Heterogeneous Bubble Nucleation Research Articles

Direct numerical simulations of microlayer formation during heterogeneous bubble nucleation

In this article, we present direct numerical simulation results for the expansion of spherical cap bubbles attached to a rigid wall due to a sudden drop in the ambient pressure. The critical pressure drop beyond which the bubble growth becomes unstable is found to match well with the predictions from classical theory of heterogeneous nucleation imposing a quasi-static bubble evolution. When the pressure drop is significantly higher than the critical value, a liquid microlayer appears between the bubble and the wall. In this regime, the interface outside the microlayer grows at an asymptotic velocity that can be predicted from the Rayleigh–Plesset equation, while the contact line evolves with another asymptotic velocity that scales with a visco-capillary velocity that obeys the Cox–Voinov law. In general, three distinctive regions can be distinguished: the region very close to the contact line where dynamics is governed by visco-capillary effects, an intermediate region controlled by inertio-viscous effects away from the contact line yet inside the viscous boundary layer, and the region outside the boundary layer dominated by inertial effects. The microlayer forms in a regime where the capillary effects are confined in a region much smaller than the viscous boundary layer thickness. In this regime, the global capillary number takes values much larger then the critical capillary number for bubble nucleation, and the microlayer height is controlled by viscous effects and not surface tension.

Radiation-enhanced precipitation and the impact on He bubble formation in V-Ti-based refractory alloys containing interstitial impurities

V-Ti-based refractory high entropy alloys (RHEAs), e.g., VTiTa and VTiTaNb, have been found with outstanding mechanical properties and irradiation resistance, granting them application potential in nuclear engineering. Nonetheless, similar to conventional V-Ti-based alloys such as V-4Ti-4Cr, these alloys are sensitive to C/N/O interstitial impurities. Under irradiation, such impurities may promote the formation of precipitates which may affect the defect evolution, especially at the appearance of helium. Previous studies have observed different helium behavior in the above alloys, including the heterogeneous distribution of helium bubbles in VTiTa, but the origin was not clear. In this work, detailed microstructural characterization is performed to analyze the irradiation induced precipitates and their impact on helium bubble formation in these alloys. Under helium ion irradiation at 700°C, the TiN precipitates with the NaCl-type structure are formed in all the three alloys. Nonetheless, the interfaces between precipitates and matrix in VTiTa and VTiTaNb are semi-coherent while those in V-4Ti-4Cr are coherent, due to the various lattice parameters of the alloys. Moreover, severe lattice distortion is also found inside the precipitates in the two RHEAs. Consequently, significant heterogeneous nucleation of He bubbles is observed in these two RHEAs but not in V-4Ti-4Cr. Among the three alloys, the growth of precipitates is the fastest in VTiTa, causing the formation of bubble clusters. Furthermore, the growth direction as well as the shape of precipitates affects the shape of bubbles, leading to the {001} cuboids shape in V-4Ti-4Cr and the truncated {110} dodecahedra shape in V-Ti-Ta and V-Ti-Ta-Nb.

Evaluating the Role of Titanomagnetite in Bubble Nucleation: Novel Applications of Low Temperature Magnetic Analysis and Textural Characterization of Rhyolite Pumice and Obsidian From Glass Mountain, California

AbstractNucleation of H2O vapor bubbles in magma requires surpassing a chemical supersaturation threshold via decompression. The threshold is minimized in the presence of a nucleation substrate (heterogeneous nucleation, <50 MPa), and maximized when no nucleation substrate is present (homogeneous nucleation, >100 MPa). The existence of explosively erupted aphyric rhyolite magma staged from shallow (<100 MPa) depths represents an apparent paradox that hints at the presence of a cryptic nucleation substrate. In a pair of studies focusing on Glass Mountain eruptive units from Medicine Lake, California, we characterize titanomagnetite nanolites and ultrananolites in pumice, obsidian, and vesicular obsidian (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336), calculate titanomagnetite crystal number densities, and compare titanomagnetite abundance with the physical properties of pumice to evaluate hypotheses on the timing of titanomagnetite crystallization. Titanomagnetite crystals with grain sizes of approximately 3–33 nm are identified in pumice samples from the thermal unblocking of low‐temperature thermoremanent magnetization. The titanomagnetite number densities for pumice are 1018 to 1020 m−3, comparable to number densities in pumice and obsidian obtained from room temperature methods (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336). This range exceeds reported bubble number densities (BND) within the pumice from the same eruptive units (average BND ∼4 × 1014 m−3). The similar abundances of nm‐scale titanomagnetite crystals in the effusive and explosive products of the same eruption, together with the lack of correlation between pumice permeability and titanomagnetite content, are consistent with titanomagnetite formation having preceded the bubble formation. Results suggest sub‐micron titanomagnetite crystals are responsible for heterogeneous bubble nucleation in this nominally aphyric rhyolite magma.

Revealing the Heterogeneous Bubble Nucleation at Individual Silica Nanoparticles.

Deep understanding of the bubble nucleation process is universally important in systems, from chemical engineering to materials. However, due to its nanoscale and transient nature, effective probing of nucleation behavior with a high spatiotemporal resolution is prohibitively challenging. We previously reported the measurement of a single nanobubble nucleation at a nanoparticle using scanning electrochemical cell microscopy, where the bubble nucleation and formation were inferred from the voltammetric responses. Here, we continue the study of heterogeneous bubble nucleation at interfaces by regulating the local nanostructures using silica nanoparticles with a distinct surface morphology. It is demonstrated that, compared to the smooth spherical silica nanoparticles, the raspberry-like nanoparticles can further significantly reduce the nucleation energy barrier, with a critical peak current about 23% of the bare carbon surfaces. This study advances our understanding of how surface nanostructures direct the heterogeneous nucleation process and may offer a new strategy for surface engineering in gas involved energy conversion systems.

Oxide nanolitisation-induced melt iron extraction causes viscosity jumps and enhanced explosivity in silicic magma

Explosivity in erupting volcanoes is controlled by the degassing dynamics and the viscosity of the ascending magma in the conduit. Magma crystallisation enhances both heterogeneous bubble nucleation and increases in magma bulk viscosity. Nanolite crystallisation has been suggested to enhance such processes too, but in a noticeably higher extent. Yet the precise causes of the resultant strong viscosity increase remain unclear. Here we report experimental results for rapid nanolite crystallisation in natural silicic magma and the extent of the subsequent viscosity increase. Nanolite-free and nanolite-bearing rhyolite magmas were subjected to heat treatments, where magmas crystallised or re-crystallised oxide nanolites depending on their initial state, showing an increase of one order of magnitude as oxide nanolites formed. We thus demonstrate that oxide nanolites crystallisation increases magma bulk viscosity mainly by increasing the viscosity of its melt phase due to the chemical extraction of iron, whereas the physical effect of particle suspension is minor, almost negligible. Importantly, we further observe that this increase is sufficient for driving magma fragmentation depending on magma degassing and ascent dynamics.

Morphological control of electrodeposited iron for fluctuating photovoltaic electrical energy

With the Paris Agreement, the installed capacity of green power, such as photovoltaic (PV) and wind power, is growing rapidly at a rate of more than 10 % per year. The practical application of these green electrical energies is limited by the relative lag in the scale of energy storage systems. In response to the pressure on energy storage systems, we explored the feasibility of supplying DC power directly from a PV plant to an industrial electrolyzer by comparing the electrodeposition behavior of Fe2+ in a sulfuric acid system from stabilized DC and fluctuating PV power sources. We found that the factor that damages the stable operation of an iron electrodeposition system the most is the presence of hydrogen evolution reactions, where the generation of hydrogen bubbles reduces the current efficiency of the electrodeposition system and, in severe cases, destroys the surface morphology of the deposited products, causing devastating damage to the system. On this basis, we designed a simple scheme that uses heterogeneous bubble nucleation instead of homogeneous bubble nucleation for rapid bubble expulsion from the cathode surface. Benefiting from the rapid transfer of bubbles, the current efficiency of the electrodeposited iron system increases by 1.25%–10 % at different current densities, and the maximum roughness decreases by 129–2,396 nm, which considerably improves the stability of the electrodeposited iron system for PV power. This work provides useful experimental support for the transformation of the traditional steel smelting industry, which is characterized by high carbon emission and energy consumption, into an environmentally friendly industry.

Influence of step-down pressure on wettability-controlled heterogeneous bubble nucleation of sparingly soluble gases in water

The liberation of dissolved gas from a liquid typically occurs either by diffusion through the gas–liquid interface and/or by bubble nucleation at the solid–liquid interface. The goal of this research was to study the influences of step-down pressure (degree of supersaturation) and wettability on the onset pressure for bubble nucleation of sparingly soluble gases, methane and nitrogen, in water. Bubble nucleation experiments were carried out with the gas-supersaturated liquids in the hydrophilic (untreated) and hydrophobic (treated) glass vials. The gas-saturated liquids were prepared by saturating water with methane or nitrogen in a glass vial placed in a pressure cell at 6,000 mbar for a minimum of 16 or 15 h, respectively. To initiate the pressure-driven bubble nucleation process, 100-, 500-, or 6,000-mbar step-down pressures were applied to the pressure cell until the bubble nucleation was initiated. Bubble nucleation did not occur in the hydrophilic vial with any of the gas-supersaturated waters, irrespective of the step-down pressures. Unexpectedly, 100-mbar step-down pressure did not cause neither methane nor nitrogen bubble nucleation in the hydrophobic vials. Whereas, both methane and nitrogen bubble nucleations occurred in the hydrophobic vials when the step-down pressure was increased to 500 mbar. The above experimental findings revealed the importance of step-down pressure and surface wetting nature on sparingly soluble gas bubble nucleation. To further understand the experimental findings, an analytical method was developed to estimate the effective supersaturations using Fick’s 2nd law of diffusion and Henry’ law, which indicated that at any given pressure the 500-mbar step-down pressure creates higher supersaturation compared to 100-mbar step-down pressure.

High-speed imaging and statistics of puffing and micro-exploding droplets in spray-flame synthesis

Thermally-induced breakup of metal-precursor-laden droplets in spray-flame synthesis occurs via a rapid and disruptive disintegration, i.e., “puffing” and “micro-explosion”. To assess the temporal evolution and statistics of droplet disruption, LED-illuminated droplet shadowgraphs were imaged with a microscope onto a high-speed camera and morphological image analysis was applied. The atomized liquid was a mixture of 35 vol.-% ethanol and 65 vol.-% 2-ethylhexanoic acid mixed with iron(III) nitrate nonahydrate (INN) as a precursor. Droplet evaporation and disruption were also simulated with a population balance model. The model finds solid precipitates forming in the droplets because of the decomposition of the precursor intermediate iron(III) 2-ethylhexanoate. The precipitates form a particle shell, which favors the superheating of the droplets’ interior, and they facilitate heterogeneous bubble nucleation. Imaging experiments and modelling find that per 10 µs lifetime of a droplet, the probability for disruption increases from 5 to 13% and 5 to 19%, respectively, when increasing the INN concentration from 0.05 to 0.5 mole/l. The probability of disruption suggests that throughout their lifetime in the spray flame, nearly all droplets will undergo disruption and many of them multiple times. In the experiment, droplets before disruption are 15% smaller than regular, non-disrupting droplets. Once disrupted, the droplets have a 45% smaller mean diameter than regular droplets. Under all conditions, disrupting and disrupted droplets are slower than regular droplets while the disruption does not significantly accelerate disrupted droplets.

The physics of dancing peanuts in beer.

In Argentina, some people add peanuts to their beer. Once immersed, the peanuts initially sink part way down into the beer before bubbles nucleate and grow on the peanut surfaces and remain attached. The peanuts move up and down within the beer glass in many repeating cycles. In this work, we propose a physical description of this dancing peanuts spectacle. We break down the problem into component physical phenomena, providing empirical constraint of each: (i) heterogeneous bubble nucleation occurs on peanut surfaces and this is energetically preferential to nucleation on the beer glass surfaces; (ii) peanuts enshrouded in attached bubbles are positively buoyant in beer above a critical attached gas volume; (iii) at the beer top surface, bubbles detach and pop, facilitated by peanut rotations and rearrangements; (iv) peanuts containing fewer bubbles are then negatively buoyant in beer and sink; and (v) the process repeats so long as the beer remains sufficiently supersaturated in the gas phase for continued nucleation. We used laboratory experiments and calculations to support this description, including constraint of the densities and wetting properties of the beer-gas-peanut system. We draw analogies between this peanut dance cyclicity and industrial and natural processes of wide interest, ultimately concluding that this bar-side phenomenon can be a vehicle for understanding more complex, applied systems of general interest and utility.

Rheology of nanocrystal-bearing andesite magma and its roles in explosive volcanism

Recent petrological and experimental studies have proposed that explosive volcanism may originate from the formation of nanoscale crystals in magma and the resultant ductile–brittle transition. However, the rheology of magma with quantified volume fractions of nanoscale crystals has not been investigated before, and thus, the formation of nanoscale crystals causing magma fragmentation that explains the origin of explosive eruptions is not conclusive. Here, we investigate the rheology of andesite magma with nanoscale crystals (magnetite). For this, a glass fibre elongation experimental apparatus with a heating furnace was developed at the synchrotron radiation X-ray system (SPring-8). During melt elongation, we observed the formation of crystals using small-angle X-ray scattering and wide-angle X-ray diffraction. Our experimental data demonstrate that magma viscosity increases with the formation of nanoscale crystals, but the degree of the increase is much lower than that predicted from analogue materials. Finally, we conclude that nanocrystal formation in intermediate composition magmas cannot explain rheological transition and other mechanisms such as nanocrystal agglomeration (not observed in our experiments) and/or heterogeneous nucleation of gas bubbles on nanocrystals are required to induce mafic to intermediate explosive volcanisms.

A molecular dynamics study on the mechanism of heterogeneous bubble nucleation of mixed liquid

The nucleate boiling of mixed boiling is widely utilized in natural gas, hydrogen liquefaction process, and other fields. In this paper, the molecular dynamics study of heterogeneous bubble nucleation of pure argon liquid and the argon‑krypton mixed liquid on the platinum surface was conducted. The nucleate boiling characteristics were presented and the variations of the density, temperature, nucleation embryo volume, and heat flux were analyzed. The difference in the nucleation mechanism of pure liquid and mixed liquid was explained based on the competition of atomic potential energy and atomic kinetic energy. The effect of the component mole fraction on the bubble nucleation of mixed fluid was also discussed. The results showed that the onset superheat of boiling of argon‑krypton mixed liquid is higher than that of pure argon. The higher the mole fraction of the component with a lower saturation temperature, the lower the onset superheat of the liquid nucleate boiling. When x < 0.5, the growth rate of the nucleation embryo is dominated by the x value, when x > 0.5, the growth rate is dominated by wall temperature. Moreover, the difference in diffusion coefficients of each component is one of the direct causes of determining the growth rate of nucleation embryos. The findings provide insights into the mechanisms of differences in boiling nucleation between mixed liquid and pure liquid, as well as guide the reasonable utilization of mixed liquid.

Direct measuring of single–heterogeneous bubble nucleation mediated by surface topology

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes ranging from nature to technology. There is excellent evidence that surface topology is important in directing heterogeneous nucleation; however, deep understanding of the energetics by which nanoscale architectures promote nucleation is still challenging. Herein, we report a direct and quantitative measurement of single-bubble nucleation on a single silica nanoparticle within a microsized droplet using scanning electrochemical cell microscopy. Local gas concentration at nucleation is determined from finite element simulation at the corresponding faradaic current of the peak-featured voltammogram. It is demonstrated that the criteria gas concentration for nucleation first drops and then rises with increasing nanoparticle radius. An optimum nanoparticle radius around 10 nm prominently expedites the nucleation by facilitating the special topological nanoconfinements that consequently catalyze the nucleation. Moreover, the experimental result is corroborated by our theoretical calculations of free energy change based on the classic nucleation theory. This study offers insights into the impact of surface topology on heterogenous nucleation that have not been previously observed.

Cerebrospinal Fluid-philic and Biocompatibility-Enhanced Soft Cranial Window for Long-Term In Vivo Brain Imaging.

Soft, transparent poly(dimethyl siloxane) (PDMS)-based cranial windows in animal models have created many opportunities to investigate brain functions with multiple in vivo imaging modalities. However, due to the hydrophobic nature of PDMS, the wettability by cerebrospinal fluid (CSF) is poor, which may cause air bubble trapping beneath the window during implantation surgery, and favorable heterogeneous bubble nucleation at the interface between hydrophobic PDMS and CSF. This may result in excessive growth of the entrapped bubble under the soft cranial window. Herein, to yield biocompatibility-enhanced, trapped bubble-minimized, and soft cranial windows, this report introduces a CSF-philic PDMS window coated with hydroxyl-enriched poly(vinyl alcohol) (PVA) for long-term in vivo imaging. The PVA-coated PDMS (PVA/PDMS) film exhibits a low contact angle θACA (33.7 ± 1.9°) with artificial CSF solution and maintains sustained CSF-philicity. The presence of the PVA layer achieves air bubble-free implantation of the soft cranial window, as well as induces the formation of a thin wetting film that shows anti-biofouling performance through abundant water molecules on the surface, leading to long-term optical clarity. In vivo studies on the mice cortex verify that the soft and CSF-philic features of the PVA/PDMS film provide minimal damage to neuronal tissues and attenuate immune response. These advantages of the PVA/PDMS window are strongly correlated with the enhancement of cortical hemodynamic changes and the local field potential recorded through the PVA/PDMS film, respectively. This collection of results demonstrates the potential for future microfluidic platforms for minimally invasive CSF extraction utilizing a CSF-philic fluidic passage.

Evolution of helium bubbles in SLM 316L stainless steel irradiated with helium ions at different temperatures

The temperature dependence of helium bubble evolution in 316L stainless steel (SS) fabricated by selective laser melting (SLM) has been studied by performing 500 keV helium ion irradiation at temperatures between 350 and 900 °C. The number density of helium bubbles decreased steadily whereas the bubble diameter gradually increased with increasing temperature. The material swelling caused by helium bubbles had no obvious change at the temperature below 800 °C. However, at the maximum temperature of 900 °C, the swelling sharply increased to 0.17% mainly due to the rapidly growing bubble diameter. In particular, heterogeneous nucleation of bubbles occurred in the vicinity of dislocation structures at high temperatures of 700–900 °C, and the formed helium bubbles rapidly grew at the oxide-matrix interface at 900 °C. It was confirmed various intrinsic structures (dislocation structures or oxide particles) differently affected the bubble evolution process. The apparent activation energy (Eact) of helium bubbles determined by the Arrhenius model were used to elucidate the most probable helium bubble evolution mechanisms at various temperatures, as it is an experimentally observed activation energy, which contained information about the activity of helium atoms. It was found that the low-temperature regime with low apparent activation energies (ECbact=0.15eV,ERbact=0.04eV) and high-temperature regime with high apparent activation energies (ECbact=1.50eV,ERbact=0.58eV) favored different helium bubble evolution mechanisms. In the low-temperature regime, helium bubble evolution was controlled by the diffusion of helium atoms via a vacancy mechanism in the SLM 316L SS. On the other hand, the migration and coalescence of helium bubbles also occurred in the high-temperature regime which caused the apparent activation energy of density was higher than the theoretical value of helium atoms diffuse via a replacement mechanism.

Alteration in Ti6Al4V implant surface properties with micro textures density

ABSTRACT In lab-developed micro milling method was utilized to machine microtextures on Ti6Al4V alloy. Subsequently, the quality and geometry of the formed textures have been analysed and characterized. The functionality of textured surfaces has been assessed by performing wetting analysis concerning area surface roughness and roughness factor. The surface free energy of textured surfaces was computed via Owen and Wendt method, and Neumann’s equation. Geometrically correct, defect-free, the machined textures are uniformly dispersed. The density of textures has enhanced the area surface roughness values of manufactured surfaces. Further, the wetting nature of fabricated surfaces is significantly altered. Heterogeneous bubble nucleation has been observed between the interface of solid surfaces and liquid droplets at high-density textured surfaces. The apparent surface free energy of textured surfaces varied with geometrical parameters. This study offers in-depth knowledge about the properties of textured Ti6Al4V implant surfaces.

Heterogeneous Bubble Nucleation by Homogeneous Crystal Nuclei in Poly(l‐Lactic Acid) Foaming

AbstractA novel method to enhance and control bubble nucleation in poly(l‐lactic acid) (PLLA) foaming, not relying on external nucleating agents, is presented. Amorphous PLLA is annealed at temperatures close to its glass transition, to allow local alignment of polymer chains in the form of nanosized aggregates, called homogeneous crystal nuclei (HCN). PLLA containing HCN is then foamed after sorption of CO2 at 120 °C and 10 MPa. Under the chosen experimental conditions, HCN can promote growth of PLLA spherulites that can be controlled by thermal‐pressure history. Foam morphology is strongly affected by the presence of HCN, which act as heterogeneous nucleation sites for bubbles. Resulting foams are characterized by a bimodal morphology, with bubbles of about 50–70 µm in diameter developed in the bulk, and much smaller cells of few µm, grown upon pre‐existing crystal aggregates. The method detailed here to attain PLLA foams with tailored morphology can be exploited also for other semicrystalline polymers.

THE MIXTURE HETEROGENEOUS BUBBLE NUCLEATION MODEL BASED ON THERMODYNAMIC PROPERTIES OF NONIDEAL FLUIDS

THE MIXTURE HETEROGENEOUS BUBBLE NUCLEATION MODEL BASED ON THERMODYNAMIC PROPERTIES OF NONIDEAL FLUIDS

The roles of microlites and phenocrysts during degassing of silicic magma

Silicic magmas span a wide range of eruptive styles between explosive and effusive, and transitions between these styles are commonplace. Yet the triggers of switches in eruptive style remain poorly understood. Eruptions are mostly driven by degassing of magmatic water and their eruption style - effusive or explosive - is likely governed by the efficiency of outgassing as well as magma ascent rate. Microlites and phenocrysts are often purported to promote heterogeneous bubble nucleation and outgassing, both key variables in the degassing dynamics that become crucial in controlling the eruptive style. Here, in order to shed light on the role of nature, size and abundance of crystals on degassing of silicic magma, we experimentally investigate (heating-induced) vesiculation in a multiphase (microlite- and phenocryst-bearing), low-water content, and bubble-free, natural rhyolite. The experiments were conducted at magmatic temperature (∼900-1100°C) and atmospheric pressure in an optical dilatometer. Our results indicate that microlites exert a large influence on bubble nucleation while the effect of phenocrysts is subordinate. Amongst the microlite phases, bubbles nucleate more easily on Fe-Ti oxides than other mineral phases. Bubble coalescence and connectivity are, in contrast, enhanced by the phenocrysts, more than microlites, in low-crystallinity magma. Comparing the bubble textures of the post-experimental samples with those produced in a phenocryst-free and microlite-poor silicic magma, we observe that the phenocryst- and microlite-bearing magma exhibits significantly more bubble coalescence and connectivity, as well as higher bubble number densities. These findings help to constrain the roles that pre- and syn-eruptive crystalline phases may play in degassing processes during ascent of silicic magma.

Persistent reduction of boiling incipience of ethanol on biphilic porous textured surfaces

Boiling of highly wetting fluids is of great interest to thermal management of high-powered electronic devices, enhancement of which conventionally relies on functional modifications of surface shape and structure so as to promote bubble generation and growth. In this paper we attempt to combine the approach of porous texturing with a novel technique of surface wettability engineering to enhance saturated pool boiling of ethanol. Thin layers of microporous and nanoporous surface topographies were chemically deposited on an aluminum substrate by anodization in phosphoric acid and sulfuric acid, respectively. Boiling on the textured surfaces recorded over-twofold increases in terms of the heat transfer coefficient compared with that on a plain smooth surface, which can be attributed to a proliferation of active nucleation sites. The boiling incipience point on both porous surfaces, however, exhibited an interesting dependence on the initial wetting state of the surface. With the application of amphiphobic coatings of fluoropolymer modified with halloysite nanotubes, we managed to engender a biphilic (i.e., spatially alternating hydrophobicity and hydrophilicity) pattern on the porous-textured surfaces. The repeat control experiments on the hybrid surfaces showed an equally efficient mode of nucleate boiling, whose inception, by contrast, seemed to occur at consistently low superheats. The remarkable reduction (by more than 80%) of the minimum superheat at the boiling onset hints at an alternative nanobubble-related mechanism for heterogeneous bubble nucleation, which is notably less affected by incondensable gas, to the classic vapor-trapping-cavity model.

Mechanism of Heterogeneous Bubble Nucleation in Polymer Blend Foaming.

A three-dimensional heterogeneous bubble nucleation model is constructed to provide a reasonable explanation at the molecular level for the foaming mechanism of polypropylene (PP) and polystyrene (PS) blends. CO2 solubilities and supersaturation rations are quantitatively calculated to help interpret the contribution of each phase of the blend in the CO2 dissolution stage. The spatial density profiles of polymer/CO2 binary melt around different polymer chains are presented to give an intuitive perspective to the thermodynamic driving force. The predicted interfacial tension and contact angles of critical bubbles provide valid evidence to distinguish the wettability of CO2 in different regions. The values of predicted free-energy barriers, critical radii, and nucleation number densities imply that bubbles that nucleate in the PP and PS blend interfacial region attached to the PS-rich phase achieve the smallest size and largest number density. The reliability of the theoretical model has been tested by partial available experimental data.