Experiments with active and driven synthetic colloids in complex fluids

In this review, we focus on recent experimental research involving active colloidal particles of non-biological origin evolving in non-Newtonian fluids. This includes self-propelling active particles and particles driven by external fields. We present different propulsion strategies that are either enabled, or strongly modified, by the presence of a complex medium. This paves the way for novel mechanisms of active transport in biofluids or in other non-Newnotian fluids. When considering the medium, we differentiate between disordered complex fluids, such as diluted polymer solutions, and liquid crystals. While the latter are also viscoelastic fluids, the ability to control their molecular orientation results in distinct colloidal driving and steering mechanisms, and enables new types of active soft matter in the form of active quasi-particles.     

甲壳质、甲壳胺衍生物保护的贵金属胶体

<正> 高分子保护金属胶体是近年来金属催化剂领域中引人注目的研究课题。它除了具有负载型金属催化剂的优点外,还具备下列新优点:a.胶体分散体系可形成“均相”溶液;b.保护高分子可以屏蔽胶体催化剂,减小毒物或空气的不良影响;c.胶体溶液的透光性能比颗粒要好得多,由此最近高分子保护金属胶体常被用于光化学研究中的催化剂;d.高分子保护金属胶体与普通金属胶体相比较,不仅具有较稳定的特点,而且尺寸小(1—10nm)、分布窄,表现出极高的催化活性和选择性;e.保护高分子对金属的修饰作用,可     

Engineering shapes of active colloids for tunable dynamics

Active particles can autonomously propel and have the tendency to organize into high-order ensembles and phases that evolve and reconfigure. They have emerged as a focused subject in contemporary colloid science, holding great promise in advancing fields, such as cargo delivery, sensing, micromachinery and microrobotics, and materials science. Realization of the full potentials of active particles requires delicate control of their dynamics in propulsion and assembly, which is challenging due to the out-of-equilibrium nature of such systems. Recently, systematically engineered colloidal shapes have been exploited as an effective means to tune and even program the dynamic behaviors of active particles. Various anisotropic particles, with controlled geometries and possessing either homogeneous or heterogeneous composition, have been fabricated, regulating how particles actively propel, interact, and assemble under several chemical and physical stimuli. In this paper, we provide an overview of these progresses. We also briefly discuss our view on the future directions and challenges.     

Multiple dynamic regimes in colloid‐polymer dispersions: New insight using X‐ray photon correlation spectroscopy

We present an X‐ray photon correlation spectroscopy (XPCS) study of dynamic transitions in an anisotropic colloid‐polymer dispersion with multiple arrested states. The results provide insight into the mechanism for formation of repulsive glasses, attractive glasses, and networked gels of colloids with weakly adsorbing polymer chains. In the presence of adsorbing polymer chains, we observe three distinct regimes: a state with slow dynamics consisting of finite particles and clusters, for which interparticle interactions are predominantly repulsive; a second dynamic regime occurring above the saturation concentration of added polymer, in which small clusters of nanoparticles form via a short‐range depletion attraction; and a third regime above the overlap concentration in which dynamics of clusters are independent of polymer chain length. The observed complex dynamic state diagram is primarily governed by the structural reorganization of a nanoparticle cluster and polymer chains at the nanoparticle‐polymer surface and in the concentrated medium, which in turn controls the dynamics of the dispersion. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 752–760     

Preparation of polyurethane dispersions by miniemulsion polymerization

With a two‐step miniemulsion polymerization, hydrophobic polyurethane (PU) dispersions were prepared with a cosurfactant, the costabilizer hexadecane (HD) in the oil phase, and sodium dodecyl sulfate (SDS) in the water phase. The first step involved the formation of NCO‐terminated prepolymers between isophorone diisocyanate and poly(propylene glycol) oligomer in toluene. Next, PU dispersions were produced by a miniemulsion method in which an oil phase containing NCO‐terminated prepolymers, HD, the chain extender 1,4‐butanediol (BD), the crosslinking agent trimethylol propane (TMP), and the catalyst dibutyltin dilaurate was dispersed in the water phase containing SDS. The influence of experimental parameters, such as the ultrasonication time, concentrations of SDS and HD, and TMP/BD and NCO/OH equivalent ratios, on the sizes of the miniemulsion droplets and polymer particles, as well as the molecular weights and thermal properties of the PU polymer, was examined. The chemical structure of the produced PU polymer was identified with a Fourier transform infrared spectrometer. The molecular weight distribution and average particle size were measured through gel permeation chromatography and dynamic light scattering, respectively. The thermal stability of the PU polymer was characterized with thermogravimetric analysis. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4870–4881, 2005     

含有不同脂肪环胺侧基的聚丙烯酰胺保护的贵金属胶体制备的研究

<正> 近年来高分子保护金属胶体的研究在金属催化剂领域中受到人们的突出关注。自Hirai等人对聚乙烯吡咯烷酮保护的铂族金属胶体的系统研究工作发表之后,一项重要的研究进展是双金属胶体的制备成功。当前制备窄分布乃至单分散胶体的努力成为众多研究的集中目标,具有重要的理论意义和实际应用价值。Bradley等报道用金属蒸汽冷凝至含有高分子稳定剂的有机溶剂中制备得到2.0—5.0nm直径的金属胶体,Schmid等报道用水溶性三苯膦磺酸钠小分子配位体作为稳定剂制备得到直径为18.6±0.1nm的金胶体。Esumi等用在有机溶剂中热解乙酸钯的方法制备得到不同粒径的均一球形的钯金属胶体。     

Active colloids on fluid interfaces

We review recent work on active colloids at interfaces, including self-propelled colloids that move by generating a propulsive force, and driven colloids that move under external fields. Features unique to fluid interfaces alter the flows generated at interfaces by active colloid motion, and hydrodynamic interactions with these layers. We emphasize recent observations of natural swimmers, like bacteria, and bio-mimetic colloids including self-propelled phoretic and Marangoni swimmers, and magnetically driven colloids. We discuss active colloid interaction with boundaries and with each other. We conclude with a discussion of open issues and opportunities to design active colloids as active surface agents that manipulate interfacial properties and the transport in the vicinity of interfaces.     

Defect structures and three-body potential of the mean force for nanoparticles in a nematic host

We have investigated the defect structures and potential of mean force (PMF) of three colloidal spheres immersed in a nematic liquid crystal (for linear and equilateral-triangular configurations), using a coarse-grained theory for the tensor order parameter. At large separations, each sphere is surrounded by an equatorial Saturn ring defect; at short separations, the theory predicts a drastic reorganization of the disclination lines: additional disclination rings are observed in planes perpendicular to the original ones. For both types of configurations, the PMF is attractive and always greater than a pairwise sum of binary PMFs. Comparing the PMFs of linear and equilateral-triangular configurations, we have found the triangular configuration to be more stable than the linear configuration. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1033-1040, 2005     

Particle monolayer formation at the air–water interface by silica particle with well‐defined grafted polymer

Particle monolayer formation at the air–water interface by polymer‐grafted colloidal silica was investigated. Methyl methacrylate (MMA) was polymerized from initiative bromide groups at colloidal silica surface by atom transfer radical polymerization. We obtained polymer‐grafted silica particle (SiO₂‐PMMA) with relative narrow polydispersity of PMMA. For the polymer‐grafted particle with high graft density, particle monolayer formation was confirmed by π‐A isotherm measurement and SEM observation. Interparticle distance was controllable by surface pressure. Furthermore, grafted polymer chains were suggested to be fairly extended at the air–water interface. However, for the polymer‐grafted particle with low graft density, monolayer structure on substrate showed aggregation and voids. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2789–2797, 2006     

Comparisons between cationic polyelectrolytes and nonionic polymers for the protection of palladium and platinum nanocatalysts

Several palladium and platinum nanocatalysts protected by cationic polyelectrolytes were prepared by the in-situ reduction of palladium chloride, PdCl₂, and dihydrogen hexachloroplatinate, H₂PtCl₆. The particle sizes and size distributions were determined by transmission electron microscopy, and the colloids were further characterized by UV-vis spectroscopy. The catalytic activity of these nanoparticles was qualitatively investigated by the hydrogenation and conversion of cyclohexene as a model reaction and compared to palladium and platinum colloids protected by a selection of water-soluble, nonionic polymers. The results show that the catalytic activity is strongly influenced by the protective polymer chosen, as well as particle size and morphology. The use of cationic polyelectrolytes decreases the catalytic activities significantly, in comparison to several water-soluble, nonionic polymers investigated. The effects depend strongly on the particular metal, as illustrated in this case by differences observed between palladium and platinum. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3151–3160, 1997     

Adsorption of flexible polymers on small colloids: complexes and gels

We describe adsorption of neutral flexible polymer chains onto small colloids that can be covered by one chain. We discuss the structure of star-shaped complexes and their abundance in dilute solution. In semi-dilute solution similar complexes form crosslinks between chains and cause gelation.     

Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution

We report an unsurpassed solution characterization technique based on analytical ultracentrifugation, which demonstrates exceptional potential for resolving particle sizes in solution with sub‐nm resolution. We achieve this improvement in resolution by simultaneously measuring UV/Vis spectra while hydrodynamically separating individual components in the mixture. By equipping an analytical ultracentrifuge with a novel multi‐wavelength detector, we are adding a new spectral discovery dimension to traditional hydrodynamic characterization, and amplify the information obtained by orders of magnitude. We demonstrate the power of this technique by characterizing unpurified CdTe nanoparticle samples, avoiding tedious and often impossible purification and fractionation of nanoparticles into apparently monodisperse fractions. With this approach, we have for the first time identified the pure spectral properties and band‐gap positions of discrete species present in the CdTe mixture.     

Nanoscale colloidal metals and alloys stabilized by solvents and surfactants Preparation and use as catalyst precursors

Solvent-stabilized organosols of the early transition metal series, e.g. Ti, Zr, Nb, and Mn, may be prepared by the reduction of the THF adducts or thioether solutions of the corresponding metal halides with K[BEt₃H]. Mono- and bimetallic organosols of Group 6–11 metals stabilized by tetraalkylammonium halides may be formed either by the reduction of the metal salts using NR₄ hydrotriorganoborates or conventional agents, e.g. H₂ or HCO₂H, after the pretreatment of the metal salts with NR₄X. The chemical reduction of transition metal salts in the presence of hydrophilic surfactants provides straightforward access to nanostructured mono- and bimetallic hydrosols. This synthesis can be performed even in water. Mono- and bimetallic nanoparticles stabilized by lipophilic or hydrophilic surfactants of the cationic, anionic or nonionic type serve as precursors for heterogeneous metal colloid catalysts effective for the hydrogenation and oxidation of organic substrates. Bimetallic precursors, e.g. Pt---Rh, have a synergic effect on the catalytic activity. A comparison of catalytic results and CO chemisorption experiments has revealed that the protecting surfactants still cover the nanoparticle surface after adsorption on supports, which markedly improves the lifetime of the catalysts. Chiral protecting agents may induce enantioselectivity in metal colloid catalysts.     

Preparation and catalytic properties of NR-stabilized palladium colloids

Palladium colloids revealing narrow particle size distributions can be obtained by chemical reduction using tetra–alkylammonium hydrotriorganoborates. Combining the stabilizing agent [NR] with the reducing agent [BEt₃H^?] provides a high concentration of the protecting group at the reduction centre. Alternatively, NR₄X (X = halogen) may be coupled to the metal salt prior to the reduction step: addition of N(octyl)₄Br to Pd(ac)₂ in THF, for example, evokes an active interaction between the stabilizing agent and the metal salt. Reduction of NR-stabilized palladium salts with simple reducing agents such as hydrogen at room temperature yields stable palladium organosols which may be isolated in the form of redispersible powders. The anion of the palladium salt is crucial for the success of the colloid synthesis. Electron microscopy shows that the mean particle size ranges between 1.8 and 4.0 nm. An X–ray–photoelectron spectrscopic examination demonstrated the presence of zerovalent palladium. These palladium colloids may serve as both homogeneous and heterogeneous hydrogenation catalysts. Adsorption of the colloids onto industrially important supports can be achieved without agglomeration of palladium particles. The standard activity of a charcoal catalyst containing 5% of colloidal palladium determined through the cinnamic acid standard test was found to exceed considerably the activity of the conventional technical catalysts. In addition, the lifespan of the catalyst containing a palladium colloid, isolated from the reduction of [N(octyl)₄]₂PdCl₂Br₂ with hydrogen, is superior to conventionally prepared palladium/charcoal (Pd/C) catalysts. For example, the activity of a conventional Pd/C catalyst is completely suppressed after 38×10³ catalytic cycles per Pd atom, whereas the colloidal Pd/C catalyst shows activity even after 96times;10³ catalytic cycles.     

Formation of disperse nanoparticles at the oil/water interface in normal microemulsions

An artificial oil/water interface was created in normal microemulsions. Various well-dispersed inorganic nanoparticles were successfully fabricated at this micelle interface, and a "hot liquid annealing" process was used to crystallize the products. Owing to the large solubility of the source materials in the water phase, the colloidal nanoparticles can easily be prepared on a large scale. Compared with traditional reverse-microemulsion methods, the method reported here yields larger amounts of colloidal particles but with the same quality.