Early Coalescence Research Articles

Molecular dynamics investigation of gold nanoparticle coalescence under realistic gas-phase synthesis conditions

Dependence of nanoparticle (NP) coalescence on various physical parameters (e.g., temperature, number of NPs, NP size, orientation, crystallinity, shape, or composition, etc.) is a very active field of investigation. However, most computational studies on NP coalescence to date are performed in vacuum, with only a handful of studies taking gas pressure into account and even fewer doing a systematic analysis. This is due to two reasons: first, many computational studies complement inert-gas condensation experiments, which typically happen at high vacuum. Second, a simulation set-up in vacuum is simpler and computationally less costly. Here we utilised classical molecular dynamics for a rigorous investigation of the effect (or lack of) of gas pressure, as well as of other parameters (namely temperature, angular momenta, and inert-gas species), on the early stages of coalescence between two metallic NPs. Our approach is relevant for both inert-gas condensation in high vacuum and aerosol synthesis in standard atmospheric conditions. Multiple linear regression analysis confirmed temperature as the key factor determining the degree of coalescence; relative angular momenta direction was revealed as yet another important contributor, whereas the effect of pressure was deemed insignificant for early coalescence stages. To shed light onto the sintering process we elaborate on interesting atomistic mechanisms. We aspire that our study may indicate potential strategies for both gas-phase synthesis methods.

Electrowetting-Induced Coalescence of Sessile Droplets in Viscous Medium

Manipulating the coalescence of microdroplets has recently gained enormous attention in digital microfluidics and biological and chemical industries. Here, coalescence between two sessile droplets is induced by spreading them due to electrowetting. The electrocoalescence dynamics is investigated for a wide range of operating parameters such as electrowetting number, Ohnesorge number, driving frequency, and drop to surrounding medium viscosity ratio. Here, the characteristic time scale from the classical lubrication theory is modified with an additional driving and resisting force due to the electrostatic pressure force and liquid-liquid viscous dissipation, respectively. With the revised characteristic time scale, a universal bridge growth is shown between the two merging droplets following a 1/3 power law during early coalescence followed by a long-range linear variation. To ensure precise control on droplet coalescence, a geometric analysis is also performed to define the initial separation distance.

In-situ monitoring of microwave plasma-enhanced chemical vapour deposition diamond growth on silicon using spectroscopic ellipsometry

The quality of polycrystalline diamond films is heavily dependent on the nucleation and early stages of growth, making the ability to monitor these early stages highly desirable. Spectroscopic ellipsometry (SE) allows for real-time monitoring of the thickness, composition, and morphology of films with sub-nanometre precision. In this work, ex-situ SE spectra were used to develop an optical model for film characterisation, which was then applied to in-situ data. The coalescence of individual crystallites into a single film was observed through a parabolic decrease in void content followed by peaks in sp2 content and surface roughness. These observations were validated using ex-situ Raman spectra and AFM images of samples grown for durations between 5 and 30 min. The model was also used to investigate the impact of varying the methane concentration, finding that a higher methane fraction resulted in earlier coalescence and a higher peak in sp2 content. This work demonstrates that SE is a powerful tool for monitoring and optimisation of the critical early stages of polycrystalline diamond growth.

Performance Degradation of Subsea Shield Tunnel Segment Accounting for Concrete Strength Loss and Steel Bar Corrosion

Subsea shield tunnels usually serve in a typical corrosive marine environment. Under the action of chloride penetration and carbonization, tunnel lining segments are often damaged because of concrete strength loss and steel bar corrosion induced concrete cracking during their service life, which seriously degrades the service performance of the tunnels. A systematical experimental and numerical investigation into the performance degradation of subsea shield tunnel segments accounting for concrete strength loss and steel bar corrosion is presented in this paper. The study demonstrates that chloride penetration decreases the peak strength and elastic modulus of the segment concrete by 42% and 46.1%, respectively. The average of the ratio of dissipated energy to the total energy of dry concrete is much smaller than that of water saturated concrete and chlorine solution saturated concrete, and chloride penetration reduces the energy storage capacity of concrete, and the ability to resist damage is weakened. When steel bars corrode for 120 days, the outer cracks continue to extend, and the concrete around the inner steel bars just begin to crack initiation; when corrode for 20 years, the length of the inner cracks gradually exceeds that of the outer cracks, and the inner cracks initiating from different steel bars coalesce with each other and form a continuous failure surface, causing great serious damage to the segment. Due to the difference in concrete strength, for the outer layer, the evolution processes of steel bar corrosion-induced cracks show the characteristics of early initiation, slow propagation, and late coalescence, and those for the inner layer have the characteristics of late initiation, rapid propagation, and early coalescence. During the whole process the propagation speed of the inner and outer cracks appears to be fast first and then slow. Moreover, the study also illustrates that the final state of segment performance degradation after crack coalescence presents the characteristics of whole lamellar exfoliation of the concrete cover.

Degassing and gas percolation in basaltic magmas

Due to their generally low eruptive melt viscosities and concomitant high diffusivities of volatiles, basaltic magmas degas relatively efficiently. This relative efficiency, combined with variations in style, extent, timing and length scales of degassing govern the range of eruptive styles observed at basaltic volcanoes. The result is a surprising complexity of degassing regimes and products in basaltic volcanism. In particular, the transition between closed- and open-system degassing at low pressure at the percolation threshold may strongly affect the type of eruption. Here we aim to better understand degassing and gas percolation processes in basaltic magmas and their implications for eruptive style. Combining new and literature data, we present a database of vesicle metrics in basaltic rocks including vesicularity, vesicle number density, vesicle size distribution (and its polydispersivity), vesicle connectivity and permeability. We combine these textural and petrophysical data with a numerical model of percolation for systems having polydisperse vesicle size distributions. Using this model, we also evaluate different definitions of vesicle connectivity inherent to different measurement techniques. Our results show that polydispersivity exerts a strong control on the percolation threshold of basaltic magmas and consequently on eruptive style. Intermediate to highly polydisperse bubble networks are more typical of Hawaiian activity and are characterized by higher values of percolation threshold. This results in delayed coalescence and an increase in magma vesicularity hindering the formation of large decoupled and buoyant bubbles, which in turn can promote magma acceleration, fragmentation by inertia below the percolation threshold and sustained fountaining activity. Bubble populations with lower polydispersivity, typical of Strombolian eruptions, promote early coalescence prior to fragmentation, which in turn may lead to the formation of large decoupled slugs or gas pockets and/or plugs at the surface via outgassing. Further, we discuss the implications of our findings for Plinian, violent Strombolian, Surtseyan, deep submarine and effusive basaltic eruptions.

Sub- μ m features patterned with laser interference lithography for the epitaxial lateral overgrowth of α-Ga2O3 via mist chemical vapor deposition

The low growth rate of mist chemical vapor deposition normally requires a long growth time to achieve coalescence in the epitaxial lateral overgrowth of α-Ga2O3 thin films on sapphire substrates. To address this issue, sub-μm features were patterned using laser interference lithography. Periodical stripes with a ∼590-nm pitch allowed the overgrowth of crack-free, void-free, and continuous thin films, while typical growth conditions using a low carrier gas flow rate and a low Ga precursor concentration were maintained. Coalescence was achieved even with a short growth time of <30 min and a low film thickness of <500 nm. Transmittance and x-ray diffraction spectra show that the film was predominantly in α-phase. Transmission electron microscopy (TEM) images reveal cup-top-like α-Ga2O3 regions of low dislocation density on the SiOx mask. Selected area electron diffraction and high-resolution TEM analyses confirm that an α-Ga2O3 layer was formed even on the top of the SiOx mask. Interestingly, the dislocations formed on the window areas did not bend toward the center of the masks; rather, a dislocation bending outward from the center was observed. This suggests the occurrence of early coalescence and/or atomic rearrangement.

A growing genetic tree in the soil of prostate.

In this issue of Cell Stem Cell, Grossmann etal. (2021) reconstructed the developmental history of the prostate gland in a 59-year-old male by 3D tracking of somatic mutations across 319 micro-dissected specimens. It provides a genetic picture of how the human prostate gland is shaped during embryogenesis, puberty, and adult life.

Improving Copper Nucleation on Liner Materials

The damascene process for fabrication of interconnects requires a copper seed conductive layer on top of a liner material, which acts as a copper diffusion barrier. Copper metal is electrochemically deposited from solutions containing additives onto the conductive seed layer, for void-free bottom up fill. However, as interconnect dimensions continue to decrease, copper damascene faces significant challenges1. As such, alternative metallization approaches are under development. These include electrodeposition of copper directly onto liner materials such as ruthenium2,3 or cobalt4-6. Direct plate onto liner materials pose new challenges that must be overcome prior to implementation. For example, ruthenium and cobalt are easily oxidized upon exposure to air7. This native oxide film inhibits nucleation, resulting in heterogeneous growth of copper islands8. The literature shows that oxide removal improves nucleation at ruthenium and that copper can successfully be deposited onto cobalt using additives6. While there are some insights on how to achieve nucleation of copper onto resistive substrates, this must be improved prior to commercialization. We present methods for pretreatment of ruthenium in order to reduce the native oxide, allowing for more uniform deposition of the copper films (as shown by Fig. 1). Upon addition of our additives, early film coalescence and uniform deposition of copper is achieved, resulting in void-free bottom up filling. These findings will be presented and discussed.Fig. 1. Nucleation of copper onto (a) as received compared to (b) pretreated ruthenium substrate plated from a typical copper electrolyte containing no additives.

Effect of external electric field on dynamics of levitating water droplets

The self-assembled clusters of regularly positioned small droplets were observed for the first time in 2004 by Alexander Fedorets. The recent results on stabilization of droplet clusters enable the laboratory study of specific parameters of chemical and biochemical reactions in the microdroplets. However, the analysis of the cluster behavior under the action of external factors is also continued. Some new experimental findings concerning behavior of a cluster of water droplets levitating over the locally heated water surface are reported in the present paper. It is shown that an external electric field leads not only to an increase in the rate of a condensational growth of droplets but also to a relatively early coalescence of small droplets with water layer. The latter is partially explained by the attraction force arising due to polarization of droplets closely spaced to the water surface. The effect of electric field on size of droplets before their coalescence with water layer is used to estimate an electrical charge of single droplets at the experimental conditions. It is shown that this electric charge does not change the aerodynamic nature of the main forces responsible for both the self-arranging and levitation of small water droplets. The results obtained are expected to be useful for further theoretical modeling of the phenomenon of levitating droplets clusters.

The Enhancement of Spray Cooling Performance in Nucleate and Transition Boiling Regimes by Using Saline Water Containing Dissolved Carbon Dioxide

Abstract In the current work, by using various additives, the spray cooling in the transition boiling regime is significantly augmented due to the vapor film instability enhancing, which helps to overcome the disadvantages reported in the open literature for the attainment of high heat flux in the aforesaid boiling regime. Saline water containing dissolved carbon dioxide produces two favorable conditions for high heat transfer rate: (1) controlled vapor bubble nucleation and (2) low entrapped vapor bubbles coalescence rate. These phenomena are the parameters defining the step-up in the heat transfer rate. Systematic spray cooling (from 900 °C) experiments were conducted on a 6-mm thick AISI 304 steel plate (100 mm × 100 mm). The heat transfer analysis indicates that the heat removal rate in case of soda added water depicts an increasing trend with the rising of the soda concentration up to 40% in water, and further increment in soda water concentration declines the heat removal rate due to the formation of the uncontrolled vapor bubbles undergoing early coalescence. In case of salt added carbonated water spray cooling, the quenching performance indicates step-up in critical heat flux up to 1.7 MW/m2. In addition to the above, the spray cooling performance of the above-stated coolant is compared with other potential coolants such as soda–surfactant–water, soda–alcohol–water and soda–salt–surfactant–water mixtures.

Viscous resistance in drop coalescence

Hydrodynamics of drop coalescence has been studied theoretically and numerically by solving the Navier Stokes equation considering a single fluid after the minimum bridge formation. Many experiments have been performed to document bridge growth over time with the use of high speed videography and electrical methods. However, internal fluid motion during coalescence has not been extensively studied, in part due to the spherical shape of the drops. This work observed overall fluid motion (except at the site of early coalescence) using particle image velocimetry for two-dimensional (sandwiched drop) coalescence. Fluid motion inside the bulk drops is inertial, and governing fluid flow in the bridge region is one dimensional. At the merging interface, incoming liquids join and coflow in the perpendicular direction. These observations were extended to a three-dimensional counterpart, and a scaling law was developed that was validated through experimentation. While flow in the bulk drops is inertial, the dominant resistance comes through a viscous effect in the merging interface region and at the lesser extent in the bridge region. Early dynamics of drop coalescence is dominated by the Ohnesorge number (Oh), and later dynamics are dependent on how drops are bounded.

Early bubble coalescence in thermoplastic foaming

In the underlying literature, bubble coalescence has been so far observed at the latest foaming stages, where bubble growing causes large extensional loads on the polymer film between two neighboring bubbles and eventually leads to the film rupture and to the merging of the two bubbles in a single one. This phenomenon is responsible for the coarsening of the foam morphology and, in turn, the worsening of the final foam properties. Here, the unanticipated, indirect, observation of a bubble coalescence phenomenon occurring at the very early stage of foaming, when most of the blowing agent is still solubilized in the polymer, is reported. Likewise, this phenomenon is responsible for a significant coarsening of the foam morphology. With the aim of investigating this phenomenon, a novel pressure vessel for foaming was utilized, capable of imposing two different cooling histories on two polymer samples, foamed at the same time. Results are presented on polystyrene foamed with carbon dioxide.

Void growth and coalescence in irradiated copper under deformation

A decrease of fracture toughness of irradiated materials is usually observed, as reported for austenitic stainless steels in Light Water Reactors (LWRs) or copper alloys for fusion applications. For a wide range of applications (e.g. structural steels irradiated at low homologous temperature), void growth and coalescence fracture mechanism has been shown to be still predominant. As a consequence, a comprehensive study of the effects of irradiation-induced hardening mechanisms on void growth and coalescence in irradiated materials is required. The effects of irradiation on ductile fracture mechanisms - void growth to coalescence - are assessed in this study based on model experiments. Pure copper thin tensile samples have been irradiated with protons up to 0.01 dpa. Micron-scale holes drilled through the thickness of these samples subjected to uniaxial loading conditions allow a detailed description of void growth and coalescence. In this study, experimental data show that physical mechanisms of micron-scale void growth and coalescence are similar between the unirradiated and irradiated copper. However, an acceleration of void growth is observed in the later case, resulting in earlier coalescence, which is consistent with the decrease of fracture toughness reported in irradiated materials. These results are qualitatively reproduced with numerical simulations accounting for irradiation macroscopic hardening and decrease of strain-hardening capability.

A case study of thin film stress evolution at a dissimilar material interface via molecular dynamics simulations

Evolution of deformation and stress in growing thin films has been studied in this work using computational simulations that resolve matter at atomic length and time scales. For the surface layers of films laying on the substrate of a dissimilar material, the stress distribution analysis around defects becomes more challenging. Herein, spatial and temporal distribution of deformation and associated stress evolution are presented for different thin film formation events including (1) sub-monolayer growth during an early film nucleation stage and (2) coalescence of adjacent monolayer “islands.” Validity of the stress computed via local computations of the virial expression for stress in a system of interacting particles was checked by comparing to results obtained from considerations of local atomic deformation in conjunction with existing expressions for epitaxial thin film growth stress. For the geometries studied here, where a monolayer of film with a highly characterized linear defect, as in the case of a stacking fault, was simulated for coalescence, fairly good agreement was found. This result demonstrates that, for similar defects at the surface layer, with sufficient sub-ensemble averaging of the standard virial expression for stress, semiquantitative spatial stress distribution information can be obtained from atomic scale simulations. Using our validated stress computation method, we reveal significant stress localization during thin film growth processes, leading to pronounced differences in maximum and minimum stress observed over very small spatial extent (of order multiple GPa over 3–6 nm distances). One prominent mechanism of stress localization revealed here is coalescence between adjacent growing islands. For geometries explored here, stress manifesting during coalescence is highly localized.

Evolution of flow patterns and the associated heat and mass transfer characteristics during flow boiling in mini-/micro-channels

A three-dimensional numerical investigation of flow boiling in mini-/micro-channels is carried out using a coupled Level-set (LS)/Volume of Fluid (VOF) method (CLSVOF) in the present study. A phase change model based on the interfacial temperature gradient is implemented and the numerical approach is first validated by the experimental and numerical results in the literature. A fabricated cavity in lieu of the traditional methods of seed bubble or local superheat is treated as the nucleation site, where boiling occurs exclusively and periodic stream of single bubble emerges. The transitions of flow patterns from bubbly flow to slug flow and annual flow are observed, and the numerical wall temperatures agree with the experimental results. The growths of the radial diameter and axial length of vapor bubble are compared with the experimental data, and good agreement is found for the radial diameter while deviation is found for the axial length due to early coalescence of bubbles near the nucleation site. Meanwhile, the local heat transfer coefficients are found to be significantly influenced by the flow patterns. The variations of the local vapor qualities at five representative locations along the flow direction are also studied quantitatively and the fluctuations are found to be consistent with the local flow patterns. Finally, we present a numerical attempt on flow boiling with two nucleation sites, which shows a good potential of the present numerical approach to deal with the complicated flow boiling process. The information presented here is very useful to the design and operation of the mini-/micro-reactors.

Collisions and coalescence in droplet streams for the production of freeze-dried powders

Streams of mono-disperse micro-droplets with diameters ranging from about 20μm to 100μm were produced from diluted aqueous solutions containing carbohydrates and proteins using a pinhole type piezoelectric generator with either a 20μm or a 50μm single-orifice diaphragm. Image sequences indicating droplet size, velocity, inter-droplet spacing at various distances from the nozzles as well as collision events and coalescence were recorded using a high-speed camera and analysed quantitatively. The size-dependent gradual deceleration of the droplets is superimposed by small scale random movements, which equally affect both large and small droplets and lead to early contacts and coalescence. The loss of mono-dispersity can be reduced by quick cooling since both the nucleation rate and the freezing rate of micro-droplets are extremely dependent upon the temperature of their gaseous environment.

Symmetric drop coalescence on an under-liquid substrate.

We have derived a modified one-dimensional lubrication equation to describe the early coalescence behavior of a symmetric sessile drop coalescence for under-liquid substrates, which takes into account the viscosities of both the drop and the surrounding medium. We found a time scale, which governs the process, and there exists a crossover time between the universal scaling of the bridge height growth h^{*}∼t^{*} (valid for both under-liquid and air) and a much slower bridge growth h^{*}∼t^{*}^{0.24} occurring at a later time. It is also found that the drop coalescence bridge profile has a self-similarity, which breaks up much earlier for under-liquid substrates as opposed to symmetric coalescence in air.

Studies on droplet size distributions during coalescence in immiscible polymer blends filled with silica nanoparticles

The effect of silica nanoparticles on the morphology of (10/90 wt%) PDMS/PBD blends during the shear induced coalescence of droplets of the minor phase at low shear rate was investigated systematically in situ by using an optical shear technique. Two blending procedures were used: silica nanoparticles were introduced to the blends by pre-blending silica particles first in PDMS dispersed phase (procedure 1) or in PBD matrix phase (procedure 2). Bimodal or unimodal droplet size distributions were observed for the filled blends during coalescence, which depend not so much on the surface characteristics of silica but mainly on blending procedure. For pure (10/90 wt%) PDMS/PBD blend, the droplet size distribution exhibits bimodality during the early coalescence. When silica nanoparticles (hydrophobic and hydrophilic) were added to the blends with procedure 1, bimodal droplet size distributions disappear and unimodal droplet size distributions can be maintained during coalescence; the shape of the different peaks is invariably Gaussian. Simultaneously, coalescence of the PDMS droplets was suppressed efficiently by the silica nanoparticles. It was proposed that with this blending procedure the nanoparticles should be mainly kinetically trapped at the interface or in the PDMS dispersed phase, which provides an efficient steric barrier against coalescence of the PDMS dispersed phase. However, bimodal droplet size distributions in the early stage of coalescence still occur when incorporating silica nanoparticles into the blends with procedure 2, and then coalescence of the PDMS droplets cannot be suppressed efficiently by the silica nanoparticles. It was proposed that with this blending protocol the nanoparticles should be mainly located in the PBD matrix phase, which leads to an inefficient steric barrier against coalescence of the PDMS dispersed phase; thus the morphology evolution in these filled blends is similar to that in pure blend and bimodal droplet size distributions can be observed during the early coalescence. These results imply that exploiting non-equilibrium processes by varying preparation protocol may provide an elegant route to regulate the temporal morphology of the filled blends during coalescence.

A Functional Interplay between the Small GTPase Rab11a and Mitochondria-shaping Proteins Regulates Mitochondrial Positioning and Polarization of the Actin Cytoskeleton Downstream of Src Family Kinases

It is believed that mitochondrial dynamics is coordinated with endosomal traffic rates during cytoskeletal remodeling, but the mechanisms involved are largely unknown. The adenovirus early region 4 ORF4 protein (E4orf4) subverts signaling by Src family kinases (SFK) to perturb cellular morphology, membrane traffic, and organellar dynamics and to trigger cell death. Using E4orf4 as a model, we uncovered a functional connection between mitochondria-shaping proteins and the small GTPase Rab11a, a key regulator of polarized transport via recycling endosomes. We found that E4orf4 induced dramatic changes in the morphology of mitochondria along with their mobilization at the vicinity of a polarized actin network typifying E4orf4 action, in a manner controlled by SFK and Rab11a. Mitochondrial remodeling was associated with increased proximity between Rab11a and mitochondrial membranes, changes in fusion-fission dynamics, and mitochondrial relocalization of the fission factor dynamin-related protein 1 (Drp1), which was regulated by the Rab11a effector protein FIP1/RCP. Knockdown of FIP1/RCP or inhibition of Drp1 markedly impaired mitochondrial remodeling and actin assembly, involving Rab11a-mediated mitochondrial dynamics in E4orf4-induced signaling. A similar mobilization of mitochondria near actin-rich structures was mediated by Rab11 and Drp1 in viral Src-transformed cells and contributed to the biogenesis of podosome rosettes. These findings suggest a role for Rab11a in the trafficking of Drp1 to mitochondria upon SFK activation and unravel a novel functional interplay between Rab11a and mitochondria during reshaping of the cell cytoskeleton, which would facilitate mitochondria redistribution near energy-requiring actin-rich structures.

UNCOVERING THE DEEPLY EMBEDDED ACTIVE GALACTIC NUCLEUS ACTIVITY IN THE NUCLEAR REGIONS OF THE INTERACTING GALAXY Arp 299

We present mid-infrared (MIR) 8-13micron spectroscopy of the nuclear regions of the interacting galaxy Arp299 (IC694+NGC3690) obtained with CanariCam (CC) on the 10.4m Gran Telescopio Canarias (GTC). The high angular resolution (~0.3-0.6arcsec) of the data allows us to probe nuclear physical scales between 60 and 120pc, which is a factor of 10 improvement over previous MIR spectroscopic observations of this system. The GTC/CC spectroscopy displays evidence of deeply embedded Active Galactic Nucleus (AGN) activity in both nuclei. The GTC/CC nuclear spectrum of NGC3690/Arp299-B1 can be explained as emission from AGN-heated dust in a clumpy torus with both a high covering factor and high extinction along the line of sight. The estimated bolometric luminosity of the AGN in NGC3690 is 3.2(+/-0.6)x10^44 erg/s. The nuclear GTC/CC spectrum of IC694/Arp299-A shows 11.3micron polycyclic aromatic hydrocarbon (PAH) emission stemming from a deeply embedded (A_V~24mag) region of less than 120pc in size. There is also a continuum-emitting dust component. If associated with th putative AGN in IC694, we estimate that it would be approximately 5 times less luminous than the AGN in NGC3690. The presence of dual AGN activity makes Arp299 a good example to study such phenomenon in the early coalescence phase of interacting galaxies.