of TiO2 and ZnO nanoparticles by hydrothermal-assisted sol-gel method followed by systematic calcination steps with the aim to shed light on the recrystallization process to engineer higher photocatalytic activity. Spherical anatase-TiO2 nanoparticles with a 20 nm diameter size present higher photocatalytic activity than 70 nm of ZnO NPs calcined at 500 °C. A photocatalytic yield of 84% within 30 min of irradiation for the degradation of model dyes was observed for the titania particles, which were also less sensitive to a gglomeration, a key challenge when designing slurry reactors. The variation in zeta potential of TiO2 and ZnO with pH exhibited isoelectric points (IEP) in aqueous media at 5.1 and 6.5, respectively, suggesting amphoteric behaviors while X ray photo-electron spectroscopy and diffusive reflectance spectroscopy data were used to characterize the changes in surface vacancies and the band gap of the materials. These data open the door to the development of advanced oxidation processes in complex industrial environments.

Molecular dynamics simulation of coalescence kinetics and neck growth in laser additive manufacturing of aluminum alloy nanoparticles

Laser additive manufacturing emerged as an advanced manufacturing process to fabricate components in a layered fashion by fusing the powder particles. This process is multifaceted and pivotal to understand the underlying physics of the coalescence of powder particles during the process, which impacts the structural and mechanical properties of the build component. In this study, a classical molecular dynamics (MD) model is developed for the coalescence of pre-alloyed aluminum alloy (AlSi10Mg) particles during the laser additive manufacturing process. The model is employed to investigate the neck growth and coalescence kinetics of different pairs of particle size with changing the laser energy density from 7 to 17J/mm2. The simulation results reveal that the unevenly sized particles undergo complete coalescence as compared with even-sized particles, and the neck growth rate of AlSi10Mg particles increases with an increase in laser energy density. Based on the present investigation, it is established that the coalescence kinetics of the AlSi10Mg nanoparticles are governed by the surface and volume diffusion and the surface energy reduction during the joining of particles. This analysis will act as a guideline to design process parameters and quality control for the printing of new components.

Microfluidic method for determining drop-drop coalescence and contact times in flow

Coalescence kinetics is an important parameter when describing the stability of dispersions. A number of methods to measure coalescence time are available, however they often take long time and do not allow working with small droplets or coalescence in flow. Here we present a new microfluidic method for recording and analysing hundreds or thousands of individual droplet interactions in flow. Our method allows to extract information about both the coalescence and the contact times of drops. In addition, we can distinguish the coalescence events by the droplet size, as well as the approach velocity of the colliding droplets. We validated the proposed methodology by systematically changing a number parameters of the experiment (salinity, oil composition, presence of surfactant, temperature). The increase of salinity lead to compression of the double layer and decreased coalescence time. A difference was found when studying coalescence of heptane, xylene and dodecane, which was attributed to their hydrophobicity and viscosity. The addition of surfactant caused a significant increase of coalescence time through additional repulsion and Marangoni effect, while higher temperature caused faster coalescence of droplets. We also found that increased approach velocity generally reduced both contact and coalescence times, up to certain (critical) value. Beyond that, no coalescence was observed.

A CFD-PBM-EMMS integrated model applicable for heterogeneous gas-solid flow

Accurate prediction of the structure-dependent interphase drag coefficient in a computational cell is of importance for the simulation of heterogeneous gas-solid flow. However, the cluster diameter in the cell was usually correlated with macroscopic hydrodynamic parameters or simply approximated as a constant value, though there is a cluster size distribution (CSD) at the sub-grid scale. Based on the energy minimization multiscale (EMMS) model and the population balance model (PBM), a CFD-PBM-EMMS integrated model was proposed to account for the effect of CSD on the interphase drag within the computational grid in this article, in which the PBM was used to describe the spatio-temporal evolution of CSD by introducing an EMMS-based cluster growth rate model. The CFD-PBM-EMMS model was validated by the experiments in a pilot-scale circulating fluidized bed riser of Geldart A and/or B particles. Comparing with the two-fluid model using the EMMS drag or homogeneous drag law, the CFD-PBM-EMMS integrated model shows the best agreement with the experiments under various operating conditions. Much effort is being devoted to incorporating the coalescence and breakage kinetics of clusters to the PBM further to improve the simulation accuracy of the CFD-PBM-EMMS integrated model, especially for dense gas-solid flow.

Assessing the arrest of coalescence due to Marangoni effects in flowing emulsions using population balance

The conditions for coalescence arrest due to Marangoni effect of surfactant enriched emulsions flowing through a microfluidic device are analyzed. For that aim, we develop a Population Balance Equation model that allows the quantification of coalescence occurrence for emulsion systems at different surfactant concentration making use of isotherms. Besides, our model requires three parameters with physical significance to describe the behavior of the emulsions under shear to include Marangoni flow. The results were benchmarked by comparison to microfluidic experiments reported in the literature. Hydrodynamic coupling was observed at intermediate shear rates wherein coalescence was favored compared to the kinetics without surfactant. The coalescence kinetics was found to depend on the intertwined roles of both surfactant coverage and flow properties. These results represent a step towards the use of Population Balance Equation models not only for the study and prediction of conditions leading to coalescence arrest determined by either the external shear, repulsive barriers or Marangoni based forces but also for product design and control.

Analysis of Non‐Isothermal Viscous Flow Coalescence at Micro Scale

The non‐isothermal coalescence of two spherical bodies caused by capillary‐induced viscous flow was analyzed. Based on this analysis, a new dimensionless number (K number) was introduced for defining thermal coalescence regimes. Based on the value of this number, coalescence may or may not be affected by thermal effects in different cases. To make this clearer, the conventional coalescence models of Frenkel‐Eshelby and Pokluda were modified by assuming viscosity as a temperature dependent variable. This was conducted by considering the effects of temperature on the viscosity of the involved material through evaluating different expressions including linear and Reynolds and Williams‐Landel‐Ferry (WLF) equations. The results of the modified models for the bridge growth rate show that temperature changes significantly affect the kinetics of coalescence, particularly when the characteristic times of coalescence and heat conduction are in the same order, i.e., moderate K numbers. This analysis is applicable for diverse situations since viscous flow coalescence occurs in various physical and industrial applications of particles or droplets.

Kinetic Modeling of Flocculation and Coalescence in the System Emulsion of Water-Xylene-Terbutyl Oleyl Glycosides

The development of a mathematical model for explaining the kinetics of flocculation and coalescence of emulsion droplets is essential to study the stability of an emulsion system of the kinetics of emulsion stability. Mathematic models was developed from the equation Van Den Tempel by modifying emulsion systems. The emulsion was made by mixing water-xylene and surfactant tert-butyl oleyl glycosides. This research studied the effect of stirrer speed on the value of flocculation rate constant (a) and coalescence rate constant (K). The model identified the emulsion development condition whether controlled by coalescence or flocculation. It was observed that under lower agitation speed (1000 rpm) the emulsion development was controlled by flocculation mechanism, while a faster agitation (2000 rpm or higher) exhibited coalescence controlled mechanism. The results confirmed that the 1st model was the most appropriate for water-xylene-TBOG emulsion system. From four models after fitting with experimental data, the most suitable model is 4th model, because it has the smallest error of 2.22 %.

Time–temperature superposition principle for the kinetic analysis of destabilization of pharmaceutical emulsions

The time–temperature superposition principle (TTSP) was applied to the destabilization kinetics of a pharmaceutical emulsion. The final goal of this study is to predict precisely the emulsion stability after long-term storage from the short-period accelerated test using TTSP. As the model emulsion, a cream preparation that is clinically used for the treatment of pruritus associated with chronic kidney disease was tested. After storage at high temperatures ranging from 30 to 45 °C for designated periods, the emulsion state was monitored using magnetic resonance imaging, and then the phase separation behaviors observed were analyzed according to the Arrhenius approach applying TTSP. The Arrhenius plot showed a biphasic change around 35 °C, indicating that the separation behaviors of the sample were substantially changed between the lower (30–35 °C) and higher (35–45 °C) temperature ranges. This study also monitored the coalescence behavior using a backscattered light measurement. The experiment verified that the destabilization was initiated by coalescence of oil droplets and then it eventually led to obvious phase separation via creaming. Furthermore, we note the coalescence kinetics agreed well with the phase separation kinetics. Therefore, in the case of the sample emulsion, the coalescence behavior has a dominant influence on the destabilization process. This study offers a profound insight into the destabilization process of pharmaceutical emulsions and demonstrates the promising applicability of TTSP to pharmaceutical research.

Metal-support interaction controlled migration and coalescence of supported particles

The particle migration and coalescence (PMC) kinetics of a supported metal are the main deactivation mechanisms restricting the successful industrialization of nanoparticles, but the theoretical insights regarding these kinetics are lacking. One key issue is the lack of a physical model to predict the effects of metal-support interaction (MSI) on PMC kinetics. In this paper, we report a theoretical study of PMC kinetics and their dependence on MSI. A new particle diffusion model is proposed based on the surface premelting hypothesis that considers the contact angle of a hemispherical particle on the support. Enhanced MSI suppresses PMC by increasing the radius of curvature and the interfacial adhesion energy, even though the accompanying reduction in the geometry factor partially promotes PMC kinetics. The increased surface energy increases the chemical potential of the atoms in the particle, which is conducive to PMC; an increased surface energy also results in enhanced MSI, which suppresses PMC. The competition between these two contradictory effects leads to a critical contact angle where the surface energy has no influence on the diffusion and resulting PMC kinetics. The proposed diffusion theory mode lincluding the effects of the support and the corresponding kinetic simulations, shed light onto the support-dependence of PMC kinetics and provide a foundation for further optimization and design of supported particles with better stability.

Modeling droplet coalescence kinetics in microfluidic devices using population balances

The coalescence kinetics of oil-in-water emulsions in a wide range of properties and flow microfluidic conditions is quantified. The conditions were chosen in order to mimic situations found in industrial processes involving liquid-liquid dispersions. With that aim, a numerical scheme based on population balance equations is proposed, applied, and validated by comparison to microfluidic experiments reported in the literature. A coalescence efficiency model accounting for colloidal and hydrodynamic interactions, and interface mobility is incorporated using the Smoluchowski collision kernel. The latter assures the accurate estimation of the droplet size evolution which governs the interfacial area and rate of mass transfer. Besides, the combined effect of interfacial tension and oil viscosity on the coalescence kinetics is properly quantified with one single fitting parameter. From the kinetics, the estimated coalescence time increases as the shear rate and volume fraction of the dispersed phase diminish. The close agreement of our results with the experimental findings substantiates the accuracy and wider application of the methodology here described as a diagnostic tool beneficial to industrial process design and control.

Surface Fluctuations and Coalescence of Nucleolar Droplets in the Human Cell Nucleus

The nucleolus is a membraneless organelle embedded in chromatin solution inside the cell nucleus. It is a site of ribosomal biogenesis, plays a key role in cell cycle progression and stress response, and acts as an indicator in many human diseases and aging. However, the physical nature of the human nucleolus remains an open question. Elucidating nucleolar dynamics, such as their shape fluctuations and fusion, could not only help answer this question, but also provide insight into the material properties of the surrounding nucleoplasm. In this work, we analyze the surface dynamics and fusion kinetics of nucleoli in live human cells[1]. Nucleolar surface fluctuations inform on the nucleolus-nucleoplasm interface, while nucleolar fusion might resolve whether nucleoli aggregate like solid particles or coalesce like liquid droplets. We find that the surface exhibits subtle, but measurable, fluctuations and that the neck connecting two fusing nucleoli grows as r(t)∼t1/2. This is consistent with liquid droplets with low surface tension ∼10−6 Nm−1 coalescing in an outer fluid of higher viscosity ∼103 Pa s, i.e. chromatin solution. We hypothesize that the high viscosity of chromatin solution slows down the nucleolar coalescence kinetics to facilitate transcription inside nucleoli. Interestingly, we find the neck velocity, dr/dt, to be comparable to the velocity of chromatin solution[2]. Surprisingly, nucleolar coalescence occurs in an active fluid, yet can be described by coalescence theory for passive liquid droplets, suggesting that the measured viscosity and surface tension might be effective quantities. Our study presents a noninvasive approach of using natural probes, such as nucleoli, and their dynamics to investigate material properties of the cell and its constituents[1]. 1. Caragine CM et al., Phys. Rev. Lett., In Press, 2018. 2. Zidovska A et al., Proc. Natl. Acad. Sci., 110(39), 15555-60, 2013.

Assessing Physical Stability of Colloidal Dispersions Using a Turbiscan Optical Analyzer.

The physical stability of aqueous colloidal dispersions containing highly concentrated droplets of poorly water-soluble drugs has recently been identified as one of the main considerations in developing amorphous solid dispersions (ASDs). The Turbiscan analyzer, an instrument based on multiple light scattering technology, was employed for the first time to assess colloidal dispersions with ritonavir as the model compound. The physical instability of ritonavir-rich droplets was monitored directly with and without the presence of candidate polymer additives at different drug concentrations and temperatures. The mechanism of the observed instability was confirmed to be coalescence of liquid droplets, based on the low glass transition temperature of water-saturated amorphous ritonavir determined using a newly developed experimental procedure. Temperature and solvent composition, within the range studied, have little influence on the kinetics of ritonavir coalescence. On the contrary, a higher concentration of drug, i.e., more droplets per unit volume, greatly accelerates the coalescence process. In addition, polymers with varying degrees of hydrophobicity resulted in different levels of effectiveness in stabilization, which is likely related to the strength of drug-polymer interactions and the corresponding differences in surface adsorption. This work demonstrates that the Turbiscan optical analyzer can be used as a rapid screening tool that provides a first-pass, high-throughput feasibility ranking of different excipients and additives to support the development of ASD formulations.

Heat Shock Factor 1 Drives Intergenic Association of Its Target Gene Loci upon Heat Shock

SUMMARYTranscriptional induction of heat shock protein (HSP) genes is accompanied by dynamic changes in their 3D structure and spatial organization, yet the molecular basis for these phenomena remains unknown. Using chromosome conformation capture and single-cell imaging, we show that genes transcriptionally activated by Hsf1 specifically interact across chromosomes and coalesce into diffraction-limited intranuclear foci. Genes activated by the alternative stress regulators Msn2/Msn4, in contrast, do not interact among themselves nor with Hsf1 targets. Likewise, constitutively expressed genes, even those interposed between HSP genes, show no detectable interaction. Hsf1 forms discrete subnuclear puncta when stress activated, and these puncta dissolve in concert with transcriptional attenuation, paralleling the kinetics of HSP gene coalescence and dissolution. Nuclear Hsf1 and RNA Pol II are both necessary for intergenic HSP gene interactions, while DNA-bound Hsf1 is necessary and sufficient to drive heterologous gene coalescence. Our findings demonstrate that Hsf1 can dynamically restructure the yeast genome.