Diffusiophoresis of colloidal spheres in nonelectrolyte gradients at small but finite Péclet numbers

The diffusiophoretic motion of a spherical particle in a uniform imposed gradient of a nonionic solute is analyzed for small but finite Péclet numbers. The range of the interaction between the solute molecules and the particle surface is assumed to be small relative to the radius of the particle, but the polarization effect of the mobile solute in the thin diffuse layer surrounding the particle caused by the strong adsorption of the solute is incorporated. A normal flux of the solute and a slip velocity of the fluid at the outer edge of the diffuse layer are used as the boundary conditions for the fluid domain outside the diffuse layer. Through the use of a method of matched asymptotic expansions along with these boundary conditions, a set of transport equations governing this problem is solved in the quasisteady situation and an approximate expression for the diffusiophoretic velocity of the particle good to O(Pe ²) is obtained analytically. The analysis shows that the first correction to the particle velocity is O(Pe ²). The normalized particle velocity is found to decrease monotonically with the Péclet number and to increase monotonically with the dimensionless relaxation coefficient. The stronger the polarization effect in the diffuse layer, the weaker the convective effect on the diffusiophoresis. Received: 25 May 2000 Accepted: 6 September 2000     

Synthesis and characterization of magnetite particles covered with α-trietoxysilil-polydimethylsiloxane

New silicon magnetite ferrofluids were prepared by dispersing siloxane-coated magnetite particles in polydimethylsiloxane with low or high molecular weights. Ferrofluids are stable colloidal dispersions of ultra fine covered magnetite particles, which may be selected for a specific application. We demonstrated new methods of stabilizing the magnetic particles by reacting the hydroxyl groups on the surface of magnetite particles with terminal ethoxy groups of polydimethylsiloxane, followed by their dispersion in silicon fluids. The new silicon ferrofluids were tested from the morphology, magnetic properties/losses, and rheological properties point of view.     

光镊系统的组建及光阱效应的观察

光镊是美国科学家Arthur Ashkin于1986年发明的,现被用来操控微小粒子和作为微小力的传感器.随着光镊技术的不断发展,光镊在生物大分子的操控和生物大分子生命过程中动力学研究方面发挥着巨大作用.本文介绍了光镊的工作原理,以及如何利用实验室现有条件,以较低成本搭建了一个简化的光镊系统,并观察了光镊对几种微小粒子的捕捉情况,证实了光阱有一定的作用范围,且其捕获能力随微粒尺寸增大而减小.     

Influence of Solvent Size on Structural Properties of Bottle-brush Polymers

We proposed a modified dissipative particle dynamics simulation model by which one can effectively avoid the bond-crossing problem, and investigated the effect of solvent size on the structural properties of bottle-brush polymers in dilute solution on the basis of this model. It was found that with the increase of solvent size, the radius of the gyration of the bottle-brush polymer decreases considerably in the athermal solvent but increases in the selective solvent favoring the backbone, respectively.     

胶体粒子的机械性能调控及其在药物递送中的应用

胶体粒子是肿瘤治疗中最常用的载体, 尽管在过去的研究中不同的胶体粒子已经被广泛报道, 但如何进一步提高胶体粒子的药物递送效率仍然存在着一些挑战. 大量的研究表明胶体粒子的尺寸、形状、结构和表面化学等物理化学性质在药物递送过程中具有重要的作用, 但胶体粒子的机械性能对药物递送过程的影响研究和综述相对较少. 本综述从不同机械性能胶体粒子的制备与表征出发, 概述了胶体粒子的机械性能对血液循环、肿瘤富集、渗透以及细胞内化过程的影响, 并对该领域存在的问题以及发展的趋势进行了展望. 该综述有助于帮助科学工作者更好地理解胶体粒子的机械性能对药物递送的影响规律, 从而优化胶体粒子的设计并提高纳米药物的递送效率和生物利用率.     

磁粒子成像示踪剂的研究进展

磁粒子成像是基于功能和断层影像技术检测磁性纳米粒子空间分布的示踪方法, 具有正向的对比信号、 较低的组织背景、 无限的组织穿透深度、 非侵入性成像以及无电离辐射等优点, 是近年来一种很有前途的生物医学成像技术. 磁粒子成像信号是通过在无场点切换磁性纳米粒子的磁自旋矢量来产生的. 磁粒子成像的灵敏度和空间分辨率都高度依赖于作为磁粒子成像示踪剂的磁性纳米粒子本身的磁性能, 因此目前的研究主要集中在磁性纳米粒子的设计和合成上. 本文重点介绍了磁粒子成像示踪剂的最新研究进展, 总结了可作为磁粒子成像示踪剂的磁性纳米粒子的种类、 合成方法、 性能以及生物医学应用, 以期为磁粒子成像的未来研究提供参考.     

Approximate analytic expressions for the diffusiophoretic velocity of a spherical colloidal particle

The general expression is derived for the diffusiophoretic velocity of a spherical colloidal particle of radius a in a concentration gradient of symmetrical electrolyte. On the basis of this expression, simple approximate analytic expressions for the diffusiophoretic velocity correct up to the order of 1/κa is derived, where κ is the Debye-Hückel parameter. It is found that the approximate expression correct to order unity can be applied for κa ≥ 50 with negligible errors, while the approximate expression correct to order 1/κa can be applied for κa ≥ 20 with negligible errors.     

Diffusiophoresis in suspensions of highly charged soft particles

Diffusiophoresis phenomenon of aoft particles suspended in binary electrolyte solutions is explored theoretically in this study based on the spherical cell model, focusing on the chemiphoresis component in absence of diffusion potential. Both the electrostatic and hydrodynamic aspects of the boundary confinement, or steric effect, due to the presence of neighboring particles are examined extensively under various electrokinetic conditions. Significant local extrema are found in mobility profiles expressed as functions of the Debye length in general, synchronized with the strength of the motion-inducing double layer polarization. Moreover, a seemingly peculiar phenomenon is observed that the soft particles may move faster in more concentrated suspensions. The competition between the simultaneous enhancement of the motion-inducing electric driving force and the motion-retarding hydrodynamic drag force from the boundary confinement effect of the neighboring particles is found to be responsible for it. The above findings are also demonstrated experimentally in a very recent study on the diffusiophoretic motion of soft particles through porous collagen hydrogels. The results presented here are useful in various practical applications of soft particles like drug delivery.     

Elastic-inertial separation of microparticle in a gradually contracted microchannel

Separation of microparticle in viscoelastic fluid is highly required in the field of biology and clinical medicine. For instance, the separation of the target cell from blood is an important prerequisite step for the drug screening and design. The microfluidic device is an efficient way to achieve the separation of the microparticle in the viscoelastic fluid. However, the existing microfluidic methods often have some limitations, including the requirement of the long channel length, the labeling process, and the low throughput. In this work, based on the elastic-inertial effect in the viscoelastic fluid, a new separation method is proposed where a gradually contracted microchannel is designed to efficiently adjust the forces exerted on the particle, eventually achieving the high-efficiency separation of different sized particles in a short channel length and at a high throughput. In addition, the separation of WBCs and RBCs is also validated in the present device. The effect of the flow rate, the fluid property, and the channel geometry on the particle separation is systematically investigated by the experiment. With the advantage of small footprint, simple structure, high throughput, and high efficiency, the present microfluidic device could be utilized in the biological and clinical fields, such as the cell analysis and disease diagnosis.     

Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as “particle ordering,” is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the ordering efficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slipping phenomenon.