Energy Barrier For Heterogeneous Nucleation Research Articles

Adhesion-Controlled Heterogeneous Nucleation of Tin Halide Perovskites for Eco-Friendly Indoor Photovoltaics.

The rapid development of the Internet of Things (IoT) has accelerated the advancement of indoor photovoltaics (IPVs) that directly power wireless IoT devices. The interest in lead-free perovskites for IPVs stems from their similar optoelectronic properties to high-performance lead halide perovskites, but without concerns about toxic lead leakage in indoor environments. However, currently prevalent lead-free perovskite IPVs, especially tin halide perovskites (THPs), still exhibit inferior performance, arising from their uncontrollable crystallization. Here, a novel adhesive bonding strategy is proposed for precisely regulating heterogeneous nucleation kinetics of THPs by introducing alkali metal fluorides. These ionic adhesives boost the work of adhesion at the buried interface between substrates and perovskite film, subsequently reducing the contact angle and energy barrier for heterogeneous nucleation, resulting in high-quality THP films. The resulting THP solar cells achieve an efficiency of 20.12% under indoor illumination at 1000 lux, exceeding all types of lead-free perovskite IPVs and successfully powering radio frequency identification-based sensors.

A numerical study on the effect of particle surface wettability on homogeneous and heterogeneous condensation processes in supersonic flows

Supersonic separators are an emerging device for processing natural gas impurities. Natural gas contains solid particulate impurities in addition to gaseous impurities, which can trigger heterogeneous condensation within the supersonic separator. There is a lack of research on the effect of particle wettability on supersonic condensation flow, although there have been studies on the effect of particle surface wettability on heterogeneous condensation in this area. Therefore, in this paper, the supersonic condensation flow is modeled considering the contact angle, and the effects of particle contact angle on heterogeneous condensation and homogeneous condensation are investigated separately. The results show that homogeneous condensation is inhibited and heterogeneous condensation is promoted by decreasing the particle contact angle. The reason is the smaller contact angle leads to the reduction of the free energy barrier for heterogeneous nucleation, which makes the conditions for heterogeneous nucleation lower and condensation more likely to occur.

Inhibition of Heterogeneous Nucleation in Water by Hydrogel Coating.

Heterogeneous nucleation plays a critical role in the phase transition of water, which can cause damage in various systems. Here, we report that heterogeneous nucleation can be inhibited by utilizing hydrogel coatings to isolate solid surfaces and water. Hydrogels, which contain over 90% water when fully swelled, exhibit a high degree of similarity to water. Due to this similarity, there is a great energy barrier for heterogeneous nucleation along the water-hydrogel interface. Additionally, hydrogel coatings, which possess polymer networks, exhibit higher fracture energy and more robust adhesion to solid surfaces compared to water. This high fracture and adhesion energy acts as a deterrent for fracture nucleation within the hydrogel or along the hydrogel-solid interface. With a hydrogel layer approximately 100 μm thick, the boiling temperature of water under atmospheric pressure can be raised from 100 to 108 °C. Notably, hydrogel coatings also result in remarkable reductions in cavitation pressure on multiple solid surfaces. We have demonstrated the efficacy of hydrogel coatings in preventing damages resulting from acceleration-induced cavitation. Hydrogel coatings have the potential to alter the energy landscape of heterogeneous nucleation on the water-solid interface, making them an exciting avenue for innovation in heat transfer and fluidic systems.

Temperature-gradient effects on heterogeneous ice nucleation from supercooled water

Investigating the nucleation and freezing behavior of supercooled water under temperature gradients is of significance for theoretical research and practical application. This paper focuses on the freezing temperature of bulk supercooled water in a rectangular container for which the temperature at two sides can be controlled to present a temperature difference. The experimental results show that the nucleation temperature under temperature differences will be lower than that under isothermal conditions. In addition, an increase in the temperature difference is shown to linearly increase the supercooling needed for ice nucleation. The temperature gradient is capable of suppressing heterogeneous ice nucleation. Based on the classical nucleation theory, the free energy barrier for heterogeneous nucleation is attributed to the main factor to affect the nucleation rate. By comparing theoretical evaluations with experimental results, correction on the free energy barrier was conducted to predict the nucleation rate in the temperature gradient. Our experimental research and theoretical correction reveal the nucleation behavior of supercooled water under nonequilibrium conditions.

Measuring the activation energy barrier for the nucleation of single nanosized vapor bubbles

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes that has received broad interest from diverse fields of physics and chemistry. While most studies focused on large microbubbles, here we employed a surface plasmon resonance microscopy to measure the nucleation rate constant and activation energy barrier of single nanosized embryo vapor bubbles upon heating a flat gold film with a focused laser beam. Image analysis allowed for simultaneously determining the local temperature and local nucleation rate constant from the same batch of optical images. By analyzing the dependence of nucleation rate constant on temperature, we were able to calculate the local activation energy barrier within a submicrometer spot. Scanning the substrate further led to a nucleation rate map with a spatial resolution of 100 nm, which revealed no correlation with the local roughness. These results indicate that facet structure and surface chemistry, rather than geometrical roughness, regulated the activation energy barrier for heterogeneous nucleation of embryo nanobubbles.

Droplet Growth Dynamics during Atmospheric Condensation on Nanopillar Surfaces

ABSTRACTThe Gibbs free energy barrier for heterogeneous nucleation of a condensed droplet on a rough surface changes significantly with changes of humidity content in the condensing environment. The influence of environmental factors (ambient temperature and relative humidity) and substrate characteristics (topology, surface chemistry, and substrate temperature) on atmospheric condensation phenomenon is very important to elucidate the condensed droplet wetting state and condensate harvesting applications. Condensation from the humid air has been reported for plain silicon and fabricated nanopillar surfaces to facilitate condensate harvesting. Droplet growth and size distributions were recorded for 90 min. Spherical droplets condensed on the silicon surfaces and irregular-shaped droplets were observed on the nanopillar surfaces due to the pinning effect of the pillars. The effect of droplet pinning on coalescence events has been described based on the energy balance for the condensed droplets. A mathematical model reveals that certain dimensional combinations (pillar pitch, pillar diameter, and pillar height) of the nanopillar geometry are required to exhibit the pinning mechanism for condensed droplets. Regeneration of droplets was observed at void spaces generated from coalescence events. The growth of individual droplets was tracked over multiple time and length scales, starting from nucleation to get further insight into the direct growth and coalescence mechanisms.Abbreviation: ESEM: Environmental Scanning Electron Microscope; HCP: Hexagonal Closed-Packed; MPL: Microsphere Photolithography; RH: Relative Humidity

The distinctive nucleation of polystyrene composites with differently shaped carbon‐based nanoparticles as nucleating agent in the supercritical CO2 foaming process

AbstractMicrocellular polystyrene (PS)/carbon‐based nanoparticle (CbN) composites were prepared by a pressure release process using supercritical CO2 as the foaming agent. Nanocomposite foams with different shapes or dimensions but with CbNs with a similar surface structure showed a quite different cell nucleation. The underlying nucleation mechanism was semi‐quantitatively analyzed by classical nucleation theory, which indicated that the energy barrier for heterogeneous nucleation was related to the shapes of the fillers. Below 80 °C and 20 MPa, the nucleation density of the composites with different fillers showed marked differences, among which the nucleation density of PS/fullerene (FE) was the highest, followed by PS/carbon nanotubes (CNTs) and PS/thermally reduced graphene (TRG). However, the difference in nucleation density of PS/CbNs decreased with increase in filler content to 0.3 wt%. With increase in foaming temperature, the PS/TRG composite foams showed the highest cell density and no cellular merging and collapsing at 100 and 120 °C. The results obtained from DSC, rheology and positron annihilation lifetime spectroscopy measurements showed that FE nanoparticles exhibited the strongest plasticization effects on the matrix followed by CNTs and TRG nanoparticles. The distinctive plasticization suggested that the shapes of CbNs have a strong influence on the interaction between the PS matrix and the nanoparticles, which could affect the energy barrier for heterogeneous nucleation. The influence of CO2 on the rheology and thermal properties of the composites was investigated using high pressure DSC and high pressure rheology measurements. The results revealed that at high temperature high viscosity became the key factor to determining the cell morphology of nanocomposite foams by preventing bubble coalescence and collapse. © 2018 Society of Chemical Industry

Gypsum scale formation on graphene oxide modified reverse osmosis membrane

Graphene oxide (GO) coatings on membranes can improve antifouling performance against a variety of microorganisms and organics. However, the effects of GO coatings on mineral scaling were not investigated. Here gypsum scaling on bare (ESPA2) and GO-modified thin-film polyamide membranes (ESPA2-GO) followed by cleaning with deionized (DI) water were investigated with a bench-scale reverse osmosis setup. The flux decline caused by gypsum scaling on ESPA2-GO was slightly reduced than on ESPA2. This is because the ESPA2-GO membrane is more hydrophilic than ESPA2, indicating a higher energy barrier for heterogeneous nucleation and/or the deposition of gypsum on it. Moreover, the more negatively charged ESPA2-GO membrane lead to stronger electrostatic repulsive forces between the membrane and the negatively charged gypsum particles and thus further inhibiting gypsum deposition onto membranes. Interestingly, during the cleaning process, smaller flux recovery was observed for ESPA2-GO. This is because ESPA2-GO surfaces have higher densities of carboxyl (–COOH) groups, which form complexes with Ca2+, building strong bonds between GO coatings and gypsum. This study provided unique insights on the physicochemical interactions among membrane, the scaling mineral, and aqueous species, which can help the rational design of coatings for better simultaneous anti-scaling and anti-fouling performances.

High-melting-point crystals of poly(l-lactic acid) (PLLA): the most efficient nucleating agent to enhance the crystallization of PLLA

Even though self-nucleation is considered to be the ideal case for polymer nucleation, it has rarely been used to enhance polymer crystallization in practical processing techniques. Inspired by self-nucleation theory and by utilizing the large difference in melting points of various poly(L-lactide) (PLLA) resins, we introduced high-melting-point PLLA (hPLLA) crystallites into a low-melting-point PLLA (lPLLA) matrix via melt blending at a processing temperature between the two melting points of the selected PLLA resins. The hPLLA crystallites turn out to be efficient nucleating agents (NAs) for lPLLA and high crystallinity (>40%) PLLA samples with a greatly accelerated crystallization rate can be easily obtained. The results of non-isothermal crystallization show that the crystallization process is remarkably accelerated with a small amount (0.1 wt%) of hPLLA. With a further increase of hPLLA content, the crystallization temperature of the blends continues to shift to higher temperature. This crystallization promoting effect results from the excellent nucleation ability of the hPLLA crystallites, as revealed by in situ optical microscopy observation. Furthermore, an incredibly high nucleation efficiency of 103.0% (exceeding 100%) was obtained for the PLLA sample with 5.0 wt% hPLLA. The nucleation mechanism for hPLLA was studied systematically. It was found that the lPLLA matrix and hPLLA crystallites possess an absolutely identical crystal structure of α-form crystals and an excellent interfacial interaction between the nucleating agent, i.e., hPLLA crystallite, and the lPLLA matrix is achieved, resulting in the reduction of the energy barrier for heterogeneous nucleation and acceleration of crystallization kinetics. Therefore, by using high-melting-point polymer crystals as NAs, the crystallization rate of their low-melting-point polymer matrix can be improved largely, thus providing a simple way to obtain high crystallinity products for semicrystalline polymers with very low crystallization rates.

Effect of Nucleation Temperature on Detecting Molecular Ions and Charged Nanoparticles with a Diethylene Glycol-Based Particle Size Magnifier

The classical nucleation theory predicts that a decrease in nucleation temperature under a constant saturation ratio increases the energy barrier for homogeneous nucleation to occur; therefore lower nucleation temperature would allow higher saturation ratio inside a condensation particle counter (CPC) while suppressing homogenous nucleation of working fluid vapor below a threshold value. On the other hand, the classical theory also predicts that a decrease in nucleation temperature increases the energy barrier for heterogeneous nucleation to occur, which potentially increases the minimum detectable size of CPC. Accordingly, it is important to investigate experimentally whether higher super-saturation under lower nucleation temperature decreases the minimum detectable size or not. Minimum detectable sizes of a diethylene glycol (DEG)-based nanoparticle size magnifier (nano-PSM) developed by Ito et al. (Ito, E., Seto, T., Otani, Y., and Sakurai, H. [2011]. Aerosol Sci. Technol. 45:1250–1259) were investigat...

Freezing of sessile water droplets on surfaces with various roughness and wettability

This paper focus on the freezing delay time and the freezing time of sessile droplet on smooth, micro-structured and micro/nano-structured surfaces, and the whole freezing process are comparatively studied. The freezing delay time of the smooth surfaces with roughness smaller than the size of the critical ice nuclei is found to be much longer than superhydrophobic surfaces with hierarchical structures. Experimental data and theoretical analysis show that the surface roughness plays a very crucial role in nucleation. The freezing delay time could not be extended further on rough surface with more superhydrophobic for sessile droplet. In addition, decreased roughness can increase the free energy barrier for heterogeneous nucleation, result in significant freezing delay. On the contrary, the freezing time from the start of nucleation to the completion of freezing increases with the contact angle. In addition, surfaces with hierarchical roughness are found to have the longest freezing time.

Investigation of the Heterogeneous Nucleation on Fractal Surfaces

Classical theory of heterogeneous nucleation has been developed with an implied hypothesis of smooth substrate surfaces; however, morphologies of any real substrate surfaces are generally complicated and demonstrate fractal characteristics. In this paper, the wettability between the embryo and the fractal substrate surface was discussed, and heterogeneous nucleation behaviors were theoretically analyzed. The result shows that the roughness factor of a fractal surface varies with the scale of the embryo. As a result, the fractal character of the substrate surface has important effects on heterogeneous nucleation behaviors. It has been shown that the energy barrier for heterogeneous nucleation of a non-wetting phase on a fractal rough surface increases with increasing fractal dimensions, and both the critical nucleus radius and the nucleation energy barrier decrease with increasing fractal dimensions for heterogeneous nucleation of a wetting phase on the fractal rough surface. For a non-wetting system, the critical nucleus radius shows a slight shift with changes of the intrinsic wetting angle, but for a wetting system, the critical nucleus radius shows an obvious change with decreasing intrinsic wetting angle, thus imposes a stronger effect on the heterogeneous nucleation behaviors.

Thermodynamic Investigation of the Barrier for Heterogeneous Nucleation on a Fluid Surface in Comparison with a Rigid Surface

When a vapor phase is in contact with a solid or nonvolatile fluid, under conditions where the vapor is thermodynamically metastable to condensation, a droplet may nucleate from the vapor either homogenously within the vapor phase, or heterogeneously at the solid or fluid substrate interface. The case where the droplet is thermodynamically favored to nucleate heterogeneously is the subject of this article. The heterogeneous nucleation of a sessile drop on a soft surface has been studied many times experimentally and theoretically. It has been observed experimentally that heterogeneous nucleation happens faster on a soft surface in comparison with a rigid surface. Here we use Gibbsian surface thermodynamics to provide a physical understanding for this observation. Due to the difficulties of considering soft-elastic surfaces, we demonstrate that by considering only the fluidity of a surface (i.e., by considering a fluid surface as an infinitely soft material and comparing a fluid surface with a rigid surface), thermodynamics will predict that heterogeneous nucleation is easier on soft surfaces compared with rigid surfaces. We first investigate the effect of contact angle on the barrier for heterogeneous nucleation on rigid substrates at constant vapor phase pressure. Then we find a lower energy barrier for heterogeneous nucleation at a fluid surface in comparison with heterogeneous nucleation at a rigid surface which explains the faster nucleation on soft surfaces compared with rigid surfaces. Finally we inspect the role of each contribution to the energy barrier.

Influence of thermo-mechanical treatment on the precipitation strengthening behavior of Inconel 740, a Ni-based superalloy

Abstract

Homogeneous and heterogeneous nucleation of Lennard-Jones liquids

The homogeneous and heterogeneous nucleation of a Lennard-Jones liquid is investigated using the umbrella sampling method. The free energy cost of forming a nucleating droplet is determined as a function of the quench depth, and the saddle point nature of the droplets is verified using an intervention technique. The structure and symmetry of the nucleating droplets are found for a range of temperatures. We find that for deep quenches the nucleating droplets become more anisotropic and diffuse with no well-defined core or surface. The environment of the nucleating droplets forms randomly stacked hexagonal planes. This behavior is consistent with a spinodal nucleation interpretation. We also find that the free energy barrier for heterogeneous nucleation is a minimum when the lattice spacing of the impurity equals the lattice spacing of the equilibrium crystalline phase. If the lattice spacing of the impurity is different, the crystal grows into the bulk instead of wetting the impurity.

Heterogeneous nucleation uniformizing cell size distribution in microcellular nanocomposites foams

Microcellular polycarbonate/nano-silica nanocomposites (PCSN) were prepared by temperature rising process using supercritical CO 2 as the blowing agent. Neat PC foam showed a quite broad distribution of cell sizes. Under the same foaming conditions, the addition of nano-silica resulted in PCSN foams having uniform cell size distribution, reduced cell size of 0.3–0.5 μm and increased cell density of 10 11–10 13 cells/cm 3. The underlying nucleation mechanism was semi-quantitatively analyzed by the classical nucleation theory. The results indicate that the energy-barrier for heterogeneous nucleation was three orders of magnitude lower than that of homogeneous one. The heterogeneous nucleation of nano-silica aggregates dramatically increased the nucleation rate, decreased the nucleation time interval, and hence facilitated the almost instantaneous growth of cell size. Combined with the well-dispersed nucleation sites, resulted from the uniform dispersion of nano-silica aggregates, the narrow-distributed cell size was obtained in PCSN foams.

Equilibrium shape and heterogeneous nucleation barrier at spherical interfaces

The conditions for the equilibrium shape of a second-phase particle nucleating either on a spherically curved substrate or at a spherical interphase boundary are derived under the assumption that the spherical interfacial structure has a torque of an appropriate strength. The results are then incorporated into the modified Gibbs-Wulff construction in order to demonstrate a graphical representation of both the equilibrium shape and the free energy barrier for heterogeneous nucleation. The finite curvature associated with the nucleation site causes a geometrical compatibility condition between the radius of the nucleation site and the radius of a critical nucleus, but the compatibility condition vanishes as the ratio of the two radii approaches infinity. Comparison of the free energy barriers for heterogeneous nucleation shows that the free energy of formation, ΔG ∗ , for a critical nucleus on a concave spherical substrate increases with an increase in the radius of the substrate, approaching that of a planar substrate case when the substrate radius becomes infinite. For a spherical interphase boundary, the effect of the curvature is also to decrease ΔG ∗ as the radius of the curvature decreases. On the other hand, ΔG ∗ for the convex spherical substrate case increases as the substrate radius decreases, becoming identical to that of a homogeneous nucleation case when the substrate radius vanishes. However, the condition of a complete wetting, i.e., the condition for a zero ΔG ∗ , remains unaffected by the presence of the finite curvature associated with the nucleation sites.

The Modified Gibbs-Wulff Construction and Critical Nucleus Morphology at an Interface

AbstractThe conditions for the equilibrium shape of a second-phase particle nucleating either at a planar interface or at a spherically curved interface are reviewed under the assumption that the interfacial structure of the nucleation site has a torque of an appropriate strength. The results are then incorporated into the modified Gibbs-Wulff construction in order to demonstrate a graphical representation of both the equilibrium shape and the free energy barrier for heterogeneous nucleation.