Wicking and evaporation of liquids in porous wicks: A simple analytical approach to optimization of wick design

Wicking and evaporation of volatile liquids in porous, cylindrical wicks is investigated where the goal is to model, using simple analytical expressions, the effects of variation in geometrical parameters of a wick, such as porosity, height and bead‐size, on the wicking and evaporation processes, and find optimum design conditions. An analytical sharp‐front flow model involving the single‐phase Darcy's law is combined with analytical expressions for the capillary suction pressure and wick permeability to yield a novel analytical approach for optimizing wick parameters. First, the optimum bead‐radius and porosity maximizing the wicking flow‐rate are estimated. Later, after combining the wicking model with evaporation from the wick‐top, the allowable ranges of bead‐radius, height and porosity for ensuring full saturation of the wick are calculated. The analytical results are demonstrated using some highly volatile alkanes in a polycarbonate sintered wick. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1930–1940, 2014     

Zur Schaumgeometrie bei Styropor®

The present paper deals with the geometrical changes occuring during blockmoulding of beads. Packing of beads and their blocks are examined, the passage of air and steam is measured and calculations are carried out for an idealized pattern assuming a densest packing which grows to irregular pentagonal dodecahedra without interspace. The remaining interparticularvolume IPV ? 1.6% has the figure of spherical tetrahedra interconnected by capillaries. These capillaries communicate with each other. The product of the number of capillaries per surface and the forth potency of the radius n · r⁴ obeys the HAGEN -POISEUILLE -rule. Its size is determined by the IPV, because the capillaries get longer and smaller when the tetrahedra decrease. The IPV is fixed by a definite diameter of the flattened sphere, an intermediate of sphere and polyhedron. The determination of the IPV by the diameter is experimentally difficult because the maximum increase of the diameter is only by a factor 1.26. If one knows the product n · r⁴ by measuring the permeability to air, then it is possible to calculate the time the steampressure in the block decreases from 1.7 at to 1.1 at.     

Effect of liquid droplet impact velocity on liquid wicking kinetics in surface V-grooves

The liquid droplet inertia effect on liquid wicking in V-groove has direct implications to the liquid sample flow in microfluidic devices using V-groove channels and to the ink wicking along the inter-fibre gaps on uncoated paper surfaces, which is critical to the ink jet print quality. In this study liquid droplet inertia and the V-groove geometry is systematically varied to understand the effect of droplet impact, V-groove apex angle and groove width on the liquid wicking rate in the groove. Our results show that both the apex angle and the groove width influence the rate of liquid wicking in V-grooves forced by liquid droplet inertia. The inertia effect lasts for only a short time and its influence to the sample delivery accuracy in V-groove microfluidic devices can be minimised or eliminated by improving microfluidic channel design. On improving ink jet printing quality of uncoated papers, this work shows that applying surface sizing to uncoated ink jet papers is likely to be an effective way to change the geometry of the inter-fibre gaps and therefore can reduce the feathering effect in ink jet printing.     

Understanding electrokinetic thermodynamics in nanochannels

Understanding the electrokinetic conversion efficiency in a nanochannel is vital for designing energy storage and conversion devices. In this paper, an analytical electrokinetic energy conversion efficiency in a nanochannel is obtained based on the linear electrokinetic response. The analytical result shows that the conversion efficiency has a maximum with the increasing of the nanochannel pore radius. Numerical solutions based on the Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations are used to confirm the analytical expressions. Besides, the influences of the pore radius and surface roughness on the conversion efficiency in nanochannels are also studied by the numerical calculations. In particular, the influences of the surface roughness on the fluid flow, streaming current and streaming potential are examined. The results show that the large bumps and grooves representing the roughness can hinder the fluid flows and ion transports in the nanochannels. The maximum efficiency in a smooth nanochannel is higher than that in a rough channel. However, the small bumps and grooves can increase the surface area of the channel, which is beneficial to improving the conversion efficiency in some cases. This research can provide theoretical guidance to design electrokinetic energy conversion devices.     

低渗透储层的微观孔隙结构特征研究

本文以鄂尔多斯盆地低渗透储层为例,在岩芯观测、岩石薄片、铸体薄片、扫描电镜以及压汞资料分析的基础上,分析了该地区低孔低渗砂岩的岩石学特征、储层物性及微观孔隙结构特征;分析了储层有效喉道半径、有效喉道体积、有效孔隙半径、有效孔隙体积及孔喉比等特征参数与孔隙度、渗透率之间具有较好的相关性。     

A new model to determine contact angles on swelling polymer particles by the column wicking method

The contact angle determination on swelling polymer particles by the Washburn equation using column wicking measurements may be problematic because swelling occurs during the wicking process. The objective of this research was to develop a new model to more accurately determine contact angles for polymer particles that undergo solvent swelling during the column wicking process. Two phenomena were observed related to the swelling effect during the wicking process: (1) a temperature rise was detected during the wicking process when the swelling polymer particles interacted with polar liquids, and (2) a smaller average capillary radius (r) was obtained when using methanol (polar liquid) compared to using hexane (non-polar liquid). The particle swelling will induce both particle geometry changes and energy loss which will influence the capillary rise rate. The model developed in this study considered the average pore radius change and the energy loss due to the polymer swelling effect. Contact angle comparisons were conducted on wood with formamide, ethylene glycol, and water as test liquids, determined by both the new model and the Washburn equation. It was shown that the contact angles determined by the new model were about 4-37° lower than those determined by the Washburn equation for water, formamide, and ethylene glycol. Todetermine whether the polymer particles are swelling, two low surface tension liquids, one polar (methanol) and the other non-polar (hexane), can be used to determine the average pore radius (r values) using the Washburn equation. If the same r values are obtained for the two liquids, no swelling occurs, and the Washburn equation can be used for the contact angle calculation. Otherwise, the model established in this study should be used for contact angle determination.     

Parametric study of droplet size in an axisymmetric flow-focusing capillary device

A conventional technique for microfluidic droplet generation is Co-axial Flow Focusing(CFF) in which a contraction zone is placed downstream of the dispersed phase nozzle. In this contraction zone, the dispersed-phase(dphase) fluid is pinched off by continuous-phase(c-phase) fluid to generate micro-droplets. Studying the influence of multiple parameters such as the fluids velocities and viscosities, the interfacial tension, and nozzle and orifice diameters on the droplet size is of great importance for the design and application of CFF devices. Thus,development of more complete numerical models is required. In this paper, we show our model is compatible with experimental data and then numerically investigate the effects of aforementioned parameters on the droplet generation in a CFF microfluidic device. Simulation results showed that the c–phase flow rate, viscosity and the interfacial tension had great impacts on the droplet size. The effect of the nozzle diameter on the generated droplet size was small compared to that of the orifice in a CFF device. Using the simulation results, a correlation was also developed and suggested which predicts the droplet size with less than 15% error in a wide range of the introduced dimensionless parameters.     

Wicking into porous polymer-derived ceramic monoliths fabricated by freeze-casting

Silicon oxycarbide monoliths of different pore size distribution were fabricated by freeze-casting. The samples revealed a lamellar pore structure with an axial anisotropy. To evaluate the capillary transport abilities we performed wicking experiments. The sample weight measurement method was applied during the imbibition. The samples show deviations in permeability from 10% to 49% at different sample orientations that quantifies the impact of the anisotropy in the axial direction. The deviations were larger for the samples with smaller pore size. For these samples we also observed larger differences in the wicking behaviour. The samples with bigger pore size demonstrated higher permeability and faster wicking. Imbibition results at both sample orientations showed a good agreement with a prediction via the Lucas–Washburn equation with gravity effects. We demonstrate hereby, that our approach of macroscopic modelling predicts wicking behaviour in anisotropic structures reasonably well, providing a simple tool for further porous material investigations.     

Pore network simulation of fluid imbibition into paper during coating—III: modelling of the two-phase flow

Extensive computer simulations have been carried out to model imbibition of a coating fluid into a paper. The microstructure of the paper's pore space is represented by a network of interconnected channels or pore throats that are formed between the paper's fibers. The geometrical characteristics of the channels, such as their effective radius and length, as well as their connectivity, are selected from the experimental data presented in Part II of this series. The imbibition process that we simulate is the result of forcing the coating fluid into the pore space by applying a time-dependent flow-driven pressure distribution to the external surface of the paper, or is driven only by capillary forces. The dynamic pressure distribution is representative of a high-speed coating process. The simulations indicate that the connectivity of the pore throats, the anisotropic structure of the paper's pore space, and the dynamic pressure distribution all have a strong influence on imbibition of a coating fluid into a paper's pore space and, hence, on the quality of the coating.     

Z2断块储层特征研究

应用恒速压汞以及核磁共振技术研究了Z2断块低渗透储层的微观孔隙结构以及可动流体特征,并分别建立了主流喉道半径、可动流体百分数与储层渗透率的关系;最终以主流喉道半径、可动流体百分数、粘土矿物含量、原油粘度为指标,依据储层评价标准,对Z2断块的三个层系进行了储层分类1,分类结果表明,三个层系均为Ⅰ-Ⅱ类储层,开发较为容易。研究成果为分析评价同类储层并指导增储上产提供了可靠的科学依据。     

Phenomenological study and modelling of wick debinding

Wick debinding employs capillary suction (via a surrounding wicking powder) to remove the liquid binder phase from powder injection moulded parts (known as a compact). Experimental measurements of binder distribution within the compact during debinding highlight flaws in previous wick debinding models. The spatially uniform distribution of binder observed consistently during debinding indicates that it is removed in order of pore size regardless of location in the compact. A model is proposed which gives good agreement with 1-D experimental data of binder distribution. Key parameters of the model are the permeability of the wicking powder and the relationship between the capillary pressure, saturation and relative permeability of the compact.     

一步法制备生物相容油核微胶囊及其可控释放

一步法可控制备生物相容油核微胶囊对工业制备微胶囊及其应用具有重要意义。通过设计微流控器件,成功实现一步法制备尺寸均一可控的生物相容油核微胶囊。利用玻璃毛细管管套管的方法制备了微流控器件。通过外相水凝胶相剪切内相油相得到油核液滴,同时油核液滴和外相水凝胶相在重力作用下脱离管口,形成油核微胶囊,再通过交联水凝胶壳层得到稳定的结构。系统研究了微流控器件结构、内相流速、外相流速等参数对油核微胶囊油核数量、微胶囊直径、壁厚等性质的影响规律。生物相容油核微胶囊作为活性物质的理想载体,可以实现pH改变触发的快速释放和壁厚调节的缓慢释放,为其实际应用奠定了基础。     

微通道内液-液多相流数值模拟研究进展

液滴微流控技术在化学化工、生物医学等领域具有良好的应用前景,而微通道内的液-液多相流动则是液滴微流控技术中最常见的流动现象,深入研究其机理及其内在规律对相关装置与过程的优化设计具有重要的指导意义。本文系统地综述了研究微通道液-液多相流常用数值研究方法,回顾了连续力学方法与介观动理学方法的研究进展,详细介绍了界面追踪方法与界面捕捉方法的特点以及常用模型,讨论了多种模型的应用情况,论述并对比了不同模型的优势与限制。为进一步开展微通道液-液多相流行为规律及其内在机理的研究提供有益借鉴。微通道内多相流动涉及多种流体与界面的相互耦合,应进一步深入研究在模型简化的基础上实现更精确的界面与流体动力学行为描述。     

Mathematical Modeling of the Dependence of the Vapor Permeability Coefficient of a Membrane on Its Pore Structure

The dependence of the vapor permeability coefficient of a distillation membrane on its pore structure is analyzed. The pore structure of the membrane is represented as a two- or three-dimensional network of capillaries that have a fixed length and originate at nodes with a constant coordination number. Three pore-size distribution functions are considered, namely, a uniform, a bimodal, and a triangular function. An explicit expression for the vapor permeability coefficient of the membrane is derived using orientation averaging and the smooth-field approximation. The effect of some structural properties (pore-size distribution, porosity, coordination number of a node, etc.) of the porous membrane on the vapor permeability coefficient is studied numerically. Irrespective of the pore-size distribution function, the vapor permeability coefficient decreases with increasing pore radius and grows with increasing coordination number and Knudsen number.     

ANALYSIS OF PERMEABILITY FOR ELLIS FLUID FLOW IN FRACTAL POROUS MEDIA

This work studies the flow characteristics of Ellis fluid in saturated porous media. A fractal model is developed for the effective permeability of Ellis fluid flow in porous media based on the assumptions that porous media consist of a bundle of tortuous capillaries, whose size distribution and tortuosity follow the fractal scaling laws. The average flow velocity and the effective permeability for Ellis fluid flow in porous media are derived. The proposed fractal model does not contain any empirical constant, and every parameter in the model has clear physical meaning. The model predictions are compared with the measured data, and good agreement between them is obtained.     

Liquid Permeability of Packed Particles: Why Perpetuate the Carmen—Kozeny Model?

Liquid transport through the interstices of packed particles is commonly described using the Carmen–Kozeny mean hydraulic radius model, which calibrates the effective pore dimension from mean macroscopic parameters. However, the experimental aqueous permeability of sets of porous powder compacts varying widely in porosity and pore structure was shown to be much better described in terms of the linear mean pore size determined from mercury penetration porosimetry. Here it is shown that the latter model is supported by studies of the permeability of porous rock and percolation theory.     

微纳米流控芯片传感器研究及其在环境检测中的应用

与传统生物传感器相比,微纳米流体生物传感器在减少样品使用剂量,实现高通量、快速检测、特异性检测,以及简化实验操作等方面都显示出无可比拟的优越性。本文所涉及的微纳米的生物传感器大体上可以分成两大类:第一类是常规的微纳米流控生物传感器(简称微流控芯片),通常以硅、玻璃以及高分子聚合物作为基材;第二类则是最近兴起的试纸条生物传感器,其基材为纸质。本文从以下几方面对当前微流体生物传感器的研究与应用进行总结:微流体生物传感器的基本理论,材料特征与制作工艺方面,以及在环境监测领域的典型性应用,最后对基于各种不同工艺技术制作的微流体生物传感器在技术方面的难点和应用上的局限性进行简要分析。     

Modelling laminar pulsed flow in rectangular microchannels

Fully developed laminar pulsed flow of an incompressible Newtonian fluid through rectangular ducts has been modelled and analyzed using Green functions. Based on the solutions for the velocity profile presented previously [Fan, C., Chao, B.-T., 1965. Unsteady, laminar, incompressible flow through rectangular ducts. Zeitschrift für Angewandte Mathematik und Physik 16, 351-369], exact analytical solutions in series form for wall shear stress and volumetric flow rate have been obtained. Various flow effects in periodic pulsed flow through rectangular microchannels, including flow reversal, phase shift and wall shear stress enhancement were calculated indicating that a substantial increase in local wall shear stress can be achieved with a modest increase of average flow rate over a cycle. The analytical solutions and the calculated results will help optimize parameters in cleaning of microfluidic devices by pulsed flow.     

LIQUID-LIQUID RELATIVE PERMEABILITY: NETWORK MODELS AND EXPERIMENTS

Two different but related two-dimensional network models are used to elucidate the concept of relative permeability in simultaneous liquid-liquid flow in porous media. The first model is designed for monosized sphere packs while the other, consisting of spherical pore chambers interconnected by capillaries of variable length and radius, is intended for more general porous media. The effects of various operating conditions and physical factors—pressure gradient, wettability, surface tension, throat and pore size distributions, imbibition or drainage—on relative permeability are studied. Experimental data are also presented to confirm various aspects of the theory.     

Bubble generation rules in microfluidic devices with microsieve array as dispersion medium

Microfluidic devices with microsieve array as the dispersion medium have been well recognized. However, few studies have been made on gas‐liquid two‐phase flow in microsieve dispersion devices. The bubble generation rules with single‐pore, radial‐array pores, axial‐array pores, and square‐array pores were systematically investigated. The rules of pore activation have been suggested by considering the capillary force, flow resistances of both dispersed phase and continuous phase. An empirical equation and a theoretical equation to predict the activation of pores in microfluidic devices were developed. An equation to correlate the average bubble diameter with parameter of channel structure, phase ratio, and Ca number of continuous phase was also developed. The strategy of design and scaling up for microsieve devices is proposed. Meanwhile, a device with dual‐size pores according to the rules derived is designed. This device achieved much better dispersion performance. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1663–1676, 2015