黏弹性表面活性剂溶液中颗粒沉降特性研究

黏弹性表面活性剂溶液悬浮颗粒流广泛存在于自然界和工业生产中,黏弹性表面活性剂溶液的非线性流变性质及应力松弛效应对其中颗粒沉降有着显著影响。采用FENE-P和Giesekus黏弹性本构模型对表面活性剂溶液中颗粒沉降特性进行研究,发现两种本构模型不仅表现出剪切稀化,而且出现拉伸硬化。颗粒在沉降初期的不稳定性主要是由溶液自身的弹性效应引起,弹性效应越强,颗粒沉降速度不稳定性越强,而剪切稀化效应会减弱颗粒沉降速度的不稳定。颗粒沉降过程中在其尾部形成一个“负尾迹”,随着剪切稀化和拉伸硬化效应增强,负尾迹区增大,弹性效应增加,负尾迹增强,负尾迹区流体内部反向速度分布导致的表面活性剂溶液中微观胶束的拉伸断裂和重构可能是引起颗粒沉降速度持续波动的原因。     

十六烷基三甲基溴化铵/油酸钠混合体系蠕虫状胶束流变性研究

阴、阳离子表面活性剂之间强烈的相互作用利于形成自由弯曲的蠕虫状胶束。本文利用阳离子表面活性剂十六烷基三甲基溴化铵(CTAB)和阴离子表面活性剂油酸钠(NaO A)制备了CTAB/NaO A蠕虫状胶束,研究了两表面活性剂的混合比和表面活性剂总浓度的变化对蠕虫状胶束体系稳态流变性及动态黏弹性的影响。结果表明,蠕虫状胶束在剪切过程中的解缠、拟网状结构的破坏以及最终沿剪切速度方向取向等是蠕虫状胶束产生剪切稀释特性的原因。两表面活性剂的混合比和表面活性剂总浓度的变化导致表面活性剂之间的静电作用、疏水作用发生较大的变化,最终引起体系内部表面活性剂聚集体形态的差异。体系内蠕虫状胶束长度、体系结构复杂程度、蠕虫状胶束形成的网络结构的致密度等都影响着体系的流变行为。在混合比R=3.6、总浓度cT=0.24 mol/L时,体系中蠕虫状胶束最长,网络结构最为紧密,体系的零剪切黏度达到最大值。表面活性剂浓度一定时,混合比的提高有助于蠕虫状胶束的定向生长,弛豫时间τR和储能模量高频区平台模量G0提高,R=3.6时两者皆达到极大值,此后由于蠕虫状胶束的分枝化及(或)胶束破裂导致τR及G0下降。在表面活性剂混合比一定(R=3.6)时,表面活性剂浓度的提高利于蠕虫状胶束的增长或者分枝化,增加了胶束网络结构缠绕(融合)点的密度,导致G0逐渐增大。Cole-Cole图证实本文研究的蠕虫状胶束体系是符合Maxwell模型的线性黏弹性流体。     

Gemini表面活性剂黏弹性流体的流变性研究

C16-4-C16阳离子Gemini表面活性剂在NaCl和NaSal两种盐的作用下,可以形成蠕虫状胶束的黏弹性流体。采用流变仪研究了浓度、温度、剪切速率、剪切时间等因素对该流体黏度的影响,结果表明,2wt%的C16-4-C16流体在120℃下的零剪切黏度仍可达60mPa·s以上,而在80℃、170s~(-1)下剪切2 h黏度维持160mPa·s基本不变,说明该流体具有较强的耐温耐剪切性。触变实验的结果清楚地显示出该流体在不同的剪切作用下,蠕虫状胶束的形成与破坏是个可逆的过程。蠕虫状胶束间相互缠绕形成的网络结构,使流体具备了优异的黏弹性,高频有利于胶束储存能量,展现弹性形变;随着温度的升高,体系由弹性为主逐渐转变成了黏性较高的黏弹体,在60℃下黏性和弹性相当,展现了理想的Maxwell黏弹性流体的行为。     

十四烷基三甲基溴化铵蠕虫状胶束的形成及其线性黏弹性

以阳离子表面活性剂十四烷基三甲基溴化铵(TTAB)溶液作为反应主剂,油酸钠(NaOA)溶液作为反应助剂,制备得到了TTAB/NaOA蠕虫状胶束,并研究了表面活性剂摩尔比、主剂质量分数对蠕虫状胶束线性黏弹性的影响。结果表明,当主剂TTAB与助剂NaOA的摩尔比为2.6∶1～4.6∶1,TTAB质量分数在7%～16%之间时,溶液中均有蠕虫状胶束形成。当主剂TTAB与助剂NaOA的摩尔比为4.2∶1,TTAB质量分数为14%时,零剪切黏度可达到最大值即为η₀=15 324 mPa·s,且溶液体系的平台模量G₀和弛豫时间τ_R也达到最大值,此时溶液体系中蠕虫状胶束长度最长,网络结构最为紧密,并呈现出良好的线性黏弹性。由上述溶液体系的实际Cole-Cole图表明本文探讨研究的蠕虫状胶束体系是符合麦克斯韦模型的线性黏弹性流体。     

表面活性剂蠕虫状胶束缔合体系研究进展

介绍了表面活性剂蠕虫状胶束体系的研究进展情况,包括蠕虫状胶束的生长,稀溶液向亚浓溶液的过渡、粘弹体系的动态特性和非线性粘弹性以及剪切诱导粘弹性。     

表面活性剂聚集体的流变性质

以流变学基础知识为出发点，系统综述了胶束、微乳液，溶致液晶（层状、六角状、立方状），囊泡、虫状胶束等表面活性剂聚集体的流变性质及其剪切诱导结构转变现象的研究现状，总结了自流变性质的特点和理论模型，对具有黏弹性的表面活性剂活性剂聚集体进行了较为详细的论述，胶束稀溶液和微乳液多为牛顿流体；溶致液晶为非牛顿流体，有应力服价值和较高的黏弹性，囊泡的弹性性质比较突出，；虫状胶束体系具有非线性黏弹性，易形成网络结构；层状液晶、囊泡和虫状胶束等结构在剪切作用下能发生变化。这些结论对指导表面活性剂的研究和应用有重要意义。     

AES/Na_2CO_3蠕虫状胶束溶液流变性研究

采用流变学方法分别考察了脂肪醇聚氧乙烯醚硫酸钠(AES)质量分数、碳酸钠(Na_2CO_3)质量分数、AES水解和温度对AES/Na_2CO_3蠕虫状胶束溶液流变性能的影响。结果表明,Na_2CO_3的加入可以促进AES蠕虫状胶束的生长。与氯化钠相比,碳酸钠对AES溶液的增黏机制更加复杂。碳酸钠加量小时,反离子Na~+作用小而增黏能力弱;加量高的碳酸钠溶液碱性强,可拟制AES水解,导致活性分子数量增加,表现出较强的增黏能力。同时发现,缩短AES酸性水解时间,有利于改善蠕虫状胶束体系的黏弹性。随着温度升高,AES/Na_2CO_3体系黏弹性逐渐减弱,超过70℃后,体系呈黏性流体。     

黏弹性表面活性剂及其在油田中的应用

介绍了黏弹性表面活性剂溶液的性质，确定黏弹性表面活性剂性质的测试方法和实验手段，以及其内部微观结构和流变特性等方面的研究成果。当黏弹性表面活性剂溶液浓度增加到某一临界浓度时，球形胶束开始向蠕虫状胶束转变，溶液黏度突然增大，随着浓度的进一步增大，蠕虫状胶束快速生长、增长并形成线型柔性棒状胶束，柔性棒状胶束相互缠绕、粘附甚至融合，形成某种超分子三维网状结构，溶液黏度急剧增加，并表现出较强的黏弹性。概述了黏弹性表面活性剂在油田中的应用，特别是在压裂液、酸液、钻井液及提高采收率等方面的应用，指出了黏弹性表面活性剂广泛运用的前景和尚未解决的问题。     

AES/Na_2CO_3蠕虫状胶束溶液流变性研究

采用流变学方法分别考察了脂肪醇聚氧乙烯醚硫酸钠(AES)质量分数、碳酸钠(Na_2CO_3)质量分数、AES水解和温度对AES/Na_2CO_3蠕虫状胶束溶液流变性能的影响。结果表明,Na_2CO_3的加入可以促进AES蠕虫状胶束的生长。与氯化钠相比,碳酸钠对AES溶液的增黏机制更加复杂。碳酸钠加量小时,反离子Na⁺作用小而增黏能力弱;加量高的碳酸钠溶液碱性强,可拟制AES水解,导致活性分子数量增加,表现出较强的增黏能力。同时发现,缩短AES酸性水解时间,有利于改善蠕虫状胶束体系的黏弹性。随着温度升高,AES/Na_2CO_3体系黏弹性逐渐减弱,超过70℃后,体系呈黏性流体。     

蠕虫状胶束溶液组成对其黏弹性影响

用阳离子表面活性剂十八烷基三甲基溴化铵(STAB)作为主剂,水杨酸钠(NaSal)/碳酸钠(Na_2CO_3)、油酸钠(NaOA)作为助剂,制备蠕虫状胶束溶液,研究组分摩尔比及助剂种类对蠕虫状胶束流变特性的影响。结果表明,溶液总质量分数为3.3%,STAB与助剂摩尔比R在2.2~2.6时,两个体系均形成了蠕虫状胶束。助剂为NaSal/Na_2CO_3,R=2.4时体系形成的胶束最长;助剂为NaOA,R=2.6时体系形成的胶束最长,数目最多。摩尔比相同时,STAB/NaOA溶液黏弹性更好。由溶液的实际模量所得Cole-Cole图在低频及中频时为半圆形,说明所制备的2种溶液均为符合麦克斯韦模型的黏弹性流体。     

Extensional flow of wormlike micellar solutions

We consider the inhomogeneous extensional response of a new constitutive model, the VCM model [Vasquez, et al., 2007. A network scission model for wormlike micellar solutions I: model formulation and homogeneous flow predictions. J. Non-Newt. Fluid Mech. 144, 122-139] that has been developed to describe concentrated solutions of wormlike micelles. The time dependent numerical analysis is carried out in a simplified slender filament formulation appropriate for transient elongational flows of complex fluids. The simulations show that, beyond a critical extension rate, elongating filaments of a micellar fluid described by the VCM model exhibit a dramatic and sudden rupture event as a result of the scission of the entangled wormlike chains. The computations capture many of the features of the high-speed rupture process observed experimentally [Bhardwaj, et al., 2007. Filament stretching and capillary breakup extensional rheometry measurements of viscoelastic wormlike micelle solutions. J. Rheol. 51, 693-719] in filament stretching experiments with wormlike micelle solutions. The highly localized rupture predicted by the VCM model and the corresponding evolution in the tensile force within the filament is contrasted with the familiar and more gradual necking responses predicted by the upper convected Maxwell and Giesekus models under equivalent kinematic boundary conditions.     

Origin of the negative wake behind a bubble rising in non-Newtonian fluids

The present work aims at understanding the behavior of individual bubbles in non-Newtonian fluids. By means of a Particle Image Velocimetry (PIV) device, the complete flow field around either a single non-spherical bubble rising in polyacrylamide (PAAm) solutions or a solid sphere settling down in the same fluids shows for the first time the similar coexistence of three distinct zones: a central downward flow behind the bubble or the sphere (negative wake), a conical upward flow surrounding the negative wake zone, and an upward flow zone in front of the bubble or the sphere. This excludes then the possible influence of the interface deformation on the negative wake. A theoretical lattice Boltzmann scheme coupled to a sixth-order Maxwell model was developed for computing the complex flow field around a solid sphere. The good agreement with the experimental measurements provides evidence that the physical mechanism responsible for the negative wake in such fluids could be related to the fluid's viscoelastic properties.     

Numerical Characterization of the Bubble Rise Behavior in Viscoelastic Liquids

The bubble rise behavior in viscoelastic media is analyzed numerically with CFD. Three different constitutive models, Giesekus, linear and exponential Phan‐Thien‐Tanner, are used to evaluate three different biopolymer solutions. The terminal rise velocity over a range of bubble sizes is validated against experimental data. The local velocity fields are compared with respect to the shape and onset of the negative wake. Furthermore, the normal and shear components of the stress fields, transformed according to the local flow direction, are given. The simulations are performed with a volume of fluid solver in OpenFOAM.     

Natural Giesekus fluids: Shear and extensional behavior of food gum solutions in the semidilute regime

The shear and extensional behavior of two aqueous gum solutions, namely (1) 1–20 g/L guar gum (Torres et al., Food Hydrocolloids. 2014;40:85–95) and (2) κ/ι‐hybrid carrageenan solutions (5–20 g/L), are shown to exhibit Giesekus‐fluid behavior when in the semidilute regime. In this regime, a common set of Giesekus fluid parameters described both shear and extensional behavior. A new analytical result describing the extension of a Giesekus fluid in the filament stretching geometry is presented. This also gave reasonable predictions of the Trouton ratio. Higher concentration guar solutions, in the entangled regime, yielded different Giesekus fluid parameters for extension to those for simple shear. The extensional data for all concentrations of both gums collapsed to a common functional form, similar to that reported for cake batters (Chesterton et al., J Food Eng. 2011;105(2):332–342); the limits of the new filament thinning expression provide insight into this behavior. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3902–3915, 2014     

Rheological properties of poly(trimethylene terephthalate) in capillary flow

The shear viscosity, extensional viscosity, and die swell of the PTT melt were investigated using a capillary rheometer. The results showed that the PTT melt was a typical pseudoplastic fluid exhibiting shear thinning and extensional thinning phenomena in capillary flow. There existed no melt fracture phenomenon in the PTT melt through a capillary die even though the shear rate was 20,000 s^?1. Increasing the shear rate would decrease the flow activation energy and decline the sensitivity of the shear viscosity to the melt temperature. The molecular weight had a significant influence on the flow curve. The flow behavior of the PTT melt approached that of Newtonian fluid even though the weight‐molecular weight was below 43,000 s^?1 at 260°C. The extensional viscosity decreased with the increase of the extensional stress, which became more obvious with increasing the molecular weight. The sensitiveness of the extensional viscosity to the melt temperature decreased promptly along with increasing the extensional strain rate. The die swell ratio and end effect would increase along with increasing the shear rate and with decreasing the temperature, which represented that the increase of the shear rate and the decrease of temperature would increase the extruding elasticity of the PTT melt in the capillary die. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 705–709, 2005     

Characterization of viscoelastic fluid flow in a periodically driven cavity: Flow structure,frequency response,and phase lag

Transport phenomena in periodically driven cavities are important because of their relevance in polymer processing and microfluidics. The transient periodic flow of viscoelastic fluids in a cuboidal cavity, with periodic motion of top plate, was studied in this work. Flow with a characteristic time scale was achieved through the simple harmonic motion of the top plate. The flow in the cavity was characterized by measuring planar velocity fields using particle image velocimetry (PIV). Temporal variation of velocity except at central vertical plane showed predominance of the plate frequency. The temporal point variations, though seemingly similar to those for Newtonian and purely viscous non‐Newtonian fluids, led to rich varieties of spatial flow structures in case of the viscoelastic fluids. The overall flow behavior was characterized using spatial variations, phase trajectories, and streamline patterns. The transition from low Reynolds number steady‐lid driven type flow to complex vortical patterned flow was observed during a cycle of periodic motion of viscoelastic fluids. The effects of elasticity and inertia on the flow fields were analyzed. Computational fluid dynamics simulations with purely viscous shear thinning fluid (power law) and Newtonian fluid showed significant differences with experimental measurements on viscoelastic fluids. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Stability of Newtonian-PTT coextrusion fiber spinning

Linear stability analysis has been applied to a coextrusion fiber spinning flow that consists of a Newtonian fluid as a core layer and a Phan-Thien/Tanner (PTT) fluid as a skin layer. These two chosen fluids show entirely different rheological behaviors. The stability of this coextrusion system was affected by the choice of three characteristic parameters: the skin layer fraction (f), the extensional parameter (ϵ), and the shear thinning parameter (ξ) in the Phan-Thien/Tanner model. The linear stability results indicate that the viscoelastic skin layer (PTT fluid) has a stabilizing effect that delays the onset of draw resonance. Under fixed compositions (f is fixed), the stability envelopes changed from upturned curves to flattened ones as extensional force dominated the system. The neutral stable curves closed to the horizontal line at a critical draw ratio of around 20, showing similar behavior to a Newtonian fluid where the system has a very high Deborah number or it is dominated by shear thinning effects.     

Drag on a sphere in a spherical dispersion containing Carreau fluid

The drag on a rigid sphere in a spherical dispersion containing Carreau fluid is investigated theoretically based on a free surface cell model for Reynolds number in the range [0.1,100], Carreau number in the range [0,10], the power-law index in the range [0.3,1], and the void fraction in the range [0.271,0.999]. The influences of the particle concentration, the nature of the Carreau fluid, and Reynolds number, on the drag coefficient are examined. We show that the drag coefficient declines with the decreasing particle concentration, and the reversal of the flow field in the rear region of a sphere is enhanced by the shear-thinning nature of the fluid. An empirical relation, which correlates the drag coefficient with the void fraction (= 1 − particle concentration), the nature of the Carreau fluid, and Reynolds number, is proposed.     

黏弹性流体基纳米流体流变学特性

黏弹性流体基纳米流体（viscoelastic fluid based nanofluid,VFBN）是一种具有湍流减阻和对流换热相对强化特性的新型换热工质,其湍流减阻机理与流变学特性关系密切。通过对以2.5×10^-3、5×10^-3、1×10^-2三种质量分数的十六烷基三甲基氯化铵/水杨酸钠水溶液为基液,粒子体积分数为0.1%、0.25%、0.5%、1.0%的铜纳米流体的剪切黏度、零剪切黏度以及松弛时间的测量,实验结果表明VFBN有明显的剪切稀变特性,同时纳米粒子的添加增大了基液的零剪切黏度,并导致基液黏弹性增强。以Giesekus本构模型为理论基础,利用实验参数得到了描述VFBN剪切黏度的实验关联式。