Boundary layer viscosity of CNT-doped liquid crystals: effects of phase behavior

Carbon nanotube (CNT)-doped liquid crystals (LCs) have attracted intensive research studies as prospective materials in optic display devices, microfluidic sensors, and lubricants due to their unique molecular structures and properties. In this paper, the interaction between the doped CNTs and the hosting 4-cyano-4 $^{\prime }$ -pentylbiphenyl (5CB) molecules (in both nematic and isotropic phases) was investigated and we focused on the boundary layer rheological properties of the CNT-doped 5CB under external electric field. The experiments were performed by using a quartz crystal microbalance (for boundary layer viscosity investigation) and a rheometer (for bulk viscosity measurement). The results indicate that the bulk viscosity of the CNT-doped 5CB presents an obvious electroviscous effect in its nematic phase while no electroviscous effect in its isotropic phase. Additionally, we found that the boundary layer viscosity of the CNT-doped 5CB demonstrated significant electroviscous effects both in its nematic phase and isotropic phase. The enhanced electroviscous effects on the boundary layer viscosity of CNT-doped 5CB can be attributed to the highly ordered structures of LC molecules and CNTs on the substrate under external electric field. The unique properties of the boundary layer rheology of CNT-doped 5CB LC were further discussed in view of the ordering of LC molecules induced by the electric field, the polarity of CNTs, and the aligning interaction between CNTs and LC molecules.     

Spreading of a liquid over a plane rigid substrate in an electric field

The influence of an electric field on spreading of a thin conducting liquid layer over a plane rigid substrate is investigated theoretically. The conductivity of the liquid is assumed to be so low that the effect of the magnetic field of the currents generated in the liquid under the action of the electric field can be neglected. The spreading is assumed to be so slow that the quasi-steady approximation can be used to calculate the electric field strength which can be considered to be equal to zero inside the liquid. Equations that describe variations in the layer shape are obtained in the lubrication theory approximation. The general formulation of the problem is considered. The solution of the problem is obtained in parametric form when the effect of the gravity force and the surface tension can be neglected. Variations in the layer thickness along the substrate are so smooth that the charge distribution over its surface can be assumed to be the same as that over the substrate surface in the absence of the liquid.     

薄膜润滑中双电层效应的理论分析与实验研究

建立了考虑双电层效应的有限宽组合滑块薄膜润滑数学模型,并利用组合滑块与圆盘的滑动摩擦试验对双电层效应进行研究,利用实验结果修正了润滑过程中双电层效应的计算,给出电粘度的计算公式并进行数值分析.结果表明:在薄膜厚度较薄的情况下,双电层效应使得流体的等效粘度随膜厚减小而迅速增加;随着膜厚增加,双电层的电粘度效应逐渐减弱;随着电场强度增加,双电层的电粘度效应增加,当电场强度达到一定程度时,双电层的电粘度效应开始减弱.     

Continuum Electromechanical Theory for Nematic Continua with Application to Freedericksz Instability

Of interest in this work are nematic continua that exhibit electromechanical coupling. The first part of this paper presents a novel variational formulation with a potential energy depending on four independent variables (the displacement, director, specific polarization and electric displacement perturbation). Variations of the potential energy with respect to each one of these variables lead to the governing mechanical equilibrium and constitutive relations plus Maxwell’s equations.The proposed variational formulation is next applied to the study of bifurcation of an infinite layer of a nematic liquid crystal confined between two parallel plates and subjected to a uniform electric field perpendicular to these plates under full anchoring boundary conditions. As the electric field exceeds a critical value, the nematic directors which are initially parallel to the plates, rotate and tend to align with the electric field orientation. This phenomenon, termed in the literature as Freedericksz transition, is treated here as a bifurcation problem using a fully 2D formulation. It is shown that the solution corresponding to the lowest applied electric field, also termed the critical load, is uniform in the direction parallel to the plates and that the corresponding bifurcated path is stable near this critical load. This result holds for arbitrary positive constants of the Frank-Oseen energy (and values of electric susceptibility constants that allow bifurcation) and justifies the 1D treatment of the Freedericksz transition in 2D settings that is widely adopted in the liquid crystal literature. An asymptotic analysis of the supercritical, stable bifurcated equilibrium path about the critical load is also presented and compared with the exact bifurcated solution.     

弹流润滑与薄膜润滑转化关系的研究

采用ＮＧＹ－２型纳米级膜厚测量仪,研究了润滑膜厚度与各工况因素之间的关系,分析了薄膜润滑的机理,探讨了膜厚、速度、润滑油粘度等各因素对弹流润滑与薄膜润滑之间转化的影响,建立了转化临界膜厚值与润滑剂表观粘度的关系。     

Orientation instability of the layer of a nematic liquid crystal

The origin of periodic structures in a layer of a lyotropic nematic liquid crystal observed in the director (vector, describing the anisotropic properties of the medium) reorientation experiment is studied. Such perturbations with the wavevector perpendicular to the initial orientation can develop in a liquid crystal layer in the unstable equilibrium state when the director is parallel to the walls under the condition that its orthogonality to the boundary corresponds to the minimum anchoring energy. It is shown that the linear dependence of the domain period on the layer thickness observed experimentally can be theoretically described when the Frank orientation elasticity energy is considered in the most general form taking the divergence terms into account and the anchoring energy of orientation is small as compared with the bulk energy. A relation between the coefficient of the divergence terms (saddlesplay elastic constant) and two other coefficients in the Frank energy is obtained.     

Surface effect on Bleustein-Gulyaev wave in a piezoelectric half-space

Surface effect, which is attributed to the different environment surrounding the surface or near-surface atoms from that embracing the bulk atoms, may become significant when the surface-to-volume ratio of a body is large. This paper considers the effect of a plane boundary of a piezoelectric body modeled as a thin layer with specified material properties, for which a transfer relation between the state vectors at the top and bottom surfaces is derived based on the state-space formulations. The equations of surface piezoelectricity for different orders without any bias field are then presented by making use of the power series expression of the transfer matrix. The surface effect is demonstrated by considering the Bleustein-Gulyaev wave existing in a horizontally polarized piezoceramic half-space.     

磨合过程中液体润滑实现超低摩擦的混合模型研究

针对球-盘滑动试验,在磨合过程中获得超低摩擦的液体润滑状态,建立耦合流体润滑、粗糙接触力学、Archard磨损方程和相关物理参数(液体黏度、表面粗糙度和磨损系数)时变函数的混合模型,研究磨合过程中液体润滑的摩擦系数演化. 通过数值模拟结果可知:在磨合过程中,润滑介质等效黏度增大,形成流体动压润滑薄膜,有效隔开粗糙表面;其次在磨合过程中,新生成的表面粗糙度降低,减少粗糙峰承载比,实现超低摩擦润滑状态;最后在适当的液体黏度和提高表界面效应减少边界摩擦系数,可进一步实现液体超低摩擦润滑状态. 为磨合过程宏观液体润滑性能演化所建立的混合数值模型对提高液体润滑超低摩擦设计效率具有重要价值意义.      

固-液复合润滑对轴承钢摩擦学性能的影响

为了提高GCr15钢的耐磨性和承载能力,采用低温离子渗硫技术在GCr15钢的表面制备了FeS固体润滑薄膜,在球-盘摩擦磨损试验机上对比研究了在基础油及2种含硫极压抗磨添加剂(ZDDP和SO)润滑条件下GCr15钢渗硫前后的摩擦学性能.利用SEM观察了磨损表面的形貌,利用XPS分析了磨损表面边界润滑膜化合物的价态及元素随深度的变化.研究表明:在摩擦过程中,表面的硫化层会阻碍润滑油中的添加剂与基体之间的作用,当添加剂与基体直接接触时其作用才能发挥.只有当硫化层破坏程度较小时,与添加剂的复合作用效果才较为明显.含硫添加剂与基体之间的反应可以补充硫化层因磨损失去的FeS,体现了显著的复合润滑效果.其中,高活性的硫烯比需要温度催化的ZDDP对硫化层的补充作用更明显.     

Skin friction measurements on reflective surfaces using nematic liquid crystal

 A new liquid crystal technique for full surface skin friction measurements is introduced. With the new technique, the transmission of light through nematic liquid crystal viewed in reflection provides a quantitative measurement of the skin friction. The measurement technique is discussed with the aid of a model which describes the rotation dynamics of the liquid crystal molecules. Calibration experiments performed in a laminar flow duct demonstrate that the model captures the essential physics of the new technique. The measurement of skin friction downstream of a three dimensional roughness element in an incompressible laminar boundary layer is then presented as a demonstration of the utility of the technique. Received: 5 June 1998/Accepted: 13 November 1998     

Joule heating effect of electroosmosis in a finite-length microchannel made of different materials

This paper presents a numerical analysis of Joule heating effect of electroosmo- sis in a finite-length microchannel made of the glass and polydimethylsiloxane （PDMS） polymer. The Poisson-Boltzmann equation of electric double layer, the Navier-Stokes equation of liquid flow, and the liquid-solid coupled heat transfer equation are solved to investigate temperature behaviors of electroosmosis in a two-dimensional microchannel. The feedback effect of temperature variation on liquid properties （dielectric constant, vis- cosity, and thermal and electric conductivities） is taken into account. Numerical results indicate that there exists a heat developing length near the channel inlet where the flow velocity, temperature, pressure, and electric field rapidly vary and then approach to a steady state after the heat developing length, which may occupy a considerable portion of the microchannel in cases of thick chip and high electric field. The liquid temperature of steady state increases with the increase of the applied electric field, channel width, and chip thickness. The temperature on a PDMS wall is higher than that on a glass wall due to the difference of heat conductivities of materials. Temperature variations are found in the both longitudinal and transverse directions of the microchannel. The increase of the temperature on the wall decreases the charge density of the electric double layer. The longitudinal temperature variation induces a pressure gradient and changes the behavior of the electric field in the microchannel. The inflow liquid temperature does not change the liquid temperature of steady state and the heat developing length.     

Origin of the echo phenomenon upon flow reversal in low molecular weight nematic liquid crystals

We analyse the transient shear stress response in tumbling nematic liquid crystals upon flow reversal within the framework of Leslie–Ericksen theory. In particular, we focus our attention on the echo phenomenon, i.e. the progressive re-emergence, upon flow reversal, of the transient oscillations observed at the flow start-up in low molecular weight (LMW) liquid crystals. We show that it is possible to interpret this phenomenon if the director distribution that develops after start-up contains a reversible and an irreversible component. In short, the formation of an echo results from the reversible component while the irreversible component attenuates its amplitude. Within this model, the relative proportion of reversible and irreversible contributions to the total director distribution determines the magnitude of the echo and its rate of decay with the increasing of elapsed time between start-up and flow reversal. The model proposed in this paper is fully analytical; in particular, we give an analytical expression for the intensity of the echo. This model is in good agreement with published experimental data for the MSHMA/5CB nematic mixture (Gu et al. J Rheol 37:985–1001, 1993; Gu and Jamieson, Macromolecules 27:337–347, 1994). We end this paper with a brief discussion contrasting the reappearance of the oscillations in liquid crystal polymers and surfactants immediately after the flow reversal, with the observation of the echo phenomenon around t?=?2t _R in LMW liquid crystals, where t _R is the time interval between the start-up and the reversal of the flow.     

微纳米结构壁面对流场及动压润滑的影响研究

固体边界具有的微纳米结构将影响流体在近壁面处的流动行为,进而由于尺度效应改变流体在整个微间隙的流动或润滑规律.将壁面可渗透微纳米结构等效为多孔介质薄膜,采用Brinkman方程来描述流体在近壁面边界渗透层内的流动,并将其与自由流动区域的不可压缩流体Navier-Stokes控制方程耦合,在界面处的连续边界条件下求解和分析了速度分布规律和压力变化规律.针对恒定法向承载力的油膜润滑条件,进一步讨论了静止表面或运动表面的微纳米结构对近壁面流动行为的影响;并揭示了考虑壁面微纳米结构的流体动压润滑的油膜厚度和摩擦系数的变化规律.论文结果为具有可渗透微结构表面的微间隙流动与润滑提供了理论参考.     

辛烷基苯酚聚氧乙烯醚与正癸醇层状液晶的结构及润滑性能研究

用小角Ｘ射线衍射和自旋标记电子自旋共振（ＥＳＲ）方法测定辛烷基苯酚聚氧乙烯醚与正癸醇体系层状液晶的结构。用高速环－块摩擦磨损试验机考察了不同组分含量的层状液晶对铝合金－钢摩擦副的润滑作用。结果表明：随着水含量的增加,层状液晶稳定性降低,润滑效果变差;随着辛烷基苯酚聚氧乙烯醚含量的增加,层状液晶的稳定性增加,润滑性能提高。     

RESISTANCE EFFECT OF ELECTRIC DOUBLE LAYER ON LIQUID FLOW IN MICROCHANNEL

Poisson-Boltzrnann equation for EDL (electric double layer) and Navier-Stokes equation for liquid flows were numerically solved to investigate resistance effect of electric double layer on liquid flow in microchannel. The dimension analysis indicates that the resistance effect of electric double layer can be estimated by an electric resistance number, which is proportional to the square of the liquid dielectric constant and the solid surface zeta potential, and inverse-proportional to the liquid dynamic viscosity, electric conductivity and the square of the channel width. An "electric current density balancing" (ECDB) condition was proposed to evaluate the flow-induced streaming potential, instead of conventional "electric current balancing" (ECB) condition which may induce spurious local backflow in neighborhood of the solid wall of the microchannel. The numerical results of the flow rate loss ratio and velocity profile are also given to demonstrate the resistance effect of electric double layer in microchannel.     

Interfacial energy effects within the framework of strain gradient plasticity

In the framework of strain gradient plasticity, a solid body with boundary surface playing the role of a dissipative boundary layer endowed with surface tension and surface energy, is addressed. Using the so-called residual-based gradient plasticity theory, the state equations and the higher order boundary conditions are derived quite naturally for both the bulk material and the boundary layer. A phenomenological constitutive model is envisioned, in which the bulk material and the boundary layer obey (rate independent associative) coupled plasticity evolution laws, with kinematic hardening laws of differential nature for the bulk material, but of nondifferential nature for the layer. A combined global maximum dissipation principle is shown to hold. The higher order boundary conditions are discussed in details and categorized in relation to some peculiar features of the boundary surface, and their basic role in the coupling of the bulk/layer plasticity evolution laws is pointed out. The case of an internal interface is also studied. An illustrative example relating to a shear model exhibiting energetic size effects is presented. The theory provides a unified view on gradient plasticity with interfacial energy effects.     

碳钢表面渗硼层在油润滑条件下的摩擦磨损性能研究

采用化学热处理工艺对 45 # 钢进行渗硼表面改性 .在 SRV摩擦磨损试验机上对比考察了渗硼层在液体石蜡及含硼添加剂 -液体石蜡润滑下的摩擦磨损性能 .采用 X射线光电子能谱仪和 X射线衍射仪对比分析了 45 #钢表面硼改性层和含硼添加剂润滑下钢球磨斑表面典型元素组成和化学状态 .结果表明 ,化学渗硼可以大幅度降低碳钢的摩擦系数和磨损体积损失 ,渗硼层主要由 Fe2 B和 B2 O3 组成 ;渗硼改性层在含硼添加剂 /液体石蜡润滑下的摩擦磨损性能进一步改善 ,这归因于渗硼改性和含硼添加剂的摩擦化学效应的双重作用     

Alterations in peristaltic pumping of Jeffery nanoliquids with electric and magnetic fields

The combined effects of electric and magnetic fields on peristaltic flow of Jeffery nanoliquids are analytically investigated. Double-diffusive convection in the asymmetric microchannel is also carried out. The walls of the microchannel are propagating with a finite phase difference in a sinusoidal manner. Rosseland diffusion flux model is employed to examine the thermal radiation effect. The zeta potential on the walls is considered very low to apply Hückel–Debye approximations. The coupled non-linear governing equations are simplified by using dimensional analysis and lubrication theory. The closed form solutions for potential function, nanoparticle fraction field, solute concentration field, temperature field, stream function, and axial velocity are derived under the appropriate boundary conditions. It is noteworthy that the pumping characteristics strongly depend on the magnetic fields, electric fields, electric double layer thickness, Jeffery parameter, thermal radiation and Grashof number. Furthermore, trapping phenomenon is analyzed under the effects of Hartmann number, Jeffrey parameter, Grashof number and Helmholtz–Smoluchowski velocity. The novelty of the present work is the amalgamation of biomimetics (peristaltic propulsion), electro-magneto-hydrodynamics and nanofluid dynamics to produce a smart pump system model for smart drug delivery systems.     

Wärmeübertragung bei freier Strömung im überkritischen Gebiet reiner Fluide

Natural convective heat transfer to carbon dioxide near its critical point was investigated. For a vertical flat plate of constant temperature the boundary layer equations were solved with variable fluid properties. The numerical results were compared with experiments and it was shown that calculated heat transfer coefficients near the critical point depend not only on the physical properties, but also on variations of these properties. Especially the extreme property variations have to be considered, if the critical point is inside the thermal boundary layer. In the vicinity of the critical point the properties are changing in a very narrow temperature range. With increasing distance this range becomes more extended and the property variations less important. Therefore the influence of physical property variations on natural convective heat transfer depends also on the temperature difference between the wall and the bulk temperature.     

Compliant interfaces: A mechanism for relaxation of dislocation pile-ups in a sheared single crystal

Discrete dislocation plasticity models and strain-gradient plasticity theories are used to investigate the role of interfaces in the elastic–plastic response of a sheared single crystal. The upper and lower faces of a single crystal are bonded to rigid adherends via interfaces of finite thickness. The sandwich system is subjected to simple shear, and the effect of thickness of crystal layer and of interfaces upon the overall response are explored. When the interface has a modulus less than that of the bulk material, both the predicted plastic size effect and the Bauschinger effect are considerably reduced. This is due to the relaxation of the dislocation stress field by the relatively compliant surface layer. On the other hand, when the interface has a modulus equal to that of the bulk material a strong size effect in hardening as well as a significant reverse plasticity are observed in small specimens. These effects are attributed to the energy stored in the elastic fields of the geometrically necessary dislocations (GNDs).