椭球与圆球混合胶体体系的玻璃化转变

以椭球与圆球混合的胶体体系为研究对象,通过增加体系的面积分数,从实验上研究了混合体系发生玻璃化转变过程中结构和动力学行为的演变规律.在结构方面,通过计算和分析径向分布函数、泰森多边形以及取向序参量,发现椭球可以有效地抑制圆球结晶,整个体系在结构上始终保持无序.在动力学方面,通过计算体系的均方位移和自散射函数,发现随着面积分数的增加,体系的动力学明显变慢,弛豫时间在接近模耦合理论预测的玻璃化转变点快速增大并发散.通过考察快速粒子参与的协同重排行为,发现协同重排区域形状、大小和位置都与椭球的存在密切关联.     

Effect of suspended particles on spreading of volatile droplet on solid substrate

Abstract  

We have been interested in behaviors of suspended particles in a volatile droplet placed on a smooth substrate. It is known that the particles gather and deposit in the vicinity of the macroscopic contact line of the droplet, which is generally called ‘coffee stain problem’. A convective flow induced by non-uniform evaporation through the interface brings suspended particles toward the pinned contact line in the drying droplet, which forms a ring stain. We have focused on the dynamics of the droplet with/without suspended particles spreading on the solid substrate and on the behaviors of particles in the evaporating droplet. Spreading process of the droplet is significantly affected by the suspended particles. We indicate flow patterns in the droplet, in which the flow exhibits a modal structure with a mode number in the azimuthal direction, and indicate particles depositions after the dryout of the droplet. Three-dimensional particle tracking velocimetry is applied to reconstruct such unique flow patterns in the spreading process of the droplet. Resultant patterns of the particles depositing on the substrate are introduced.     

Evaporation of saline colloidal droplet and deposition pattern

The dynamic process of the evaporation and the desiccation of sessile saline colloidal droplets, and their final deposition are investigated. During the evaporation, the movement of the colloidal particles shows a strong dependence on the salt concentration and the droplet shape. The final deposition pattern indicates a weakened coffee-ring effect in this mixed droplet system. The microscopic observation reveals that as evaporation proceeds, the particle motion trail is affected by the salt concentration of the droplet boundary. The Marangoni flow, which is induced by surface tension gradient originating from the local evaporative peripheral salt enrichment, suppresses the compensation flow towards the contact line of the droplet. The inhomogeneous density and concentration field induced by evaporation or crystallization can be the major reason for various micro-flows. At last stage, the distribution and crystallization of Na Cl are affected by the colloidal particles during the drying of the residual liquid film.     

受限胶体液滴蒸发过程中胶体颗粒沉积过程观察

亲水玻璃基片在掩模板的保护下, 通过喷涂超疏水层, 得到了被疏水层包围的圆形亲水区域. 胶体液滴在这一区域被很好地限制, 并且液滴体积可以在较大范围内变化, 体积的变化可以改变液滴与基片的表观接触角. 通过显微观察手段原位观察了表观接触角为疏水的受限胶体液滴蒸发过程中粒子沉积行为. 在整个蒸发过程中, 受限液滴边界被钉扎在亲疏水交界处. 粒子沉积过程中, 驱动粒子的液滴内部流动会发生变化. 粒子沉积图案形成过程由三种流体行为控制, 最初, Marangoni效应占主导作用, 驱动粒子在液滴表面聚集, 随之沉积到液滴边缘; 随着蒸发进行, 当接触角变小(<60°)时, 由于边界蒸发速度更快导致的毛细补偿流使得粒子直接向边界沉积. 在干燥的最后阶段, 亲水区域内的液层变得很薄, 只有一单层粒子存在于这一薄液层中, 蒸发继续进行时, 薄液层发生失稳使得粒子迅速聚集而形成网络化图案, 由于粒子间距变小, 球间的液桥毛细力也会参与到这一聚集过程中.     

溶液液滴蒸发变干的环状沉积

液体蒸发驱动的颗粒自组装现象在许多的工业技术中有重要应用. 本文利用显微镜观测含有颗粒物质的液滴变干后留在固体表面的颗粒形成的环状沉积图案. 采用微米粒径的SiO₂小球水溶液液滴蒸发变干模拟咖啡环的形成过程, 结果发现液滴蒸发过程中接触线的钉扎是环状沉积的必要条件. 在液滴蒸发过程中颗粒随着补偿流不断的向液滴边缘移动, 聚集在接触线处形成环. 液滴蒸发变干后残留在液滴内部的颗粒数随颗粒质量分数的增加而增加, 可以达到单层的颗粒排列. 而玻璃衬底上的颗粒环在颗粒质量分数很小时, 形成单层排列, 且一排一排地生长. 蒸发过程中颗粒环由于液滴边缘的尺寸限制向液滴中心缓慢移动. 这会导致液滴中不同大小颗粒的分离. 关键词： 液滴 接触线 蒸发 颗粒     

Local crystalline order in a 2D colloidal glass former

A mixture of two types of super-paramagnetic colloidal particles with long-range dipolar interaction is confined by gravity to a flat interface of a hanging water droplet. The particles are observed by video microscopy and the dipolar interaction strength is controlled by an external magnetic field. The local structure as obtained by pair correlation functions and bond order statistics is investigated as a function of system temperature and relative concentration. Although the system has no long-range order and exhibits glassy dynamics, different types of stable crystallites coexist. The local order of the globally disordered structure is explained by a small set of specific crystal structures. The statistics of crystal unit cells show a continuous increase of local order with decreasing system temperature as well as a dependence on sample history and local composition.     

Solid-liquid transition induced by the anisotropic diffusion of colloidal particles

We numerically study the phase behaviors of colloids with anisotropic diffusion in two dimensions. It is found that the diffusion anisotropy of colloidal particles plays an important role in the phase transitions. A strong diffusion anisotropy induces the large vibration of particles, subsequently, the system goes into a disordered state. In the presence of the strong-coupling, particles with weak diffusion anisotropy can freeze into hexagonal crystals. Thus, there exists a solid-liquid transition. With the degree of diffusion anisotropy increasing, the transition points are shifted to the stronger-coupled region. A competition between the degree of diffusion anisotropy and coupling strength widens the transition region where the heterogeneous structures coexist, which results in a broad-peak probability distribution curve for the local order parameter. Our study may be helpful for the experiments related to the phase behavior in statistical physics, materials science and biophysical systems.     

Dynamic heterogeneity of a colloidal solution near the sol-gel transition

The results of the simulation of the dynamics of particles in a colloidal solution in the vicinity of the sol-gel transition are presented. The Van Hove correlation function, incoherent scattering function, and mean-square displacement of particles at different temperatures and volume densities have been calculated from the simulation data. The effects of dynamic heterogeneity have been evaluated numerically and the gelation temperature at different volume densities has been determined using the non-Gaussian parameter. It has been shown that the specific features observed in the dynamics of particles in the colloidal solution near the sol-gel transition are explained by the conventional separation of the particles in the system into fast particles, which contribute to translational diffusion, and slow particles, which participate predominantly in vibrational processes.     

Buckling-driven morphological transformation of droplets of a mixed colloidal suspension during evaporation-induced self-assembly by spray drying

Morphological transformation during evaporation-induced self-assembly of a mixed colloidal suspension in micrometric droplets has been investigated. It has been demonstrated that a buckling-driven shape transition of drying droplets of mixed colloidal suspension takes place during evaporation-induced self-assembly. Further, it is also shown that the distortion modulations get significantly amplified with enhancement in volume fraction of anisotropic soft colloidal component of the mixed colloids. It has been argued that the reduction in elastic modulus of formed shell, at the boundary of a drying droplet, and the anisotropic nature of one of the colloidal components facilitate the deformation process. Hierarchical structures of these assembled colloidal grains have been probed using electron microscopy and scattering techniques.     

Robust and Flexible Colloidal Photonic Crystal Films with Bending Strain–Independent Structural Colors for Anticounterfeiting

Colloidal photonic crystals (PCs) possess iridescent and metallic structural color, making them an attractive candidate for anticounterfeiting. However, traditional colloidal PC-based anticounterfeiting materials usually have bending-induced color-switching characteristics or poor flexible stability, significantly affecting their color reproducibility and durability. Here, a novel robust colloidal PC film with bending strain–independent structural color and high flexible stability has been developed through the self-assembly of SiO₂ particles into the poly(ethylene glycol) diacrylate (PEGDA) matrix. The unique microstructure of the colloidal PC film contains ordered and disordered arrays of SiO₂ nanoparticles embedded into the flexible PEGDA matrix, which is crucial to achieving bending strain–independent structural color. In fact, during the bending process, the colorless disordered arrays act as the buffer space, avoiding deformation of the colored ordered arrays and thus maintaining its original structural color. Remarkably, the film retains its structural and optical integrity after 10 000 times bending, supporting its high flexible stability and robustness. In addition, the film shows high transparency so that it can easily achieve an invisible and visible state transition under switch of weak and strong light. The film is potentially useful for applications in anticounterfeiting and encryption.     

Ripening of ordered breath figures

We have investigated the ripening of breath figures with variable initial order. A dramatic impact of the degree of order on the coalescence behavior is observed. As opposed to the two-droplet coalescence events common to the usual disordered droplet arrays, four-droplet coalescence cascades predominate in a perfectly hexagonal breath figure. Upon introduction of disorder, a gradual transition to a regime dominated by three-droplet cascades is observed. The statistics of coalescence cascades allows for detailed conclusions on the microscopic droplet dynamics.     

Basic physics of colloidal plasmas

Colloidal plasma is a distinct class of the impure plasmas with multispecies ionic composition. The distinction lies in the phase distribution of the impurity-ion species. The ability to tailor the electrostatic interactions between these colloidal particles provides a fertile ground for scientists to investigate the fundamental aspects of the Coulomb phase transition behavior. The present contribution will review the basic physics of the charging mechanism of the colloidal particles as well as the physics of the collective normal mode behavior of the general multi-ion species plasmas. Emphasis will be laid on the clarification of the prevailing confusing ideas about distinct qualities of the various acoustic modes, which are likely to exist in colloidal plasmas as well as in normal multi-ion species plasmas. Introductory ideas about the proposed physical models for the Coulomb phase transition in colloidal plasma will also be discussed. An erratum to this article is available at .     

Fluid–triangular solid phase transitions in a system of two-dimensional nematic quadrupoles

Density functional theory of freezing is used to study the phase transitions in a system of spherical colloidal particles dispersed in nematic host confined to two dimensions. We have considered both the one-component and two-component systems of the colloidal dispersions. Particles are assumed to interact via director distortion-mediated purely repulsive potential which scales as the fifth power of the inverse interparticle separation. The pair correlation functions needed as input information in the density functional theory are calculated by solving Roger–Young integral equation theory. In one-component system, a triangular crystalline phase is found to be stable. On the other hand, considering the freezing of the fluid phase of the binary mixture into a substitutionally disordered triangular solid, the temperature–composition phase diagram is found to have spindle shape for the ratio of quadrupole moment of the particles of the components being 0.9 and 0.8. The phase diagram changes to an azeotrope at a ratio 0.7. The results are verifiable in real-space experiments on nematic quadrupoles confined to a two-dimensional plane.     

Wetting-Induced Aggregation of Colloids

More than two decades ago, in a seminal paper John Cahn proposed scaling arguments for the possibility of a wetting transition in two coexisting fluid phases near the critical point. Since then, Cahn's model has been tested in many fluid systems and further refined by including the real interactions between the fluid and the solid wall. A fascinating consequence of the existence of a wetting transition is the possibility for a transition from weak to strong adsorption in the homogeneous phase. The situation is further enriched in nonstandard geometries having special geometrical constraints. The subject of this review concerns one such situation, where charge-stabilized colloidal particles are suspended in the homogeneous region of a binary liquid mixture. In this case, the preferential adsorption of one of the liquid components on to the colloid surface completely modifies the stability of the particles leading to an aggregation process. Although the exact mechanism underlying the adsorption phenomenon is still debated, it is closely related to the wetting transition. Recent experimental developments concerning the static and dynamic aspects of this phenomenon are reviewed. In addition, the main findings of a theoretical model based on the adsorption-modified electrostatic interactions between the colloidal particles are discussed.     

Phonon spectra, nearest neighbors, and mechanical stability of disordered colloidal clusters with attractive interactions

We investigate the influence of morphology and size on the vibrational properties of disordered clusters of colloidal particles with attractive interactions. Spectral features of the vibrational modes are found to depend strongly on the average number of nearest neighbors, NN, but only weakly on the number of particles in each glassy cluster. In particular, the median phonon frequency, ω(med), is constant for NN<2 and then grows linearly with NN for NN>2. This behavior parallels concurrent observations about local isostatic structures, which are absent in clusters with NN<2 and then grow linearly in number for NN>2. Thus, cluster vibrational properties appear to be strongly connected to cluster mechanical stability, and the scaling of ω(med) with NN is reminiscent of the jamming transition. Simulations of random networks of springs corroborate observations.     

Two scenarios for colloidal phase transitions

A two-dimensional assembly of charged colloidal particles induced by an alternating electric field was studied in real space by means of digital video microscopy. Phase transitions occur from a highly ordered colloidal monolayer to an isotropic suspension by changing the field strength or frequency (in the appropriate range). In particular, it is found that the strength-dependent phase transition is an infinite-order phase transition, in contrast with the frequency-dependent phase transition, which is a second-order phase transition.     

Absence of the liquid phase when the attraction is not pairwise additive

Recent work on charged colloidal suspensions with very low levels of added salt has suggested that although pairs of the colloidal particles repel, clusters of the particles attract. Motivated by this, we study simple model particles which have many-body attractions. These attractions are generic many-body attractions and are not calculated for any specific colloidal suspension. We find that many-body attractions can stabilize solid phases at low pressures but that the liquid phase is either completely absent from the equilibrium phase diagram or present only within a small region of parameter space.     

Field-induced phases of an orientable charged particle in a dilute background of point charges

We report numerical simulations of a two-dimensional dynamical model comprised of a rodlike particle surrounded by a cloud of smaller particles of the same charge, in the presence of an alternating electric field inside a box. We show that this system displays a remarkable dynamical effect; at low forcing frequencies the rod tends to align perpendicularly to the external field, whereas for higher field frequencies the standard orientation (parallel to the field) prevails. Interestingly, the transition between orientations is abrupt enough to resemble a phase transition. The fact that the "anomalous" orientation (perpendicular to the field) takes place is also interesting in the light of some recent laboratory experiments on colloidal solutions, where anomalous orientation at low frequencies was observed. Our toy model suggests that future physically realistic simulations of these systems should explore whether the anomalous orientation may be due to the collective dynamics of the colloidal particles, without necessarily involving more sophisticated electro-osmotic effects.