Few fully natural and biocompatible materials are available for the effective particle-stabilization of emulsions since strict requirements, such as insolubility in both fluid phases and intermediate wettability, need to be met. In this paper, we demonstrate the first use of water-insoluble proteins, employing the corn protein zein as a representative of this family, as effective particle-stabilizers of oilin-water emulsions of natural oils and water. For this purpose, we synthesized zein colloidal particles through an anti-solvent precipitation procedure and demonstrated their use in the formation of stable oil-in-water Pickering emulsions as a function of particle concentration, pH and ionic strength. We confirmed that the wetting properties of zein, studied as a function of pH and ionic strength, strongly favor interfacial particle adsorption with oil-in-water three-phase contact angles q ow close to 90. We found that unmodified zein colloidal particles can produce stable, surfactant-free o/w emulsions with droplet sizes in the range 10-200 mm under experimental mixing conditions (2 min with Ultra Turrax homogenizer at 13 500 rpm) at pH above and below the isoelectric point of zein, for low to moderate ionic strengths (1-10 mM). Under conditions where the particle volume fraction is low (<0.2 wt%) or at low pH, the resulting emulsions are not stable against coalescence. At a higher ionic strength, the zein particles have a tendency to aggregate and the resulting emulsions flocculate, forming an emulsion-gel phase.

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 k B T 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.

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.

Self-assembly is ubiquitous in nature, science, and technology and provides a general route to achieve order from disorder at various length scales. [1] Extensive effort has been exerted to molecular and colloidal self-assembly, where molecules and colloids, respectively, organize into larger-scale ordered structures. Although these two research areas have developed separately to a great extent, their combination would be very promising. Nature, for instance, utilizes hierarchical selfassembly across different length scales to construct complex, dynamic functional entities such as cells. Here we bridge the nano-and microscale by the hierarchical co-assembly between molecules and colloids, where molecular self-assembly induces the self-assembly of colloids into ordered structures.Colloidal self-assembly is widely employed in analogues of molecular systems and processes encountered in chemistry, physics, and biology. [2][3][4][5][6][7][8][9][10][11] Colloids mimic naturally occurring systems such as microorganisms, [10] micelles, [3] molecules, [6] and polymers. [7] The directed organization into such specific ordered structures is fuelled by the rapidly advancing availability of colloidal building blocks that are asymmetric in shape and chemical functionality. [2][3][4][5][6]12] Of particular interest is the creation of colloidal helical structures, for instance, as models of the DNA helix. Colloidal structures with a helical twist have been assembled from complex anisotropic magnetic colloids [4] and amphiphilic Janus spheres. [5] These sophisticated building blocks are believed to be essential for inducing directionality and chirality in self-assembly. [13] Here we demonstrate that the simplest of building blocks, namely the isotropic sphere, already suffices to generate a library of ordered structures, including helical sphere chains. These structures form through the spontaneous co-assembly of colloidal spheres and confining surfactant-cyclodextrin microtubes, [14] thereby coupling molecular and colloidal selfassembly. We introduce these microtubes as a novel versatile platform for the self-assembly of colloid-in-tube structures, as depicted in Figure 1 a. Microtube precursors sodium dodecyl sulfate (SDS) and b-cyclodextrin (b-CD) are mixed with colloidal particles at elevated temperatures to obtain isotropic mixtures (see the Supporting Information for experimental details). The microtubes form upon cooling to room temperature and, simultaneously, the colloids co-assemble inside the microtubes into ordered, chain-like structures throughout the sample volume. The elementary building block of the straight and rigid microtubes consists of one SDS molecule and two b-cyclodextrin molecules, forming an aqueous inclusion complex (Figure 1 a). These building blocks in turn assemble into multiple curved SDS/b-CD bilayers with in-plane order, thereby forming a set of coaxial hollow cylinders. The thus formed microtubes exhibit a rather uniform pore diameter of 0.9 mm and prefer to align parallel with each other (Figu...

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 colloid-stabilized meso-emulsions with a potentially high impact on industrial emulsification processes due to the associated large energy savings.

The co‐assembly of spherical colloids and surfactant–cyclodextrin microtubes yields a library of dynamic colloid‐in‐tube structures, including helices. In situ observations of these structures, including their thermo‐reversible assembly and disassembly, demonstrate the potential of the interplay between molecular and colloidal self‐assembly, thereby providing a novel route to temperature‐sensitive particle alignment and release.

Abstract.Mixtures of colloids and polymers display a rich phase behavior, involving colloidal gas (rich in polymer, poor in colloid), colloidal liquid (poor in polymer, rich in colloid) and colloidal crystal phases (poor in polymer, highly ordered colloids). Recently, the colloidal gas-colloidal liquid interface received considerable attention as well. Due to the colloidal length scale the interfacial tension is much lower than in the atomic or molecular analog (nN/m instead of mN/m). This ultra-low interfacial tension has pronounced effects on the kinetics of phase separation, the colloidal gas-liquid profile near a single wall and the thermally induced fluctuations of the interface. The amplitudes of these thermally excited capillary waves are restrained by the interfacial tension and are for that reason of the order of the particle diameter. Therefore, in molecular systems, the capillary waves can only be seen indirectly in scattering experiments. In colloidal systems, however, the wave amplitudes are on a (sub) micrometer scale. This fact enables the direct observation of capillary waves in both real space and real time using confocal scanning laser microscopy. Moreover, the real space technique enables us to demonstrate the strong influence of interface fluctuations on droplet coalescence and droplet break up. PACS

Structural transformations of superparamagnetic colloids confined within self-assembled microtubes are studied by systematically varying tube-colloid size ratios and external magnetic field directions. A magnetic field parallel to microtubes may stretch non-linear chains like zigzag chains into linear chains. Non-parallel fields induce new structures including repulsive chains of single colloids, kinked chains and repulsive dimers, which are not observed for unconfined magnetic colloids in the bulk. The formed colloidal structures are confirmed via model calculations which account for tube-colloid size ratio effects and their reconfigurability with the field direction. Furthermore, structures are formed that allow controllable switching between a helical and a non-helical state. All observed field-induced transformations in microtubes are reversible provided the microtubes are not completely filled with colloids. In addition, we demonstrate magnetic field-responsive 2D crystallization by extending control over colloidal configurations in single microtubes to multiple well-aligned microtubes.