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Hexagonal sheets of colloidal particles self‐assemble in a biaxial electric field and can be made permanent by thermal annealing. One can also rapidly switch the suspension structure from isotropic, to 1D ‘strings’ and 2D ‘sheets’, which is useful for applications that require anisotropic suspension properties. Thus, multiaxial fields offer a flexible way to manipulate colloidal interactions and their self‐assembled structures.

Active systems such as microorganisms and self-propelled particles show a plethora of collective phenomena, including swarming, clustering, and phase separation. Control over the propulsion direction and switchability of the interactions between the individual self-propelled units may open new avenues in designing of materials from within. Here, we present a self-propelled particle system, consisting of half-gold coated titania (TiO 2) particles, in which we can fast and on-demand reverse the propulsion direction, by exploiting the different photocatalytic activities on both sides. We demonstrate that the reversal in propulsion direction changes the nature of the hydrodynamic interaction from attractive to repulsive and can drive the particle assemblies to undergo both fusion and fission transitions. Moreover, we show these active colloids can act as nucleation sites, and switch rapidly the interactions between active and passive particles, leading to reconfigurable assembly and disassembly. Our 1 arXiv:2012.00107v1 [cond-mat.soft] 30 Nov 2020 experiments are qualitatively described by a minimal hydrodynamic model. Designing novel artificial microswimmers or self-propelled particle systems is currently a subject of vast interest in active matter for a variety of reasons. First, they provide us with model systems to study the collective behaviour of their more complex natural counterparts. 1-4 Second, they display a variety of non-equilibrium phenomena, such as active clustering, segregation, and anomalous density fluctuations. 1-4 Several experimental studies have been reported on artificial microswimmers, invoking and using different swimming strategies. 1-10 One class consists of internally driven systems of which Janus particles are an example, inducing motion by converting chemical energy on one side of the particle from its local environment. 1-4, 11 In these synthetic systems, rotational diffusion of the particle randomizes the propulsion direction, but its directionality i.e., the velocity vector with respect to the metal-coated hemisphere, remains constant. 4, 11 On the other hand, the position and/or the orientation of driven particles can be controlled using external stimuli such as magnetic, and electric fields. 6, 7, 12 However, unavoidable fieldinduced (magnetic/electric) long-range dipole-dipole interactions between the particles by external fields 13-15 usually preclude studies of collective behavior controlling solely the activity. 12 We are only aware of recent proof-of-principle studies that considered a single internally-driven particle achieving propulsion direction reversal using a wettability contrast on both sides of Janus particles at different temperatures, 8, 16 but as these are limited by heat transfer rates, they are inherently slow. To study the effect on collective behaviour, the dynamics of switching need to be faster than the intrinsic timescale of the systems, e.g., rotational diffusion. In nature, some microorganisms do indeed display rather fast periodic reversals in the directi...

We describe a facile and flexible approach for synthesizing uniform non-spherical micron sized PMMA (poly(methyl methacrylate)) colloids with well-controlled protrusions. When homogeneously crosslinked PMMA spheres were used as seeds in a swelling process using again a methyl methacrylate monomer, they were found to transform into non-spherical particles with a single or multiple protrusions mainly depending on the cross-link density of the seeds. Alternatively, if core-shell PMMA spheres bearing a highly cross-linked shell around an uncross-linked 'soft' core were employed as seed particles, they always developed just a single protrusion. Precise control over the anisotropy of the particles was achieved by varying the amount and composition of the swelling mixture as well as the concentration of the stabilizer. Subsequently, the phase separation was enhanced and protrusions could be readily polymerized through temperature elevation of the system, yielding PMMA 'snowman'-like or dumbbell-like colloids. Furthermore, these particles could be labeled with fluorescent dyes either before or after the polymerization, and transferred into apolar, refractive index and density matching liquids (cyclohexyl bromide (CHB) and/or decalin), enabling their use in quantitative confocal fluorescence microscopy studies in concentrated systems. Some examples of the use of these particles as a model system for real space analysis are given. These examples include the formation of plastic crystals, a special form of a colloidal crystal where the particles are positionally ordered but orientationally disordered. Additionally, the non-spherical particles could be organized into semiflexibly bonded colloidal chains aided by an electric field in a polar solvent (formamide). .nl; A.Imhof@uu.nl; Fax: +31 030 2532706; Tel: +31 030 2532423 † Electronic supplementary information (ESI) available: Electric field; semi-flexible strings made of nonspherical particles. See

Colloidal particles with a dielectric constant (magnetic susceptibility) mismatch with the surrounding solvent acquire a dipole moment in a homogeneous external electric (magnetic) field. The resulting dipolar interactions can lead to aggregation of the particles into string-like clusters. Recently, several methods have been developed to make these structures permanent. However, especially when multiple particle sizes and/or more complex shapes than single spheres are used, the parameter space for these experiments is enormous. We therefore use Monte Carlo simulations to investigate the structure of the self-assembled string-like aggregates in binary mixtures of dipolar hard and charged spheres, as well as dipolar hard asymmetric dumbbells. Binary mixtures of spheres aggregate in different types of clusters depending on the size ratio of the spheres. For highly asymmetric systems, the small spheres form ring-like and flame-like clusters around strings of large spheres, while for size ratios closer to 1, alternating strings of both large and small spheres are observed. For asymmetric dumbbells, we investigate both the effect of size ratio and dipole moment ratio, leading to a large variety of cluster shapes, including chiral clusters.

A facile method is demonstrated for bonding assembled colloids without loss of colloidal stability by thermal annealing. Examples include both close-packed and non-close-packed structures. The confocal microscopy image shows a cross-section of a 3D labyrinthine structure after it was made permanent. The 3D network is completely preserved after the annealing step.

In recent years, there is a growing interest in designing artificial analogues of living systems, fueled not only by potential applications as 'smart micro-machines', but also by the demand for simple models that can be used to study the behavior of their more complex natural counterparts. Here, we present a facile, internally driven, experimental system comprised of fluorescently labeled colloidal silica rods of which the self-propulsion is powered by the decomposition of HO catalyzed by a length-wise half Pt coating of the particles in order to study how shape anisotropy and swimming direction affect the collective behavior. We investigated the emerging structures and their time evolution for various particle concentrations in (quasi-)two dimensional systems for three aspect ratios of the rods on a single particle level using a combination of experiments and simulations. We found that the dynamic self-organization relied on a competition between self-propulsion and phoretic attractions induced by phoresis of the rods. We observed that the particle clustering behavior depends on the concentration as well as the aspect ratio of the rods. Our findings provide a more detailed understanding of dynamic self-organization of anisotropic particles and the role the propulsion direction plays in internally driven systems.