Thermoresponsive microgels find widespread use as colloidal model systems, because their temperature-dependent size allows facile tuning of their volume fraction in situ. However, an interaction ...potential unifying their behavior across the entire phase diagram is sorely lacking. Here we investigate microgel suspensions in the fluid regime at different volume fractions and temperatures, and in the presence of another population of small microgels, combining confocal microscopy experiments and numerical simulations. We find that effective interactions between microgels are clearly temperature dependent. In addition, microgel mixtures possess an enhanced stability compared to hard colloid mixtures - a property not predicted by a simple Hertzian model. Based on numerical calculations we propose a multi-Hertzian model, which reproduces the experimental behavior for all studied conditions. Our findings highlight that effective interactions between microgels are much more complex than usually assumed, displaying a crucial dependence on temperature and on the internal core-corona architecture of the particles.
Charged fluorescent bowl‐shaped colloids consisting of a polystyrene core surrounded by a poly(N‐isopropylmethacrylamide) shell are obtained by nanoengineering spherical composite microgels. The ...phase diagram of these soft bowl‐shaped colloids interacting through long‐range Yukawa‐type interactions is investigated using confocal laser scanning microscopy. The bowl‐shaped structure leads to marked differences in phase‐behavior compared to their spherical counterpart. With increasing number density, a transition from a fluid to a plastic crystal phase, with freely rotating particles, followed by a glass‐like state is observed. It is found that the anisotropic bowl shape frustrates crystallization and slows down crystallization kinetics and causes the glass‐like transition to shift to a significantly lower volume fraction than for the spheres. Quantitative analysis of the positional and orientational order demonstrates that the plastic crystal phase exhibits quasi‐long range translational order and orientational disorder, while in the disordered glass‐like phase the long‐range translational order vanishes and short‐range rotational order appears, dictated by the specific bowl shape. It is further shown that the different structural transitions are characterized by decoupling of the translational and orientational dynamics.
The phase behavior of bowl‐shaped colloids interacting via a long‐range Yukawa potential is compared to their spherical counterpart. The formation of a plastic crystal and glass‐like phase is observed at low‐volume fractions. Using a novel tracking algorithm, the order and dynamics of these anisotropic colloids in the phases is investigated in detail.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Despite the omnipresence of colloidal suspensions, little is known about the influence of colloid shape on phase transformations, especially in nonequilibrium. To date, real-space imaging results at ...high concentrations have been limited to systems composed of spherical colloids. In most natural and technical systems, however, particles are nonspherical, and their structural dynamics are determined by translational and rotational degrees of freedom. Using confocal microscopy of fluorescently labeled core-shell particles, we reveal that suspensions of ellipsoidal colloids form an unexpected state of matter, a liquid glass in which rotations are frozen while translations remain fluid. Image analysis unveils hitherto unknown nematic precursors as characteristic structural elements of this state. The mutual obstruction of these ramified clusters prevents liquid crystalline order. Our results give insight into the interplay between local structures and phase transformations. This helps to guide applications such as self-assembly of colloidal superstructures and also gives evidence of the importance of shape on the glass transition in general.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
The color of materials usually originates from a combination of wavelength‐dependent absorption and scattering. Controlling the color without the use of absorbing dyes is of practical interest, not ...only because of undesired bleaching properties of dyes but also regarding minimization of environmental and health issues. Color control without dyes can be achieved by tuning the material's scattering properties in controlling size and spatial arrangement of scatterers. Herein, calibrated photonic glasses (PGs), which are isotropic materials made by random aggregation of nonabsorbing, monodisperse colloidal polystyrene spheres, are used to generate a wide spectral range of purely structural, angular‐independent colors. Experimental reflectance spectra for different sized spheres compare well with a recent theoretical model, which establishes the latter as a tool for color mapping in PGs. It allows to determine the range of visible colors accessible in PGs as function of size, packing fraction, and refractive index of scatterers. It also predicts color saturation on top of the white reflectance as function of the sample's optical thickness. Blue, green, and red are obtained even with low index, while saturated green, cyan, yellow, and magenta can be reached in higher index PGs over several orders of magnitude of sample thickness.
The range of achievable isotropic structural colors in photonic glasses is controlled by tuning size, spatial arrangement, and refractive index of spherical colloidal scatterers. Experimental reflectance spectra compare well with a recent theoretical model establishing the latter as a tool for color mapping. It further predicts color saturation as function of the sample's optical thickness.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
The structure of self-assembled materials is determined by the shape and interactions of the building blocks. Here, we investigate the self-assembly of colloidal 'superballs', i.e. cubes with rounded ...corners, by temperature-tunable critical Casimir forces to obtain insight into the coupling of a cubic shape and short range attractions. The critical Casimir force is a completely reversible and controllable attraction that arises in a near-critical solvent mixture. Using confocal microscopy and particle tracking, we follow the self-assembly dynamics and structural transition in a quasi-2D system. At low attraction, we observe the formation of small clusters with square symmetry. When the attraction is increased, a transition to a rhombic Λ
-lattice is observed. We explain our findings by the change in contact area at faces and corners of the building blocks combined with the increase in attraction strength and range of the critical Casimir force. Our results show that the coupling between the rounded cubic shape and short-range attraction plays a crucial role for the superstructures that form and provide new insights for the active assembly control of micro and nanocubes.
Small-angle X-ray scattering (SAXS) is an indispensable tool in structural investigations of self-assembled colloidal crystals and colloidal liquid crystals. This paper reviews recent studies of the ...particle shape effects on the crystal structure as revealed by SAXS. Rod-like, plate-like, biaxial board-like as well as cubic-like shapes are discussed. Since relatively large, (sub)micron particles are often used in these studies, we describe the principles of the microradian X-ray diffraction technique that allows detailed characterisation of the periodic order including the determination of the intrinsic width of the Bragg peaks.
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•The structure of colloidal crystals can be affected by the shape of the particles.•Highly anisometric colloids yield a rich variety of liquid crystalline phases.•Small-angle X-ray scattering is an excellent tool to unravel colloidal structures.•Microradian resolution can be achieved using compound refractive optics.•SAXS must be further promoted in the colloid community.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
For magnetite spherical nanoparticles, the orientation of the dipole moment in the crystal does not affect the morphology of either zero field or field induced structures. For non-spherical particles ...however, an interplay between particle shape and direction of the magnetic moment can give rise to unusual behaviors, in particular when the moment is not aligned along a particle symmetry axis. Here we disclose for the first time the unique magnetic properties of hematite cubic particles and show the exact orientation of the cubes' dipole moment. Using a combination of experiments and computer simulations, we show that dipolar hematite cubes self-organize into dipolar chains with morphologies remarkably different from those of spheres, and demonstrate that the emergence of these structures is driven by competing anisotropic interactions caused by the particles' shape anisotropy and their fixed dipole moment. Furthermore, we have analytically identified a specific interplay between energy, and entropy at the microscopic level and found that an unorthodox entropic contribution mediates the organization of particles into the kinked nature of the dipolar chains.
Although single-particle level studies on prolate ellipsoidal colloids are relatively abundant, similar studies on oblate ellipsoids are rare because suitable model systems are scarcely available. ...Here, we present the preparation of monodisperse hard core–shell oblate ellipsoids that can be imaged and tracked in 3D with confocal laser scanning microscopy. Using a thermomechanical squeezing method, we transform spherical core–shell polymethyl-methacrylate (PMMA) particles into oblate ellipsoids. We show how the shape polydispersity as well as the aspect ratio of the obtained oblate ellipsoids can be controlled. In addition, we discuss how the core–shell geometry limits the range of aspect ratios because of the different viscoelastic properties of the cross-linked PMMA core and linear PMMA shell. We further demonstrate imaging of the core–shell oblate dispersions on a single-particle level in real space and time and the tracking of position and orientation using our recently developed tracking algorithm for anisotropic core–shell colloids. Our results thus provide the tools for the future investigation of the behavior of oblate ellipsoids, especially in dense suspensions.
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IJS, KILJ, NUK, PNG, UL, UM
Suspensions of hard ellipsoidal particles exhibit complex phase behavior as shown by theoretical predictions and simulations of phase diagrams. Here, we report quantitative confocal microscopy ...experiments of hard prolate colloidal ellipsoids with different aspect ratio
a
/
b
. We studied different volume fractions
of ellipsoids in density and refractive index matched suspensions. Large 3D sample volumes were investigated and the positions as well as the orientations of all ellipsoids were extracted by image analysis routines. By evaluating the translational and orientational order in the system we determined the presence of isotropic and nematic phases. For ellipsoids with
a
/
b
= 2.0 we found that isotropic phases form at all
, while ellipsoids with
a
/
b
= 7.0 formed nematic phases at high
, as expected from theory and simulations. For
a
/
b
= 3.5 and
a
/
b
= 4.1, however, we observed the absence of long-range orientational order even at
where nematic phases are expected. We show that local orientational order formed with the emergence of nematic precursors for
a
/
b
= 3.5 and short-ranged nematic domains for
a
/
b
= 4.1. Our results provide novel insight into the phase behavior and orientational order of ellipsoids with different aspect ratios.
Particle resolved measurements of the phase behavior of hard colloidal ellipsoids with aspect ratios from 2 to 7 reveal that for intermediate aspect ratios the predicted nematic phase is absent and instead nematic precursors and domains are formed.
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