Homogeneous Nucleation Process Research Articles

Molecular insight into the dual effect of salts: Promoting or inhibiting the nucleation and growth of carbon dioxide clathrate hydrates

Facing the huge gap between the growing CO2 emissions and net-zero emissions target to mitigate global warming, the long-term storage of excess CO2 becomes an intractable issue. Sequestering CO2 in the oceans as clathrate hydrates is a promising solution. Many unanswered questions still exist at the molecular level regarding how ions in seawater kinetically affect the formation of CO2 hydrates, particularly the dual effect: as the concentration rises, salt ions shift from being a promoter of hydrate formation to becoming an inhibitor. Herein, we report the homogeneous nucleation process of CO2 hydrates in saline solutions via molecular dynamics simulations. Consistent with experimental findings, our results show that nucleation is promoted within certain salt concentration ranges; however, the evidence suggests that ions do not act as nucleation sites. Multiple ion-induced variations in the aqueous phase are analyzed, including changes in the mobility, structure, and CO2 solubility. We find that the presence of ions strengthens the hydrophobic interactions among the dissolved CO2 molecules, which are associated with the salting-out phenomenon, accounting for the promoting effect of salts at low concentrations. However, as the salt concentration increases, this advantage is offset, leading to the inhibiting effect at high salt concentrations. We demonstrate that anions are inevitably involved in the construction of the solid hydrate phase. Their presence disrupts the native tetrahedral connectivity of water molecules and imparts negative charges to the solid phase, thereby attracting cations. In addition to advancing our understanding of the intricate interactions among water, guest, and additive molecules to germinate ordered solid structures, these findings can accelerate the development and utilization of sustainable hydrate-based applications.

A uniform understanding of polymer crystallization models: Diffusion and conformational transition model

In the last 100 years, the study of polymer crystallization theory has achieved many breakthroughs, and a large number of models and theories have been proposed. However, each of them can only partially explain the crystallization behavior of polymers. A uniform understanding of polymer crystallization remains a huge challenge. In this paper, we try to understand the crystallization of polymers from the motion of polymer chains, and a universal model, the diffusion and conformational transition (DCT) model, is proposed. The DCT model can provide a unified understanding of the current polymer crystallization models. We propose that the nature of polymer crystallization is the synergy and competition between the diffusion (D) and the conformational transition (CT) processes. Moreover, by dividing the types of D and CT into D1 to D3 and CT1 to CT6, respectively, the homogeneous nucleation process and the growth of polymer crystals on the surface of an existing crystal are discussed. Some important results on the number of stems for critical nuclei, the type of nucleation, the origin of the conformation of chain segments on the end surface of lamellar crystals, etc. have been achieved. These results not only provide a good explanation of the experimental findings in the literature, but also contain some new findings. The DCT model may further be used to resolve disputes, explain and predict new phenomena, and teach beginners in polymer crystallization in the future.

Phase and Morphology of Water-ice Grains Formed in a Cryogenic Laboratory Plasma

Grains of ice are formed spontaneously when water vapor is injected into a weakly ionized laboratory plasma in which the background gas has been cooled to cryogenic temperatures comparable to those of deep space. These ice grains are levitated indefinitely within the plasma so that their time evolution can be observed under free-floating conditions. Using microscope imaging, ice grains are shown to have a spindle-like fractal structure and grow over time. Both crystalline and amorphous phases of ice are observed using Fourier transform infrared spectroscopy. A mix of crystalline and amorphous grains coexists under certain thermal conditions, and a linear mixing model is used on the ice absorption band surrounding 3.2 μm to examine the ice phase composition and its temporal stability. The extinction spectrum is also affected by inelastic scattering as grains grow, and characteristic grain radii are obtained from Mie scattering theory and compared to size measurements from direct imaging. Observations are used to compare possible ice nucleation mechanisms, and it is concluded that nucleation is likely catalyzed by ions, as ice does not nucleate in the absence of plasma and impurities are not detected. Ice grain properties and infrared extinction spectra show similarity to observations of some astrophysical ices observed in protoplanetary disks, implying that the fractal morphology of the ice and observed processes of homogeneous ice nucleation could occur as well in such astrophysical environments with weakly ionized conditions.

In-situ observation of homogeneous nucleation of colloidal crystals formed with attractive interactions under density-matching conditions

Colloidal crystals formed with attractive interactions are used for in-situ observations of homogeneous nucleation processes in density-matched dispersion. We found globular and plate-like polycrystalline clusters stably suspended in the dispersion. Both clusters rotated randomly and moved freely with suspending in the dispersion. They were probably formed through homogeneous nucleation processes.

An entropic theory of homogeneous ice nucleation in non-ionic aqueous solutions.

The nucleation of ice from aqueous solutions is a process essential to myriad environmental and industrial processes, but the physical factors affecting the capacity of different solutes to depress the homogeneous nucleation temperature of ice are yet poorly understood. In this work, we demonstrate that for many binary aqueous solutions of non-ionic solutes, this depression is dominated by the entropy of the liquid phase. Employing the classic Turnbull interpretation of the interfacial free energy γ∼TSliquid-Ssolid and estimating solution entropies with a Flory-style modification of the ideal entropy of mixing that accounts for solute size effects, we demonstrate that mixing entropy alone predicts experimental homogeneous nucleation temperatures across a wide variety of non-ionic solutions. We anticipate that this physical insight will not only enhance a fundamental understanding of homogeneous nucleation processes across fields but also open new avenues to the rational design of aqueous solutions for desired nucleation behaviors.

Molecular Simulation of the Impact of Defects on Electrolyte Intrusion in Zeolites.

We have investigated through molecular simulation the intrusion of electrolytes in two representative pure-silica zeolites, silicalite-1 and chabazite, in which point defects were introduced in varying amounts. We distinguish between two types of defects, considering either "weak" or "strong" silanol nest defects, resulting in different hydration behaviors. In the presence of weak defects, the hydration process occurs through a homogeneous nucleation process, while with strong defects, we observe an initial adsorption followed by a filling of the nanoporous volume at a higher pressure. However, we show that electrolytes do not penetrate the zeolites, and these defects appear to have only marginal influence on the thermodynamics of electrolyte intrusion. While replacing pure water by the electrolyte solution shifts the intrusion pressure toward higher values because of the drop of water saturation vapor pressure, an increase in hydrophilicity of the framework due to point defects has the opposite effect, showing that controlling the amount of defects in zeolites is crucial for storage energy applications.

Trace Water in Lead Iodide Affecting Perovskite Crystal Nucleation Limits the Performance of Perovskite Solar Cells.

The experimental replicability of highly efficient perovskite solar cells (PSCs) is a persistent challenge faced by laboratories worldwide. Although trace impurities in raw materials can impact the experimental reproducibility of high-performance PSCs, the in situ study of how trace impurities affect perovskite film growth is never investigated. Here, light is shed on the impact of inevitable water contamination in lead iodide (PbI2 ) on the replicability of device performance, mainly depending on the synthesis methods of PbI2 . Through synchrotron-based structure characterization, it is uncovered that even slight additions of water to PbI2 accelerate the crystallization process in the perovskite layer during annealing. However, this accelerated crystallization also results in an imbalance of charge-carrier mobilities, leading to a degradation in device performance and reduced longevity of the solar cells. It is also found that anhydrous PbI2 promotes a homogenous nucleation process and improves perovskite film growth. Finally, the PSCs achieve a remarkable certified power conversion efficiency of 24.3%. This breakthrough demonstrates the significance of understanding and precisely managing the water content in PbI2 to ensure the experimental replicability of high-efficiency PSCs.

Electric Field Driven Reversible Spinodal Dewetting of Thin Liquid Films on Slippery Surfaces

AbstractThe stability of thin liquid films on a surface depends on the excess free energy of the system involving various short‐range and long‐range interactions. When forced to wet an unfavorable surface, thin liquid films dewet into multiple small‐sized droplets via spinodal, homogeneous, or heterogeneous nucleation process. However, if the total excess free energy of the system can be manipulated using external stimuli, for example, electric field, temperature, or light, one can control the stability of thin liquid films on demand. Here the electric‐field induced reversible dewetting and rewetting of thin liquid films underneath aqueous drops on slippery surfaces is studied. Upon applying voltage, hundreds of nanometer thick stable liquid films dewet in a manner identical to the spinodal dewetting of few nanometer‐thick liquid films following the linear stability analysis. Upon removing the applied voltage, the dewetted droplets spread, coalesce with neighboring ones, and form a uniform film again, albeit taking significantly longer times. The characteristic features defined via spatial and temporal evolution of the dewetting and rewetting processes are present over multiple cycles suggesting the complete reversibility of the process.

Spatial and temporal regulation of homogeneous nucleation and crystal growth for high-flux electrochemical water softening

Electrochemical softening is an effective technology for the treatment of circulating cooling water, but its hardness removal efficiency is limited because that nucleation and growth of scale crystals depended on cathode surface. In this study, a novel method was proposed to break through this limit via spatiotemporal management of nucleation and growth processes. A cube reactor was divided into cathodic chamber and anodic chamber via installing a sandwich structure module composed of mesh cathode, nylon nets, and mesh anode. Using this continuous-flowing electrochemical reactor, OH ̄ generated by water electrolysis was rapidly pushed away from cathode surface by water flow and hydrogen bubbles movement. As a result, a wide range of strongly alkaline regions was rapidly constructed in cathodic chamber to play a nucleation region, and homogeneous nucleation in liquid phase replaced heterogeneous nucleation on cathodic surface. Furthermore, the growth process of scale crystals in alkaline regions was monitored in situ. It took only 150 s of residence time to grow to 500 nm, which may be easily separated from water by a microfiltration membrane. With this new method, the precipitation rate was 290.8 g/(hˑm2) and corresponding energy consumption was 2.1 kW·h/kg CaCO3, both were superior to those reported values. Therefore, this study developed an efficient electrochemical softening method by spatial and temporal regulation of homogeneous nucleation and crystal growth processes.

Homogeneous nucleation of crystalline methane hydrate in molecular dynamics transition paths sampled under realistic conditions.

Methane hydrates are important from a scientific and industrial perspective, and form by nucleation and growth from a supersaturated aqueous solution of methane. Molecular simulation is able to shed light on the process of homogeneous nucleation of hydrates, using straightforward molecular dynamics or rare event enhanced sampling techniques with atomistic and coarse grained force fields. In our previous work [Arjun, T. A. Berendsen, and P. G. Bolhuis, Proc. Natl. Acad. Sci. U. S. A. 116, 19305 (2019)], we performed transition path sampling (TPS) simulations using all atom force fields under moderate driving forces at high pressure, which enabled unbiased atomistic insight into the formation of methane hydrates. The supersaturation in these simulations was influenced by the Laplace pressure induced by the spherical gas reservoir. Here, we investigate the effect of removing this influence. Focusing on the supercooled, supersaturated regime to keep the system size tractable, our TPS simulations indicate that nuclei form amorphous structures below roughly 260K and crystalline sI structures above 260K. For these temperatures, the average transition path lengths are significantly longer than in our previous study, pushing the boundaries of what can be achieved with TPS. The temperature to observe a critical nucleus of certain size was roughly 20K lower compared to a spherical reservoir due to the lower concentration of methane in the solution, yielding a reduced driving force. We analyze the TPS results using a model based on classical nucleation theory. The corresponding free energy barriers are estimated and found to be consistent with previous predictions, thus adding to the overall picture of the hydrate formation process.

Improvements of the Double-Moment Bulk Cloud Microphysics Scheme in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM)

Abstract This study revises the collisional growth, heterogeneous ice nucleation, and homogeneous ice nucleation processes in a double-moment bulk cloud microphysics scheme implemented in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). The revised cloud microphysical processes are tested by 10-day global simulations with a horizontal resolution of 14 km. It is found that both the aggregation of cloud ice with smaller diameters and the graupel production by riming are overestimated in the current schemes. A new method that numerically integrates the collection kernel solves this issue, and consequently, the lifetime of cloud ice is reasonably extended in reference to satellite observations. In addition, the results indicate that a reduction in graupel modulates the convective intensity, particularly in intense rainfall systems. The revision of both heterogeneous and homogeneous ice nucleation significantly increases the production rate of cloud ice number concentration. With these revisions, the new version of the cloud microphysics scheme successfully improves outgoing longwave radiation, particularly over the intertropical convergence zone, in reference to satellite observations. Therefore, the revisions are beneficial for both long-term climate simulations and representing the structure of severe storms. Significance Statement Very high-resolution global atmospheric models have been developed to simultaneously address global climate and regional weather. In general, cloud microphysics schemes used in such global models are introduced from regional weather forecasting models to realistically represent mesoscale cloud systems. However, a cloud microphysics scheme that was originally developed with the aim of weather forecasting can cause unexpected errors in global climate simulations because such a cloud microphysics scheme is not designed for interdisciplinary usage across spatiotemporal scales. This study focuses on systematic model biases in evaluating the terminal velocity of ice cloud particles and proposes a method to accurately calculate the growth rate of ice cloud particles. Improvements in ice cloud modeling successfully reduce model biases in the global energy budget. In addition, the internal structure of intense rainfall systems is modified using the new cloud model. Therefore, improvements in ice cloud modeling could further increase the reliability of weather forecasting, seasonal prediction, and climate projection.

Change in evaporation flux due to homogeneous condensation of vapour near the interfacial surface

An approach that allows to estimate the necessity of homogeneous nucleation process accounting/not accounting for evaporation intensity and to predict the direction (increase or decrease) of mass flux density change of evaporating matter has been proposed. A dimensionless parameter making it possible to determine the areas of increase/decrease of evaporation intensity in the presence of a homogeneous nucleation process is proposed. It is shown that for some substances (methanol, ethanol) the direction of process significantly depends on the nature of heat exchange between the formed droplets and the surrounding vapor droplets. Keywords: homogeneous nucleation, evaporation, supersaturation degree.

Изменение величины испарительного потока в результате объемной конденсации пара вблизи межфазной поверхности

An approach that allows to estimate the necessity of homogeneous nucleation process accounting/not accounting for evaporation intensity and to predict the direction (increase or decrease) of mass flux density change of evaporating matter has been proposed. A dimensionless parameter making it possible to determine the areas of increase/decrease of evaporation intensity in the presence of a homogeneous nucleation process is proposed. It is shown that for some substances (methanol, ethanol) the direction of process significantly depends on the nature of heat exchange between the formed droplets and the surrounding vapor droplets.

Carbon quantum dots with high quantum yield prepared by heterogeneous nucleation processes

Homogeneous and heterogeneous nucleation processes are employed to synthesize carbon quantum dots (CDs) by a solvothermal method, with o-phenylenediamine used as the precursor. For the heterogeneous process, titanium dioxide (TiO2) nanoparticles were employed as the nucleation seeds. The ultraviolet-visible (UV–vis) and photoluminescence (PL) properties of CDs prepared by homogeneous (CDs) and TiO2-induced heterogeneous (T-CDs) methods are compared herein. The experimental results show that, in comparison to the CDs, the n-π * transition peak in the UV–vis absorbance spectra of T-CDs red-shifts slightly and becomes broad, implying smaller and multiple band gaps of the T-CDs. The PL spectra demonstrate that the emission wavelengths of CDs and T-CDs are about 530 and 496 nm, respectively, which are independent of the reaction conditions. By tuning the operation conditions, the maximum quantum yield (QY) obtained by the CDs is 18 %, while a value of 68 % can be accomplished by the T-CDs. If N-doped TiO2 nanoparticles were utilized as the nucleation seeds, an even higher QY (85.9 %) could be achieved. Fourier-transform infrared spectroscopy and x-ray photoelectron spectroscopy analysis demonstrated that the T-CDs contain a higher percentage of N atoms and a lower fraction of pyrrolic state N, which is inferred to be the reasons causing the shorter emission wavelength and higher QYs of the T-CDs.

Molten salt synthesis of mullite whiskers entirely derived from fly ash for electronic packaging toughening ceramic applications

Preparation of mullite from high-alumina coal fly ash (CFA) has become one of the most important ways for high-value utilization. However, low Al2O3/SiO2 ratio, poor reaction activity and various impurities in fly ash led to poor quality of mullite products. Conventional sintering method requires addition of aluminum sources and sintering additives, and high synthesis temperature. This paper proposes a new technical route for preparation of mullite whiskers entirely from fly ash by molten salt method based on additive activation-acid leaching process. The growth process and synthesis mechanism of mullite whiskers in Na2SO4 molten salt from fly ash were systematically investigated, and the application of fly ash-derived mullite whiskers in electronic packaging toughening ceramic was also explored. The results show that leaching liquor containing Al3+, Na+, and SO42−, and amorphous silica slag can be separated and obtained after acid leaching of activated fly ash at 80 °C, and the fly ash-derived mullite whiskers with a length of about 1 μm can be synthesized from the mixture of crystalline products and silica slag by the proposed method under the optimal synthesis conditions. Using amorphous silica slag with higher dissolution activity as the silicon source, the nucleation of mullite whiskers with homogeneous nucleation process is promoted and highly monodispersed mullite whiskers derived from fly ash form. The fly ash-derived mullite whiskers can improve the mechanical strength, reduce the dielectric constant and dielectric loss of the composite ceramic substrates, and the dielectric constant and dielectric loss of the composite ceramic are the lowest 5.16 and 0.006, respectively, when the mullite addition content is 20 wt.%, indicating that they could be used as electronic packaging materials. This paper provides a novel pathway for synthesis of mullite whiskers and high-value utilization of fly ash.

Geopolymerization kinetics of steel slag activated gasification coal fly ash: A case study for amorphous-rich slags

The geopolymer processing of amorphous-rich slag required excessive soluble alkali usage severely limiting the application of geopolymer materials. In this study, industrial solid waste steel slag (SS) was introduced as alkali substitution for stimulating the potential activity of Shell gasification coal fly ash (SFA). Result shows that a 15%–20% addition of SS can reduce 14.5–23.3% NaOH consumption for the preparation of high-strength geopolymers. SS accelerates the dissolution rate of SFA promoting the early strength of geopolymer (ES) via alkalinity adjustment and reducing the wetting angle of refactoring globular particles in the slurry. The long-term strength of geopolymer (LS) is increased to 68.5 MPa with the higher devotion of SFA (67 ω%). During the geopolymerization course, Ca-rich hydrates growth are competed with the polymerization of amorphous minerals. The geopolymerization kinetics enlightens that the soluble tetrahedral SiO4 or AlO4 and hydrates in pozzolanic phase initially generated in paste then transformed to a more stable form of amorphous phase after a series of repeated processes of dissolution, homogeneous nucleation, and heterogeneous growth. This work paves a environmentally friendly way for disposing amorphous-rich slags with low activity and discloses the detailed mechanism of related geopolymerization.

Activation energy of homogeneous nucleation of Zr hydride: Density functional theory calculation

Considering the nucleation process of Zr hydrides as phase transformation from hexagonal closed-packed (HCP) to face-centered tetragonal (FCT) structure, we calculated the activation energy of the homogeneous nucleation process of Zr hydrides and atomic rearrangement during nucleation for Zr4H, Zr2H, ZrH and ZrH2 using density functional theory calculations and minimum energy path detection. At 0 K limit, although ZrH and ZrH2 have lower chemical potentials and are more energetically stable than Zr4H and Zr2H, the latter have lower activation energies for nucleation. At finite temperatures, the crossover of activation energies occurs around 300 K, where ZrH becomes the most possible candidate with the lowest activation energy. This was explained by the difference in the atomic rearrangement and change in phonon frequency during phase transformation.

Stiff and Tough Hydrogels Prepared Through Integration of Ionic Cross-linking and Enzymatic Mineralization

Enzymatic mineralization has become an effective approach to enhancing the stiffness of hydrogels for bone tissue engineering, but generally with limited toughness. On the other hand, double network cross-linking provides hydrogel with enhanced toughness. In this study, we integrated double cross-linking method with enzymatic mineralization to synthesize stiff and tough hydrogels. We have synthesized three kinds of sodium alginate-polyacrylamide (SA-PAM) double-network hydrogels and systematically compared the composition and structure differences, mechanical properties, and biological properties of the different hydrogels in the absence and presence of mineralization. In particular, we examined the role of specific cross-linking ions, i.e., calcium, zinc and strontium ions, in modulating the mineralization process. Synergistic effect of ionic cross-linking and enzymatic mineralization was clearly observed with dramatic increase in compressive modulus. In particular, mineralized hydrogel cross-linked with Sr2+ showed the highest compressive Young's modulus of 17.28 ± 3.56 MPa, which was 37 times of that of the original hydrogel. In addition, it had the highest tensile Young's modulus at 2.60 ± 0.25 MPa and 84 ± 5.5% elongation at break. Such synergistic effect from Sr2+ was attributed to a more uniformed mineralization process due to the early initiation of a more homogeneous nucleation process and subsequent denser mineralized structure. Cellular study also suggested that cell proliferation, adhesion and osteogenic differentiation were improved as a result of enzymatic mineralization. Our results provided an effective way for the preparation of stiff and tough hydrogels with osteogenesis, and demonstrated potential in bone tissue engineering applications. Statement of significanceHydrogels with excellent stiffness, stability and biocompatibility have attracted significant attentions in the bone tissue engineering applications. Our results suggested that the synergistic effect of ionic cross-linking and enzymatic mineralization rendered more enhancement of the compressive and tensile stiffness of SA-PAM DN hydrogels, as well as the toughness, swelling stability and cellular response. In particular, mineralized hydrogel cross-linked with Sr2+ showed the highest compressive Young's modulus of 17.28 ± 3.56MPa, which was 37 times of that of the original hydrogel. Such synergistic effect from Sr2+ was attributed to a more uniformed mineralization process. The cell proliferation, adhesion and osteogenic differentiation were greatly improved as a result of enzymatic mineralization, where the MSCs cultured on strontium ion cross-linked mineralized hydrogel showed the best performance.

Assessment of meta-atom scheme for nucleation of dislocation loops in TWIP steel

We use atomistic calculations to explore the process of homogeneous nucleation of dislocation loops in TWIP steel with the nudged-elastic-band method and meta-atom interatomic potential. Energy barriers to the nucleation process have been estimated at different applied shear stresses, and an atomistically informed nucleation model is fitted to the results for both Shockley and twinning dislocation loops. Besides exhibiting excellent agreement with the direct atomistic results, the nucleation model also provides the critical size of the dislocation loop. Comparison of the critical loop diameters with the intrinsic spatial scale of the meta-atom potential reveals that the loop nucleation is sensitive to the local compositional heterogeneity. As a result, the simulation results obtained with a meta-atom potential are shown to require careful interpretation in terms of their statistical ramifications.

New investigations on homogeneous ice nucleation: the effects of water activity and water saturation formulations

Abstract. Laboratory measurements at the AIDA cloud chamber and airborne in situ observations suggest that the homogeneous freezing thresholds at low temperatures are possibly higher than expected from the so-called “Koop line”. This finding is of importance, because the ice onset relative humidity affects the cirrus cloud coverage and, at the very low temperatures of the tropical tropopause layer, together with the number of ice crystals also the transport of water vapor into the stratosphere. Both the appearance of cirrus clouds and the amount of stratospheric water feed back to the radiative budget of the atmosphere. In order to explore the enhanced ice onset humidities, we re-examine the entire homogeneous ice nucleation process, ice onset, and nucleated crystal numbers, by means of a two-moment microphysics scheme embedded in the trajectory-based model (CLaMS-Ice) as follows: the well-understood and described theoretical framework of homogeneous ice nucleation includes certain formulations of the water activity of the freezing aerosol particles and the saturation vapor pressure of water with respect to liquid water. However, different formulations are available for both parameters. Here, we present extensive sensitivity simulations testing the influence of three different formulations for the water activity and four for the water saturation on homogeneous ice nucleation. We found that the number of nucleated ice crystals is almost independent of these formulations but is instead sensitive to the size distribution of the freezing aerosol particles. The ice onset humidities, also depending on the particle size, are however significantly affected by the choices of the water activity and water saturation, in particular at cold temperatures ≲205 K. From the CLaMS-Ice sensitivity simulations, we here provide combinations of water saturation and water activity formulations suitable to reproduce the new, enhanced freezing line.