Colloidal Dumbbells Research Articles

Dumbbell Impurities in 2D Crystals of Repulsive Colloidal Spheres Trap Dislocations.

Impurity-induced defects play a crucial role for the properties of crystals, but little is known about impurities with anisotropic shape. Here, we study how colloidal dumbbells distort and interact with a hexagonal crystal of charged colloidal spheres at a fluid interface. We find that subtle differences in the dumbbell length determine whether it induces a local distortion of the lattice or traps a dislocation, and determine how the dumbbell moves inside the repulsive hexagonal lattice. Our results provide new routes toward controlling material properties and understanding fundamental questions in phase transitions through particle-bound dislocations.

Memory-induced alignment of colloidal dumbbells

When a colloidal probe is forced through a viscoelastic fluid which is characterized by a long stress-relaxation time, the fluid is excited out of equilibrium. This is leading to a number of interesting effects including a non-trivial recoil of the probe when the driving force is removed. Here, we experimentally and theoretically investigate the transient recoil dynamics of non-spherical particles, i.e., colloidal dumbbells. In addition to a translational recoil of the dumbbells, we also find a pronounced angular reorientation which results from the relaxation of the surrounding fluid. Our findings are in good agreement with a Langevin description based on the symmetries of a director (dumbbell) as well as a microscopic bath-rod model. Remarkably, we find an instability with amplified fluctuations when the dumbbell is oriented perpendicular to the direction of driving. Our results demonstrate the complex behavior of non-spherical objects within a relaxing environment which are of immediate interest for the motion of externally but also self-driven asymmetric objects in viscoelastic fluids.

Linear growth of colloidal dumbbells into three-lobed polymer nanoparticles mediated by a gradient in surface wettability

Anisotropy is a deciding factor in determining the hydrodynamics and self-assembly of colloidal particles. Linking particle morphology to said behaviors promoted the development of strategies to obtain anisotropic particles exhibiting defined shapes and symmetries. Dumbbell-shaped polymer particles made by phase separation during seeded polymerization are prominent examples. Phase separation among monomer and seed particle yields a liquid protrusion of monomer on the seed. This protrusion is then polymerized, becoming solid and yielding a solid spherical lobe. When this process is performed with spherical seeds, two-lobed particles, known as colloidal dumbbells, are obtained. Repeating this process of lobe formation one or more times could pave the way to tailored particle morphologies. Given the higher degree of anisotropy, multi-lobed particles can expand the rich phase behavior already found for dumbbells. We propose a new route in making anisotropic polymer particles by directing phase separation in a linear direction, thus permitting linear growth. Colloidal particles composed of three individual polymer lobes with the potential for site-specific modifications are obtained. Triggering of the phase separation is done complementary to prior efforts in fabricating three-lobed polymer particles based on cross-linked precursor particles. We will show that tailored surface properties of anisotropic seed particles can prove as an effective tool not only to promote the monomer-polymer phase separation, but also to guide it in a linear direction. Such gradients in surface functionalization open perspectives for making polymer colloids on a large scale in whose custom-tailored shapes their phase behavior and superstructure formation are already established.Graphical

Sorting of heterogeneous colloids by AC-dielectrophoretic forces in a microfluidic chip with asymmetric orifices

HypothesisThe synthesis of compositionally heterogeneous particles is central to the development of complex colloidal units for self-assembly and self-propulsion. Yet, as the complexity of particles grows, synthesis becomes more prone to “errors”. We hypothesize that alternating-current dielectrophoretic forces can efficiently sort Janus particles, as a function of patch size and material, and colloidal dumbbells by size. ExperimentsWe prepared Janus particles with different patch size and material by physical vapor deposition and colloidal dumbbells via capillarity-assisted particle assembly. We then performed sorting experiments in a microfluidic chip comprising electrodes with asymmetric orifices, specifically exploiting the dielectric contrast between different portions of the particles or their size difference to steer them towards different outlets. FindingsWe calculated that the DEP force for Janus particles may switch from positive to negative as a function of composition at a critical AC frequency, thus enabling sorting different particles crossing the electrodes’ region. The predictions are confirmed by optical microscopy experiments. We also show that intact and “broken” dumbbells can be simply separated as they experience different DEP forces. The integration of multiple asymmetric orifices leads a larger zone with high field gradient to increase separation efficiency and makes it a promising tool to select precise particle populations, isolating fractions with narrowly distributed characteristics.

Height distribution and orientation of colloidal dumbbells near a wall.

Geometric confinement strongly influences the behavior of microparticles in liquid environments. However, to date, nonspherical particle behaviors close to confining boundaries, even as simple as planar walls, remain largely unexplored. Here, we measure the height distribution and orientation of colloidal dumbbells above walls by means of digital in-line holographic microscopy. We find that while larger dumbbells are oriented almost parallel to the wall, smaller dumbbells of the same material are surprisingly oriented at preferred angles. We determine the total height-dependent force acting on the dumbbells by considering gravitational effects and electrostatic particle-wall interactions. Our modeling reveals that at specific heights both net forces and torques on the dumbbells are simultaneously below the thermal force and energy, respectively, which makes the observed orientations possible. Our results highlight the rich near-wall dynamics of nonspherical particles and can further contribute to the development of quantitative frameworks for arbitrarily shaped microparticle dynamics in confinement.

Structure and phase transitions of two-dimensional core-softened colloidal dumbbells: a molecular dynamics study

Formation of two-dimensional (2D) quasi-crystalline structure by colloidal dumbbells interacting via a core-softened potential is studied using molecular dynamics simulations. Owing to the presence of two length scales associated with the rigid bond length of the dumbbell and the minimum of the core-softened repulsive force well, the dumbbell system exhibits rich complex 2D patterns ranging from low-density triangular ordered lattice, coupled stripe-ordered triangular lattice structure, hexagonal lattice, Kagome patterns, to high density clusters as density increases. At very high density, the 2D dumbbells form compact triangular ordered lattice. We have also identified the order-to-disorder transition temperature at which the ordered phase transitions to disordered structure by examining the potential energy, heat capacity and radial distribution functions of the system. In addition to 2D dimeric dumbbells, we studied the structures formed by 2D monomeric spheres that interact through the same core-softened potential. We found that although the monomeric spheres can produce most of the quasi crystalline patterns found for the dumbbell system, there are notable differences. The interplay of the two spatial length scales present in the dumbbells can produce a unique coupled stripe-triangular lattice structure at intermediate densities that is not observed for monomeric particles. The disparity between the two spatial lengths also has the effect of disrupting the formation of regular patterns leading to irregular and defective hexagons and Kagome lattices. This study shows that the two spatial length scales, present in the dumbbells, can be utilized to generate unique structures and demonstrates the potential for using dumbbells as building blocks for creating 2D structures.

Energy landscape of colloidal dumbbells in a periodic distribution of light.

Using a ray tracing calculation, the energy landscape of dumbbells, made of spherical colloidal particles, interacting with a periodic distribution of light is calculated. As shown previously [E. Sarmiento-Gomez, J. A. Rivera-Moran and J. L. Aruaz-Lara, Soft Matter, 2018, 14, 3684], planar aggregates of spherical particles adopt discrete configurations in such light distribution. Here we focus on the case of colloidal dumbbells both symmetric and asymmetric from an experimental and theoretical point of view. It has been shown that the direct calculation using the ray tracing approximation is in excellent agreement with the experiment in spite of the fact that the particles size and the wavelength of the trapping light are comparable. We also corroborate, at least for the more simple case of a single particle in a parabolic light distribution, that the simple method used here provides the same results as the more complex and general Lorenz-Mie approach giving a more simple yet reliable method for the calculation of the energy landscape of colloidal aggregates in periodic light distributions.

Curvature dynamics and long-range effects on fluid-fluid interfaces with colloids.

We investigate the dynamics of a phase-separating binary fluid, containing colloidal dumbbells anchored to the fluid-fluid interface. Extensive lattice Boltzmann-immersed boundary method simulations reveal that the presence of soft dumbbells can significantly affect the curvature dynamics of the interface between phase-separating fluids, even though the coarsening dynamics is left nearly unchanged. In addition, our results show that the curvature dynamics exhibits distinct non-local effects, which might be exploited for the design of new soft mesoscale materials. We point out that the inspection of the statistical dynamics of the curvature can disclose new insights into local inhomogeneities of the binary fluid configuration, as a function of the volume fraction and aspect ratio of the dumbbells.

Template–assisted assembly of asymmetric colloidal dumbbells into desirable cluster structures

In this report, using Monte Carlo simulations, we investigate the cluster structure of colloidal dumbbells with asymmetric properties in sizes via emulsion droplet evaporation. By tuning the diameter ratio of two spherical colloids, $q=\sigma _{l}/\sigma _{s}$ with $\sigma _{l}$ the diameter of the larger sphere and $\sigma _{s}$ the diameter of the smaller sphere, we obtain clusters with both the compact and open structures. For $q<1.2$ , we found a unique category of cluster isomers that minimize the second moment of the mass distribution. The uniqueness is lost when q ranges from 1.2 to 1.7. A further increase in the size ratio leads to a variety of different isomers with more complex configurations.

Self-Assembly of Two-Dimensional Patchy Colloidal Dumbbells

We study the self-assembly of two-dimensional patchy colloidal dumbbells, which are composed of attractive and repulsive circles. The shape of a colloidal dumbbell is characterized by the ratio of ...

Modelling critical Casimir force induced self-assembly experiments on patchy colloidal dumbbells.

Colloidal particles suspended in a binary liquid mixture can interact via solvent mediated interactions, known as critical Casimir forces. For anisotropic colloids this interaction becomes directional, which leads to rich phase behavior. While experimental imaging and particle tracking techniques allow determination of isotropic effective potentials via Boltzmann inversion, the modeling of effective interaction in anisotropic systems is non-trivial precisely because of this directionality. Here we extract effective interaction potentials for non-spherical dumbbell particles from observed radial and angular distributions, by employing reference interaction site model (RISM) theory and direct Monte Carlo simulations. For colloidal dumbbell particles dispersed in a binary liquid mixture and interacting via induced critical Casimir forces, we determine the effective site-site potentials for a range of experimental temperatures. Using these potentials to simulate the system for strong Casimir forces, we reproduce the experimentally observed collapse, and provide a qualitative explanation for this behavior.

Assembly of open clusters of colloidal dumbbells via droplet evaporation.

We investigate the behavior of a mixture of asymmetric colloidal dumbbells and emulsion droplets by means of kinetic Monte Carlo simulations. The evaporation of the droplets and the competition between droplet-colloid attraction and colloid-colloid interactions lead to the formation of clusters built up of colloid aggregates with both closed and open structures. We find that stable packings and hence complex colloidal structures can be obtained by changing the relative size of the colloidal spheres and/or their interfacial tension with the droplets.

Nonequilibrium structure of colloidal dumbbells under oscillatory shear

We investigate the nonequilibrium behavior of dense, plastic-crystalline suspensions of mildly anisotropic colloidal hard dumbbells under the action of an oscillatory shear field by employing Brownian dynamics computer simulations. In particular, we extend previous investigations, where we uncovered nonequilibrium phase transitions, to other aspect ratios and to a larger nonequilibrium parameter space, that is, a wider range of strains and shear frequencies. We compare and discuss selected results in the context of scattering and rheological experiments. Both simulations and experiments demonstrate that the previously found transitions from the plastic crystal phase with increasing shear strain also occur at other aspect ratios. We explore the transition behavior in the strain-frequency phase and summarize it in a nonequilibrium phase diagram. Additionally, the experimental rheology results hint at a slowing down of the colloidal dynamics with higher aspect ratio.

Au@Ag core/shell cuboids and dumbbells: Optical properties and SERS response

Recent studies have conclusively shown that the plasmonic properties of Au nanorods can be finely controlled by Ag coating. Here, we investigate the effect of asymmetric silver overgrowth of Au nanorods on their extinction and surface-enhanced Raman scattering (SERS) properties for colloids and self-assembled monolayers. Au@Ag core/shell cuboids and dumbbells were fabricated through a seed-mediated anisotropic growth process, in which AgCl was reduced by use of Au nanorods with narrow size and shape distribution as seeds. Upon tailoring the reaction rate, monodisperse cuboids and dumbbells were synthesized and further transformed into water-soluble powders of PEGylated nanoparticles. The extinction spectra of AuNRs were in excellent agreement with T-matrix simulations based on size and shape distributions of randomly oriented particles. The multimodal plasmonic properties of the Au@Ag cuboids and dumbbells were investigated by comparing the experimental extinction spectra with finite-difference time-domain (FDTD) simulations. The SERS efficiencies of the Au@Ag cuboids and dumbbells were compared in two options: (1) individual SERS enhancers in colloids and (2) self-assembled monolayers formed on a silicon wafer by drop casting of nanopowder solutions mixed with a drop of Raman reporters. By using 1,4-aminothiophenol Raman reporter molecules, the analytical SERS enhancement factor (AEF) of the colloidal dumbbells was determined to be 5.1×106, which is an order of magnitude higher than the AEF=4.0×105 for the cuboids. This difference can be explained by better fitting of the dumbbell plasmon resonance to the excitation laser wavelength. In contrast to the colloidal measurements, the AEF=5×107 of self-assembled cuboid monolayers was almost twofold higher than that for dumbbell monolayers, as determined with rhodamine 6G Raman reporters. According to TEM data and electromagnetic simulations, the better SERS response of the self-assembled cuboids is due to uniform packing and more efficient generation of electromagnetic hot spots, as compared to the dumbbell monolayers.

Equilibrium structure and fluctuations of suspensions of colloidal dumbbells

We investigate the structure and equilibrium linear-response dynamics of suspensions of hard colloidal dumbbells using Brownian dynamics computer simulations. The focus lies on the dense fluid and plastic crystal states of the colloids with investigated aspect (elongation-to-diameter) ratios varying from the hard sphere limit up to 0.39, which is roughly the stability limit of the plastic crystal phase. We find expected structural changes with larger elongation with respect to the hard sphere reference case and very localised orientational correlations, typically just involving next-neighbour couplings. These relatively weak correlations are also reflected in only minor effects on the translational and rotational diffusion coefficients for most of the investigated elongations. However, the linear response shear viscosity exhibits a dramatic increase at high packing fractions (φ ≳ 0.5) beyond a critical anisotropy factor of about L* ≃ 0.15 which is surprising in view of the relatively weak changes found before on the level of colloidal self-dynamics. We suspect that even for the small investigated anisotropies, newly occurring, collective rotational–translational couplings must be made responsible for the slow time scales appearing in the plastic crystal.

Reference interaction site model and optimized perturbation theories of colloidal dumbbells with increasing anisotropy.

We investigate thermodynamic properties of anisotropic colloidal dumbbells in the frameworks provided by the Reference Interaction Site Model (RISM) theory and an Optimized Perturbation Theory (OPT), this latter based on a fourth-order high-temperature perturbative expansion of the free energy, recently generalized to molecular fluids. Our model is constituted by two identical tangent hard spheres surrounded by square-well attractions with same widths and progressively different depths. Gas-liquid coexistence curves are obtained by predicting pressures, free energies, and chemical potentials. In comparison with previous simulation results, RISM and OPT agree in reproducing the progressive reduction of the gas-liquid phase separation as the anisotropy of the interaction potential becomes more pronounced; in particular, the RISM theory provides reasonable predictions for all coexistence curves, bar the strong anisotropy regime, whereas OPT performs generally less well. Both theories predict a linear dependence of the critical temperature on the interaction strength, reproducing in this way the mean-field behavior observed in simulations; the critical density—that drastically drops as the anisotropy increases—turns to be less accurate. Our results appear as a robust benchmark for further theoretical studies, in support to the simulation approach, of self-assembly in model colloidal systems.

Cluster formation and phase separation in heteronuclear Janus dumbbells

We have recently investigated the phase behavior of model colloidal dumbbells constituted by two identical tangent hard spheres, with the first being surrounded by an attractive square–well interaction (Janus dumbbells, Munaó et al 2014 Soft Matter 10 5269). Here we extend our previous analysis by introducing in the model the size asymmetry of the hard-core diameters and study the enriched phase scenario thereby obtained. By employing standard Monte Carlo simulations we show that in such ‘heteronuclear Janus dumbbells’ a larger hard-sphere site promotes the formation of clusters, whereas in the opposite condition a gas–liquid phase separation takes place, with a narrow interval of intermediate asymmetries wherein the two phase behaviors may compete. In addition, some peculiar geometrical arrangements, such as lamellæ, are observed only around the perfectly symmetric case. A qualitative agreement is found with recent experimental results, where it is shown that the roughness of molecular surfaces in heterogeneous dimers leads to the formation of colloidal micelles.

Self-assembly of patchy colloidal dumbbells.

We employ Monte Carlo simulations to investigate the self-assembly of patchy colloidal dumbbells interacting via a modified Kern-Frenkel potential by probing the system concentration and dumbbell shape. We consider dumbbells consisting of one attractive sphere with diameter σ1 and one repulsive sphere with diameter σ2 and center-to-center distance d between the spheres. For three different size ratios, we study the self-assembled structures for different separations l = 2d/(σ1 + σ2) between the two spheres. In particular, we focus on structures that can be assembled from the homogeneous fluid, as these might be of interest in experiments. We use cluster order parameters to classify the shape of the formed structures. When the size of the spheres is almost equal, q = σ2/σ1 = 1.035, we find that, upon increasing l, spherical micelles are transformed to elongated micelles and finally to vesicles and bilayers. For size ratio q = 1.25, we observe a continuously tunable transition from spherical to elongated micelles upon increasing the sphere separation. For size ratio q = 0.95, we find bilayers and vesicles, plus faceted polyhedra and liquid droplets. Our results identify key parameters to create colloidal vesicles with attractive dumbbells in experiments.

Colloidal Plastic Crystals in a Shear Field.

We study the structure and viscoelastic behavior of 3D plastic crystals of colloidal dumbbells in an oscillatory shear field based on a combination of small-angle neutron scattering experiments under shear (rheo-SANS) and Brownian dynamics computer simulations. Sterically stabilized dumbbell-shaped microgels are used as hard dumbbell model systems which consist of dumbbell-shaped polystyrene (PS) cores and thermosensitive poly(N-isopropylacrylamide) (PNIPAM) shells. Under increasing shear strain, a discontinuous transition is found from a twinned-fcc-like crystal to a partially oriented sliding-layer phase with a shear-molten state in between. In the novel partially oriented sliding-layer phase, the hard dumbbells exhibit a small but finite orientational order in the shear direction. We find that this weak correlation is sufficient to perturb the nature of the nonequilibrium phase transition as known for hard sphere systems. The discontinuous transition for hard dumbbells is observed to be accompanied by a novel yielding process with two yielding events in its viscoelastic shear response, while only a single yielding event is observed for sheared hard spheres. Our findings will be useful in interpreting the shear response of anisotropic colloidal systems and in generating novel colloidal crystals from anisotropic systems with applications in colloidal photonics.

Self-consistent electric field-induced dipole interaction of colloidal spheres, cubes, rods, and dumbbells

When calculating the interaction between electric field-induced dipoles, the dipole moments are often taken to be equal to their polarizability multiplied by the external electric field. However, this approach is not exact, since it does not take into account the fact that particles with a dipole moment affect the local electric field experienced by other particles. In this work, we employ the Coupled Dipole Method to calculate the electric-field-induced dipole pair interaction self-consistently: that is, we take into account many-body effects on the individual induced dipole moments. We calculate interactions of particles with nonvanishing dimensions by splitting them up into self-consistently inducible “chunks” of polarizable matter. For point dipoles, spheres, cubes, rods, and dumbbells, we discuss the differences and commonalities between our self-consistent approach and the aforementioned approach of pre-assigning dipole moments to either the point dipoles or, in the case of spatially extended particles, to the chunks making up the particle.