Coalescence Of Cores Research Articles

Controllable Double‐Emulsion Droplet Manipulation Actuated by a Triboelectric Nanogenerator

AbstractElectrically controlled droplet manipulation is attractive for lots of applications ranging from material synthesis and drug delivery. However, the present techniques using bulky and expensive power supplies to generate high‐voltage electrical signals are easy to cause electrode electrolysis, bubble formation in buffer solution, and are incapable for point‐of‐care testing. Herein, a novel and portable electrical method for self‐powered emulsion droplet core coalescence and release based on triboelectric nanogenerator (TENG) is presented. The electrical signal with a 2.25 kV open‐voltage and a 35 µA short‐circuit current can be conveniently generated via frictional electrification effect, and the harmful electrochemical reaction in solutions can be avoided. The high transient pulse voltage and stable sinusoidal signal between two electrodes in microfluidic device can generate a strong electric field, which causes apparent Maxwell electrical stresses at droplet interfaces and subsequently promotes the core coalescence and release of double‐emulsions. By adjusting the voltage magnitude, the core coalescence and release behaviors of double‐emulsion droplets can be controllably achieved, followed by the synthesis copper‐based metal–organic framework (Cu‐MOF) nanoparticles. Thus, this portable, effective and self‐powered droplet manipulation technique can be attractive for those droplet‐based applications.

Numerical study of a hollow pileup yielded by deposition of successive hollow droplets

Understanding the pileup formation process of sequentially deposited droplets is vital in advancing droplet-based printing technologies. While pileups of simple droplets have been extensively studied, knowledge of the hollow pileup formation is inadequate. This paper presents a fully resolved numerical analysis of the pileup formed by successively depositing incoming hollow droplets on a pre-solidified (or base) droplet on a supercool surface. An axisymmetric front-tracking method is used to handle the simulations. The pileup height increases as the incoming droplets coalesce, while the hollow cores may or may not merge. The pileup shape and its hollow configuration depend on parameters such as the Stefan number, Peclet number, Weber number, Fourier number, and the size and number of hollow cores. Varying these parameters does not affect the peak formation at the top of the pile caused by volume expansion during phase change, although the Fourier number has a strong influence on the mean aspect ratio and solidification time of the pileup. Increasing the deposition rate enhances the coalescence of hollow cores and reduces the mean aspect ratio of the pileup. Reducing the Stefan number also promotes hollow core coalescence, which decreases the mean aspect ratio. However, the size of the hollow core and the Peclet and Weber numbers have almost no influence on the outer shape of the hollow pileup. The effect of the number of incoming droplets on the pileup formation is also revealed.

Sequential Coalescence Enabled Two-Step Microreactions in Triple-Core Double-Emulsion Droplets Triggered by an Electric Field.

Advances in microfluidic emulsification have enabled the generation of exquisite multiple-core droplets, which are promising structures to accommodate microreactions. An essential requirement for conducting reactions is the sequential coalescence of the multiple cores encapsulated within these droplets, therefore, mixing the reagents together in a controlled sequence. Here, a microfluidic approach is reported for the conduction of two-step microreactions by electrically fusing three cores inside double-emulsion droplets. Using a microcapillary glass device, monodisperse water-in-oil-in-water droplets are fabricated with three compartmented reagents encapsulated inside. An AC electric field is then applied through a polydimethylsiloxane chip to trigger the sequential mixing of the reagents, where the precise sequence is guaranteed by the discrepancy of the volume or conductivity of the inner cores. A two-step reaction in each droplet is ensured by two times of core coalescence, which totally takes 20-40 s depending on varying conditions. The optimal parameters of the AC signal for the sequential fusion of the inner droplets are identified. Moreover, the capability of this technique is demonstrated by conducting an enzyme-catalyzed reaction used for glucose detection with the double-emulsion droplets. This technique should benefit a wide range of applications that require multistep reactions in micrometer scale.

Role of dislocation pile-ups in nucleation-controlled size-dependent strength of Fe nanowires

We studied the strength of Fe faceted nanowires under diametrical compression using molecular dynamics simulations. We show that a series of consecutive events of dislocation nucleations occurs at the vertices of the nanowire, and two edge dislocation pile-ups are formed before a catastrophic event occurs, previously named a cross-split of edge dislocations. The compressive stress at which dislocations are nucleated depends on the specimen's size and obeys a power-law, with an exponent that depends on the number of dislocations already nucleated into the pile-up. The role of the dislocation pile-up is discussed and, on the basis of classical nucleation theory, we examine the contribution of the dislocation pile-up to the nucleation conditions at the vertices of the nanowire. We found that three contributions need to be accounted for: the stress gradient at the vertices, the back stress from the nucleated pile-up, and the image stresses due to the confined volume of the nanowire. In addition, the proposed model agrees with the distribution of the dislocations inside the pile-up. We ratify that cross-splitting occurs at the head of the pile-up when the distance between the two leading dislocations is about twice their Burgers vectors. Considering this value as the limit for dislocation core coalescence, the results of our model and molecular dynamic simulations for the stresses needed for cross-splitting were in excellent agreement. Based on these results, we found the dependency of the critical stress needed for cross-splitting on the size. Finally, we extend the discussion to the general role of dislocation pile-ups in nucleation-controlled plasticity.

Continuously Electrotriggered Core Coalescence of Double-Emulsion Drops for Microreactions.

Microfluidically generated double emulsions are promising templates for microreactions, which protect the reaction from external disturbance and enable in vitro analyses with large-scale samples. Controlled combination of their inner droplets in a continuous manner is an essential requirement toward truly applications. Here, we first generate dual-cored double-emulsion drops with different inner encapsulants using a capillary microfluidic device; next, we transfer the emulsion drops into another electrode-integrated polydimethylsiloxane microfluidic device and utilize external AC electric field to continuously trigger the coalescence of inner cores inside these emulsion drops in continuous flow. Hundreds of thousands of monodisperse microreactions with nanoliter-scale reagents can be conducted using this approach. The performance of core coalescence is investigated as a function of flow rate, applied electrical signal, and core conductivity. The coalescence efficiency can reach up to 95%. We demonstrate the utility of this technology for accommodating microreactions by analyzing an enzyme catalyzed reaction and by fabricating cell-laden hydrogel particles. The presented method can be readily used for the controlled triggering of microreactions with high flexibility for a wide range of applications, especially for continuous chemical or cell assays.

Understanding the primary and secondary aggregation states of sputtered silver nanoparticles in thiolate matrix and their immobilization in resin

Abstract We report detailed aggregation mechanisms of sputtered silver nanoparticles in a viscous thiolate liquid matrix. Fluorescent silver nanoparticles were prepared by the sputtering deposition over pentaerythritol tetrakis(3-mercaptopropionate) (PEMP). The dependences of extinction, fluorescence and particle size on time were discussed in terms of two possible aggregation mechanisms. The main aggregation process was secondary aggregation, with primary aggregation (core coalescence) making a small contribution. The four thiol groups in the PEMP structure are the driving force determining the predominance of secondary aggregation. Finally, we successfully prepared a thiourethane resin by the polymerization of a thiolate matrix, and complete immobilization of nanoparticles in the resin resulted in no observable degradation of the photophysical properties. The demonstrations presented here will be beneficial in understanding the detailed aggregation mechanism and optical properties of nanoparticles.

Thermal annealing as an easy tool for the controlled arrangement of gold nanoparticles in block-copolymer thin films

Thermal annealing was used for the bottom-up fabrication of morphologically controlled gold–block-copolymer (Au–BC) nanocomposites. Three different blends formed by polystyrene (PS) homopolymer and PS-coated gold nanoparticles (PSSH@Au) were used as modifiers of asymmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA): PS26/PS26SH@Au, PS75/PS75SH@Au and PS167/PS167SH@Au (where the subscripts refer to the number of styrene monomeric units). The results indicated that all three blends used as modifiers (PSn/PSnSH@Au) were successfully located in the PS phase during thermally induced BC self-assembly for a composition range from 5 to 43 wt% without macro-phase separation. The PSnSH@Au moiety experienced molecular desorption, nanocrystal core coalescence and partial molecular re-encapsulation processes during thermal annealing, leading to sphere-like gold NPs with a larger average size (without exceeding an interdomain distance). Ligand chain length regulated the degree of coalescence and re-encapsulation, defining ultimate core size. Furthermore, proper combination of chain length and composition enabled tuning of NP partitioning and arrangement on different length scales through thermally activated cooperative assembly processes. These results have not only significant impact for establishing thermal processing as a useful tool for the precise control of NP size and distribution, but also much broader implications for many nanoparticle-based technologies.

A Simple Strategy for Fabrication of “Plum-Pudding” Type Pd@CeO2 Semiconductor Nanocomposite as a Visible-Light-Driven Photocatalyst for Selective Oxidation

The Pd@CeO2 semiconductor nanocomposite with “plum-pudding” structure has been fabricated successfully via a facile low-temperature hydrothermal reaction of polyvinylpyrrolidone (PVP)-capped Pd colloidal particles and cerium chloride precursor followed by a calcination process in air. Different characterization techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission scanning electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), UV–vis diffuse reflectance spectra (DRS), X-ray photoelectron spectra (XPS), photoluminescence spectra (PL), nitrogen adsorption–desorption, and electron spin resonance spectra (ESR), have been used to investigate the structure and properties of the Pd@CeO2 nanocomposite. It is found that the nanosized Pd particles are evenly dispersed into the matrix of CeO2, thus forming a plum-pudding structure, i.e., multi-Pd core@CeO2 shell configuration. This unique nanostructure endows the Pd@CeO2 nanocomposite with enhanced activity and selectivity toward the visible-light-driven oxidation of various benzylic alcohols to corresponding aldehydes using dioxygen as oxidant at room temperature and ambient pressure compared with a supported Pd/CeO2 nanocomposite and nanosized CeO2 powder. The formation of the multi-Pd core@CeO2 shell structure can be understood by a synergistic interaction of heterogeneous seeded growth process, monolayer-capped core coalescence, and shell re-encapsulation. Together with the previous report, it can be concluded that the intrinsic structure nature of noble metal colloids is able to play a key role in affecting the formation process of noble metal core@semiconductor shell nanocomposites, by which we can realize the design and preparation of different specific core–shell nanostructures with atomic scale accuracy. It is hoped that our current work could open promising prospects of the fabrication of multimetal core@semiconductor shell nanocomposites and their application to visible-light-driven selective organic transformations.

Core Mass Function: The Role of Gravity

We analyze the mass distribution of cores formed in an isothermal, magnetized, turbulent, and self-gravitating nearly critical molecular cloud model. Cores are identified at two density threshold levels. Our main results are that the presence of self-gravity modifies the slopes of the core mass function (CMF) at the high-mass end. At low thresholds, the slope is shallower than the one predicted by pure turbulent fragmentation. The shallowness of the slope is due to the effects of core coalescence and gas accretion. Most importantly, the slope of the CMF at the high-mass end steepens when cores are selected at higher density thresholds, or alternatively, if the CMF is fitted with a lognormal function, the width of the lognormal distribution decreases with increasing threshold. This is due to the fact that gravity plays a more important role in denser structures selected at higher density threshold and leads to the conclusion that the role of gravity is essential in generating a CMF that bears more resemblance to the IMF when cores are selected with an increasing density threshold in the observations.

The origin of the Arches stellar cluster mass function

Abstract We investigate the time-evolution of the mass distribution of pre-stellar cores (PSCs) and their transition to the initial stellar mass function (IMF) in the central parts of a molecular cloud (MC) under the assumption that the coalescence of cores is important. Our aim is to explain the observed shallow IMF in dense stellar clusters such as the Arches cluster. The initial distributions of PSCs at various distances from the MC centre are those of gravitationally unstable cores resulting from the gravo-turbulent fragmentation of the MC. As time evolves, there is a competition between the rates of coalescence and collapse of the PSCs. Whenever the local rate of collapse is larger than the rate of coalescence in a given mass bin, cores are collapsed into stars. With appropriate parameters, we find that the coalescence–collapse model reproduces very well all the observed characteristics of the Arches stellar cluster IMF: namely, the slopes at high- and low-mass ends and the peculiar bump observed at ∼ 5–6M⊙. Our results suggest that today's IMF of the Arches cluster is very similar to the primordial one and is little affected by the mass segregation due to dynamical effects.

Manipulating core–shell reactivities for processing nanoparticle sizes and shapes

This paper describes new findings of an investigation of thermally-activated core–shell reactivities of nanoparticles in solutions for processing size, shape and surface properties. Gold nanoparticles of ≈2 nm core sizes with thiolate monolayer encapsulation were chosen as a model system for the manipulation of reaction parameters, including annealing effect, core composition and shell structure. It is revealed that, upon an evolution of particle sizes, annealing treatment of the solution in the presence of encapsulating thiols can lead to the formation of highly monodispersed nanoparticles. New insights into shell desorption, core coalescence and shell re-encapsulation have been provided by dependencies of the evolution temperature on the capping thiolate chain length and the core alloy composition. Transmission electron microscopic, FTIR and UV-Visible techniques were used to characterize the morphological and chemical properties. The implication of the results for the development of abilities in chemical processing core–shell nanoparticles is discussed.