Electric-Field-Driven Assembly of Dipolar Spheres Asymmetrically Confined between Two Electrodes.

Externally applied electric fields have previously been utilized to direct the assembly of colloidal particles confined at a surface into a large variety of colloidal oligomers and nonclose-packed honeycomb lattices (J. Am. Chem. Soc. 2013, 135, 7839-7842). The colloids under such confinement and fields are observed to spontaneously organize into bilayers near the electrode. To extend and better understand how particles can come together to form quasi-two-dimensional materials, we have performed Monte Carlo simulations and complementary experiments of colloids that are strongly confined between two electrodes under an applied alternating current electric field, controlling field strength and particle area fraction. Of particular importance, we control the fraction of particles in the upper vs lower plane, which we describe as asymmetric confinement, and which effectively modulates the coordination number of particles in each plane. We model the particle-particle interactions using a Stockmayer potential to capture the dipolar interactions induced by the electric field. Phase diagrams are then delineated as a function of the control parameters, and a theoretical model is developed in which the energies of several idealized lattices are calculated and compared. We find that the resulting theoretical phase diagrams agree well with simulation. We have not only reproduced the structures observed in experiments using parameters that are close to experimental conditions but also found several previously unobserved phases in the simulations, including a network of rectangular bands, zig zags, and a sigma lattice, which we were then able to confirm in experiment. We further propose a simple way to precisely control the number ratio of particles between different planes, that is, superimposing a direct current electric field with the alternating current electric field, which can be implemented conveniently in experiments. Our work demonstrates that a diverse collection of materials can be assembled from relatively simple ingredients, which can be analyzed effectively through comparison of simulation, theory, and experiment. Our model further explains possible pathways between different phases and provides a platform for examining phases that have yet to be observed in experiments.

Colloidal molecules assembled from binary spheres under an AC electric field.

Colloidal particles are envisioned as analogues of atoms and molecules, however they often lack the complexities present in their counterparts. In this work, we report the assembly of colloidal molecules from a binary mixture of polystyrene spheres (1, 1.6, 2, and 4 µm) under an alternating current electric field. The rich family of assembled oligomers typically consists of a large sphere that is closely surrounded by a number of smaller petal particles, driven by the dipolar attraction between large and small particles. In deionized water, the number of satellite particles, i.e., the coordination number increases with the increasing size ratio of the constituent particles. For a given size ratio, the coordination number decreases with the increasing frequency of the applied field. These trends have also been correctly captured by computing the electric energy of different oligomers based on induced dipolar and double-layer interactions. By suspending the particles in polyvinylpyrrolidone aqueous solution, we can further tune the bond length of the oligomers independent of their coordination numbers. The addition of polyvinylpyrrolidone also allows us to lock the assembled colloidal molecules so that they remain intact after the electric field is turned off. Our method provides a robust way to produce a family of colloidal molecules with well-defined geometry and high yield.

Inducing Propulsion of Colloidal Dimers by Breaking the Symmetry in Electrohydrodynamic Flow.

We show that dielectric colloidal dimers with broken symmetry in geometry, composition, or interfacial charges can all propel in directions that are perpendicular to the applied ac electric field. The asymmetry in particle properties ultimately results in an unbalanced electrohydrodynamic flow on two sides of the particles. Consistent with scaling laws, the propulsion direction, speed, and orientation of dimers can be conveniently tuned by frequency. The new propulsion mechanism revealed here is important for building colloidal motors and studying collective behavior of active matter.

Electric-field-induced assembly and propulsion of chiral colloidal clusters.

Chiral molecules with opposite handedness exhibit distinct physical, chemical, or biological properties. They pose challenges as well as opportunities in understanding the phase behavior of soft matter, designing enantioselective catalysts, and manufacturing single-handed pharmaceuticals. Microscopic particles, arranged in a chiral configuration, could also exhibit unusual optical, electric, or magnetic responses. Here we report a simple method to assemble achiral building blocks, i.e., the asymmetric colloidal dimers, into a family of chiral clusters. Under alternating current electric fields, two to four lying dimers associate closely with a central standing dimer and form both right- and left-handed clusters on a conducting substrate. The cluster configuration is primarily determined by the induced dipolar interactions between constituent dimers. Our theoretical model reveals that in-plane dipolar repulsion between petals in the cluster favors the achiral configuration, whereas out-of-plane attraction between the central dimer and surrounding petals favors a chiral arrangement. It is the competition between these two interactions that dictates the final configuration. The theoretical chirality phase diagram is found to be in excellent agreement with experimental observations. We further demonstrate that the broken symmetry in chiral clusters induces an unbalanced electrohydrodynamic flow surrounding them. As a result, they rotate in opposite directions according to their handedness. Both the assembly and propulsion mechanisms revealed here can be potentially applied to other types of asymmetric particles. Such kinds of chiral colloids will be useful for fabricating metamaterials, making model systems for both chiral molecules and active matter, or building propellers for microscale transport.

Colloidal structures of asymmetric dimers via orientation-dependent interactions.

We apply an AC electric field to induce anisotropic interactions among asymmetric colloidal dimers. These anisotropic interactions, being shape-specific and orientation-dependent, can create complex and unique structures that are not possible for spherical particles or symmetric dimers. More specifically, we show a series of novel structures that closely resemble one- and two-dimensional antiferromagnetic lattices, including small clusters, linear chains, square lattices, and frustrated triangular arrays. All of them are uniquely formed by alternating association between dimers with opposite orientations. Our theoretical model attributes those patterns to an exquisite balance between electrostatic (primarily dipolar) and electrohydrodynamic interactions. Although similarly oriented dimers are strongly repulsive, the oppositely oriented dimers possess a concave shoulder in the pair interaction, which favors clustering to minimize the number of overlaps between neighboring particles. By combining the anisotropy in both particle geometry and field-induced interaction, our work suggests a new way to tailor colloidal interactions on anisotropic particles, which is important for both scientific understanding and practical applications.

Bulk synthesis of metal-organic hybrid dimers and their propulsion under electric fields.

Metal-organic hybrid particles have great potential in applications such as colloidal assembly, autonomous microrobots, targeted drug delivery, and colloidal emulsifiers. Existing fabrication methods, however, typically suffer from low throughput, high operation cost, and imprecise property control. Here, we report a facile and bulk synthesis platform that makes a wide range of metal-organic colloidal dimers. Both geometric and interfacial anisotropy on the particles can be tuned independently and conveniently, which represents a key advantage of this method. We further investigate the self-propulsion of platinum-polystyrene dimers under perpendicularly applied electric fields. In 1 × 10(-4) M KCl solution, the dimers exhibit both linear and circular motion with the polystyrene lobes facing toward the moving direction, due to the induced-charge electroosmotic flow surrounding the metal-coated lobes. Surprisingly, in deionized water, the same dimers move in an opposite direction, i.e., the metallic lobes face the forward direction. This is because of the impact of another type of electrokinetic flow: the electrohydrodynamic flow arising from the induced charges on the conducting substrate. The competition between the electrohydrodynamic flow along the substrate and the induced-charge electroosmotic flow along the metallic lobe dictates the propulsion direction of hybrid dimers under electric fields. Our synthetic approach will provide potential opportunities to study the combined impacts of the geometric and interfacial anisotropy on the propulsion, assembly, and other applications of anisotropic particles.

Formation of colloidal molecules induced by alternating-current electric fields.

We report a versatile method for building colloidal molecules from particles that are isotropic in geometry and interfacial properties. When an external alternating-current electric field is applied, the particles experience anisotropic interactions that lead to the formation of colloidal oligomers via different assembly pathways that strikingly resemble chemical reactions of real molecules. We propose a mechanism for the formation of colloidal molecules that agrees well with the experiments. Our method can be used to build colloidal analogues of molecules using spherical particles with isotropic properties, which offers considerable advantages over existing methods. Moreover, our approach does not rely on material-specific properties and thus could have potential applications to a broad range of particles with different chemical properties.