Job H. J. Thijssen

Microradian X‐ray diffraction in colloidal photonic crystals

Ultra-high-resolution small-angle X-ray scattering in various colloidal photonic crystals is reported. It is demonstrated that an angular resolution of about two microradians is readily achievable at a third-generation synchrotron source using compound refractive optics. The scheme allows fast acquisition of two-dimensional X-ray diffraction data and can be realised at sample-detector separations of only a few metres. As a result, diffraction measurements in colloidal crystals with interplanar spacings larger than a micrometre, as well as determination of the range of various order parameters from the width of the Bragg peaks, are made possible.

Fabrication of large binary colloidal crystals with a NaCl structure

Binary colloidal crystals offer great potential for tuning material properties for applications in, for example, photonics, semiconductors and spintronics, because they allow the positioning of particles with quite different characteristics on one lattice. For micrometer-sized colloids, it is believed that gravity and slow crystallization rates hinder the formation of high-quality binary crystals. Here, we present methods for growing binary colloidal crystals with a NaCl structure from relatively heavy, hard-sphere-like, micrometer-sized silica particles by exploring the following external fields: electric, gravitational, and dielectrophoretic fields and a structured surface (colloidal epitaxy). Our simulations show that the free-energy difference between the NaCl and NiAs structures, which differ in their stacking of the hexagonal planes of the larger spheres, is very small (≈0.002 kBT). However, we demonstrate that the fcc stacking of the large spheres, which is crucial for obtaining the pure NaCl structure, can be favored by using a combination of the above-mentioned external fields. In this way, we have successfully fabricated large, 3D, oriented single crystals having a NaCl structure without stacking disorder.

Colloidal particles at the interface between an isotropic liquid and a chiral liquid crystal

We create an interface between a cholesteric liquid crystal (CLC) and an isotropic liquid (silicone oil) at which homeotropic anchoring leads to a well aligned cholesteric layer and the formation of the fingerprint texture. Fluorescent colloidal particles with planar surface anchoring are dispersed in the CLC and subsequently imaged using confocal microscopy. A majority of these particles decorate the interface between the CLC and the silicone oil. We present a detailed study of the position of the particles along the direction perpendicular to the interface: the final distribution of particles perpendicular to the interface has a clear dependence on the ratio between the particle size and the pitch of the CLC. This suggests, supported by simulations, that there is a particle size and pitch length dependent drive to expel particles, due to the elastic energy cost of remaining in the CLC. We use polarizing optical microscopy to observe changes to the fingerprint texture as the particles perturb the interface. This is combined with a qualitative study of the in-plane ordering of the particles. Chains of particles form perpendicular to the helical axis (parallel to the cholesteric layers), whereas disordered aggregates are seen where the direction of the helical axis is not uniform.

Particle-size effects in the formation of bicontinuous Pickering emulsions

We demonstrate that the formation of bicontinuous emulsions stabilized by interfacial particles (bijels) is more robust when nanoparticles rather than microparticles are used. Emulsification via spinodal demixing in the presence of nearly neutrally wetting particles is induced by rapid heating. Using confocal microscopy, we show that nanospheres allow successful bijel formation at heating rates two orders of magnitude slower than is possible with microspheres. In order to explain our results, we introduce the concept of mechanical leeway, i.e., nanoparticles benefit from a smaller driving force towards disruptive curvature. Finally, we suggest that leeway mechanisms may benefit any formulation in which challenges arise due to tight restrictions on a pivotal parameter, but where the restrictions can be relaxed by rationally changing the value of a more accessible parameter.

How do (fluorescent) surfactants affect particle-stabilized emulsions?

We present the first confocal-microscopy study of synergistic effects in emulsions stabilized by both colloidal particles and a common fluorescent dye that acts as a surfactant. In situ microscopic imaging reveals surfactant adsorption onto the liquid–liquid interface and onto the colloidal particles, which changes the interfacial tension and the particle contact angle. This leads to emulsions that are more stable, more polydisperse and can incorporate more of the dispersed phase.

Particle-stabilized oscillating diver: a self-assembled responsive capsule

We report the experimental discovery of a self-assembled capsule, with density set by interfacial glass beads and an internal bubble, that automatically performs regular oscillations up and down a vial in response to a temperature gradient. Similar composites featuring interfacial particles and multiple internal compartments could be the solution to a variety of application challenges.

Squeezing particle-stabilized emulsions into biliquid foams - equation of state†

Using a centrifuge, we measure the (pressure vs. density) equation of state of Pickering emulsions stabilized by hard-sphere colloids, in order to elucidate the particle contribution to their mechanical properties. Moreover, we have developed a transparent Pickering emulsion, allowing us to determine local volume fraction as a function of distance within the sediment using confocal microscopy, thus extracting an entire equation of state from one centrifugation cycle. We can explain and predict trends in our data using a quantitative model incorporating interdroplet films with a thickness on the scale of the (micron-sized) particles and repulsive interactions across these films. We suggest that the effective repulsion between droplets is due to the deformation of the liquid–liquid interface between particles on one droplet due to compression against a neighbouring droplet.

Dynamics of hard sphere suspensions using dynamic light scattering and X-ray photon correlation spectroscopy: Dynamics and scaling of the intermediate scattering function

Intermediate scattering functions are measured for colloidal hard sphere systems using both dynamic light scattering and x-ray photon correlation spectroscopy. We compare the techniques, and discuss the advantages and disadvantages of each. Both techniques agree in the overlapping range of scattering vectors. We investigate the scaling behavior found by Segré and Pusey [Phys. Rev. Lett. 77, 771 (1996)] but challenged by Lurio et al. [Phys. Rev. Lett. 84, 785 (2000)]. We observe a scaling behavior over several decades in time but not in the long-time regime. Moreover, we do not observe long-time diffusive regimes at scattering vectors away from the peak of the structure factor and so question the existence of long-time diffusion coefficients at these scattering vectors.

Hybrid electrolytes with 3D bicontinuous ordered ceramic and polymer microchannels for all-solid-state batteries

Hybrid solid electrolytes, composed of 3D ordered bicontinuous conducting ceramic and insulating polymer microchannels are reported. The ceramic channels provide continuous, uninterrupted pathways, maintaining high ionic conductivity between the electrodes, while the polymer channels permit improvement of the mechanical properties from that of the ceramic alone, in particular mitigation of the ceramic brittleness. The conductivity of a ceramic electrolyte is usually limited by resistance at the grain boundaries, necessitating dense ceramics. The conductivity of the 3D ordered hybrid is reduced by only the volume fraction occupied by the ceramic, demonstrating that the ceramic channels can be sintered to high density similar to a dense ceramic disk. The hybrid electrolytes are demonstrated using the ceramic lithium ion conductor Li1.4Al0.4Ge1.6(PO4)3 (LAGP). Structured LAGP 3D scaffolds with empty channels were prepared by negative replication of a 3D printed polymer template. Filling the empty channels with non-conducting polypropylene (PP) or epoxy polymer (epoxy) creates the structured hybrid electrolytes with 3D bicontinuous ceramic and polymer microchannels. Printed templating permits precise control of the ceramic to polymer ratio and the microarchitecture; as demonstrated by the formation of cubic, gyroidal, diamond and spinodal (bijel) structures. The electrical and mechanical properties depend on the microarchitecture, the gyroid filled with epoxy giving the best combination of conductivity and mechanical properties. An ionic conductivity of 1.6 × 10−4 S cm−1 at room temperature was obtained, reduced from the conductivity of a sintered LAGP pellet only by the volume fraction occupied by the ceramic. The mechanical properties of the gyroid LAGP–epoxy electrolyte demonstrate up to 28% higher compressive failure strain and up to five times the flexural failure strain of a LAGP pellet before rupture. Notably, this demonstrates that ordered ceramic and polymer hybrid electrolytes can have superior mechanical properties without significantly compromising ionic conductivity, which addresses one of the key challenges for all-solid-state batteries.

Interfacial rheology of sterically stabilized colloids at liquid interfaces and its effect on the stability of Pickering emulsions

Particle-laden interfaces can be used to stabilize a variety of high-interface systems, from foams over emulsions to polymer blends. The relation between the particle interactions, the structure and rheology of the interface, and the stability of the system remains unclear. In the present work, we experimentally investigate how micron-sized, near-hard-sphere-like particles affect the mechanical properties of liquid interfaces. In particular, by comparing dried and undried samples, we investigate the effect of aggregation state on the properties of the particle-laden liquid interface and its relation to the stability of the corresponding Pickering emulsions. Partially aggregated suspensions give rise to a soft-solid-like response under shear, whereas for stable PMMA particulate layers a liquid-like behavior is observed. For interfacial creep-recovery measurements, we present an empirical method to correct for the combined effect of the subphase drag and the compliance of the double-wall ring geometry, which makes a significant contribution to the apparent elasticity of weak interfaces. We further demonstrate that both undried and dried PMMA particles can stabilize emulsions for months, dispelling the notion that particle aggregation, in bulk or at the interface, is required to create stable Pickering emulsions. Our results indicate that shear rheology is a sensitive probe of colloidal interactions but is not necessarily a predictor of the stability of interfaces, e.g., in quiescent Pickering emulsions, as in the latter the response to dilatational deformations can be of prime importance.