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Strain engineering is an alternative to chemical modification in manipulating functionalities of materials. Here, we report the lattice strain could relax the abrupt phase transition of (Ni1−xFex)1−δS and lead to an excellent negative thermal expansion (NTE) effect. The high-resolution scanning transmission electron microscope and extended x-ray absorption fine structure unambiguously demonstrate that parallel Ni3S4-type nanosheets could be introduced into the matrix lattice by increasing the δ value. By forming the Ni(Fe)-S-Ni(Fe) bonds, the (1–11) plane of nanosheets epitaxially grows on the (002) plane of the hexagonal matrix. Those bonds are strong enough to bear the large lattice mismatch along the interface as the phase transition occurs. As the concentration of nanosheets increases, the weak and inhomogeneous strain in matrix lattice becomes strong and uniform, expanding the NTE window in different ways. This result opens an unconventional avenue for designing NTE materials and extends the research scope of lattice strain as well. [ABSTRACT FROM AUTHOR]

The propagation characteristics of sub-terahertz (sub-THz) waves through the sharp-coned plasma sheath are investigated, revealing a frequency selection phenomenon. Two significant electron density gradients within the sharp-coned plasma sheath, which result in high reflection coefficients, are identified. These strong reflective interfaces divide the plasma into distinct regions, and the frequency selection mechanism is analyzed using the improved scattering matrix method. This research finds that the combination of these reflective interfaces and the intervening plasma forms a "resonator structure," leading to the observed frequency selection. A quantitative relationship between plasma parameters and the frequency selection phenomenon is analyzed. The results indicate that the reflection coefficients of the reflective interfaces increase, making the frequency selection more pronounced, when the thickness of the interfaces decreases or the peak electron density increases. In addition, a lower collision frequency leads to reduced absorption effects and a more pronounced frequency selection. The phenomenon suggests that enhancing transmissivity at lower frequencies may be feasible, providing a theoretical insight into the application of sub-THz waves in mitigating communication blackouts. [ABSTRACT FROM AUTHOR]

A small-angle neutron scattering (SANS) instrument at the China Spallation Neutron Source (CSNS) is an operating instrument for studying structures and inhomogeneities with dimensions ranging from 1 to 100 nm. Preparing multiple samples at once and measuring them sequentially is a common approach in SANS experiments to reduce neutron beamline wastes and increase experimental efficiency. We present the development of an automatic sample changer for the SANS instrument, including system design, thermal simulation, optimization analysis, structure design details, and temperature control test results. It features a two-row construction that can hold 18 samples on each row. The controllable temperature range is −30 to 300 °C. Furthermore, neutron scattering experiments on SANS at CSNS proved that this instrument has good temperature control performance and low background. This automatic sample changer is optimized for usage at SANS and will be offered to other researchers through the user program. [ABSTRACT FROM AUTHOR]

Polymers and surfactants are often employed simultaneously to control droplet dynamics with higher flexibility in many applications, such as droplet microfluidics and chemical enhanced oil recovery. However, the coupling effects of polymer-induced fluid viscoelasticity and surfactant have not been fully uncovered yet. To facilitate studies in this area, we present a systematic investigation on the transport of a surfactant-laden viscoelastic droplet through a confined microchannel by using our own three-dimensional front-tracking finite-difference methodology. Of particular interest is the droplet-induced additional pressure loss, which is important to deeply understand the flow rate–pressure loss relation of droplet-laden flows. We have found that either the fluid viscoelasticity or surfactant tends to enlarge the additional pressure loss, while their co-occurrence induces a further increase. Notably, negative synergistic effects are indicated between fluid viscoelasticity and surfactant; that is, their combined effect to increase the additional pressure loss is smaller than the sum of their individual effects. This synergistic effect primarily results from mutual inhibition of the viscoelastic stress and the surfactant-induced Marangoni stress to reduce the droplet surface mobility, no matter whether the surfactant is soluble or insoluble. Particularly, when the surfactant is soluble to the viscoelastic fluid phase, its transport and the consequent Marangoni stress is suppressed by the bulk viscoelastic stress via two mechanisms: the weakened surface convection by direct impact of the viscoelastic stress on the droplet surface mobility and the weakened bulk convection by the flow modification effect. [ABSTRACT FROM AUTHOR]

Nanoparticle surfactants, formed at liquid–liquid interfaces by the interactions between functional groups on nanoparticles and polymers having complementary end-functionality, have been recently proposed as an excellent interface stabilizer to cover liquid droplets for applications of substance encapsulation and delivery. However, the effects of nanoparticle surfactants on the production of liquid droplets in a microfluidic channel have not been comprehensively understood yet, which is a key prerequisite for achieving various functions in real applications. In this study, we have performed a systematic investigation on the effects of nanoparticle surfactants on droplet formation in a flow-focusing microchannel by using microfluidic experiments and theoretical analysis. We have found that simultaneously adding carboxylated nanoparticles into the dispersed phase and amino-terminated polymers into the continuous phases significantly decreases the droplet size but increases the production rate. More importantly, we have indicated that the combined effect of nanoparticles and polymers is much greater than the sum of their individual effects, which is mainly attributed to the significant reduction of the oil–water interfacial tension by the formation of nanoparticle surfactants. Besides, via analyzing the competition between hydrodynamic and interfacial forces acting on the droplet, we have established a theoretical criterion for the prediction of the droplet size with considering the effects of nanoparticle surfactants, which shows a good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Despite its significance in understanding behaviors of biological cells with nucleus or designing functions of complex artificial capsules in applications, the dynamics of elastic capsules enclosing complicated internal structures in flow is still largely unexplored. In this study, by using our own three-dimensional front-tracking finite-difference model, we present a numerical investigation into the dynamics of a compound capsule in a simple shear flow whose inner and outer membranes have the same prolate ellipsoidal shape at the rest state. Particular interest is focused on the unsynchronized motion of the inner and outer membranes. Regarding the dynamical regime, both the inner and outer capsules can undergo either synchronized or unsynchronized dynamical regimes (i.e., swinging or tumbling), which strongly depends on the inner-to-outer capillary number ratio Cain/Caout, the inner-to-outer volume ratio ϕ, and the prolate aspect ratio a/b. Particularly, via establishing a phase diagram based on a/b and ϕ at Cain/Caout = 1, we find that the inner and outer membranes can exist simultaneously in different dynamical regimes, even if they have the same deformability and the same shape. More importantly, if the detailed oscillation behavior is also concerned besides the capsule's dynamical regime, such as the transient shape and the oscillating period, unsynchronization is always obvious between the inner and outer capsules. Specifically, the inner capsule exhibits a slower oscillation than the outer capsule no matter if they lie in the swinging or tumbling regime. [ABSTRACT FROM AUTHOR]

Despite its significance in droplet-based microfluidic technologies with the use of thermal stimuli and surfactants, coupling effects of thermal- and surfactant-induced Marangoni stresses on the transport of droplets in microchannels are not fully uncovered yet. To facilitate studies in this area, we present a three-dimensional numerical study on the thermocapillary migration of an insoluble-surfactant-laden droplet under Poiseuille flow in a microchannel. This work is realized via our own front-tracking finite-difference method with further integration of the energy conservation equation and the surface surfactant transport equation. Our numerical results agree well with the previously reported analytical results for ambient conditions with negligible thermal convection. In this study, we mainly focus on the effects of the thermal convection at high thermal Peclet numbers and find that it induces a significant change in the thermal Marangoni stress. As a consequence, the migration of surfactant-laden droplets in the microchannel is significantly retarded by the thermal convection, which is observed for two different ambient conditions, i.e., the imposed temperature increasing or decreasing along the main flow direction. To understand the mechanism underlying the effects of the thermal convection, we analyze the distributions of the temperature, surfactant concentration, and the thermal- and surfactant-induced surface tension variations over the droplet surface. Notably, the surfactant-induced Marangoni stress always opposes the thermal-induced Marangoni stress for the entire range of thermal Peclet numbers considered in this study, but the competition between them is significantly alternated by the thermal convection in a quantitative manner. [ABSTRACT FROM AUTHOR]

Despite its significance in droplet microfluidics and enhanced oil recovery, the dynamics of single droplets in confined microchannels still remains an unsolved fundamental problem, especially when the droplet is covered with nanoparticles. In this study, we present a numerical investigation on the transport of a nanoparticle-covered droplet through a confined microchannel with a square cross section. This work is realized via developing a level-set-based computational methodology with the nanoparticle–fluid, nanoparticle–nanoparticle, and nanoparticle–interface interactions all taken into account. We find that the interfacial convection drags the nanoparticles to the droplet rear and induces their local jamming and assembly into closely packed structures. More importantly, the interfacial jamming of the nanoparticles leads to nearly complete immobilization of the droplet surface, where the surface velocity is decreased by about two orders of magnitude compared to the clean surface. As a final consequence, the transport of the whole droplet through the confined microchannel is significantly retarded; specifically, the droplet-induced extra pressure loss in the microchannel is remarkably enlarged. Although the nanoparticle-induced surface immobilization is similar to the results from the surfactant-induced Marangoni stress, the immobilization-induced reduction in the surface velocity for the nanoparticle-covered droplet is much more significant than the surfactant-laden droplet. As a result, the effects of interfacial jamming of nanoparticles on the droplet motion, e.g., the extra pressure loss in the microchannel, are much stronger than that of the surfactant-induced Marangoni stress. [ABSTRACT FROM AUTHOR]

The neutral gas temperature of fluorocarbon plasmas in a remote toroidal transformer-coupled source was measured to be greater than 5000 K, under the conditions of a power density greater than 15 W/cm3 and pressures above 2 torr. The rovibrational bands of C2 molecules (swan bands, d 3Πg→a 3Πu) were fitted to obtain the rotational temperature that was assumed to equal the translational temperature. This rotational-translational temperature equilibrium assumption was supported by the comparison with the rotational temperature of second positive system of added N2. For the same gas mixture, the neutral gas temperature is nearly a linear function of plasma power, since the conduction to chamber wall and convection are the major energy-loss processes, and they are both proportional to neutral gas temperature. The dependence of the neutral gas temperature on O2 flow rate and pressure can be well represented through the power dependence, under the condition of constant current operation. An Arrhenius type of dependence between the etching rate of oxide film and the neutral gas temperature is observed, maybe indicating the importance of the pyrolytic dissociation in the plasma formation process when the temperature is above 5000 K. [ABSTRACT FROM AUTHOR]

In recent years, microfluidic channels with narrow constrictions are extensively proposed as a new but excellent possibility for advanced delivery technologies based on either natural or artificial capsules. To better design and optimize these technologies, it is essential and helpful to fully understand the releasing behavior of the encapsulated solute from capsules under various flow conditions which, however, remains an unsolved fundamental problem due to its complexity. To facilitate studies in this area, we develop a numerical methodology for the simulation of solute release from an elastic capsule flowing through a microfluidic channel constriction, in which the tension-dependent permeability of the membrane is appropriately modeled. Using this model, we find that the release of the encapsulated solute during the capsule's passage through the constriction is enhanced with the increase in the capillary number and constriction length or the decrease in the constriction width. On the other hand, a large variation in the channel height, which is generally larger than the capsule diameter, generates little effect on the released amount of the solute. We reveal that the effects of the capillary number and constriction geometry on the solute release are generally attributed to their influence on the capsule deformation. Our numerical results provide a reasonable explanation for previous experimental observations on the effects of constriction geometry and flow rate on the delivery efficiency of cell-squeezing delivery systems. Therefore, we believe these new insights and our numerical methodology could be useful for the design and optimization of microfluidic devices for capsule-squeezing delivery technologies. [ABSTRACT FROM AUTHOR]

Surfactants are widely used in the manipulation of drop motion in microchannels, which is commonly involved in many applications, e.g., surfactant assisted oil recovery and droplet microfluidics. This study is dedicated to a crucial fundamental problem, i.e., the effects of a soluble surfactant on drop motion and their underlying mechanisms, which is an extension of our previous work of an insoluble-surfactant-covered droplet in a square microchannel [Z. Y. Luo, X. L. Shang, and B. F. Bai, "Marangoni effect on the motion of a droplet covered with insoluble surfactant in a square microchannel," Phys. Fluids 30, 077101 (2018)]. We make essential improvements to our own three-dimensional front-tracking finite-difference model, i.e., by further integrating the equation governing surfactant transport in the bulk fluid and surfactant mass exchange between the drop surface and bulk fluid. We find that the soluble surfactant generally enlarges the droplet-induced extra pressure loss compared to the clean droplet, and enhancing surfactant adsorption tends to intensify such an effect. We focus specifically on the influences of four soluble-surfactant-relevant dimensionless parameters, including the Biot number, the dimensionless adsorption depth, the Damkohler number, and the bulk Peclet number. Most importantly, we discuss the mechanisms underlying the soluble surfactant effect, which consists of two aspects similar to the insoluble case, i.e., the reduced surface tension to decrease droplet-induced extra pressure loss and the enlarged Marangoni stress playing the opposite role. Surprisingly, we find that the enlarged Marangoni stress always makes the predominant contribution over the reduced surface tension in the effects of above-mentioned four soluble-surfactant-relevant dimensionless parameters on drop motion. This finding explains why the droplet-induced extra pressure loss increases with the film thickness, which is opposite to that observed for clean droplets. [ABSTRACT FROM AUTHOR]

Despite its significance in various applications, e.g., droplet microfluidics and chemical enhanced oil recovery, the motion of surfactant-laden droplets in non-circular microchannels remains an unsolved fundamental problem. To facilitate studies in this area, we present a systematic investigation on the motion of a droplet covered with an insoluble surfactant in a square microchannel. This work is realized via our three-dimensional front-tracking finite-difference model with integration of the convection-diffusion equation for surfactant transport on a deforming drop surface. Our results indicate significant effects of the surfactant on steady-state characteristics of droplet motion, especially the droplet-induced additional pressure loss in the channel. More particularly, the surfactant-induced reduction in drop surface tension remarkably lowers the additional pressure loss, but this effect can be fully counteracted by the effect of surface tension gradient induced Marangoni stress (i.e., to enlarge the additional pressure loss). The increasing effect of the Marangoni stress is primarily determined by two surfactant-related dimensionless parameters, i.e., the surface Peclet number and the elasticity number. The additional pressure loss significantly increases with either of them increasing. Besides, the Marangoni effect on the droplet-induced additional pressure loss also strongly depends on three other independent parameters, i.e., it is inhibited by decreasing the size ratio of the drop to the channel, increasing the viscosity ratio of the drop to the surrounding fluid, or increasing the capillary number. Finally, we discuss the mechanism of the Marangoni effect on drop motion via analyzing the distributions of the surfactant concentration and drop surface velocity on a three-dimensional drop surface. [ABSTRACT FROM AUTHOR]

In this article the major kinetics models for plasma-surface interactions were reviewed highlighting their strengths and limitations. As a subset of reactive-site modeling, mixing-layer kinetics model was developed based upon the assumption of random atomic mixing in the top surface layer. The translation of the layer enabled the modeling of both etching and deposition. A statistical concept, nearest-neighbor bonding probability, was defined to express the concentration of any surface moieties with the surface elemental composition. A lumped set of reactions was adopted to carry on the overall physichemical processes including ion incorporation, neutral adsorption, physical sputtering, ion-enhanced etching, dangling bond generation and annihilation, and spontaneous etching. The rate coefficients were fitted to the experimental etching yields at various beam etching conditions. The good match between the kinetics modeling and the experimental results verified the capability of the mixing-layer model of predicting the poly-Si etching in chlorine plasma at various operating conditions. Then the kinetics model was incorporated into the three-dimensional Monte Carlo profile simulator. The concept of the mixing layer was simulated by a cellular-based model through composition averaging among neighboring cells. The reactions were sorted out in terms of ion initiated and neutral initiated, respectively, as discrete events. The reaction rates were calculated based upon the cellular composition and used as probabilities to remove particles from the cell. Results showed that the profile simulation combined with the kinetics, the numeric kinetics model, and the experimental etching yields are in quantitative agreement, which demonstrated the accuracy of kinetics after incorporation into the profile simulation. The simulation was compared to the published research work comprehensively including the etching yields, surface compositions, and dominant product distributions. [ABSTRACT FROM AUTHOR]

Silicon oxide etching was modeled using a translating mixed-layer model, a novel surface kinetic modeling technique, and the model showed good agreement with measured data. Carbon and fluorine were identified as the primary contributors to deposition and etching, respectively. Atomic fluorine flux is a major factor that determines the etching behavior. With a chemistry having a small amount of atomic fluorine (such as the C4F8 chemistry), etching yield shows stronger dependence on the composition change in the gas flux. [ABSTRACT FROM AUTHOR]

Detailed measurements of gold plasmas produced with 0.351 μm laser light are reported. The temporal and spatial variations of Z¯T[sub e] are obtained in the case that pump intensities are 6.5×10[sup 13] and 1.4×10[sup 14] W/cm[sup 2]. The experimental results are qualitatively consistent with two nonlocalized absorption models. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In the electron beam Penning trap, electron focusing at the trap center by means of injecting low-canonical-angular-momentum electrons has been identified. It is found that the focusing occurs instantaneously under the condition that the injection current is larger than a threshold and the effective electric potential well is spherical. A self-consistent model is proposed to describe the setup process of the dense core. According to the model the instantaneous core density is evaluated to be about one order greater than the corresponding Brillouin density. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]