Experimental Study of Bubble Growth and Departure at the Tip of Capillary Tubes with Various Wettabilities in a Stagnant Liquid

In the present paper, the bubble growth and departure at the tip of capillary tubes with different wall wettabilities in a stagnant fluid is experimentally investigated by using a high-speed visual system. The visual experiments show that the bubble growth experienced three typical stages: the initial growth, the speed-up growth, and the speed-down growth, with distinct varying behaviors of tip contact angle and size of bubble. The formation mechanism of each growth stage is discussed individually. It can be deduced from the experimental results that the bubble breakthrough point for a hydrophilic capillary tube is resulting from the triple contact line rapidly advancing from the inner wall slightly beneath tube tip to the inside surface edge of the tube tip when the contact angle of the bubble on the inner wall approaches the receding contact angle. The wall wettability has a significant effect on bubble growth and departure. The departure size and growth cycle period of bubble for a Teflon tube with hydrophobic wall is obviously smaller than these for glass tube with hydrophilic wall. Furthermore, the triple contact line of the bubble locates at the inside surface edge of the tube tip for glass tube, while one locates at the outside surface edge for Teflon tube before bubble departure. The liquid incursion into the tube tip for glass tube has never arisen for the Teflon tube after bubble departing from the tube tip.     

Experimental Studies of Liquid Droplet Coalescence on the Gradient Surface

Visual experiments were conducted to study the behaviors of liquid droplet coalescence on the surface with gradient surface energy. The microscopic contour of the gradient energy surface, which was fabricated on the base of a silicon chip by diffusion-controlled silanization of alkyltrichlorosilanes, was characterized by atom force microscopy. The effects of droplets on the three-phase contact line and the contact angle were obtained. It was observed that the process of droplet coalescence accelerates the droplet speed on the gradient surface. On the high-energy surface, the triple-phase contact line of the coalescing droplet is prone to pinning. The interfacial energy released from coalescence is the main source for promoting the motion. Furthermore, the nonequilibrium capillary pressure gradient inside coalescence droplets is considered to aid in accelerating the coalescence.     

Thermo-hydrodynamic transport phenomena in partially wetting liquid plugs moving inside micro-channels

Single-phase as well as two-phase fluid flows inside mini/micro-channels and capillary tubes are of practical importance in many miniaturized engineering systems. While several issues related to single-phase transport are fairly well understood, two-phase systems still pose challenges for engineering design. The presence of gas–liquid interfaces, dominance of surface forces, moving contact lines, wettability, dynamic contact angle hysteresis and flow in confined geometries are some of the unique features of two-phase systems, which manifest into complex transport phenomena. While Taylor plug/bubble flow is a fairly common flow pattern in several micro-fluidic devices operating at low Bond number, the ensuing transport characteristics are complex and still not fully discerned. This review paper aims at highlighting the nuances and features of a unit cell of a Taylor plug flow, especially focusing on partially wetting systems, which are more common in engineering applications. Emphasis is given to a ‘unit cell’ flow system consisting of an isolated liquid Taylor plug with adjacent gas phase, confined in a capillary tube. Such a seemingly simple flow condition poses considerable challenges for discerning and modelling local thermo-hydrodynamic transport coefficients. Relevant background information and fundamentals are carefully scrutinized while summarizing the state-of-the-art. The role of wettability and dissipation near the contact line is highlighted via available experimental and simulation results. Local momentum and heat transfer exchange processes during the motion of an isolated plug of partially wetting liquid moving inside a capillary tube are delineated.     

Electric line source illumination of a chiral cylinder placed in another chiral background medium

An electric line source illumination of a chiral cylinder embedded in a chiral background medium is considered. The field expressions inside and outside of a chiral cylinder have been derived using the wave field decomposition approach. The effects of various chiral cylinders, chiral background media and source locations upon the scattering gain pattern have been investigated. It is observed that the chiral background reduces the backward scattering gain as compared to the free space background for a dielectric cylinder. It is also studied that by moving a line source away from a cylinder reduces the backward scattering gain for a chiral cylinder placed in a chiral background under some specific conditions. A unique phenomenon of reduced scattering gain has been observed at a specific observation angle for a chiral cylinder placed in a chiral background having an electric line source location of unity free space wavelength. An isotropic scattering gain pattern is observed for a chiral nihility background provided that if cylinder is chiral or chiral nihility type. It is also observed that this isotropic behaviour is independent of background and cylinder chirality.     

Experimental study on a simple Ranque-Hilsch vortex tube

The Ranque-Hilsch vortex tube is a device by which cold gas can be generated using compressed gas. To understand the cooling mechanism of this device, it is necessary to know the pressure, temperature, and velocity distributions inside the tube. In order to investigate this, a simple vortex tube is built and nitrogen is used as its working fluid. A special Pitot tube is used for the measurement of the pressure and velocity. This Pitot tube consists of a capillary which has only one hole in the cylinder wall. With this Pitot tube, the pressure and velocity fields inside the tube were measured. In the same way, the temperature field was measured with a thermocouple. The results of three different entrance conditions are compared here. With the measurements results, the analysis based on the two thermodynamic laws has been made. It is found that rounding off the entrance has influence on the performance of the vortex tube. The secondary circulation gas flow inside the vortex tube can be enhanced and enlarged, the performance of the Ranque-Hilsch vortex tube improved.     

Determination of surface heterogeneity by contact angle measurements on glassfibres coated with different sizings

This study has been achieved on industrial products (glassfibres coated with different sizings). Contact angle measurements and contact angle hysteresis were obtained with three liquids (glycerol, tricresylphosphate and mineral oil). Fibres are perfectly wetted with tricresylphosphate and mineral oil contrary to glycerol. For this last liquid a large distribution of contact angle and contact angle hysteresis were observed. Three parameters were used to analyse the results: places where filaments were extracted in the roving (inside or at the periphery), roughness and heterogeneity of the filaments surfaces (chemical heterogeneity). Statistical measurements of contact angles have shown heterogeneity in the roving. Atomic force microscopy (AFM) measurements have shown that roughness has only a weak contribution to the large distribution observed on the wetting results with glycerol. This large distribution and the contact angle hysteresis were due to the chemical heterogeneity of the fibres' surfaces.     

Visual Experiments of Bubble Growth and Departure at the Tip of Inclined Capillary Tubes in a Stagnant Liquid

The bubble growth and departure at the tip of inclined glass capillary tubes in stagnant fluid is experimentally investigated by using a high-speed visual system. The visual experiments show that the bubble growth experiences a sphere-like growth stage and the asymmetric growth stage at the tip of an inclined capillary tube. In the asymmetric growth stage, the bubble firstly gets an upward deflection under action of buoyancy force and then the triple contact line of the bubble detaches from the downside of the tube tip and withdraws into the tube. In the period of bubble departure, the neck of the bubble shrinks towards the upside of the tube tip and finally breaks up at the far upper portion of inner surface edge. The incursion of liquid into the tube tip has been detected after the bubble departing from tube tip. It is found that the departure size and the growth cycle period of the bubble as well as the volume of liquid incursion into the tube are decreased with an increase in the inclined angle of the tube. A smaller inner diameter of the inclined capillary tube leads to a smaller bubble departure size and a shorter cycle period of the bubble growth.     

Capillary Stability in a Tilted Circular Cylinder

The classical capillary stability problem in a vertical circular cylinder is a special case of the more general problem of the stability of liquid above a capillary surface in a circular cylinder with arbitrary orientation of gravity. This problem can, of course, also be viewed as arbitrary cylinder orientation in a steadily accelerating spacecraft. The general (tilted) circular cylinder capillary stability problem is solved numerically by use of the Surface Evolver code for general tilt and general contact angle. Tens of thousands of combinations of contact angle, tilt angle, and Bond number are solved for with a global volunteer computing network running Surface Evolver. The results appear to be symmetric about 90 degree contact angle, as in the previous vertical cylinder studies, and not symmetric about 45 degree tilt.     

FLOW PATTERNS THROUGH TUBE BUNDLES AND PARTICULATE FOULING ONTO CYLINDERS

The shear-stress distributions around cylinders in tube bundles were measured by means of the limiting current method and the flow inside the bundle was also visualized. In the case of the staggered bundle, the shear-stress distributions had two peaks by the effect of the jet streams generated at the front row spacing and the adjacent cylinder. While in the bundle with in-line arrangement, the front stagnant point was shifted about 30° downward and the traverse streams among the rows were observed. This traverse streams caused asymmetry of the shear-stress distributions with respect to the main flow axis.

These results were discussed in connection with the fouling experiments investigated in dusty air. Particle deposition around cylinders could be qualitatively predicted from the shear-stress distributions, namely particles tend to deposit at positions where are less shear-stress in both tube arrangements. The size of particle deposited on the wake zone was coarser than that near the stagnant point behind the third row in both arrangements.     

Drop Tower Experiments on Non-isothermal Reorientation of Cryogenic Liquids

Capillary driven surface oscillations of liquid argon (T_sat = 87.3 K at 1,013 hPa) have been investigated in a partly filled right circular cylinder under non-isothermal boundary conditions. The oscillations take place during the reorientation from the normal gravity surface position towards a new position upon step reduction of gravity. The situation is similar to the end of thrust in a rocket tank when the cold propellant moves along the warmer tank wall driven by capillary forces. The aim was to investigate the influence of the temperature difference between the slightly subcooled cryogenic liquid and the superheated cylinder wall on the oscillations and their characteristics in a single-component, two-phase system. Axial wall temperature gradients of averaged 0.15 K/mm ? 1.93 K/mm above the normal gravity surface position were implemented. A general dependence of the reorientation behavior on the gradient value was observed, concerning the apparent contact line behavior, the frequency and damping of the oscillations of the free surface center point, and the apparent contact angle. The behavior of the ullage pressure was found to follow the behavior of the contact line.     

Surface adsorption and wetting properties of amorphous diamond-like carbon thin films for biomedical applications

Diamond-like carbon (DLC) thin films have unique properties for biological and medical applications due to their excellent bio-compatibility, chemical inertness, and superior mechanical properties. It is important to understand the surface properties of DLC thin films for these applications. In this work, we showed that after DLC deposition, NiTi surfaces became much smoother by choosing suitable deposition conditions. Adsorption and wetting properties of DLC films were studied. The adsorption properties of DLC films were unusual in that a hysteresis was found in the adsorption/desorption isotherms, which cannot be interpreted using the conventional theory of capillary condensation in pores. The model proposed in this work for this unusual hysteresis characteristic is that the hysteresis results from the non-wetting property of DLC surfaces in the nano-scale. The nano-sized droplets formed on the DLC surfaces may require significantly higher energy to evaporate than the formation energy. Argon plasma treatment resulted in a small decrease of the contact angles. After oxygen plasma treatment, the wetting contact angles reduced significantly due to the increase of carbon-oxygen sites on the surfaces, suggesting that the low concentration of carbon-oxygen sites on the surfaces of DLC films contributed to the adsorption hysteresis observed in this work.     

风偏角对方形断面结构驰振不稳定性影响

方形断面的细长结构广泛的应用在工程领域,其在特定风向角和特定的风速范围下会发生具有较大危害性的驰振,目前斜置或者斜风向作用下的细长方柱驰振不稳定性的规律尚不清楚.采用刚性模型测压风洞试验,通过对斜置方柱气动力的分析,讨论了风偏角对方柱气动力的影响,基于准定常假设,分析了考虑风偏角的方柱驰振不稳定性.结果表明:方形断面是...     

1.4 Fast volume determination using a differential capacitance manometer

The reliable pressure readings with a capacitance manometer are the basis for a new type of instrument designed for accurate and rapid measurements of capillary volumes. The basic measuring principle is based on expansion of air atmospheric pressure enclosed in a known volume into a pre-evacuated cylinder so that the enclosed volume will cause a reduced pressure in the cyllinder. The same is done with the air in an unknown volume into another identical pre-evacuated cylinder. A differential capacitance manometer is then used to measure the difference in pressure between the two cylinders with a high degree of accuracy. This difference in pressure is shown to be ainear function of the difference in volume between the unknown and the known volume. The accuracy and repeatability of the instrument is better than 0.5%.     

Formation and microstructural characterization of coaxial carbon cylinders consisting of aligned carbon nanotube stacks

The macroscopic coaxial carbon cylinders (dia. approximately 0.5 cm with varying lengths approximately 2-5 cm) consisting of aligned carbon nanotube (CNT) stacks have been prepared by spray pyrolysis of benzene-ferrocene solution in argon atmosphere at approximately 850 degrees C-900 degrees C temperature. The coaxial carbon cylinders of CNT stacks have been formed directly inside the quartz tube. We attempted to prepare superimposed multi carbon cylinder configurations, each consisting of ordered and aligned CNTs stacked over each other. For this, we have terminated the spray of precursor after run of about 25 minutes, for a short interval (approximately 5 min), and then the solution was sprayed again over the already deposited hot CNT stack. Gross structural characterization of CNTs was done through X-ray diffraction technique (XRD). Microstructural characterizations of as prepared coaxial carbon cylinders with CNT stacks were done by scanning electron microscopic (SEM) techniques. SEM studies show that the CNTs are well aligned along the periphery of the cross section of coaxial carbon cylinder, each consisting of CNTs of the type described in the above. Comparisons have been made between the present macroscopic coaxial carbon cylinders with CNT stacks studied earlier by several other workers. Plausible explanation for the synthesis of CNT stacks will be put forward.     

FLOW PATTERNS THROUGH TUBE BUNDLES AND PARTICULATE FOULING ONTO CYLINDERS

ABSTRACT

The shear-stress distributions around cylinders in tube bundles were measured by means of the limiting current method and the flow inside the bundle was also visualized. In the case of the staggered bundle, the shear-stress distributions had two peaks by the effect of the jet streams generated at the front row spacing and the adjacent cylinder. While in the bundle with in-line arrangement, the front stagnant point was shifted about 30° downward and the traverse streams among the rows were observed. This traverse streams caused asymmetry of the shear-stress distributions with respect to the main flow axis.

These results were discussed in connection with the fouling experiments investigated in dusty air. Particle deposition around cylinders could be qualitatively predicted from the shear-stress distributions, namely particles tend to deposit at positions where are less shear-stress in both tube arrangements. The size of particle deposited on the wake zone was coarser than that near the stagnant point behind the third row in both arrangements.     

Stokes flow past a swarm of porous circular cylinders with Happel and Kuwabara boundary conditions

The problem of creeping flow past a swarm of porous circular cylinders with Happel and Kuwabara boundary conditions is investigated. The Brinkman equation for the flow inside the porous cylinder and the Stokes equation outside the porous cylinder in their stream function formulations are used. The force experienced by each porous circular cylinder in a cell is evaluated. Explicit expressions of stream functions are obtained for both the inside and outside flow fields. The earlier results reported by Happel and Kuwabara for flow past a solid cylinder in Happel’s and Kuwabara’s cell model, have been deduced. Analytical expressions for the velocity components, pressure, vorticity and stresstensor are also obtained     

Lattice Boltzmann simulations of contact line motion in a liquid-gas system

We use a lattice Boltzmann algorithm for liquid-gas coexistence to investigate the steady-state interface profile of a droplet held between two shearing walls. The algorithm solves the hydrodynamic equations of motion for the system. Partial wetting at the walls is implemented to agree with Cahn theory. This allows us to investigate the processes which lead to the motion of the three-phase contact line. We confirm that the profiles are a function of the capillary number and a finite-size analysis shows the emergence of a dynamic contact angle, which can be defined in a region where the interfacial curvature tends to zero.     

毛细管气体流量特性研究

通过实验研究,得出了气体在毛细管中流动时的沿程能量损失规律,经过数据处理,拟合出了毛细管的气体流量随毛细管内径、长度的变化曲线及其变化率曲线,得出了其韦泊分布函数。拟合出了毛细管的气体流量随压力差变化的曲线及其直线函数。为制冷系统的设计匹配以及毛细管批量生产的质量检验提供了一些非常有益的参考。     

Evaporation-induced evolution of the capillary force between two grains

The evolution of capillary forces during evaporation and the corresponding changes in the geometrical characteristics of liquid (water) bridges between two glass spheres with constant separation are examined experimentally. For comparison, the liquid bridges were also tested for mechanical extension (at constant volume). The obtained results reveal substantial differences between the evolution of capillary force due to evaporation and the evolution due to extension of the liquid bridges. During both evaporation and extension, the change of interparticle capillary forces consists in a force decrease to zero either gradually or via rupture of the bridge. At small separations between the grains (short & wide bridges) during evaporation and at large volumes during extension, there is a slight initial increase of force. During evaporation, the capillary force decreases slowly at the beginning of the process and quickly at the end of the process; during extension, the capillary force decreases quickly at the beginning and slowly at the end of the process. Rupture during evaporation of the bridges occurs most abruptly for bridges with wider separations (tall and thin), sometimes occurring after only 25 % of the water volume was evaporated. The evolution (pinning/depinning) of two geometrical characteristics of the bridge, the diameter of the three-phase contact line and the “apparent” contact angle at the solid/liquid/gas interface, seem to control the capillary force evolution. The findings are of relevance to the mechanics of unsaturated granular media in the final phase of drying.     

长管拖车气瓶鼓包失效分析

采用化学成分分析、室温拉伸试验、低温冲击试验、布氏硬度试验以及金相检验等方法分析了某长管拖车气瓶产生鼓包的原因。结果表明:气瓶鼓包是由于热处理不当,导致显微组织异常,出现较多铁素体,降低了气瓶的强度和硬度;当气瓶局部工作压力超过其屈服强度时,便会产生鼓包现象。