Interplay between Colloids and Interfaces : Emulsions, Foams and Microtubes

The central theme of this thesis is the interplay between colloids and interfaces. The adsorption of colloids at fluid-fluid interfaces is the main topic and covers Chapters 2-6. Pickering emulsions where colloidal particles act as emulsion stabilizers in the absence of surfactants are studied in a number of systems with different colloids, particle shapes and oils. Interfacial particle adsorption is widely employed in food, cosmetic and pharmaceutical applications and plays a pivotal role in oil recovery and metallurgical refining. The conditions under which Pickering emulsions may form spontaneously are discussed in Chapter 2. The spontaneous formation and the stability of the Pickering emulsions of varying composition are achieved by a collective effect of solid particles, amphiphilic ions and interfacial tensions of the bare oil-water interface of ~10 mN/m or below. The observed stability and formation of emulsions of different composition point to a new class of solid-stabilized meso-emulsions. The effect of particle shape anisotropy in the stabilization of emulsions is investigated in Chapter 3 by employing for the first time cubic, ellipsoidal and peanut-type hematite microparticles. The interfacial packing and orientation of these anisotropic microparticles are revealed at the single particle level by direct microscopic observations. Emulsions are stable against further coalescence for at least one year. The creation of surfactant-free Pickering foams with anisotropic hematite microparticles and their dependence on ionic strength are studied in Chapter 4. The microparticles cover bubbles by densely packed interfacial monolayers that provide high stability against disproportionation and coalescence. Moreover, solid-stabilized air bubbles are used as scaffolds for the creation of free-standing particle films that are, in fact, inorganic bilayers that only consist of cubes. In Chapter 5 the self-assembly of colloidal silica cubes at oil-water interfaces is discussed. A combination of optical and laser scanning confocal microscopy enables the in-situ study of both the packing as well as the orientation of the colloidal cubes at the interface. Single layers of cubic particles arrange in ordered domains, displaying a packing intermediate between cubic and hexagonal. Moreover, the cubes show a preference for orienting parallel with the interface. Solid-stabilized emulsions from natural resources are presented in Chapter 6 by using colloids, prepared from the water-insoluble corn protein zein, and soy bean oil. Zein colloids are synthesized via an anti-solvent precipitation procedure and employed in the formation of stable oil-in-water Pickering emulsions as a function of particle concentration, pH and ionic strength. Finally, a thermo-reversible colloid-in-tube co-assembly approach that couples molecular self-assembly with colloidal self-assembly is introduced in Chapter 7. While surfactant and cyclodextrin molecules form microtubes, colloids assemble into a library of dynamic colloidal structures within those microtubes. Isotropic spheres form straight, zigzag, and zipper chains depending on the tube-sphere size ratio. Double and triple helical structures, a common occurrence in nature, were even generated from colloidal spheres. Moreover, we demonstrate that the co-assembly of microtubes and colloids is generic for colloids with different shapes and materials. The hierarchical colloid-in-tube co-assembly provides a novel route to temperature-sensitive particle alignment and their release near human-body temperature

Magnetic nanoparticles-induced anisotropic shrinkage of polymer emulsion droplets

We here report magnetic nanoparticles (NPs)-induced buckling instability and anisotropic shrinkage behavior of polymer emulsion droplets. The oil-in-water emulsion is stabilized by the surfactant, and NPs are dispersed into the oil phase. The surface ligands (oleic acid and oleylamine) number of the NPs is an important factor to affect the shrinkage process. When a part of the ligands of the NPs is removed, the NPs show good interface attachment at the oil– water interface even with the presence of a large amount of surfactant. The increase of the interfacial viscoelasticity resulting from the attachment of NPs induced the occurrence of a buckling process. The mechanism is explored and the effect of the concentration of polystyrene and NPs is investigated in detail. The results could be helpful to understand and solve problems related with coating techniques and elastic instabilities in nature

Rigid sphere transport through a colloidal gas–liquid interface

In this paper we report on the gravity-driven transport of rigid spheres of various sizes through the fluid–fluid interface of a demixed colloid–polymer mixture. Three consecutive stages can be distinguished: (i) the sphere approaches the interface by sedimenting through the polymer-rich phase, (ii) it is subsequently transported to the colloid-rich phase and (iii) it moves away from the interface. The spheres are covered by a thin wetting film of the colloid-rich phase, to which they are eventually transported. The ultralow interfacial tension in these phase-separating mixtures results in very small capillary forces so that the process takes place in the low Reynolds regime. Moreover, it enables the investigation of the role of capillary waves in the process. Depending on the Bond number, the ratio between gravitational force and capillary force acting on the sphere, different transport configurations are observed. At low Bond numbers, the drainage transport configuration, with a dominant capillary force, is encountered. At high Bond numbers, spheres are transported through the tailing configuration, with a dominant gravitational force. By varying the sphere diameter, we observe both transport configurations as well as a crossover regime in a single experimental system

Evolution of equilibrium pickering emulsions: a matter of time scales

A new class of equilibrium solid-stabilized oil-in-water emulsions harbors a competition of two processes on disparate time scales that affect the equilibrium droplet size in opposing ways. The aim of this work is to elucidate the molecular origins of these two time scales and demonstrate their effects on the evolution of the emulsion droplet size. First, spontaneous emulsification into particle-covered droplets occurs through in situ generation of surface-active molecules by hydrolysis of molecules of the oil phase. We show that surface tensions of the oil-water interfaces in the absence of stabilizing colloidal particles are connected to the concentration of these surface-active molecules, and hence also to the equilibrium droplet size in the presence of colloids. As a consequence, the hydrolysis process sets the time scale of formation of these solid-stabilized emulsions. A second time scale is governing the ultimate fate of the solid-stabilized equilibrium emulsions: by condensation of the in situ generated amphiphilic molecules onto the colloidal particles, their wetting properties change, leading to a gradual transfer from the aqueous to the oil phase via growth of the emulsion droplets. This migration is observed macroscopically by a color change of the water and oil phases, as well as by electron microscopy after polymerization of the oil phase in a phase separated sample. Surprisingly, the relative oil volume sets the time scale of particle transfer. Phase separation into an aqueous phase and an oil phase containing colloidal particles is influenced by sedimentation of the emulsion droplets. The two processes of formation of surface-active molecules through hydrolysis and condensation thereof on the colloidal surface have an opposite influence on the droplet size. By their interplay, a dynamic equilibrium is created where the droplet size always adjusts to the thermodynamically stable state

Particle Shape Anisotropy in Pickering Emulsions: Cubes and Peanuts

We have investigated the effect of particle shape in Pickering emulsions by employing, for the first time, cubic and peanut-shaped particles. The interfacial packing and orientation of anisotropic microparticles are revealed at the single-particle level by direct microscopy observations. The uniform anisotropic hematite microparticles adsorb irreversibly at the oil–water interface in monolayers and form solid-stabilized o/w emulsions via the process of limited coalescence. Emulsions were stable against further coalescence for at least 1 year. We found that cubes assembled at the interface in monolayers with a packing intermediate between hexagonal and cubic and average packing densities of up to 90%. Local domains displayed densities even higher than theoretically achievable for spheres. Cubes exclusively orient parallel with one of their flat sides at the oil–water interface, whereas peanuts preferentially attach parallel with their long side. Those peanut-shaped microparticles assemble in locally ordered, interfacial particle stacks that may interlock. Indications for long-range capillary interactions were not found, and we hypothesize that this is related to the observed stable orientations of cubes and peanuts that marginalize deformations of the interface

Residence and waiting times of Brownian interface fluctuations

We report on the residence times of capillary waves above a given height h and on the typical waiting time in between such fluctuations. The measurements were made on phase-separated colloid-polymer systems by laser scanning confocal microscopy. Due to the Brownian character of the process, the stochastics vary with the chosen measurement interval Δt. In experiments, the discrete scanning times are a practical cutoff and we are able to measure the waiting time as a function of this cutoff. The measurement interval dependence of the observed waiting and residence times turns out to be solely determined by the time-dependent height-height correlation function g(t). We find excellent agreement with the theory presented here along with the experiments

Conditions for equilibrium solid-stabilized emulsions

Particular types of solid-stabilized emulsions can be thermodynamically stable as evidenced by their spontaneous formation and monodisperse droplet size, which only depends on system parameters. Here, we investigate the generality of these equilibrium solid-stabilized emulsions with respect to the basic constituents: aqueous phase with ions, oil, and stabilizing particles. From systematic variations of these constituents, we identify general conditions for the spontaneous formation of monodisperse solid-stabilized emulsions droplets. We conclude that emulsion stability is achieved by a combination of solid particles as well as amphiphilic ions adsorbed at the droplet surface, and low interfacial tensions of the bare oil-water interface of order 10 mN/m or below. Furthermore, preferential wetting of the colloidal particles by the oil phase is necessary for thermodynamic stability. We demonstrate the sufficiency of these basic requirements by extending the observed thermodynamic stability to emulsions of different compositions. Our findings point to a new class of colloidstabilized meso-emulsions with a potentially high impact on industrial emulsification processes due to the associated large energy savings

Dipolar structures in colloidal dispersions of PbSe and CdSe quantum dots

We show by cryogenic transmission electron microscopy that PbSe and CdSe nanocrystals of various shapes in a liquid colloidal dispersion self-assemble into equilibrium structures that have a pronounced dipolar character, to an extent that depends on particle concentration and size. Analyzing the cluster-size distributions with a one-dimensional (1D) aggregation model yields a dipolar pair attraction of 8-10 kBT at room temperature. This accounts for the long-range alignment of the crystal planes of individual nanocrystals in self-assembled superstructures and for anisotropic nanostructures grown via oriented attachment