Wettability and Applications of Nanochannels

Wettability in nanochannels is of great importance for understanding many challenging problems in interface chemistry and fluid mechanics, and presents versatile applications including mass transport, catalysis, chemical reaction, nanofabrication, batteries, and separation. Recently, both molecular dynamic simulations and experimental measurements have been employed to study wettability in nanochannels. Here, wettability in three types of nanochannels comprising 1D nanochannels, 2D nanochannels, and 3D nanochannels is summarized both theoretically and experimentally. The proposed concept of “quantum‐confined superfluid” for ultrafast mass transport in nanochannels is first introduced, and the mostly studied 1D nanochannels are reviewed from molecular simulation to water wettability, followed by reversible switching of water wettability via external stimuli (temperature and voltage). Liquid transport and two confinement strategies in nanochannels of melt wetting and liquid wetting are also included. Then, molecular simulation, water wettability, liquid transport, and confinement in nanochannels are introduced for 2D nanochannels and 3D nanochannels, respectively. Based on the wettability in nanochannels, broad applications of various nanochannels are presented. Finally, the perspective for future challenges in the wettability and applications of nanochannels is discussed.     

Two-Phase Flow Analyses During Throttling Processes

This study presents an experimental investigation of the throttling process of saturated fluorocarbon refrigerants (such as R116, R218, and R610) inside capillary tubes. For the detailed two-phase flow analyses, refrigerant R218 was selected. A divided capillary tube was prepared with a set of precise pressure and temperature sensors providing detailed information about the refrigerant flow behavior inside the tube. The metastable flow regions of the superheated liquid and of the two-phase vapor-liquid mixture were clearly detected. A correlation for the ‘underpressure’ of vaporization applicable for capillary flow of fluorocarbon refrigerants was determined. New experimental data were compared with a modified numerical model simulating all four capillary flow regions. A negative effect of non-condensing gases present within the cooling circuit on the overall capillary tube performance was experimentally noted.     

液氢加注系统竖直管道内Taylor气泡的行为特性

针对液氢加注系统竖直管道内气液两相流实验化困难的问题,运用建模仿真的方法建立了竖直管道内Taylor气泡的运动模型,对Taylor气泡的形成过程、大小以及充分发展的Taylor气泡上升速度进行了研究.采用VOF方法对气液两相的交界面进行追踪,并引入CSF模型对两相间的表面张力进行计算.仿真结果表明:Taylor气泡是由...     

Visualization of refrigerant flow at the capillary tube inlet of a high-efficiency household refrigerator

The subcooled condition at the condenser outlet ensures complete condensation, which is necessary in vapor compression systems to increase the cooling capacity and ensure the liquid conditions at the expansion device inlet. However, in household refrigerators, recent works indicate the presence of two-phase flow at the capillary tube inlet. These systems behave quite differently from other refrigeration systems due to the extremely low capacity. In the present work, a test bench was built to visualize the refrigerant flow at the condenser outlet and at the capillary tube inlet of a commercial household refrigerator. A transparent tube replaced the end of the condenser and three transparent filters were installed with different orientations. Different positions of the capillary tube within the filters were also tested. Despite measuring a certain subcooling, all the reported visualizations showed that the capillary tube was steadily drawing in two-phase flow.     

脉动热管的力学分析

脉动热管是涉及多物理学科、多参数的气液两相流系统,研究和分析微小尺度下脉动热管内部系统的受力是进行脉动热管理论建模和机理分析的基础。作用在脉动热管内液塞上的力有液塞两侧气泡的压力、壁面与液塞间的摩擦剪切力、液塞两端的毛细作用力和液塞重力。以无水乙醇为例计算了各个作用力的大小,发现液塞两侧气泡的压力和液塞两端的毛细作用力是影响液塞脉动的主要因素。     

玻纤表面动态润湿行为

提出了采用线性回归处理分析玻纤与浸润液体动态润湿的新方法,结合高精度电子天平,表征了玻纤表面动态润湿性能。研究结果表明：在玻纤表面动态润湿过程中,随着润湿速度的增加,动态接触角有增大的趋势,玻纤与去离子水、乙二醇、760E环氧、CYD128环氧的接触角分别由66.04°、42.21°、51.31°、73.90°增加到69.05°、46.95°、74.58°、170.06°,玻纤表面可润湿性能下降。玻纤表面动态润湿过程中,黏度越大,随着润湿速度增加,可润湿性能下降越快,即玻纤与CYD128环氧体系的接触角下降96.16°,而与760E环氧树脂和乙二醇的接触角下降分别为23.27°和4.74°。基于新方法的玻纤表面动态润湿系统中,玻纤所受作用力随三相接触线移动速率和浸润液体黏度的增加而增大。     

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.     

Distributed and non-steady-state modelling of an air cooler

The refrigerant flow inside the coils of a dry expansion plate-finned air cooler can be distinguished into two completely different types: two-phase flow and single-phase flow. The most difficult part of non-steady-state modelling of an air cooler is to describe the liquid and vapour mass transport phenomena occurring in the two-phase flow region, as this determines the boundary position between the two regions and then the superheat temperature, which is in turn the feedback signal of the thermostatic expansion valve. In fact, the mass transport is mainly governed by the momentum exchange between refrigerant liquid and vapour, which is usually called slip-effect. Because the momentum or force equilibrium is so fast compared to the thermal equilibrium, the slip-effect can be considered as a steady-state phenomenon. With this assumption, the mass transport in an air cooler can be described by using a simple propagation equation. The steady-state slip-effect, however, is found by solving the momentum equations for one-dimensional two-phase flow using advanced computer packages such as . This paper presents the derivation of the equations in non-steady-state modelling of an air cooler as well as the results obtained from the model. Because the model is purely distributed, it is applicable to various kinds of tube circuit arrangements of air coolers. The purpose of the model is studying and optimization of non-steady-state behaviour of refrigerating systems with capacity control.     

A Novel Device Addressing Design Challenges for Passive Fluid Phase Separations Aboard Spacecraft

Capillary solutions have long existed for the control of liquid inventories in spacecraft fluid systems such as liquid propellants, cryogens and thermal fluids for temperature control. Such large length scale, ‘low-gravity,’ capillary systems exploit container geometry and fluid properties—primarily wetting—to passively locate or transport fluids to desired positions for a variety of purposes. Such methods have only been confidently established if the wetting conditions are known and favorable. In this paper, several of the significant challenges for ‘capillary solutions’ to low-gravity multiphase fluids management aboard spacecraft are briefly reviewed in light of applications common to life support systems that emphasize the impact of the widely varying wetting properties typical of aqueous systems. A restrictive though no less typifying example of passive phase separation in a urine collection system is highlighted that identifies key design considerations potentially met by predominately capillary solutions. Sample results from novel scale model prototype testing aboard a NASA low-g aircraft are presented that support the various design considerations.     

Self-localizing stabilized mega-pixel picoliter arrays with size-exclusion sorting capabilities

We report on a liquid self-localizing process capable of producing Mega-pixel arrays of picoliter volumes on a 1 cm(2) area, within seconds, for high throughput sampling. The chip is based on principles of spatially varying wetting and stabilization. The key is to develop differential surface contact regions, which lead to both localization of the solution and increasing the surface adsorption energy to further pin the liquid to the surface, as highlighted by other studies. By exploiting surface roughness for enhanced wettability, we demonstrate wetting of wells with the aspect ratio of 100. The high precision of reactive ion etching (RIE) of silicon substrates allows for an extremely reproducible method of preparing the array of identical well structures and increased contact area to increase surface adsorption in the wells. "Dynamic wetting" is then readily achieved through inducing contact line instability by simply moving a drop of liquid on the top surface of the array. Liquid samples self-localize into the array pattern with the associated liquid flow leading to self-localization of suspended particles or analyte. The resulting picoliter volumes are both spatially ordered and stable for long periods of time, even for such small volumes, to permit selective measurements of the contents. This development will be particularly important in the assembly of the massive amounts of crystalline particles needed for atomically resolved structural dynamics using the latest advances in high number density electron and X-ray sources.     

Numerical simulation of microscopic flow in a fiber bundle using the moving particle semi-implicit method

This paper simulated the microscopic flow in a fiber bundle using the moving particle semi-implicit (MPS) method. Two phases (resin and air) were directly modeled to clarify the detailed mechanism of air entrapments in a fiber bundle. An external force was then introduced into the Navier–Stokes equation using a quasi-potential term to express the wettability between fiber and resin. To validate the MPS method for application to resin flow, we simulated a droplet of resin and the capillary flow of resin between the fibers. To validate the present approach, we simulated water-and-air two-phase flow and compared the simulation results with experiment results. The simulated results for water flow agreed well with the experiment results. Based on these validations, resin-and-air two-phase flow in a fiber bundle was simulated to analyze void formation in a fiber bundle. The simulation indicates that void formation depends on fiber arrangement as well as wettability.     

Visualization and void fraction measurement of decompressed boiling flow in a capillary tube

A capillary tube is often used as a throttle for a refrigerating cycle. Subcooled refrigerant usually flows from a condenser into the capillary tube. Then, the refrigerant is decompressed along the capillary tube. When the static pressure falls below the saturation pressure for the liquid temperature, spontaneous boiling occurs. A vapor-liquid two-phase mixture is discharged from the tube. In designing a capillary tube, it is necessary to calculate the flow rate for given boundary conditions on pressure and temperature at the inlet and exit. Since total pressure loss is dominated by frictional and acceleration losses during two-phase flow, it is first necessary to specify the boiling inception point. However, there will be a delay in boiling inception during decompressed flow. This study aimed to clarify the boiling inception point and two-phase flow characteristics of refrigerant in a capillary tube. Refrigerant flows in a coiled copper capillary tube were visualized by neutron radiography. The one-dimensional distribution of volumetric average void fraction was measured from radiographs through image processing. From the void fraction distribution, the boiling inception point was determined. Moreover, a simplified CT method was successfully applied to a radiograph for cross-sectional measurements. The experimental results show the flow pattern transition from intermittent flow to annular flow that occurred at a void fraction of about 0.45.     

Modeling and experiments in dissolutive wetting: a review

Wetting, phase change, and reaction in high temperature systems (e.g., a liquid metal on a metal substrate) are complex phenomena that are only partially understood. These phenomena occur in joining processes, thin film processing and sintering among others. Dissolutive wetting is characterized by chemical and physical processes that span a broad range of spatial and temporal scales. While experiments are difficult to conduct, there have been a number of experimental investigations of dissolutive wetting in metal–metal systems and a short review of these studies is presented. Although limited, recently there have been studies comparing results, such as spreading rate and dissolution depth, from experiments to those from computational simulations. For dissolutive wetting in metal systems it is difficult to observe much of the spreading process experimentally. Computational models may provide better understanding of many aspects of dissolutive wetting. Models of dissolutive wetting incorporate knowledge from chemical thermodynamics, phase transformations, capillary behavior, and multi-phase transport. A number of computational models have appeared in the literature over the last 10?years. Dissolutive wetting has been studied using a broad range of approaches from molecular dynamics to continuum based models at the drop scale that include hydrodynamic transport using different levels of sophistication. We present a comprehensive review of the modeling approaches that have been used to study dissolutive wetting.     

Wetting in infiltration of alumina particle preforms with molten copper

The high-temperature wettability of alumina particulate preforms by copper is investigated by means of infiltration experiments conducted at 1473 K under low oxygen partial pressure. Wetting is quantified in terms of drainage curves, which plot the volume fraction of metal in the porous medium vs. the applied pressure. Mercury porosimetry is also used on similar preforms for comparison. The effect of volume fraction, particle geometry and capillary parameters on the drainage curve are studied and compared with the expression proposed by Brooks and Corey. The influence of the particle volume fraction and capillary parameters characterizing wetting in the two systems is discussed to derive an effective contact angle for wetting of alumina particles by molten copper.     

The Effect of Fluid Properties on Two-Phase Regimes of Flow in a Wide Rectangular Microchannel

We have experimentally studied a two-phase flow in a microchannel with a height of 150 μm and a width of 20 mm. Different liquids have been used, namely, a purified Milli-Q water, an 50% aqueous-ethanol solution, and FC-72. Before and after the experiment, the height of the microchannel was controlled, as well as the wettability of its walls and surface tension of liquids. Using the schlieren method, the main characteristics of two-phase flow in wide ranges of gas- and liquid-flow rates have been revealed. The flow regime-formation mechanism has been found to depend on the properties of the liquid used. The flow regime has been registered when the droplets moving along the microchannel are vertical liquid bridges. It has been shown that, when using FC-72 liquid, a film of liquid is formed on the upper channel wall in the whole range of gas- and liquid-flow rates.     

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.     

Capillary driven flow along interior corners formed by planar walls of varying wettability

Closed-form analytic solutions useful for the design of capillary flows in a variety of containers possessing interior corners were recently collected and reviewed. Low-g drop tower and aircraft experiments performed at NASA to date show excellent agreement between theory and experiment for perfectly wetting fluids. The analytical expressions are general in terms of contact angle, but do not account for variations in contact angle between the various surfaces within the system. Such conditions may be desirable for capillary containment or to compute the behavior of capillary corner flows in containers consisting of different materials with widely varying wetting characteristics. A simple coordinate rotation is employed to recast the governing system of equations for flows in containers with interior corners with differing contact angles on the faces of the corner. The result is that a large number of capillary driven corner flows may be predicted with only slightly modified geometric functions dependent on corner angle and the two (or more) contact angles of the system. A numerical solution is employed to verify the new problem formulation. The benchmarked computations support the use of the existing theoretical approach to geometries with variable wettability. Simple experiments may be performed to confirm the theoretical findings. Favorable agreement between such experiments and the present theory may argue well for the extension of the analytic results to predict fluid performance in future large length scale capillary fluid systems for spacecraft as well as for small scale capillary systems on Earth.     

Influential depth of moisture transport in concrete subject to drying–wetting cycles

An analytical approach is used to investigate the influential depth over which moisture transport occurs within initially saturated concrete subjected to drying–wetting cycles. During drying the moisture transport is modelled as an evaporation–diffusion process with instantaneous evaporation at the moving gas–liquid interface, while the wetting of dried concrete surface zone is described by capillary absorption. Based on the water loss and intake balance during drying and wetting, an equilibrium drying–wetting time ratio is identified. By this ratio, the drying–wetting cycles are classified as drying-dominated, wetting-dominated and equilibrium ones. The corresponding moisture influential depths are expressed explicitly in terms of environmental factors and material transport properties. With the available concrete sorptivity data, the equilibrium time ratio and influential depth are calculated and discussed in depth.     

Programmable Microfluidics Enabled by 3D Printed Bionic Janus Porous Matrics for Microfluidic Logic Chips

Numerous structures have been functionally optimized for directional liquid transport in nature. Inspired by lush trees’ xylem that enable liquid directional transportation from rhizomes to the tip of trees, a new kind of programmable microfluidic porous matrices using projection micro-stereolithography (PµSL) based 3D printing technique is fabricated. Structural matrices with internal superhydrophilicity and external hydrophobicity are assembled for ultra-fast liquid rising enabled by capillary force. Moreover, the unidirectional microfluidic performance of the bionic porous matrices can be theoretically optimized by adjusting its geometric parameters. Most significantly, the successive programmable flow of liquid in a preferred direction inside the bionic porous matrices with tailored wettability is achieved, validating by a precisely printed liquid displayer and a microfluidic logic chip. The programmable and functional microfluidic matrices promise applications of patterned liquid flow, displayer, logic chip, cell screening, gas–liquid separation, and so on.