Dynamic Photocontrol of the Coffee‐Ring Effect with Optically Tunable Particle Stickiness

When a colloidal drop dries on a surface, most of the particles accumulate at the drop periphery, yielding a characteristic ring‐shaped pattern. This so‐called coffee‐ring effect (CRE) is observed in any pinned evaporating drop containing non‐volatile solutes. Here, the CRE is dynamically controlled for the first time by using light, and an unprecedented reconfigurability of the deposit profile is demonstrated. This is achieved through a new mechanism where particle stickiness is optically tuned on demand, thus offering reliable modulation of the deposition pattern. The system consists of anionic nanoparticles and photosensitive cationic surfactants dispersed in water. It is shown that light‐dependent modulation of surfactant–particle interactions dictates particle attraction and trapping at the liquid–gas interface, which allows us to direct particle deposition into a wide range of patterns from rings to homogeneous disks. Patterning from single drops is photoreversible upon changing the wavelength whereas spatial control in multiple drop arrays is achieved using a photomask.     

Active colloidal particles at fluid-fluid interfaces

This review explores the intersection between two important fields of colloid and interface science – that of active colloidal particles and of (passive) particles at fluid-fluid interfaces. The former uses energy input at the particle level to propel particle motions and direct dynamic assemblies. The latter relies on the spontaneous adsorption of particles at fluid interfaces to modify the interfacial energy, rheology, and permeability of biphasic materials. Here, we address two key questions that connect these otherwise distinct fields of study. How do liquid interfaces influence the dynamics of active or driven colloidal particles? How can particle activity influence the dynamics of liquid interfaces? These questions motivate the pursuit of active particle surfactants that move and organize at fluid interfaces to perform useful functions such as enhancing mass transport or modulating interfacial properties. Drawing examples from the literature, we discuss how fluid interfaces can provide a unique environment for the study of active colloids, how surface tension can be harnessed to propel particle motions, and how capillary interactions can be activated to achieve dynamically tunable emulsions and foams. We highlight opportunities for the future study and application of active particles at liquid interfaces.     

Effect of nanoparticles on the interfacial properties of liquid/liquid and liquid/air surface layers

An investigation is reported on the interfacial properties of nanometric colloidal silica dispersions in the presence of a cationic surfactant. These properties are the result of different phenomena such as the particle attachment at the interface and the surfactant adsorption at the liquid and at the particle interfaces. Since the latter strongly influences the hydrophobicity/lipophilicity of the particle, i.e., the particle affinity for the fluid interfacial environment, all those phenomena are closely correlated. The equilibrium and dynamic interfacial tensions of the liquid/air and liquid/oil interfaces have been measured as a function of the surfactant and particle concentration. The interfacial rheology of the same systems has been also investigated by measuring the dilational viscoelasticity as a function of the area perturbation frequency. These results are then crossed with the values of the surfactant adsorption on the silica particles, indirectly estimated through experiments based on the centrifugation of the dispersions. In this way it has been possible to point out the mechanisms determining the observed kinetic and equilibrium features. In particular, an important role in the mixed particle-surfactant layer reorganization is played by the Brownian transport of particles from the bulk to the interface and by the surfactant redistribution between the particle and fluid interface.     

胶体颗粒稳定的界面及胶体颗粒在界面的相互作用

Particle-stabilized dispersions such as emulsions, foams and bubbles are catching increasing attentions across a number of research areas. The adsorption mechanism and role of these colloidal particles in stabilizing the oil-water or gas-water interfaces and how these particles interact at interfaces are vital to the practical use of these dispersion systems. Although there have been intensive investigations, problems associated with the stabilization mechanisms and particle-particle interactions at interfaces still remain to explore. In this paper, we first systematically review the historical understanding of particle-stabilized emulsions or bubbles and then give an overview of the most important and well-established progress in the understanding of particle-stabilized systems, including emulsions, foams and liquid marbles. The particle-adsorption phenomena have long been realized and been discussed in academic paper for more than one century and a quantitative model was proposed in the early 1980s. The theory can successfully explain the adsorption of solid particles onto interface from energy reduction approaches. The stability of emulsions and foams can be readily correlated to the wettability of the particles towards the two phases. And extensive researches on emulsion stability and various strategies have been developed to prepared dispersion systems with a certain trigger such as pH and temperature. After that, we discuss recent development of the interactions between particles when they are trapped at the interface and highlight open questions in this field. There exists a huge gap between theoretical approaches and experimental results on the interactions of particles adsorbed at interfaces due to demanding experimental devices and skills. In practice, it is customary to use flat surfaces/interfaces as model surfaces to investigate the particle-particle at interfaces although most of the time interfaces are produced with a certain curvature. It is shown that the introduction of particles onto interfaces can generate charges at the interfaces which could possibly account for the long range electrostatic interactions. Finally, we illustrate that particle-stabilized dispersions have been found wide applications in many fields and applications such as microcapsules, food, biomedical carriers, and dry water. One of the most investigated areas is the microencapsulation of actives based on Pickering emulsion templates. The particles adsorbed at the interface can serve as interfacial stabilizers as well as constituting components of shells of colloidal microcapsules. Emulsions stabilized by solid particles derived from natural and bio-related sources are promising platforms to be applied in food related industries. Emulsion systems stabilized by solid particles of the w/w (water-in-water) feature are discussed. This special type of emulsion is attracting increasing attentions due to their all water features. Besides of oil-water interface, particle stabilized air-water interface share similar stabilization mechanism and several applications reported in the literature are subsequently discussed. We hope that this paper can encourage more scientists to engage in the studies of particle-stabilized interfaces and more novel applications can be proposed based on this mechanism     

Drying dissipative structures of Chinese black ink on a cover glass and in a dish

Macroscopic and microscopic dissipative structural patterns are formed in the course of drying a suspension of Chinese black ink on a cover glass and in a dish. The time for the drying and the pattern area increased as the particle concentration increased. The broad ring patterns of the hills accumulated with the particles formed around the outside edges on a macroscopic scale. The height and the width of the broad ring increased as the particle concentration increased. The spokelike patterns of the rims accumulated with particles were also formed on a macroscopic scale. Microscopic patterns of colloidal accumulation were observed over the whole region of the pattern area. Various types of convection cells were observed on a cover glass and in a dish at 25–80 °C. A time-resolved observation of the drying process was also made. The convections of water and the colloidal particles at different rates under gravity and the translational and rotational Brownian movement of the particles were important for the macroscopic pattern formation. Microscopic patterns were determined by the translational Brownian diffusion of the particles and the electrostatic and the hydrophobic interactions between the particles and/or between the particles and the cell wall in the course of the solidification of the particles.     

Spontaneous formation of mesostructures in colloidal monolayers trapped at the air-water interface: a simple explanation

The spontaneous formation of loosely bound ordered aggregates, foam, voids, chains, striations, and loops (see Figure 1a), called mesostructures hereafter, has been observed in colloidal monolayers trapped at the air-water interface. The distance between particles in these mesostructures is of the order of the particle radius (micrometers), implying that the colloidal interaction potential has a minimum at such distances, which could induce the phase separation of colloidal monolayers in dense and dilute regions. This is at odds with the accepted theory (Derjaguin-Landau-Verwey-Overbeek (DLVO)) of colloidal interactions, which predicts a secondary minimum at distances of nanometers between pairs of interacting particles. Moreover, the introduction of capillary, hydrophobic, and dipolar interactions between particles in an extended DLVO theory is not able to explain the spontaneous formation of mesostructures either. Recently, a great deal of effort has focused on understanding the mechanism behind the phenomenon of long-range attraction between colloidal particles confined in interfaces. In particular, this attraction has been employed to explain the spontaneous formation of mesostructures. Here, we show that the appearance of our mesostructures is due to the contamination of colloidal monolayers by silicone oil (poly(dimethylsiloxane)), which arises from the coating of the needles and syringes used to deposit and spread the particle solution at the air-water interface. The difference in the interfacial tension of water and silicone oil accounts for the formation of the experimentally observed mesostructures.     

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier‐free emulsion polymerization. Using a stress‐controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999     

Fabrication, Characterization, and Surface-Enhanced Raman Activity Study of Silver Coated Gold Nanoparticulate Films

This paper reports a study on the preparation of Ag-clad Au colloidal monolayer films by a combination of colloid self-assembly and liquid phase microwave high-pressure technique. Firstly, monodisperse Au nanoparticles prepared by microwave heating method were assembled onto a quartz slide. Then, these Au colloidal particles on the quartz surface acted as seeds for growing the Ag-clad Au composite particulate films. The obtained particulate films were characterized by UV-Vis spectra and atomic force microscopy. It was found that the thickness of the shell and thus the size of particles in the composite colloidal films could be controlled by deposition of Ag on the preformed Au colloidal particle film in the microwave reaction system, and such films significantly increased the surface-enhanced Raman scattering enhancement （SERS） ability compared with Au colloidal particle films. Their strong enhancement ability may mainly stem from relatively large particle consisting of Ag cladding as well as effective coupling among particles in the Ag-clad Au particle ftlms.     

Stabilization of oil-in-water emulsions by colloidal particles modified with short amphiphiles

Emulsions stabilized through the adsorption of colloidal particles at the liquid-liquid interface have long been used and investigated in a number of different applications. The interfacial adsorption of particles can be induced by adjusting the particle wetting behavior in the liquid media. Here, we report a new approach to prepare stable oil-in-water emulsions by tailoring the wetting behavior of colloidal particles in water using short amphiphilic molecules. We illustrate the method using hydrophilic metal oxide particles initially dispersed in the aqueous phase. The wettability of such particles in water is reduced by an in situ surface hydrophobization that induces particle adsorption at oil-water interfaces. We evaluate the conditions required for particle adsorption at the liquid-liquid interface and discuss the effect of the emulsion initial composition on the final microstructure of oil-water mixtures containing high concentrations of alumina particles modified with short carboxylic acids. This new approach for emulsion preparation can be easily applied to a variety of other metal oxide particles.     

Neutron reflectivity and small angle neutron scattering: An introduction and perspective on recent progress

In recent years both neutron reflectivity, NR, and small angle neutron scattering, SANS, have been applied to the study of increasingly complex surface and interfacial properties. The methodology and advantages of using these neutron scattering techniques to probe surfaces and interfaces will be briefly outlined. Some recent examples of surfactant, polymer and polymer–surfactant adsorption at the air–water, liquid–solid, and liquid–liquid planar interfaces are highlighted. These are contrasted by some examples of the nature of adsorption on colloidal sol particles and emulsion droplets.     

Forces between a rigid probe particle and a liquid interface. II. The general case

The semianalytic theory developed previously (Chan, D. Y. C., Dagastine, R. R., and White, L. R., J. Colloid Interface Sci. 236, 141 (2001)) to predict the force curve of an AFM measurement at a liquid interface using a colloidal probe has been expanded to incorporate a general force law with both attractive and repulsive forces. Expressions for the gradient of the force curve are developed to calculate the point at which the probe particle on the cantilever will spontaneously jump in toward the liquid interface. The calculation of the jump instability is reduced to a straightforward embroidery of the simple algorithms presented in Chan et al. In a variety of sample calculations using force laws including van der Waals, electrostatic, and hydrophobic forces for both oil/water and bubble/water interfaces, we have duplicated the general behaviors observed in several AFM investigations at liquid interfaces. The behavior of the drop as a Hookean spring and the numerical difficulties of a full numerical calculation of F(deltaX) are also discussed.     

Colloidal Microcapsules: Self‐Assembly of Nanoparticles at the Liquid–Liquid Interface

Colloidal microcapsules (MCs) are highly modular, inherently multiscale constructs of capsules stabilized by nano‐/microparticle shells, with applications in many areas of materials and biological sciences, such as drug delivery, encapsulation, and microreactors. Until recently, fabrication of colloidal MCs focused on the use of submicron‐sized particles because the smaller nanoparticles (NPs) are inherently unstable at the interface owing to thermal disorder. However, stable microcapsules can now be obtained by tuning the interactions between the nanometer‐sized building blocks at the liquid–liquid interface. This Review highlights recent developments in the fabrication of colloidal MCs using NPs.     

On the effect of thermophoretic force,gravitational force,and particle–particle interactions in field-flow fractionation

The theoretical calculations confirmed that the gravitational force cannot be neglected in all field-flow fractionation techniques separating nanometer-sized colloidal particles whenever particle diameter is approximately 200?nm and larger. Particle–particle repulsive interactions, mostly electrostatic repulsions, influence substantially concentration distribution established by any effective field acting across the fractionation channel, as confirmed explicitly for thermophoretic force generated by temperature gradient in microthermal field-flow fractionation. The ionic strength of the carrier liquid causes the screening of the electrostatic double layer around the dispersed particles and thus influences the retention. The attractive particle–particle forces occur when the zeta potential of the particles approaches to 0?mV, the electrostatic repulsions are screened, and the aggregation of the particles is observed. The pH influences differently the size and zeta potential of the plain polystyrene latex particles and of the particles modified on the surface by the groups –COOH and –NH₂. The role of a detergent in carrier liquid is non-negligible, as demonstrated by its presence or absence in carrier liquid.     

Particle bridging between oil and water interfaces

Particle bridging between a water drop and a flat oil-water interface has been observed when the drop is brought into contact with the interface, leading to the formation of a dense particle monolayer of disc shape (namely, particle disc) that prevents the drop from coalescing into the bulk water phase. Unlike previous observations where particles from opposite interfaces appear to register with each other before bridging, the present experiment demonstrates that the particle registry is not a necessity for bridging. In many cases, the particles from one of the interfaces were repelled away from the contact region, leaving behind the particles from the other interface to bridge the two interfaces. This is confirmed by particle bridging experiments between two interfaces covered with different sized particles, and between a particle-covered interface and a clean interface. The dynamics associated with the growth of the particle disc due to particle bridging follows a power law relationship between the radius of the disc and time: r proportional, variant t0.32+/-0.03. A scaling analysis assuming capillary attraction as the driving force and a hydrodynamic resistance leads to the power law r proportional, variant t1/3, in good agreement with the experiment. In addition, we found that binary mixtures of two different sized particles can undergo phase segregation driven by the particle bridging process.     

Concentrating particles on drop surfaces using external electric fields

We propose to use an externally applied uniform electric field to alter the distribution of particles on the surface of a drop immersed in another immiscible liquid. Specifically, we seek to generate well-defined concentrated regions at the drop surface while leaving the rest of the surface particle free. Experiments show that when the dielectric constant of the drop is greater than that of the ambient liquid the particles for which the Clausius-Mossotti factor is positive move along the drop surface to the two poles of the drop. Particles with a negative Clausius-Mossotti factor, on the other hand, move along the drop surface to form a ring near the drop equator. The opposite takes place when the dielectric constant of the drop is smaller than that of the ambient liquid, namely particles for which the Clausius-Mossotti factor is positive form a ring near the equator while those for which such a factor is negative move to the poles. This motion is due to the dielectrophoretic force that acts upon particles because the electric field on the surface of the drop is nonuniform, despite the uniformity of the applied electric field. Experiments also show that when small particles collect at the poles of a deformed drop the electric field needed to break the drop is smaller than without particles. These phenomena could be useful to concentrate particles at a drop surface within well-defined regions (poles and equator), separate two types of particles at the surface of a drop or increase the drop deformation to accelerate drop breakup.     

Regulating Nonclassical Pathways of Silicalite‐1 Crystallization through Controlled Evolution of Amorphous Precursors

Differentiating mechanisms of zeolite crystallization is challenging owing to the vast number of species in growth solutions. The presence of amorphous colloidal particles is ubiquitous in many zeolite syntheses, and has led to extensive efforts to understand the driving force(s) for their self‐assembly and putative roles in processes of nucleation and growth. In this study, we use a combination of in situ scanning probe microscopy, particle dissolution measurements, and colloidal stability assays to elucidate the degree to which silica nanoparticles evolve in their structure during the early stages of silicalite‐1 synthesis. We show how changes in precursor structure are mediated by the presence of organics, and demonstrate how these changes lead to significant differences in precursor–crystal interactions that alter preferred modes of crystal growth. Our findings provide guidelines for selectively controlling silicalite‐1 growth by particle attachment or monomer addition, thus allowing for the manipulation of anisotropic rates of crystallization. In doing so, we also address a longstanding question regarding what factors are at our disposal to switch from a nonclassical to classical mechanism.     

Structural analysis of anisometric colloidal iron(III)-hydroxide particles and particle-aggregates incorporated in poly(vinyl-acetate) networks

Anisometrical colloidal iron(III)hydroxide particles and particle aggregates were incorporated in elastic poly(vinyl acetate) networks. A novel method has been developed to fix the colloidal structure of deformed samples. Digitalized image analysis has been applied in order to evaluate the micrographs. The rod-like particles allow for studying the local deformation and orientation due to uniaxial and triaxial deformations. The density correlation function as well as the micrographs show that the structure of aggregates is not influenced by the strain. Due to strong attractive interactions between the colloidal particles the developing strain is not enough to destroy the aggregate structure. The orientation behavior of the model filled networks can be satisfactorily described by using the affinity principle.     

Oil-coated bubbles in particle suspensions,capillary foams,and related opportunities in colloidal multiphase systems

The interaction of solid particles with fluid interfaces in colloidal multiphase systems can lead to a wide range of fascinating and sometimes useful phenomena. Most of the research in this area has focused on ternary systems with two immiscible fluids and one particle species. After a brief review of some common characteristics, this article discusses recent insights and future opportunities regarding systems that confront particles with multiple types of fluid interfaces, focusing in particular on the interaction of particles with oil-coated bubbles in aqueous media. Relevant examples include the particle-assisted reconfiguration of bubble–droplet morphologies, the separation of hydrophilic particles from aqueous slurries by oily bubble flotation, and the formation of capillary foams, a promising new class of foam materials.     

Amphiphilic Janus Gold Nanoparticles Prepared by Interface‐Directed Self‐Assembly: Synthesis and Self‐Assembly

Materials with Janus structures are attractive for wide applications in materials science. Although extensive efforts in the synthesis of Janus particles have been reported, the synthesis of sub‐10 nm Janus nanoparticles is still challenging. Herein, the synthesis of Janus gold nanoparticles (AuNPs) based on interface‐directed self‐assembly is reported. Polystyrene (PS) colloidal particles with AuNPs on the surface were prepared by interface‐directed self‐assembly, and the colloidal particles were used as templates for the synthesis of Janus AuNPs. To prepare colloidal particles, thiol‐terminated polystyrene (PS‐SH) was dissolved in toluene and citrate‐stabilized AuNPs were dispersed in aqueous solution. Upon mixing the two solutions, PS‐SH chains were grafted to the surface of AuNPs and amphiphilic AuNPs were formed at the liquid–liquid interface. PS colloidal particles decorated with AuNPs on the surfaces were prepared by adding the emulsion to excess methanol. On the surface, AuNPs were partially embedded in the colloidal particles. The outer regions of the AuNPs were exposed to the solution and were functionalized through the grafting of atom‐transfer radical polymerization (ATRP) initiator. Poly[2‐(dimethamino)ethyl methacrylate] (PDMAEMA) on AuNPs were prepared by surface‐initiated ATRP. After centrifugation and dissolving the colloidal particles in tetrahydrofuran (THF), Janus AuNPs with PS and PDMAEMA on two hemispheres were obtained. In acidic pH, Janus AuNPs are amphiphilic and are able to emulsify oil droplets in water; in basic pH, the Janus AuNPs are hydrophobic. In mixtures of THF/methanol at a volume ratio of 1:5, the Janus AuNPs self‐assemble into bilayer structures with collapsed PS in the interiors and solvated PDMAEMA at the exteriors of the structures.