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 colloidrich 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. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

Revealing three-dimensional structure of individual colloidal crystal grain by coherent x-ray diffractive imaging

We present results of a coherent x-ray diffractive imaging experiment performed on a single colloidal crystal grain. The full three-dimensional (3D) reciprocal space map measured by an azimuthal rotational scan contained several orders of Bragg reflections together with the coherent interference signal between them. Applying the iterative phase retrieval approach, the 3D structure of the crystal grain was reconstructed and positions of individual colloidal particles were resolved. As a result, an exact stacking sequence of hexagonal close-packed layers including planar and linear defects were identified.Comment: 8 pages, 5 figure

Nanoassembly of Polydisperse Photonic Crystals Based on Binary and Ternary Polymer Opal Alloys

Ordered binary and ternary photonic crystals, composed of different sized polymer-composite spheres with diameter ratios up to 120%, are generated using bending induced oscillatory shearing (BIOS). This viscoelastic system creates polydisperse equilibrium structures, producing mixed opaline colored films with greatly reduced requirements for particle monodispersity, and very different sphere size ratios, compared to other methods of nano-assembly

Fuel-Mediated Transient Clustering of Colloidal Building Blocks

Fuel-driven assembly operates under the continuous influx of energy and results in superstructures that exist out of equilibrium. Such dissipative processes provide a route toward structures and transient behavior unreachable by conventional equilibrium self-assembly. Although perfected in biological systems like microtubules, this class of assembly is only sparsely used in synthetic or colloidal analogues. Here, we present a novel colloidal system that shows transient clustering driven by a chemical fuel. Addition of fuel causes an increase in hydrophobicity of the building blocks by actively removing surface charges, thereby driving their aggregation. Depletion of fuel causes reappearance of the charged moieties and leads to disassembly of the formed clusters. This reassures that the system returns to its initial, equilibrium state. By taking advantage of the cyclic nature of our system, we show that clustering can be induced several times by simple injection of new fuel. The fuel-mediated assembly of colloidal building blocks presented here opens new avenues to the complex landscape of nonequilibrium colloidal structures, guided by biological design principles

Synthesis and Polyelectrolyte Functionalization of Hollow Fiber Membranes Formed by Solvent Transfer Induced Phase Separation

Ultrafiltration membranes are important porous materials to produce freshwater in an increasingly water-scarce world. A recent approach to generate porous membranes is solvent transfer induced phase separation (STrIPS). During STrIPS, the interplay of liquid-liquid phase separation and nanoparticle self-assembly results in hollow fibers with small surface pores, ideal structures for applications as filtration membranes. However, the underlying mechanisms of the membrane formation are still poorly understood, limiting the control over structure and properties. To address this knowledge gap, we study the nonequilibrium dynamics of hollow fiber structure evolution. Confocal microscopy reveals the distribution of nanoparticles and monomers during STrIPS. Diffusion simulations are combined with measurements of the interfacial elasticity to investigate the effect of the solvent concentration on nanoparticle stabilization. Furthermore, we demonstrate the separation performance of the membrane during ultrafiltration. To this end, polyelectrolyte multilayers are deposited on the membrane, leading to tunable pores that enable the removal of dextran molecules of different molecular weights (>360 kDa, >60 kDa, >18 kDa) from a feed water stream. The resulting understanding of STrIPS and the simplicity of the synthesis process open avenues to design novel membranes for advanced separation applications

Large-scale ordering of nanoparticles using viscoelastic shear processing

This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by Nature Publishing Group.Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear (BIOS) technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong structural colour. With oscillatory strain amplitudes of 300%, crystallisation initiates at the wall and develops quickly across the bulk within only 5 oscillations yielding sharp intense reflectance peaks of tunable colour. The resulting structure of randomly stacked hexagonal close-packed layers parallel to the shear plane, is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonics materials, and forms a generic tool for ordering nanoparticles.We acknowledge EPSRC grants EP/G060649/1, EP/H027130/1, EP/E040241, EP/L027151/1 and EU ERC grants LINASS 320503 and FP7 291522-3DIMAGE

Colloidal superballs

This thesis is organized in four parts as follows. Part 1 focuses on the synthetic aspects of the colloidal model systems that will be used throughout the work described in this thesis. In Chapter 2 we describe synthetic procedures for the preparation of polycrystalline hematite superballs and superellipsoids. The internal structure of the particles is also investigated and will be used later to understand the magnetic properties of colloidal hematite. The same hematite particles are used as templates for the preparation of silica hollow superballs and superellipsoids as described in Chapter 3. The particles are coated with a layer of silica that is porous and permits the dissolution of the internal hematite cores by acidic treatment. The technique is convenient to obtain lighter micron sized superballs and superellipsoids useful for the study of anisotropic shape interactions. In Chapter 4, we employ the hematite colloids that possess a permanent dipole moment and therefore behave as micro-magnets, in order to prepare spherical colloids with centered and shifted magnetic dipoles. To this purpose, the magnetic hematite is encapsulated just below the surface of polymer droplets that can be subsequently polymerized. When the polymer droplet is small, they can be coated with silica to obtain centered dipolar spheres. In Part 2 we focus our attention on non-magnetic silica superballs and particularly how the superball shape influences the phase behavior of the colloidal particles. In Chapter 5 we show that silica superballs with relatively high shape parameters (m) readily crystallize into the rare simple cubic crystal structure when they are dispersed in the presence of small non-adsorbing depletants. In Chapter 6, we extend the study to superballs with different shape parameters (m) focusing on their interaction to non-adsorbing polymers of various sizes. The result of this work is presented in the first experimental phase diagram of colloidal superballs in the presence of depletants. Part 3 deals with the study of magnetic colloids. In Chapter 7 we focus our attention in the magnetic behavior of dipolar hematite superballs and superellipsoids under different conditions. We studied the structure formation at low and high particle concentrations, in the Earth’s magnetic field as well as an externally applied magnetic field. To perform the experiment we devised a magnetic set-up that allows precise control on the direction and strength of the applied field. Using this magnetic set-up, we have developed a technique that allows cancellation of any residual magnetic fields in the environment to ensure that dipolar structure formation can really be studied in zero field. In Chapter 8 we study the self-assembly behavior of colloids with magnetic-patches (spheres with shifted dipoles). The self-assembly of the patchy colloids can be tuned by changing the size of the polymer particles, the salt concentration in solution and by application of an external field. In Part 4, we explore the preparation and behavior of food-grade colloids specifically designed for application as food-additives. In Chapter 9 we study the synthesis of colloidal pyrophosphates nanoparticles as possible additives for iron-fortification in food. Because of the novelty of the material, we have performed extensive characterization of the physico-chemical properties of the nanoparticles. In Chapter 10 we focus our attention on the control of the shape and size of the colloidal pyrophosphate. We employ the porous hollow silica colloids prepared in Chapter 3 as templates for the synthesis of pyrophosphate in their inner hollow part. In Chapter 11 we develop another kind of colloidal particles using phytosterol molecules. The particles are synthesized, characterized and preliminary in vitro experiments are performed to study their capability to lower the adsorption of cholesterol during digestion. Because the synthetic method used for the particle synthesis produces phytosterol particles with a characteristic rod-like shape, in Chapter 12 we study their phase behavior at different concentrations. We show that at certain concentrations the particles self-assemble to form a cholesteric liquid crystalline phase

Self-assembly of colloids with anisotropic shape and interactions

In this thesis the self-assembly of anisotropic polystyrene colloidal particles is studied using optical microscopy. These particles consist of different lobes with attractive and non-attractive interactions. This anisotropy in inter particle interaction is induced by depletion attraction combined with a difference in surface roughness between the lobes. The shape of the particles that are used as building blocks has a profound effect on the structures formed by self-assembly. Snowman or dumbbell-shaped particles consisting of one attractive (smooth) and one non-attractive (rough) lobe self-assemble into spherical micelle-like structures. These particles can also be used to encapsulate and stabilize larger spherical particles. Triangular particles on the other hand, consisting of one attractive and two non-attractive lobes, resembling a “Mickey Mouse” head, self-assemble into elongated tube-like structures. These structures are observed with optical microscopy in the experimental system and supported by Monte Carlo simulation results. Understanding this effect of building block shape on the resulting structure is important for the design of building blocks for the formation of new, functional structures by self-assembly. These structures could for instance be used as vehicles for targeted drug delivery. The geometry of dumbbell-shaped particles also has an effect on the crystalline ordering of these particles by convective assembly. A larger particle length (less overlap between the lobes) results in reduced crystalline order, while crystals of these particles have interesting optical properties with possible application as photonic crystals

Dynamics of active droplets and freely-jointed colloidal trimers

In this thesis we have investigated how the dynamics of particle are affected by surface activity, which is the property of particles to locally alter the solute concentration through for example a surface reactions or dissolution. We found that surface activity can have three effects on the particle dynamics. First, it can cause the particles to self-propel. Surprisingly a heterogeneous surface activity is no prerequisite for this and also particles with isotropic surface activity can swim due to a hydrodynamic instability, provided the activity is larger than a threshold value. Particles that move due to this instability are called isotropic swimmers. In Chapter 2 we studied the swimming dynamics of droplet that slowly dissolve in surfactant solution as a model for such isotropic swimmers. We found that their persistence time can be tuned through droplet size and the surfactant concentration. This finding suggests that stochastic character in the motion of active materials on granular length scales is not only caused by Brownian rotation of these active particles. Rather we think that fluctuations in the fluid flow or spatial inhomogeneities in the dissolution rate cause stochastic turning. Second, we found that even below the onset of swimming, the dynamics of particles with homogeneous surface activity are enhanced or attenuated by the activity, depending on whether solute is consumed or produced. In Chapter 3 we investigated theoretically the instability that gives rise to self-propulsion for isotropic particles. We found that particles with a surface activity just below the swimming threshold can coast as if they were inertial, even though they are in the low Reynolds number regime. We made an attempt to test this finding experimentally, but the results remain inconclusive. Third, surface activity induces effective interactions between particles. We measured such solute-mediated interactions between two dissolving oil droplets in Chapter 4 and found that the interaction scales with inter-particle distance as $1/r^2$. Moreover the interaction strength increases with droplet size and surfactant concentration. Because solute-mediated interactions are dissipative and involve the solvent, they can have the unique property that particle 1 is attracted by particle 2, while particle 2 is repelled by particle 1. This asymmetry in solute-mediated interactions can lead to chemotactic chasing, when the interaction strengths are properly tuned, as we show in Chapter 5. We also show that clusters of chasing droplets can move translationally, rotationally or reorganize depending on their geometry. We made a step in the direction of applying the knowledge of the phenomena that we learned for dissolving droplets to solid colloids. The biggest hurdle standing in the way of that comparison is that the phoretic mobility for solid particles is unknown for many solute gradients and not easy to measure experimentally. In Chapter 6 we first reproduced earlier measurements of diffusiophoretic mobilities of solid particles using microfluidic devices. Then we set out to improve this technique so that it requires fewer particles and no longer relies on the particles being fluorescent