Pinch Off Research Articles

Dynamic swarms regulate the morphology and distribution of soft membrane domains.

We study the dynamic structure of lipid domain inclusions embedded within a phase-separated reconstituted lipid bilayer in contact with a swarming flow of gliding filamentous actin. Passive circular domains transition into highly deformed morphologies that continuously elongate, rotate, and pinch off into smaller fragments, leading to a dynamic steady state with ≈23× speedup in the relaxation of the intermediate scattering function compared with passive membrane domains driven by purely thermal forces. To corroborate experimental results, we develop a phase-field model of the lipid domains with two-way coupling to the Toner-Tu equations. We report phase domains that become entrained in the chaotic eddy patterns, with oscillating waves of domains that correlate with the dominant wavelengths of the Toner-Tu flow fields.

The transport mechanism and barrier height inhomogeneity in Ag-ZnSnN2 Schottky barrier solar cells

To address the difference between the barrier height and the open circuit voltage of ZnSnN2 Schottky barrier solar cells prepared with Ag as the contact metal by sputtering deposition, the temperature-dependent current (I)-voltage (V) curves at darkness are studied. The IV properties are found to be controlled by the Schottky junction with thermionic emission as the major transport mechanism. The Schottky barrier height is inhomogeneous and obeys a double Gaussian distribution in different temperature ranges, very possibly due to the spatial structural inhomogeneity of the Ag-ZnSnN2 interface. Barrier height inhomogeneity results in positive temperature coefficient while the temperature coefficient of the bandgap remains negative. The linear relation between barrier height and ideality factor suggests that the IV curves also agree with the pinch off model.

Auto-ejection of liquid from a nozzle.

Auto-ejection of liquid is an important process in engineering applications, and is also very complicated since it involves interface moving, deforming, and jet breaking up. In this work, a theoretical velocity of meniscus at nozzle exit is first derived, which can be used to analyze the critical condition for auto-ejection of liquid. Then a consistent and conservative axisymmetric lattice Boltzmann (LB) method is proposed to study the auto-ejection process of liquid jet from a nozzle. We test the LB model by conducting some simulations, and find that the numerical results agree well with the theoretical and experimental data. We further consider the effects of contraction ratio, length ratio, contact angle, and nozzle structure on the auto-ejection, and observe some distinct phenomena during the ejection process, including the deformation of meniscus, capillary necking, and droplet pinch off. Finally, the results reported in the present work may play an instructive role on the design of droplet ejectors and the understanding of jetting dynamics in microgravity environment.

Flame Dynamics Modelling Using Artificially Thickened Models

Thickened flame models are prolific in the literature and offer an effective method of resolving flame dynamics on coarse LES meshes. The current state of the art relies heavily on the use of efficiency functions to compensate for impaired wrinkling of the thickened flame. However in practice these functions can involve parameters that are difficult to determine, perform poorly outside of certain ranges or require a posteriori analysis to evaluate performance. An alternative based on a generalised thickening is evaluated across a range of canonical configurations. The approach is demonstrated to perform well across a large range of thickening factors in capturing phenomena such as localised quenching and pinch off as well as generation of flame surface. Including good performance even in the case of large flame dynamics under acoustic forcing where the model has a clear advantage over DNS in achieving grid independence. Finally the approach is unified into an Large Eddy Simulation/Adaptive Mesh Refinement framework and applied to a turbulent Bunsen flame. The results show that even if the internal flame structure is poorly resolved on the original mesh, the global system behaviour is well predicted and compares favourably with other approaches.

Circulation production model and unified formation number of compressible vortex rings generated by a shock tube

To investigate the formation number of compressible vortex rings (CVRs), a shock-tube apparatus with an open end is designed to generate CVRs and the flow structures are measured by using particle image velocity (PIV) and time-resolved schlieren techniques. A series of experiments were designed by varying the two governing factors: Mach numbers of the incident shock (Ms= 1.28, 1.48, and 1.59) and driven section length (DL = 100, 200, and 300 mm). By theoretically analyzing the shock diffraction problem, a slipstream model is proposed to predict the circulation generation of CVRs. Comparing with the PIV results, this model well predicts the circulation for Ms=1.28 but slightly underestimates the circulation for Ms= 1.48 and 1.59. Then, an alternative model based on the variation of Ms is proposed and well predicts the circulation generation. Based on the general definition of the vortex formation time and the circulation production model, we newly define the physical formation time of CVRs and then determine the formation number (denoted by F*) when CVRs pinch off. The formation number of CVRs (F*≈3.5) is found to coincide with the optimal vortex formation number originated from incompressible vortex rings (ICVRs). This consistency generalizes the principle of optimal vortex formation into compressible flows. However, both the PIV and schlieren results demonstrate that the CVRs for different Ms pinch off in different modes. With the aim of modulating F* of CVRs, a converging nozzle is designed, and we found that F*≈3.5 is remained for Ms = 1.28 but F* = 5.5 and 6.0 is obtained for Ms= 1.48 and 1.59. Furthermore, an extension of the Kelvin–Benjamin variational principle is explored to explain the unified formation number of CVRs and ICVRs.

Endpoint of the Gregory-Laflamme instability of black strings revisited

We reproduce and extend the previous studies of Lehner and Pretorius of the endpoint of the Gregory-Laflamme instability of black strings in five space-time dimensions. We consider unstable black strings of fixed thickness and different lengths, and in all cases we confirm that at the intermediate stages of the evolution the horizon can be interpreted as a quasistationary self-similar sequence of black strings connecting spherical black holes on different scales. However, we do not find any evidence for a global timescale relating subsequent generations. The endpoint of the instability is the pinch off of the horizon in finite asymptotic time, thus confirming the violation of the weak cosmic censorship conjecture around black string spacetimes.

Formation characteristics of Taylor bubbles in a T-junction microchannel with chemical absorption

This study focuses on the effect of chemical absorption on the formation dynamic characteristics and initial length of Taylor bubbles. The temporal evolutions of neck width and length of gaseous thread and initial length with and without chemical absorption were investigated with the Capillary number and Hatta number between 0.0010–0.0073 and 1.8–5.8 respectively. The squeezing regime with typical three stages, expansion, squeezing and pinch off is observed for both two processes. Compared with the non-absorption process, the increase of formation time in the chemical absorption process arises mainly from the expansion stage, and the decrease of initial length is from the necking stage. In addition, the temporal length evolution satisfies the power-law scale with the same exponent but a smaller pre-exponential factor. The correlations of neck width for stage transition and initial length with Hatta number demonstrate the enhancement effect of chemical absorption on bubble formation dynamics and initial length at relatively high chemical reaction rates and long formation time. This study provides insight into the bubble formation mechanism and helps to regulate the bubble initial size with chemical absorption.

Abnormal Two-Stage Degradation on P-Type Low-Temperature Polycrystalline-Silicon Thin-Film Transistor Under Hot Carrier Conditions

In this study, an abnormal two-stage degradation of low-temperature polycrystalline-silicon (LTPS) thin-film transistors (TFTs) on a polyimide flexible substrate after hot carrier stress was investigated. The degradation mechanism was divided into two stages. In the first stage, the increases in capacitance in the off region and transconductance are caused by impact ionization induced electron trapping into the gate insulator (GI) at the drain edge. Furthermore, the threshold voltage (<inline-formula> <tex-math notation="LaTeX">$\text{V}_{\text {th}}$ </tex-math></inline-formula>) shift in the positive direction is caused by electrons flowing back to the source side and trapping into the buffer that induces source barrier lowing. The second stage of degradation, including a <inline-formula> <tex-math notation="LaTeX">$\text{V}_{\text {th}}$ </tex-math></inline-formula> shift in the negative direction and a decrease in the transconductance is caused by Joule heating induced negative bias temperature instability (NBTI). Furthermore, NBTI hardly occurs behind the pinch off in the channel and fixed oxide charge does not compensate the trapped electron at drain side which is induced in the first stage.

Solidification of liquid metal droplet during impact in the presence of vertical magnetic field

We report a liquid metal droplet impacting onto a cold substrate under the influence of vertical magnetic field numerically. During the impacting dynamics, the spreading and the solidification of the droplet are seriously influenced by the magnetohydrodynamic effects. The numerical methodology is implemented by coupling the volume of fluid method and the implicit enthalpy approach, the former is used to track the liquid/solid–gas interface, while the latter is employed to simulate the solidification process. At first, the numerical method is validated against a series of benchmark problems. Then, by varying the impacting velocities, the thermal contact resistance and the magnetic strengths, the variations of the maximum spreading diameter against different dimensionless parameters are reported. An interpolation scheme between the impacting effect, the thermal effect, and the magnetohydrodynamic effect is proposed to predict the maximum spreading factor, and very good agreement is observed compared to our numerical results. After that, we identify different impacting behaviors in different parameter regimes. For non-isothermal cases, we find that the solidification makes the droplet transit from full rebound to adhesion on the cold substrate, and the participation of the magnetic field promotes the pinch off phenomena during the retraction of the liquid drop. Mechanisms for the transitions between different impacting regimes are discussed, and the comparisons with the available experimental results and analytical solutions are also delivered. At last, we identify that the thickness growth of the solidified splat can be predicted by solving the simple one-dimensional Stefan problem, implying that the thermal dynamics is dominating over the hydrodynamic or the magnetohydrodynamic effects during the melting process of the spreading droplet. Our work therefore provides a general framework to model and study more complex configurations, such as the droplet impacting problems in the metallurgical industry and Tokamak devices, in which environment the droplet dynamics significantly depend on the non-isothermal magnetohydrodynamic effects.

Evidence for violations of Weak Cosmic Censorship in black hole collisions in higher dimensions

We study collisions of boosted rotating black holes in D = 6 and 7 spacetime dimensions with a non-zero impact parameter. We find that there exists an open set of initial conditions such that the intermediate state of the collision is a black hole with a dumbbell-like horizon which is unstable to a local Gregory-Laflamme-type instability. We are able to provide convincing numerical evidence that the evolution of such an instability leads to a pinch off of the horizon in finite asymptotic time thus forming a naked singularity, as in similar unstable black holes. Since the black holes in the initial state are stable, this is the first genuinely generic evidence for the violation of the Weak Cosmic Censorship Conjecture in higher dimensional asymptotically flat spacetimes.

Synaptosomes and Metamodulation of Receptors.

Synaptosomes are re-sealed pinched off nerve terminals that maintain all the main structural and functional features of the original structures and that are appropriate to study presynaptic events. Because of the discovery of new structural and molecular events that dictate the efficiency of transmitter release and of its receptor-mediated control in the central nervous system, the interest in this tissue preparation is continuously renewing. Most of these events have been already discussed in previous reviews, but few of them were not and deserve some comments since they could suggest new functional and possibly therapeutic considerations. Among them, the "metamodulation" of receptors represents an emerging aspect that dramatically increased the complexity of the presynaptic compartment, adding new insights to the role of presynaptic receptors as modulators of chemical synapses. Deciphering the mechanism of presynaptic metamodulation would permit indirect approaches to control the activity of presynaptic release-regulating receptors that are currently orphans of direct ligands/modulators, paving the road for the proposal of new therapeutic approaches for central neurological diseases.

Analytical model for 2DEG charge density in β-(Al x Ga1−x )2O3/Ga2O3 HFET

In this paper, a physics-based analytical model for two-dimensional electron gas (2DEG) charge density (n s) and pinch-off voltage (V off) in -(Al x Ga1−x )2O3/Ga2O3 heterostructure field effect transistor (HFET) is reported. The modeling approach includes solving the Poisson–Schrödinger equation in -(Al x Ga1−x )O3 barrier layer followed by using standard Fermi–Dirac statistics equations for calculating 2DEG charge density in the potential well. The developed model is then used to analyze the effect of Al composition (x) and barrier thickness (d) of -(Al x Ga1−x )2O3 layer on 2DEG charge density. The obtained parameters from our model for charge density and pinch-off voltages are validated with the experimental results in the literature demonstrating a good accuracy. It was observed that the 2DEG charge density increases with Al composition and Al x Ga1−x O barrier layer thickness, but at the cost of increased pinch off voltage. A trade-off between both is necessary to achieve parametric optimization of -(Al x Ga1−x )2O3/Ga2O3 HFET. This model will be useful to study the various parametric constraints for device optimization in terms of 2DEG charge density and pinch off voltage.

Resistively Detected NMR Lineshapes in a Local Filling ν&lt;1 Quantum Hall Breakdown

Resistively‐detected nuclear magnetic resonance (RDNMR) is a unique characterization method enabling highly sensitive NMR detection for a single quantum nanostructure, such as a quantum point contact (QPC). In many studies, dynamic nuclear polarization and RDNMR detection are used in a quantum Hall breakdown regime of a local QPC filling factor of 1 (νqpc = 1). However, the RDNMR lineshapes are complicated and still not fully understood yet. Herein, the nuclear spins are systematically polarized by current pumping from the close vicinity of the νqpc = 1 conductance plateau all the way down to pinch‐off point, providing clear evidence that the spin‐flip scattering between two edge channels at the lowest Landau level still occurs in the constriction even when it is close to the pinch off point. The collected RDNMR spectra reveal two sets of distinguished features. First, in a strong to intermediate tunneling regime, we observe an ordinary resistance dip lineshape with snake‐like transition frequencies, indicative of spatial modulation of electron density in the QPC. Second, in a weak tunneling regime, the spectrum turns into a dispersive lineshape, which is interpreted due to the build‐up of two sets of nuclear spin polarization that are in contact with different electron spin polarization.

Vertical magnetic field aided droplet-impact- magnetohydrodynamics of ferrofluids

In this present article, we have investigated the magnetohydrodynamics of ferrofluid droplets impacting on different wettability surfaces, in the presence of a vertical magnetic field. The spreading dynamics was studied for a wide spectrum of magnetic Bond number (Bom), Hartmann number (Ha) and Weber number (We). In absence of any magnetic field, the droplets exhibited secondary droplet pinch-off during rebound. In the presence of magnetic field, the field modulated Rayleigh-Plateau instability delays the droplet pinch off as Bom increases. For a fixed We, the rebound of the droplet was suppressed on a superhydrophobic (SH) surface for varying the magnetic field strength (manifested through Bom). An analytical model based on the principle of conservation of energy was formulated to explain the magnetic field modulated droplet pinch-off. We have also investigated the influence of Bom on the temporal spreading dynamics of different Ha ferrofluid droplets impacting on hydrophilic and SH surfaces. With an increase in Bom, the magneto-visco-capillarity of high Ha droplets inhibits the capillary waves and motion of the contact line after impingement onto hydrophilic surface, compared to low Ha ferrofluid drops. Instead, the droplet rim fragments into secondary droplets during retraction event, leading to the onset of rim instability for low Ha ferrofluid drops with an increase in the magnetic field strength. We have also proposed a theoretical formulation based on energy conservation principle to predict the experimentally measured maximum spreading diameters under influence of magnetic field. The findings may hold significance in ferrofluid based droplet microfluidics systems with magnetic control or actuation.

Ectosome biogenesis and release processes observed by using live-cell dynamic imaging in mammalian glial cells.

Ectosomes are recognized as shedding from the plasma membranes into the extracellular environment. Recent research has demonstrated that ectosomes are surrounded by phospholipid membranes containing lipid rafts and caveolae. Some ectosomes contain cytokines in the lumen and have high levels of phosphatidylserine exposed to the outer membrane. Intracellular vesicles share both characters with ectosomes. Why the plasma membrane-derived ectosomes have the same characteristics as intracellular vesicles remain largely unknown. Using live-cell dynamic imaging, we recorded the process of ectosome biogenesis and release in primary cultured neural cells. Our results show two different ectosome release methods: slow-releasing and fast-releasing. In the slow-releasing, multiple ectosomes emerge almost simultaneously on the cell surface and are released by outward budding from the plasma membrane. In the fast releasing, ectosomes squeeze out of the membrane domain and pinch off from a cell's surface. Using ER-tracker for live-cell imaging, we directly observed the process that intracellular vesicles jump out of the plasma membrane for release. This type of ectosomes has a reverse array of membrane proteins and phospholipids compared to the plasma membrane. So ectosomes should be divided into two groups: plasma membrane-derived and intracellular membrane-derived ectosomes. Both slow releasing and fast releasing EVs imply mechanisms of human diseases and for diagnostics and drug delivery.

Chemoport Fracture due to Catheter Pinch Off Syndrome: A Rare Complication of Subclavian Vein Approach Revisited.

Chemoports are routinely used for administering chemotherapeutic agents, drugs, blood, and blood products. Chemoport insertion is associated with inherent complications. Fracture of chemoport due to pinch off syndrome is a rare life-threatening complication. We report our experience of fracture chemoport in patients with carcinoma breast and its management. We also present a detailed review of literature about this complication, clinical features, warning signs, diagnostic workup, management, and prevention. From a prospectively maintained database of chemoport insertion patients, a retrospective analysis was done from 2017 to 2020. During this period, the incidence of fracture chemoport was evaluated and their management. Out of 560 chemoport insertions, there were 3 patients with chemoport fracture, with an incidence of 0.5%. All the three patients were hemodynamically stable, with no clinical signs of pulmonary embolism. The chemoports were non-functional and on radiologic evaluation fracture of chemoport with embolization of distal segment was demonstrated. All the patients were managed by retrieval of the embolized catheters by a snare and removal of port chambers under local anesthesia. Choose internal jugular vein over subclavian vein for placing central venous access devices. When subclavian vein is chosen, point of entry should be lateral part of costochondral space. The incidence of chemoport fractures is 0.5% which present as non-functioning chemoports. Identify pinch off sign, especially with an upright check X-ray after chemoport placement. Consider repositioning of chemoport if pinch off sign is present.

Impact of surface viscosity on the stability of a droplet translating through a stagnant fluid

This study examines the impact of interfacial viscosity on the stability of an initially deformed droplet translating through an unbounded quiescent fluid. The boundary-integral formulation is employed to investigate the time evolution of a droplet in the Stokes flow limit. The droplet interface is modelled using the Boussinesq–Scriven constitutive relationship having surface shear viscosity $\eta _\mu$ and surface dilatational viscosity $\eta _\kappa$ . We observe that, below a critical value of the capillary number, $Ca_C$ , the initially perturbed droplet reverts to its spherical shape. Above $Ca_C$ , the translating droplet deforms continuously, growing a tail at the rear end for initial prolate perturbations and a cavity for initial oblate perturbations. We find that surface shear viscosity inhibits the tail/cavity growth at the droplet's rear end and increases the $Ca_C$ compared with a clean droplet. In contrast, surface dilatational viscosity increases tail/cavity growth and lowers $Ca_C$ compared with a clean droplet. Surprisingly, both shear and dilatational surface viscosity appear to delay the time at which pinch off occurs, and hence satellite droplets form. Lastly, we explore the combined influence of surface viscosity and surfactant transport on droplet stability by assuming a linear dependence of surface tension on surfactant concentration and exponential dependence of interfacial viscosities on the surface pressure. We find that pressure-thinning/thickening effects significantly affect the droplet dynamics for surface shear viscosity but play a small role for surface dilatational viscosity. We lastly provide phase diagrams for the critical capillary number for different values of the droplet's viscosity ratio and initial Taylor deformation parameter.

Between droplets and fluid thread—the role of gravity in meso-scale flow

How gravity affects immiscible liquid co-flow is best illustrated through experiments in inclined conduits. In the macro-domain, gravity leads to flow stratification while in the microscale, the phase distribution is practically insensitive to conduit tilt. The influence of flow orientation in the intermediate scale conventionally known as meso-domain or milli-channel, although noted, has not been discussed earlier. In the present study, flow morphology is experimentally investigated during up, down, and horizontal co-flow of a biphasic liquid mixture in a glass conduit of diameter 2.38 mm. In all orientations, the dispersed phase flows either as droplets/plugs or as a continuous thread. Gravity modulates the process of thread pinch off and regulates the domain of thread/droplet flow. Apart from flow orientation, we also note entry arrangement to influence droplet detachment in horizontal conduit. The experimental observations are explained from a simplified analysis based on momentum and energy considerations; the defining parameters are fluid properties and flow rates, conduit dimension, and flow orientation. The proposed analysis, albeit the approximations, has successfully predicted thread pinch off for the present experiments. Pinch off from the thread tip is noted to be cyclic and comprises several steps, of which inception of necking to its completion is only a part.

Tip Streaming of a Lipid-Stabilized Double Emulsion Generated in a Microfluidic Channel.

Water/oil/water (w/o/w) double emulsions (DEs) are multicompartment structures which can be used in many technological applications and in fundamental studies as models of cell like microreactors or templates for other materials. Herein, we study the flow dynamics of water/oil/water double emulsions generated in a microfluidic device and stabilized with the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). We show that by varying the concentration of lipids in the oil phase (chloroform) or by modulating the viscosity of the aqueous continuous phase, the double emulsions under flow exhibit a rich dynamic behavior. An initial deformation of the double emulsions is followed by tube extraction at the rear end, relative to the flow direction, resulting in pinch off at the tube extremity by which small aqueous compartments are released. These compartments are phospholipid vesicles as deduced from fluorescence experiments. The overall process can thus be of help to shed light on the mechanical aspects of phenomena such as the budding and fusion in cell membranes.

Fabrication and characterization of a nano-convex-embedded Si MOSFET for nano-scale electrical discrimination

To demonstrate electric discrimination of the nano-pattern for nano-artifact metrics, we fabricated and characterized a nano-convex-embedded Si MOSFET. The concept of electrical discrimination is to embed the nanostructure between the gate oxide and the Si channel of the MOSFET, and reflect the structure in the drain current. Spatial resolution in the channel direction is achieved by the drain voltage dependence of the channel pinch off position. The fabricated device with a nano-convex showed the increase of the on-resistance in the linear region and the increase of the drain conductance in the saturation region. These behaviors could be reproduced by the device simulation. The transfer characteristics in the subthreshold region showed the shift of the drain current curve to the positive voltage side by embedding a nano-convex. The overall behaviors were explained by the formation of a potential barrier in the channel under the nano-convex and its drain voltage dependence.