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

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

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

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

Self-Assembly of Colloidal Spheres into One, Two, and Three Dimensional Structures

The main goal of this thesis is to increase our understanding of colloidal self-assembly processes and develop new strategies to assemble colloidal building blocks into more sophisticated and well-defined super-structures. Self-assembly is a spontaneous process in which a disordered system of pre-existing building blocks forms an ordered structure without human intervention. For example, virus capsid proteins can self-assemble into virus microcapsules. However, direct investigation of the self-assembly process of, for examples, proteins and other molecules in situ, is difficult since those objects are too small and move too fast to be tracked directly by techniques such as electron and optical microscopy. Herein, we employ colloids as models of (macro) molecules to study self-assembly. This thesis divides into two parts. In the first part, we show various methods to tune the properties of the colloids, including shape, charges, morphology, size and surface properties. In the second part, we focus on the self-assembly of spherical colloids into one, two and three-dimensional colloidal aggregates using different principles. We show that the delicate balance of short-range hydrophobic attraction and relatively longer-range electrostatic repulsion can result in the formation of a Bernal spiral-like structure. While the use of a good solvent of colloids during the vertical deposition process allows for the formation of floating colloidal crystal monolayers. We also show that pH reversible encapsulation of oppositely charged colloids with a vast size difference can be achieved in the presence of pH-responsive polyelectrolytes in solution

Polymer-mediated phase stability of colloids

In this thesis, we have isolated some key parameters governing the (in)stability of colloid--polymer mixtures. The term `colloid' refers to a state of matter in which a certain amount of material (with one of its dimensions between {one nanometre to one micrometre}) is dispersed in another medium. Polymers are macromolecules constituted of many repeating units called segments; depending on the number and nature of these segments, polymers may dissolve or phase-separate in solution. Colloid--polymer mixtures are widespread in biological systems (including blood, the cytoplasm of a living cell, and plant sap), as well as in man-made products (such as paints, drinking yogurt, and printing inks). Better control over the stability limits during product development is possible via a fundamental understanding of the effect of some relevant parameters in the system at hand. In the examples given above, multiple colloids, polymers, and other components are often present. Building knowledge on the interactions between components of the same nature, and pairs of different components is a logical starting point. We focus on three kind of colloids (spherical, anistropic, and associative) and study how the phase stability is affected upon addition of polymers

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

Fabrication of floating colloidal crystal monolayers by convective deposition

Hypothesis: Well-defined two-dimensional colloidal crystal monolayers (CCM) have numerous applications, such as photonic crystal, sensors, and masks for colloidal lithography. Therefore, significant effort was devoted to the preparation of preparing CCM. However, the fabrication of CCM that can float in the continuous phase and readily transfer to other substrate remains an elusive challenge. Experiments: In this article a facile approach to prepare floating CCM from polymeric colloids as building blocks is reported. The key to obtain floating CCM is the selection of an appropriate solvent to release the formed CCM from the substrate. There are two steps involved in the preparation of floating CCM: formation and peeling off. Findings: First, colloids are dispersed in a solvent. Evaporation of this solvent results in the formation of a meniscus structure of the air–liquid interface between the colloids that are on the substrate. The deformation of the meniscus gives rise to capillary attraction, driving the colloids together in a dense monolayer. Once a crystallization nucleus is formed, a convective flow containing additional colloids sets in, resulting in the formation of CCM on the substrate. Second, the remaining bulk dispersion is replaced by an extracting solvent that wets the substrate and peels the formed CCM off. The influence of the several solvents, the substrate materials, and the types of colloids on the CCM formation are investigated systematically. The robustness of the approach facilitates the preparation of CCM. Furthermore, the floating feature of the CCM in principle makes transfer of the CCM to other substrates possible, which broadens its applications