Interfacial Bubbles Research Articles

Photo‐Induced Electrical Gating with the Inserting of an Interfacial Carrier Blocking Layer for Organic/Graphene Phototransistors

AbstractAlthough ultrahigh photoconductive gain is achieved in organic semiconductor‐sensitized graphene phototransistors, the response speed is limited by the photo‐induced carrier injection at the organic/graphene interface. In this work, an insulating hexagonal boron nitride layer is inserted between the organic heterostructure and graphene to block the interfacial carrier transport. Above the h‐BN layer, the organic heterostructure is adopted as the light‐absorbing layer, and the photoresponse of graphene is realized through electrical gating of the atop h‐BN dielectric layer, by the accumulated carriers in the organic heterostructure. Under this “photo‐induced electrical gating” mechanism, the recombination of nonequilibrium carriers takes place only within the organic heterostructure. Furthermore, monolayer perylene‐3,4,9,10‐tetracarboxylic dianhydride (PTCDA) is deposited on graphene before transfer of the h‐BN, by which the interfacial encapsulated bubbles are effectively reduced. As a step further, the trap states are pre‐filled with background light illumination, then the carrier recombination follows the band‐to‐band pathway. Finally, the response time of the organic/h‐BN/graphene phototransistor is reduced to 7.15 ms, and a high photoresponsivity is achieved even though the carrier multiplication is sacrificed, which is reconciled by the improvement of the device quantum efficiency.

Controlled Preparation of High Quality Bubble-Free and Uniform Conducting Interfaces of Vertical van der Waals Heterostructures of Arrays.

Sharp and clean interfaces of van der Waals (vdW) heterostructures are highly demanded in two-dimensional (2D) materials-based devices. However, current assembly methods usually cause interfacial bubbles and wrinkles, hindering carrier interlayer transport. The preparation of a large-scale vdW heterostructure with a bubble-free interface is still a challenge. Although many efforts have been made to eliminate bubbles, the evolution processes of the interfacial bubbles are rarely studied. Here, the interface bubble formation and evolution of the transferred 2D materials and their vdW heterostructure are systemically studied by the atomic force microscopy (AFM) technique and high-resolution surface current mapping. A thermal annealing procedure is developed to reduce the number of bubbles and to improve the quality of interfaces. In addition, influences of the interface residues and nanosteps on bubble evolution are also discussed. Further, we develop the polystyrene (PS)-mediated polydimethylsiloxane (PDMS) transfer technique to realize the high-quality transfer of heterostructure arrays. Finally, high-resolution surface current mapping results confirm that we can now produce highly uniform electrical conduction interfaces of heterojunctions. This study provides guidance for assembling high quality interfaces and paves the way for production of bubble-free heterostructure-based electronic devices with high performance and good uniformity.

Experimental study on combustion characteristics of water-in-oil emulsified diesel pool fire with water sublayer

Study on the combustion of emulsified diesel facilitates the application of ISB method and enriches the theory of thermal engineering. The unsteady combustion behavior of emulsified diesel fuel with water sublayer is investigated using pool of 100,150 and 200 mm, oil thickness of 5, 7.5, 10, 12.5 and 15 mm, and water content of 0%, 5%, 10%, 15% and 20%. The boilover occurs once the temperatures of the fuel-water interface and the water in the fuel layer exceed the boiling temperature of water. An optimal water content to remove spilled emulsified diesel fuel floating on water is suggested. The liquid-phase convection heat transfer involving the boilover process of emulsified diesel fuel floating on water is analyzed. The nucleate boiling and interfacial layer bubbles in water promote the mixing of fluids, and the characteristic diameter of the bubble is attained according to the relative magnitude of liquid buoyancy (causing the bubble to detach) and surface tension (causing the bubble to attach to the surface). The maximal boilover intensity is quantitative calculate under various initial thicknesses and ratios of water content, thus the potential applications of emulsion boilover in the field of thermal engineering and fire prevention are proposed.

How the distribution and morphology of interfacial gas bubbles enhancing the boundary slip of fluid flow

A comprehensive investigation on how distribution and morphology of interfacial gas bubbles (IGBs) affecting the boundary slip of fluid flow was conducted by using finite element simulation. Results showed that the IGB dispersion, protrusion angle, coverage area, lateral size, height and quantity play crucial roles in determining the slip length. The slip length shows an decrease with the increase of dispersion index. Especially for IGB spacing distance smaller than 0.1 times IGB lateral size, the slip length shows an obvious increase. Moreover, a larger IGB lateral size and fewer IGB quantity are more effective for drag reduction (protrusion angle θ smaller than critical angle θc) or for drag increase (θ > θc) when IGB coverage area and protrusion angle are fixed.

Transmissive laser lock-in thermography for highly sensitive and online imaging of real interfacial bubbles in wafer bonding

Online and non-destructive detection of interfacial bubbles in wafer bonding is a critical approach to enhance the yield of advanced packaging. However, the commonly used scanning acoustic microscopy (SAM) is time-consuming and may cause additional risk due to soaking in water. The conventional thermography is suffering from low sensitivity and resolution. In this work, we demonstrate for the first time an effective method for highly sensitive and online imaging of real interfacial bubbles in wafer bonding using transmissive laser lock-in thermography (LLT). The transmissive LLT system projects a modulated laser beam onto target wafer surface and records the thermal image sequence on the other side. The bubbles distinctly emerge with clear edges in the extracted lock-in phase images. To enhance the detectivity, the relationship between phase image contrast and LLT parameters was explored. As a result, we can visualize and detect the bubbles larger than 500 μm at depth of 725 μm even under the interference of metal pattern with a speed of 130 mm2/10 s, which is 38% faster than that of SAM. Besides, the LLT phase images contain both bubbles and metal patterns information. The ratio between bubble depth and width reaches 1.45, which is about 2-fold of the latest infrared researches. Above all, this work paves the way for high-sensitive imaging of various tiny internal defects in advanced packaging using infrared thermography.

Fundamental study on the growth process of interfacial microbubbles in an air-supersaturated solution

Interfacial microbubbles widely exist and play important roles in nature and industries, while their growth process remained unknown in many aspects. In this study, the interfacial microbubbles are created in an air-supersaturated solution utilizing a sudden drop of ambient pressure, and the effects of different influential parameters on the growth process of interfacial microbubbles were studied by a self-made observation system. The investigations confirmed that the growth process of interfacial microbubbles under all conditions had experienced three stages, including the floating stage, transition stage and expansion stage. The experimental results also show that the magnitude of depressurization amplitude, surface wettability, surface roughness, dissolved air content, and solution surface tension have significant effects on the nucleation and growth of interfacial microbubbles. In addition, the modified diffusion theoretical model of dissolved air was successfully used to predict the changes in contact angle and bubble radius for interfacial bubbles during the growth process.

The research on the interfacial area transport equation in rectangular channels: Interfacial characteristics distribution and bubbles interaction mechanism

The interfacial area transport equation (IATE) is employed to describe the evolution of interfacial structure and evaluate the interfacial area concentration of two-phase flow. However, the interaction mechanisms among bubbles change in different rectangular channels, significantly impacting interfacial characteristics and source terms in IATE. In order to study the interface characteristics and mechanism of bubble interaction in rectangular channels with different gap sizes, an experimental study was conducted on the upward mixture flow of air and water in an adiabatic vertical channel under atmospheric pressure. Two channels with cross-sections of 4 × 66 mm and 8 × 66 mm were selected. The superficial velocities of water and air were 0.5–3 and 0–1.12 m/s, respectively, almost at the transitional region of bubbly to slug flow. Flat electrodes and the four-sensor probes measured the interfacial characteristics. This study analyzes the development of interface and mechanism of interaction among bubbles. The result shows that the evolution in the distribution of void fraction and interfacial area concentration significantly depends on the flow regime, flow rate, and cross-sectional geometric size of channels. Under the condition of a lower liquid flow rate, the gap size of the channel plays a vital role in the efficiency of bubble coalescence, influencing the mechanisms of the random collision and wake entrainment among bubbles. However, the interfacial parameter distribution under higher liquid rate conditions is dominated by turbulence intensity and flow regime, and the mechanisms of bubble interaction become identical. Finally, the mechanisms of interaction among bubbles (random collision and wake entrainment) are analyzed based on the development of interfacial characteristics. In a word, based on the conclusions specified in this research, the new models of source terms of ITAE in rectangular channels will be predictably established in the future.

Analysis on the Shear Behavior of Steel-Asphalt Interface Using Discrete Element Method on the Mesoscopic Scale

Shear failure is a major distress in steel bridge deck pavement (SBDP). To investigate the shear behavior of steel-asphalt interface (SAI), experimental study and numerical analysis were proposed and performed. First, the direct shear tests at two temperatures of 30°C and 60°C were carried out to provide verification for numerical analysis. Second, the virtual tests were conducted under the same condition as laboratory test using discrete element (DE) method to evaluate the shear behavior of SAI and analyze the influence of temperature on the development of interlaminar mesocracks. Third, factors affecting the interlaminar mesomechanical properties, including interface roughness and interfacial bubbles, were proposed and analyzed at 30°C. Results show that with the increase of shear displacement lots of interlaminar mesocracks occur rapidly with the decreasing of shear stress. The mesocracks occur more easily under high temperature. The shear strength of SAI increases at first and then decreases with the increase of interface roughness and decreases with the increase of interfacial bubbles.

Exceptional Mineral Scaling Resistance from the Surface Gas Layer: Impacts of Surface Wetting Properties and the Gas Layer Charging Mechanism.

Mineral scaling is a phenomenon that occurs on submerged surfaces in contact with saline solutions. In membrane desalination, heat exchangers, and marine structures, mineral scaling reduces process efficiency and eventually leads to process failure. Therefore, achieving long-term scaling resistance is beneficial to enhancing process performance and reducing operating and maintenance costs. While evidence shows that superhydrophobic surfaces may reduce mineral scaling kinetics, prolonged scaling resistance is limited due to the finite stability of the entrained gas layer present in a Cassie-Baxter wetting state. Additionally, superhydrophobic surfaces are not always feasible for all applications, but strategies for long-term scaling resistance with smooth or even hydrophilic surfaces are often overlooked. In this study, we elucidate the role of interfacial nanobubbles on the scaling kinetics of submerged surfaces of varied wetting properties, including those that do not entrain a gas layer. We show that both solution conditions and surface wetting properties that promote interfacial bubble formation enhances scaling resistance. In the absence of interfacial bubbles, scaling kinetics decrease as surface energy decreases, while the presence of bulk nanobubbles enhances the scaling resistance of the surface with any wetting property. The findings in this study allude to scaling mitigation strategies that are enabled by solution and surface properties that promote the formation and stability of interfacial gas layers and provide insights to surface and process design for greater scaling resistance.

Dynamics of Interfacial Bubble Controls Adhesion Mechanics in Van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (vdWH) can result in novel functionality that crucially depends on interfacial structure and disorder. Bubbles at the vdWH interface can modify the interfacial structure. We probe the dynamics of a bubble at the interface of a graphene-hBN vdWH by using it as the drumhead of a NEMS device because nanomechanical devices are exquisite sensors. For drums with different interfacial bubbles, we measure the evolution of the resonant frequency and spatial mode shape as a function of electrostatic pulling. We show that the hysteretic detachment of layers of vdWH is triggered by the growth of large bubbles. The bubble growth takes place due to the concentration of stress resembling the initiation of fracture. The small bubbles at the heterostructure interface do not result in delamination as they are smaller than a critical fracture length. We provide insight into frictional dynamics and interfacial fracture of vdWH.

Convolutional neural network model for synchrotron radiation imaging datasets to automatically detect interfacial microstructure: An in situ process monitoring tool during solar PV ribbon fabrication

Designing means and methods to detect the presence of interfacial bubbles and intermetallic compounds (IMCs) during hot dipping solder coating of Cu ribbon, can help in the production of defect-free PV ribbons. A mechanistic study of Cu6Sn5 IMC grain growth and bubble morphology evolution at the solder-substrate interface is performed with phase field simulation. A machine learning model is utilized to identify the occurrence of bubble(s) and IMC at the material interface of liquid solder and solid Cu. The datasets for the microstructural images consisting of bubble(s), IMC and planar solder/Cu interface are generated using in situ synchrotron radiation (SR) imaging experiment techniques. The integration of in situ SR radiography based non-destructive testing experiments with convolutional neural network model to intelligently detect the interfacial microstructures paves the path for potential industrial application of this technique in the smart manufacturing of defect free and reliable PV ribbon material.

Critical role of nanocomposites at air–water interface: From aqueous foams to foam-based lightweight functional materials

The study of aqueous foams has long been an interest of the scientific community. A developing area is the use of foams to prepare lightweight, functional solid foam materials. To design such porous materials, abundant foams should first be generated (good foamability), followed by foam destabilization processes being ceased or significantly retarded (excellent foam stability) to avoid the foam decay upon drying, which can be achieved by finely controlling interfacial behaviour of the foaming agents. The current review considers the interfacial behaviour and foaming potential of emerging nanocomposites such as: surfactant-nanoparticle composites; polymer-nanoparticle composites; and surfactant-graphene oxide (GO) composites. Compared with using only surfactants, polymers or nanoparticles, composite stabilizers are of great interest and importance since they enrich the interfacial behaviour of the bubble-stabilizing film (e.g. mechanical and structural properties), ensuring excellent foamability and long-term foam stability, and novel applications of the solid foam materials. Preparation mechanisms, properties and applications of solid foams prepared via direct foaming methods using the nanocomposite stabilizers are presented and highlighted. For the surfactant-NPs composites, the influences of the property of the foamed suspension and the bubble-stabilizing interface, on the quality of the wet and solid foams, is still elusive. As to the polymer-NPs composites, functional solid foams preparation using their stimuli-responsive performance remains an interesting topic for future research. The fundamental colloidal properties of surfactant-GO composites at the air–water interface and in stabilizing foams remain incompletely understood. Future studies are desired, combining experiments at various scales: interfacial films, Plateau borders, bulk liquid, bubbles, and foams, etc.

Jetting behavior as a bubble bursts in free space

The phenomenon of bubble bursting is very common in nature and is of prime importance in various technologies and industrial processes. Similar to interfacial bubbles, the process of a bubble bursting in free space, that is, the rupture of bubbles surrounded by air, often results in jet flows. However, the location and mechanism of the jet flows are different from those produced by interfacial bubbles. This paper describes the results of several experiments conducted to investigate the behavior of a bubble bursting in free space, especially the jet flows that occur at the end of the process. The results show that viscosity has a strong inhibitory effect on both the droplets (film drops and jet drops) and the jet resulting from bubble bursting. Based on experimental results, we establish a phase diagram for the jetting behavior in terms of the Reynolds number (Re) and the Ohnesorge number (Oh) and discover the existence of threshold conditions. Jetting occurs in the zone where Oh is less than some threshold value Ohc and Re is greater than some threshold value Rec, whereas a liquid clump appears in the zone where Oh > Ohc and Re < Rec. For 0.002 < Oh < 0.272 and 65 < Re < 52 633, we find that Rec = 503 ± 136 and Ohc = 0.079 ± 0.001. A schematic of the events that occur during bubble bursting depicting the forces at play is subsequently analyzed, and the role of viscosity at the moment of jetting is highlighted. The results of this study can be used to inhibit or increase the formation of droplets in numerous applications.

PArallel, Robust, Interface Simulator (PARIS)

Paris (PArallel, Robust, Interface Simulator) is a finite volume code for simulations of immiscible multifluid or multiphase flows. It is based on the “one-fluid” formulation of the Navier–Stokes equations where different fluids are treated as one material with variable properties, and surface tension is added as a singular interface force. The fluid equations are solved on a regular structured staggered grid using an explicit projection method with a first-order or second-order time integration scheme. The interface separating the different fluids is tracked by a Front-Tracking (FT) method, where the interface is represented by connected marker points, or by a Volume-of-Fluid (VOF) method, where the marker function is advected directly on the fixed grid. Paris is written in Fortran95/2002 and parallelized using MPI and domain decomposition. It is based on several earlier FT or VOF codes such as Ftc3D, Surfer or Gerris. These codes and similar ones, as well as Paris, have been used to simulate a wide range of multifluid and multiphase flows. Program summaryProgram Title: PArallel Robust Interface Simulator — ParisCPC Library link to program files:https://doi.org/10.17632/5cb2yrfx7r.1Licensing provisions: GPLv3.Programming language: Fortran95/2002. Parallelized using MPI and domain decomposition.Nature of problem:Paris is a free code, or software, for computational fluid dynamics (CFD) of multiphase flows (or computational multiphase fluid dynamics (CMFD)), specialized in the numerical simulation of interfacial fluid flows, involving droplets, bubbles and waves, as described in the book by Tryggvason, Scardovelli and Zaleski [1]. It solves the Euler or Navier–Stokes equations in the one-fluid formulation of two-phase flow, including a surface tension force. It computes complex flows such as fast atomizing jets or droplets, expanding cavitation bubble clusters and multiphase flow through porous media.Solution method: The code mostly implements the methods described in the book by Tryggvason, Scardovelli and Zaleski [1]. Time stepping is performed using a first-order or a second-order in time predictor–corrector method with an explicit projection step for the pressure. Spatial discretization is by finite volumes on a regular cuboid grid. Interface tracking is performed with the Front-Tracking (FT) method or the Volume-of-Fluid (VOF) method. In the VOF version Paris uses either the Lagrangian-Explicit (LE) advection method or the exactly mass-conserving method of Weymouth and Yue [2]. The normal computation is performed using the Mixed-Youngs-Centered (MYC) scheme. A mass–momentum advection method has been also implemented that is consistent with the VOF advection [3]. Curvature is computed with the Height Function (HF) method. This is combined with the balanced Continuous Surface Force (CSF) method to compute surface tension forces.If the dynamics of a phase can be neglected, Paris can also run as a free-surface code by specifying a homogeneous pressure, at most varying with time, in the neglected phase. In the case of atomizing jets, an algorithm has been implemented in Paris that can detect isolated droplets, advects them as Lagrangian point-particles and possibly merge them again with the main streamAdditional comments:Paris is extended from or inspired by the following codes: •Ftc3D: Front Tracking code for 3D simulations by Gretar Tryggvason and Sadegh Dabiri.•Surfer: VOF code for 3D simulations by Stephane Zaleski, Jie Li, Ruben Scardovelli and others.•Gerris: multiphase flow solver with Adaptive Mesh Refinement (AMR) by Stephane Popinet.

Effective Photoluminescence Imaging of Bubbles in hBN-Encapsulated WSe2 Monolayer.

Interfacial bubbles are unintentionally created during the transfer of atomically thin 2D layers, a required process in the fabrication of van der Waals heterostructures. By encapsulating a WSe2 monolayer in hBN, we study the differing photoluminescence (PL) properties of the structure resulting from bubble formation. Based on the differentiated absorption probabilities at the bubbles compared to the pristine areas, we demonstrate that the visibility of the bubbles in PL mapping is enhanced when the photoexcitation wavelength lies between the n = 1 and n = 2 resonances of the A-exciton. An appropriate choice of detection window, which includes localized exciton emission but excludes free exciton emission, further improves bubble imaging capability. The interfacial position dependence of the bubbles, whether they are located above or below the WSe2 monolayer, gives rise to measurable consequences in the PL shape.

Finite element analysis of effect of interfacial bubbles on performance of epoxy coatings under alternating hydrostatic pressure

The stresses around bubbles formed on a coating/substrate interface under hydrostatic pressure (HP) and alternating hydrostatic pressure (AHP) were calculated using the finite element method. The results reveal that HP promotes coating failure but does not mechanically destroy the interface, whereas AHP can provide tensile stress on bubbles formed at the interface and accelerate disbonding of the coating. Because of water resistance, a lag time exists for the coating that serves in an AHP environment. The coating can have a better protective performance if the lag time suits the AHP to minimize the impact of the AHP on the interface.

Impact of Heat Treatment on a Hetero-Stacked MoS2/${h}$ -BN Field-Effect Transistor

We investigated device properties before and after heat treatment for a hetero-stacked two-dimensional field-effect transistor (FET). In dual-gated monolayer MoS2 FET using a top h-BN layer and bottom SiO2 substrate, careful but harsh heat treatment is implemented in high vacuum at 200 °C for 18 h. Under the top-gate bias sweep, the field-effect mobility increases by ~9 times, and the channel carrier density doubles after the treatment. The heat treatment effect is more noticeable in the top-transferred h-BN than in the bottom SiO2 layer, because it leads to homogeneous adhesion between the layers by diminishing the adverse effects of interfacial bubbles or adsorbates. A top-gate dielectric capacitance for h-BN of 55 fF is increased to ~70 fF after the treatment, which is comparable to the theoretical value. This indicates that strong capacitive coupling for the top gate is formed, as confirmed by the capacitance-voltage measurement.

Pinpoint pick-up and bubble-free assembly of 2D materials using PDMS/PMMA polymers with lens shapes

The key to achieving high-quality van der Waals heterostructure devices made by stacking 2D layered materials lies in having a clean interface without interfacial bubbles and wrinkles. In this study, the pinpoint pick-up and transfer system of 2D crystals is constructed using polymers with lens shapes. We report the bubble-free and clean interface assembly of 2D crystals in which unidirectional sweep of the transfer interface precisely controlled with the help of the inclined substrate pushes the bubbles away from the interface.

Unexpected Homogeneous Bubble Nucleation near a Solid–Liquid Interface

We report a quasi-three-dimensional observation of electron-beam-induced nanobubbles inside a 1000 nm thick layer of water using the liquid cell electron microscopy. In the early stage of observation, heterogeneous bubble nucleation occurred, and small bubbles coalesced with the adjacent bubbles when they come in contact with each other. However, for the first time, we found that after prolonged electron beam irradiation heterogeneous nucleation did not occur more, and then homogeneous nucleation started even though a solid surface was available for heterogeneous nucleation. We conclude that the Ostwald ripening effect prevents heterogeneous nucleation from occurring and that the lower surface tension due to the generation of ions and radicals boosts the homogeneous nucleation. It was also discovered that the generation sites of homogeneous nucleation are beneath the three-phase contact lines of existing interfacial bubbles.

Minimizing residues and strain in 2D materials transferred from PDMS

Integrating layered two-dimensional (2D) materials into 3D heterostructures offers opportunities for novel material functionalities and applications in electronics and photonics. In order to build the highest quality heterostructures, it is crucial to preserve the cleanliness and morphology of 2D material surfaces that come in contact with polymers such as PDMS during transfer. Here we report that substantial residues and up to ∼0.22% compressive strain can be present in monolayer MoS2 transferred using PDMS. We show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation significantly reduces organic residues on transferred MoS2 flakes. An additional 200 ◦C vacuum anneal after transfer efficiently removes interfacial bubbles and wrinkles as well as accumulated strain, thereby restoring the surface morphology of transferred flakes to their native state. Our recipe is important for building clean heterostructures of 2D materials and increasing the reproducibility and reliability of devices based on them.