Depletion-Induced Chiral Chain Formation of Magnetic Spheres

Experimental evidence is presented for the spontaneous formation of chiral configurations in bulk dispersions of magnetized colloids that interact by a combination of anisotropic dipolar interactions and isotropic depletion attractions. The colloids are superparamagnetic silica spheres, magnetized and aligned by a carefully tuned uniform external magnetic field; isotropic attractions are induced by using poly(ethylene oxide) polymers as depleting agents. At specific polymer concentrations, sphere chains wind around each other to form helical structures–of the type that previously have only been observed in simulations on small sets of unconfined dipolar spheres with additional isotropic interactions

Catalytically propelled 3D printed colloidal microswimmers

Synthetic microswimmers are widely employed model systems in the studies of out-of-equilibrium phenomena. Unlike biological microswimmers which naturally occur in various shapes and forms, synthetic microswimmers have so far been limited almost exclusively to spherical shapes. Here, we exploit 3D printing to produce microswimmers with complex shapes in the colloidal size regime. We establish the flexibility of 3D printing by two-photon polymerisation to produce particles smaller than 10 microns with a high-degree of shape complexity. We further demonstrate that 3D printing allows control over the location of the active site through orienting the particles in different directions during printing. We verify that particles behave colloidally by imaging their motion in the passive and active states and by investigating their mean square displacement. In addition, we find that particles exhibit shape-dependant behavior, thereby demonstrating the potential of our method to launch a wide-range of in-depth studies into shape-dependent active motion and behaviour.Biological and Soft Matter Physic

Chiral and Active Colloids

Chirality is a geometric property of objects the mirror image of which cannot be brought to coincide with the original. Whereas some properties, such as evaporation temperature and colour, of left- and right-handed molecules are identical, other chemical, biological and optical properties are not. The absence of mirror symmetry leads to captivating effects such as the coupling of translational and rotational degrees of freedom and chiral liquid crystal formation. Unfortunately, it is not feasible to visualize the effects of chirality on the molecular level. Therefore, we investigated preparation routes for chiral colloids which, just as molecules, display thermal diffusion but with the crucial advantage that they can be imaged with optical microscopy. Two methods were employed to synthesize chiral colloids from isotropic spheres. The first method relies on the confinement of colloidal spheres in hollow microtubes. Microtubes self-assemble from sugar and surfactant molecules as characterized in situ with high resolution small-angle x-ray scattering spanning more than three orders of magnitude of spatial scales. The tubular microstructures consist of equally spaced curved bilayers forming a collection of concentric hollow cylinders with a pore diameter of roughly one micrometre. By adding colloidal spheres to the microtubes, colloid-in-tube assemblies are obtained. Depending on the colloid-to-tube diameter ratio, various structures are formed. Upon removing the microtubes these structures fall apart. Therefore, photo-responsive polystyrene colloids were developed that can photo-crosslink triggered by UV-light. These coumarin-modified spheres form covalent bonds holding the spheres in place even after dissolution of the microtubes. Aqueous suspensions with bulk quantities of a library of ordered structures, including helical sphere chains, have eventually become available. The colloidal equivalents of chiral molecules were used to study their Brownian dynamics. The second preparation route for chiral colloids combines dipolar hard spheres and depletion interaction. Superparamagnetic silica spheres were grafted with polymer hairs and end-functionalized with coumarin molecules. In an external magnetic field, these spheres align while forming linear sphere chains. Upon UV-irradiation, permanent magnetisable sphere chains in bulk are obtained. When a magnetic field is applied in the presence of depletion polymer, the linear sphere chains wind around each other transforming into helical chains. The number of neighbours is maximized in a helical configuration compared to a linear one, lowering the energy of the system. The application of a homogeneous field appeared to be crucialleadingtothedevelopmentofaHelmholtzcube. The final Part of this Thesis describes the realization of cubic microswimmers. Superball-shaped colloids were coated with platinum that catalyses the decomposition of hydrogen peroxide into water and oxygen propelling the colloids via self-diffusiophoresis. The speed of the active particles increases with both fuel concentration and temperature. At short times the particles undergo directed motion, whereas at longer times a random walk with an enhanced diffusion coefficient is observed

Chiral and Active Colloids

Chirality is a geometric property of objects the mirror image of which cannot be brought to coincide with the original. Whereas some properties, such as evaporation temperature and colour, of left- and right-handed molecules are identical, other chemical, biological and optical properties are not. The absence of mirror symmetry leads to captivating effects such as the coupling of translational and rotational degrees of freedom and chiral liquid crystal formation. Unfortunately, it is not feasible to visualize the effects of chirality on the molecular level. Therefore, we investigated preparation routes for chiral colloids which, just as molecules, display thermal diffusion but with the crucial advantage that they can be imaged with optical microscopy. Two methods were employed to synthesize chiral colloids from isotropic spheres. The first method relies on the confinement of colloidal spheres in hollow microtubes. Microtubes self-assemble from sugar and surfactant molecules as characterized in situ with high resolution small-angle x-ray scattering spanning more than three orders of magnitude of spatial scales. The tubular microstructures consist of equally spaced curved bilayers forming a collection of concentric hollow cylinders with a pore diameter of roughly one micrometre. By adding colloidal spheres to the microtubes, colloid-in-tube assemblies are obtained. Depending on the colloid-to-tube diameter ratio, various structures are formed. Upon removing the microtubes these structures fall apart. Therefore, photo-responsive polystyrene colloids were developed that can photo-crosslink triggered by UV-light. These coumarin-modified spheres form covalent bonds holding the spheres in place even after dissolution of the microtubes. Aqueous suspensions with bulk quantities of a library of ordered structures, including helical sphere chains, have eventually become available. The colloidal equivalents of chiral molecules were used to study their Brownian dynamics. The second preparation route for chiral colloids combines dipolar hard spheres and depletion interaction. Superparamagnetic silica spheres were grafted with polymer hairs and end-functionalized with coumarin molecules. In an external magnetic field, these spheres align while forming linear sphere chains. Upon UV-irradiation, permanent magnetisable sphere chains in bulk are obtained. When a magnetic field is applied in the presence of depletion polymer, the linear sphere chains wind around each other transforming into helical chains. The number of neighbours is maximized in a helical configuration compared to a linear one, lowering the energy of the system. The application of a homogeneous field appeared to be crucialleadingtothedevelopmentofaHelmholtzcube. The final Part of this Thesis describes the realization of cubic microswimmers. Superball-shaped colloids were coated with platinum that catalyses the decomposition of hydrogen peroxide into water and oxygen propelling the colloids via self-diffusiophoresis. The speed of the active particles increases with both fuel concentration and temperature. At short times the particles undergo directed motion, whereas at longer times a random walk with an enhanced diffusion coefficient is observed

Phase behaviour of colloids plus weakly adhesive polymers

The phase behaviour of a colloidal dispersion mediated by weakly adhesive polymers is considered. The polymers are depleted but are weakly adhesive and hence comprise a non-zero polymer concentration at the colloid's surface, in contrast to the classical assumption in depletion theories involving a zero polymer concentration at the surface. The theory is composed of a generalized free-volume theory for colloid-polymer mixtures and a self-consistent mean-field theory for polymers at surfaces. It is found that the weak adhesion of the polymers shifts the phase stability of the colloid-polymer mixtures to higher polymer concentrations as compared to assuming a full depletion effect. The predicted phase diagrams employing the new theory are consistent with experiments on mixtures of silica spheres coated with stearyl alcohol and polydimethylsiloxane in cyclohexane and with Monte Carlo simulation results

Observation of solid-solid transitions in 3D crystals of colloidal superballs

Self-organization in anisotropic colloidal suspensions leads to a fascinating range of crystal and liquid crystal phases induced by shape alone. Simulations predict the phase behaviour of a plethora of shapes while experimental realization often lags behind. Here, we present the experimental phase behaviour of superball particles with a shape in between that of a sphere and a cube. In particular, we observe the formation of a plastic crystal phase with translational order and orientational disorder, and the subsequent transformation into rhombohedral crystals. Moreover, we uncover that the phase behaviour is richer than predicted, as we find two distinct rhombohedral crystals with different stacking variants, namely hollow-site and bridge-site stacking. In addition, for slightly softer interactions we observe a solid-solid transition between the two. Our investigation brings us one step closer to ultimately controlling the experimental self-assembly of superballs into functional materials, such as photonic crystals

In situ observation of self-assembly of sugars and surfactants from nanometres to microns

The hierarchical self-assembly of sugar and surfactant molecules into hollow tubular microstructures was characterized in situ with high resolution small-angle X-ray scattering spanning more than three orders of magnitude of spatial scales. Scattering profiles reveal that aqueous host-guest inclusion complexes self-assemble into multiple equally spaced curved bilayers forming a collection of concentric hollow cylinders. Scattering data can be described by a simple theoretical model of the microtubes. The interlamellar distance was found to be surprisingly large. Moreover, we report that the multi-walled structure of the microtubes swells as the concentration or the temperature is varied

Wet-chemical synthesis of chiral colloids

We disclose a method for the synthesis of chiral colloids from spontaneously formed hollow sugar-surfactant microtubes with internally confined mobile colloidal spheres. Key feature of our approach is the grafting of colloid surfaces with photoresponsive coumarin moieties, which allow for UV-induced, covalent clicking of colloids into permanent chains, with morphologies set by the colloid-to-tube diameter ratio. Subsequent dissolution of tube confinement yields aqueous suspensions that comprise bulk quantities of a variety of linear chains, including single helical chains of polystyrene colloids. These colloidal equivalents of chiral (DNA) molecules are intended for microscopic study of chiral dynamics on a single-particle level

Wet-chemical synthesis of chiral colloids

\u3cp\u3eWe disclose a method for the synthesis of chiral colloids from spontaneously formed hollow sugar-surfactant microtubes with internally confined mobile colloidal spheres. Key feature of our approach is the grafting of colloid surfaces with photoresponsive coumarin moieties, which allow for UV-induced, covalent clicking of colloids into permanent chains, with morphologies set by the colloid-to-tube diameter ratio. Subsequent dissolution of tube confinement yields aqueous suspensions that comprise bulk quantities of a variety of linear chains, including single helical chains of polystyrene colloids. These colloidal equivalents of chiral (DNA) molecules are intended for microscopic study of chiral dynamics on a single-particle level.\u3c/p\u3