When a crystal melts into a liquid both long-ranged positional and orientational order are lost, and long-time translational and rotational self-diffusion appear. Sometimes, these properties do not change at once, but in stages, allowing states of matter such as liquid crystals or plastic crystals with unique combinations of properties. Plastic crystals/glasses are characterized by long-ranged positional order/frozen-in-disorder but short-ranged orientational order, which is dynamic. Here we show by quantitative three-dimensional studies that charged rod-like colloidal particles form three-dimensional plastic crystals and glasses if their repulsions extend significantly beyond their length. These plastic phases can be reversibly switched to full crystals by an electric field. These new phases provide insight into the role of rotations in phase behaviour and could be useful for photonic applications.

The non-equilibrium phase behavior of a colloidal hard-sphere fluid under oscillatory shear was investigated in real-space with experiments on poly(methyl methacrylate) (PMMA) colloidal suspensions and Brownian Dynamics computer simulations. All the samples in both experiments and the simulation are below the coexistence density of hard-sphere freezing, so the shear-induced crystals are out-of-equilibrium and melt after cessation of the shear. The physics is therefore fundamentally different from shear-induced crystallization in jammed or glassy systems. Although the computer simulations neglect hydrodynamic interactions and impose a linear flow, the results are in good agreement with the experiments. Depending on the amplitude and frequency of the oscillation, four regimes with different structures, hereafter referred to as phases, were identified: an oscillating twinned face-centered-cubic (fcc) phase, a sliding layer phase, a string phase and a phase that has not been reported previously in experiments, which we identify as tilted layers. This phase consists of lanes of particles that order in a hexagonal-like array (in the gradient-vorticity plane) which has lines of particles under an angle with the horizontal. Phases similar to the sliding layers, string phase and tilted layer phase were reported in Brownian and Molecular Dynamics simulations (systematically called string formation) but the validity of these simulations has been questioned. We demonstrate the experimental existence of these string-like phases and elucidate their structural differences in real-space.

In this paper, the characterization and fluorescent labeling of silica rods are reported. These rods are synthesized following a recently reported method. Material properties of the silica rods measured with NMR, elemental analysis, TGA, and porosimetry are compared with those of well‐established Stöber silica spheres. Additionally, silica rods are made suitable for quantitative real‐space studies by confocal microscopy. Several methods of fluorescent labeling to prepare rods with different fluorescent patterning, ranging from uniform fluorescence levels to gradients from one rod‐end to the other, and even patterns of several colors are presented and discussed.

Confocal microscopy in combination with real-space particle tracking has proven to be a powerful tool in scientific fields such as soft matter physics, materials science and cell biology. However, 3D tracking of anisotropic particles in concentrated phases remains not as optimized compared to algorithms for spherical particles. To address this problem, we developed a new particle-fitting algorithm that can extract the positions and orientations of fluorescent rod-like particles from three dimensional confocal microscopy data stacks. The algorithm is tailored to work even when the fluorescent signals of the particles overlap considerably and a threshold method and subsequent clusters analysis alone do not suffice. We demonstrate that our algorithm correctly identifies all five coordinates of uniaxial particles in both a concentrated disordered phase and a liquid-crystalline smectic-B phase. Apart from confocal microscopy images, we also demonstrate that the algorithm can be used to identify nanorods in 3D electron tomography reconstructions. Lastly, we determined the accuracy of the algorithm using both simulated and experimental confocal microscopy data-stacks of diffusing silica rods in a dilute suspension. This novel particle-fitting algorithm allows for the study of structure and dynamics in both dilute and dense liquid-crystalline phases (such as nematic, smectic and crystalline phases) as well as the study of the glass transition of rod-like particles in three dimensions on the single particle level.

Accurate distance measurement in 3D confocal microscopy is important for quantitative analysis, volume visualization and image restoration. However, axial distances can be distorted by both the point spread function (PSF) and by a refractive-index mismatch between the sample and immersion liquid, which are difficult to separate. Additionally, accurate calibration of the axial distances in confocal microscopy remains cumbersome, although several high-end methods exist. In this paper we present two methods to calibrate axial distances in 3D confocal microscopy that are both accurate and easily implemented. With these methods, we measured axial scaling factors as a function of refractive-index mismatch for high-aperture confocal microscopy imaging. We found that our scaling factors are almost completely linearly dependent on refractive index and that they were in good agreement with theoretical predictions that take the full vectorial properties of light into account. There was however a strong deviation with the theoretical predictions using (high-angle) geometrical optics, which predict much lower scaling factors. As an illustration, we measured the PSF of a correctly calibrated point-scanning confocal microscope and showed that a nearly index-matched, micron-sized spherical object is still significantly elongated due to this PSF, which signifies that care has to be taken when determining axial calibration or axial scaling using such particles.

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.

Colloidal particles in geometrical confinement display a complex variety of packing structures different from their three-dimensional (3D) bulk counterpart. Here, we confined charged rodlike colloids with longranged repulsions to a thin wedge-shaped cell and show, by quantitative 3D confocal microscopy, that not only their positional but also their orientational order depends sensitively upon the slit width. Synchronized with transitions in lattice symmetry and number of layers confinement induces plastic crystal-to-crystal transitions. A model analysis suggests that this complex sequence of more or less rotationally ordered states originates from the subtle competition between the electrostatic repulsion of a rod with the wall and with its neighbors. DOI: 10.1103/PhysRevLett.115.078301 PACS numbers: 82.70.Dd, 64.75.Yz When a colloidal suspension is confined to a quasi-twodimensional geometry, many fascinating crystal structures appear due to the partial restriction of translational degrees of freedom in the third dimension [1][2][3]. In the case of particles interacting with a hard potential the structure greatly depends on their packing efficiency in the confined geometry. One of the first studies in this limit was performed by Pieranski et al., who used nearly hard spherical colloids confined in a wedge-shaped cell. At increasing slit width they found a sequence of transitions that can be represented as:where n denotes the number of crystal layers and △ and □ denote layers with hexagonal and square symmetry, respectively [4]. More detailed investigations since then disclosed that many intermediate phases exist, such as a buckling phase in between 1△ → 2□, a rhombic phase in between n□ → n△, and prismatic, hexagonal close packing (hcp)-like, hcp(100), hcp⊥, and pre-square phases in between n△ → ðn þ 1Þ□ [3,[5][6][7]. All these structures have been verified by computer simulations [8][9][10][11][12][13], and the consistency between experiments and simulations gives us confidence in our understanding of the nature of the transitions for hard spheres.Fewer intermediate phases are found for charged spheres, which interact via long-ranged electrostatic interactions, both with each other and with the walls [14][15][16][17]. Using colloids with κa ¼ 0.79 and 0.37 (where κ is the inverse Debye screening length characterizing the range of the repulsive interactions, and a is the particle radius) we found a phase sequence very similar to Ref. [16]: 1△ → 2□ → 2R → 2△ → 3□ → 3R → 3△ → 4□ → 4R → 4△ (R denotes a rhombic phase) [17]. These observations imply that long-ranged electrostatic interactions play an important additional role in the packing of charged particles.The packing behavior of shape anisotropic particles has also begun to be explored. For nearly hard dimer-shaped particles so-called "side" and "out-of-plane" structures were reported [18,19]. This leads to questions about the effect of confinement on particle orientational order in a system where this effect can be separated from packingdominated changes ...

Self-assembly of binary particle systems offers many new opportunities for materials science. Here, we studied sedimentation equilibria of silica rods and spheres, using quantitative 3D confocal microscopy. We determined not only pressure, density and order parameter profiles, but also the experimental phase diagram exhibiting a stable binary smectic liquid-crystalline phase (Sm). Using computer simulations we confirmed that the Sm-phase can be stabilized by entropy alone, which opens up the possibility of combining new materials properties at a wide array of length scales.