Periodically microstructured composite films made by electric- and magnetic-directed colloidal assembly

Magnetoresponsive Artificial Cilia Self-Assembled with Magnetic Micro/Nanoparticles

Biological cilia have exquisitely organized dynamic ultrafine structures with submicron diameters and exceptional aspect ratios, which are self-assembled with ciliary proteins. However, the construction of artificial cilia with size and dynamic functions comparable to biological cilia remains highly challenging. Here, we propose a self-assembly technique that generates magnetoresponsive artificial cilia with a highly ordered 3D structural arrangement using vapor-phase magnetic particles of varying sizes and shapes. We demonstrate that both monodispersed Fe₃O₄ nanoparticles and Fe microparticles can be assembled layer-by-layer vertically in patterned magnetic fields, generating both "nanoscale" or "microscale" artificial cilia, respectively. The resulting cilia display several structural features, such as diameters of single particle resolution, controllable diameters and lengths spanning from nanometers to micrometers, and accurate positioning. We further demonstrate that both the magnetic nanocilia and microcilia can dynamically and immediately actuate in response to modulated magnetic fields while providing different stroke ranges and actuation torques. Our strategy provides new possibilities for constructing artificial nano- and microcilia with controlled 3D morphology and dynamic field responsiveness using magnetic particles of varied sizes and shapes.

1D Colloidal chains: recent progress from formation to emergent properties and applications

Integrating nanoscale building blocks of low dimensionality (0D; i.e., spheres) into higher dimensional structures endows them and their corresponding materials with emergent properties non-existent or only weakly existent in the individual building blocks. Constructing 1D chains, 2D arrays and 3D superlattices using nanoparticles and colloids therefore continues to be one of the grand goals in colloid and nanomaterial science. Amongst these higher order structures, 1D colloidal chains are of particular interest, as they possess unique anisotropic properties. In recent years, the most relevant advances in 1D colloidal chain research have been made in novel synthetic methodologies and applications. In this review, we first address a comprehensive description of the research progress concerning various synthetic strategies developed to construct 1D colloidal chains. Following this, we highlight the amplified and emergent properties of the resulting materials, originating from the assembly of the individual building blocks and their collective behavior, and discuss relevant applications in advanced materials. In the discussion of synthetic strategies, properties, and applications, particular attention will be paid to overarching concepts, fresh trends, and potential areas of future research. We believe that this comprehensive review will be a driver to guide the interdisciplinary field of 1D colloidal chains, where nanomaterial synthesis, self-assembly, physical property studies, and material applications meet, to a higher level, and open up new research opportunities at the interface of classical disciplines.

Magnetically driven in-plane modulation of the 3D orientation of vertical ferromagnetic flakes

External magnetic fields are known to attract and orient magnetically responsive colloidal particles. In the case of 2D microplatelets, rotating magnetic fields are typically used to orient them parallel to each other in a brick-and-mortar fashion. Thanks to this microstructure, the resulting composites achieve enhanced mechanical and functional properties. However, parts with complex geometries require their microstructure to be specifically tuned and controlled locally in 3D. Although the tunability of the microstructure along the vertical direction has already been demonstrated using magnetic orientation combined with sequential or continuous casting, controlling the particle orientation in the horizontal plane in a fast and effective fashion remains challenging. Here, we propose to use rotating magnetic arrays to control the in-plane orientation of ferromagnetic nickel flakes distributed in curable polymeric matrices. We experimentally studied the orientation of the flakes in response to magnets rotating at various frequencies and precessing angles. Then, we used COMSOL to model the magnetic field from rotating magnetic arrays and predicted the resulting in-plane orientations. To validate the approach, we created composites with locally oriented flakes. This work could initiate reverse-engineering methods to design the microstructure in composite materials with intricate geometrical shapes for structural or functional applications.

Patterned Electrode Assisted One-Step Fabrication of Biomimetic Morphing Hydrogels with Sophisticated Anisotropic Structures

Anisotropic structures are ubiquitous in nature, affording fascinating morphing behaviors. Biomimetic morphing materials can be developed by spatially controlling the orientations of molecules or nanofillers that produce anisotropic responses and internal stresses under external stimuli. However, it remains a serious challenge to fabricate materials with sophisticated anisotropic architectures. Here, a facile strategy to fabricate morphing hydrogels with elaborately ordered structures of nanosheets, which are oriented under distributed electric field and immobilized by polymerization to form a poly(N-isopropylacrylamide) matrix, is proposed. Diverse sophisticated anisotropic structures are obtained by engineering the electric field through the patterns and relative locations of the electrodes. Upon heating, the monolithic hydrogels with through-thickness and/or in-plane gradients in orientation of the nanosheets deform into various three-dimensional configurations. After incorporating gold nanoparticles, the hydrogels become photoresponsive and capable of programmable motions, for example, dynamic twisting and flipping under spatiotemporal stimuli. Such a strategy of using patterned electrodes to generate distributed electric field should be applicable to systems of liquid crystals or charged particles/molecules to direct orientation or electrophoresis and form functional structures. The biomimetically architectured hydrogels would be ideal materials to develop artificial muscles, soft actuators/robots, and biomedical devices with versatile applications.

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.

Top-Down Heterogeneous Colloidal Engineering Using Capillary Assembly of Liquid Particles

Capillary assembly of liquid particles (CALP) is a microfabrication strategy for engineering arbitrarily shaped polymer colloids. The method entails depositing emulsion particles into patterned microarrays within a fluidic cell: coalescence, polymerization, and extraction of the deposited material engender faceted colloids. Herein, the versatility of CALP is demonstrated by using both consecutive assembly and heterogeneous coassembly to engineer geometrically diverse Janus and patchy colloids. Liquid particles (LPs) can be patterned laterally across the plane of the template by manipulating the capillary immersion force, liquid particle hardness, and rate of coalescence. Bilayers of different polymeric LPs and patchy microarrays are fabricated, comprising solid colloids made from various materials including poly(styrene), p-styryltrimethoxysilane, and iron oxide. Eleven different structures including concentric Janus squares, triblock ellipsoids, and planar tetramer and pentagonal patchy particles are described. All particles are fluorescently labeled, resist flocculation, withstand extended heating, and endure dispersion in organic solvent. Further crystallization and processing into colloid-based microscale devices is therefore anticipated. Heterogeneous CALP combines top-down microfabrication with bottom-up synthesis to engineer nonequilibrium particle structures that cannot be made with wet chemistry. CALP enables the design and fabrication of colloids with complex internal construction to target hierarchical functional materials. Ultimately, the integration of colloidal building blocks comprising multiple components that are independently addressable is crucial for the development of nano/micromaterials such as filtration devices, sensors, diagnostics, solid-state catalysts, and optical electronics.

Shape-encoded dynamic assembly of mobile micromachines

Field-directed and self-propelled colloidal assembly have been used to build micromachines capable of performing complex motions and functions. However, integrating heterogeneous components into micromachines with specified structure, dynamics and function is still challenging. Here, we describe the dynamic self-assembly of mobile micromachines with desired configurations through pre-programmed physical interactions between structural and motor units. The assembly is driven by dielectrophoretic interactions, encoded in the three-dimensional shape of the individual parts. Micromachines assembled from magnetic and self-propelled motor parts exhibit reconfigurable locomotion modes and additional rotational degrees of freedom that are not available to conventional monolithic microrobots. The versatility of this site-selective assembly strategy is demonstrated on different reconfigurable, hierarchical and three-dimensional micromachine assemblies. Our results demonstrate how shape-encoded assembly pathways enable programmable, reconfigurable mobile micromachines. We anticipate that the presented design principle will advance and inspire the development of more sophisticated, modular micromachines and their integration into multiscale hierarchical systems.

Assembly and Dynamic Analysis of Square Colloidal Crystals via Templated Capillary Assembly

Capillary assembly has the ability to engineer centimeter-sized regions of discrete colloidal superstructures and microarrays. However, its use as a tool for directing crystallization of colloids into surface-bound nonclose-packed arrays is limited. Furthermore, the use of quantitative particle tracking tools to investigate evaporative assembly dynamics is rarely employed. In this contribution, we use templated capillary assembly to fabricate square-packed lattices of spherical, organosilica colloids using designed patterned boundaries. Particle tracking algorithms reveal that the assembly of square-packed regions is controlled by the interplay between confinement-driven nuclei formation and osmotic pressure-driven restructuring. We find that the incorporation of a square template increases the yield of particles bearing four nearest neighbors (Z_n = 4) from 4 to 39%, obtained using a heavier and more viscous solvent. Maximal square-packed domains occur at specific initial particle concentrations (1.75-2.25 wt % or φ = 0.013-0.017), indicating that rearrangements are a function of osmotic force. We use particle tracking methods to dynamically monitor conversions between square and hexagonal packing, revealing a cyclical transition between 4 and 6 coordinated particles throughout meniscus recession. Our method is highly scalable and inexpensive and can be adapted for use with different particle sizes and compositions, as well as for targeted open-packed geometries. Our findings will inform the large area, defect-free assembly of nonclose-packed lattices of unexplored varieties that are necessary for the continued expansion of colloid-based materials with vast applications in optical electronics.

Power Generation by Selective Self-Assembly of Biocatalysts

Through a careful chemical and bioelectronic design we have created a system that uses self-assembly of enzyme-nanoparticle hybrids to yield bioelectrocatalytic functionality and to enable the harnessing of electrical power from biomass. Here we show that mixed populations of hybrids acting as catalyst carriers for clean energy production can be efficiently stored, self-assembled on functionalized stationary surfaces, and magnetically re-collected to make the binding sites on the surfaces available again. The methodology is based on selective interactions occurring between chemically modified surfaces and ligand-functionalized hybrids. The design of a system with minimal cross-talk between the particles, outstanding selective binding of the hybrids at the electrode surfaces, and direct anodic and cathodic electron transfer pathways leads to mediator-less bioelectrocatalytic transformations which are implemented in the construction of a fast self-assembling, membrane-less fructose/O₂ biofuel cell.

Design of textured multi-layered structures via magnetically assisted slip casting

Multi-layered composites in nature often show functional properties that are determined by the specific orientation of inorganic building blocks within each layer. The shell of bivalve molluscs and the exoskeleton of crustaceans constitute prominent examples. An effective approach to artificially produce textured microstructures inspired by such complex composites is magnetically assisted slip casting (MASC). MASC is a colloidal process in which anisotropic particles are magnetically oriented at arbitrarily defined angles and collected at the surface of a porous mould to grow the material in an additive manner. Whereas a number of proof-of-concept studies have established the potential of the technique, the full design space available for MASC-fabricated structures, and the limits of the approach, have so far not been explored systematically. To fill this gap, we have studied both theoretically and experimentally the various torques that act on the particles at the different stages of the assembly process. We define the boundary conditions of the MASC process for magnetically responsive alumina platelets suspended in a low-viscosity aqueous suspension, considering the composition of the colloidal suspension and the dynamics of the particle alignment process under a rotating magnetic field. These findings lead to design guidelines for the fabrication of bio-inspired composites with customized multi-scale structures for a broad range of applications.

Alternating electric-field-induced assembly of binary mixtures of soft repulsive ionic microgel colloids

An external alternating electric field is used to study the assembly of a binary mixture of Poly(N-isopropylacrylamide-co-acrylic acid) microgels in their swollen form at hydrodynamic size ratio 2:1 under deprotonated state. The AC field experiments were carried out at a fixed frequency of 100 kHz in the fluid regime for three number density ratios 1:3, 1:1 and 3:1 of big-to-small microgels using a confocal microscope. Strings with different types of co-assembly structures such as buckled, ring, flame and sandwich have been observed at low and intermediate field strengths at ratio 1:3, 1:1. In buckled and ring type, one or two small particles sit at the contact of two big particles and in the flame type, small particles arrange like a cone at end of the string. In the sandwich structure, several double small particle layers lie in between big particles. At high field strength, aggregation of strings and a phase separation into individual aggregates of strings from both big and small microgels have been observed. At higher ratio 3:1, the string formation is mostly dominated by big particles. Our experimental results are discussed with the recent simulation and experimental works on AC field induced structures in binary hard sphere mixtures.

External Field Assisted Freeze Casting

Freeze casting under external fields (magnetic, electric, or acoustic) produces porous materials having local, regional, and global microstructural order in specific directions. In freeze casting, porosity is typically formed by the directional solidification of a liquid colloidal suspension. Adding external fields to the process allows for structured nucleation of ice and manipulation of particles during solidification. External control over the distribution of particles is governed by a competition of forces between constitutional supercooling and electromagnetism or acoustic radiation. Here, we review studies that apply external fields to create porous ceramics with different microstructural patterns, gradients, and anisotropic alignments. The resulting materials possess distinct gradient, core–shell, ring, helical, or long-range alignment and enhanced anisotropic mechanical properties.

Transition of Dielectrophoresis-Assembled 2D Crystals to Interlocking Structures under a Magnetic Field

Aspherical cubic hematite colloids with cylindrical arms protruding from each face, referred to as "hexapods", were assembled via negative dielectrophoresis and then manipulated using an applied magnetic field. Upon application of an ac electric field, the hexapods aligned in close-packed linear chains parallel to the field direction. The chains then aggregated to the center of the device, with adjacent chains separated by distances approximately equal to twice the arm length. The resulting open packing structure exhibited cmm plane group symmetry due to the obstruction of arms, with a high density of incorporated defects. Subsequent application of a magnetic field to the dielectrophoresis (DEP)-assembled structure was found to anneal the colloidal crystal by reorienting the hexapods to align their intrinsic magnetic dipoles with the magnetic field direction. During reorganization, the colloidal packing density was found to decrease by more than 10% at both the center and edges of the crystal, accompanied by a significant loss of ordering, prior to redensification of the 2D lattice with fewer defects. Reorganization at the edge was 1.5 times faster than at the center, consistent with the need for cooperative colloidal motion to remove defects at the centers of the crystals.

Bioinspired Wear-Resistant and Ultradurable Functional Gradient Coatings

For mechanically protective coatings, the coating material usually requires sufficient stiffness and strength to resist external forces and meanwhile matched mechanical properties with the underneath substrate to maintain the structural integrity. These requirements generate a conflict that limits the coatings from achieving simultaneous surface properties (e.g., high wear-resistance) and coating/substrate interfacial durability. Herein this conflict is circumvented by developing a new manufacturing technique for functional gradient coatings (FGCs) with the material composition and mechanical properties gradually varying crossing the coating thickness. The FGC is realized by controlling the spatial distribution of magnetic-responsive nanoreinforcements inside a polymer matrix through a magnetic actuation process. By concentrating the reinforcements with hybrid sizes at the surface region and continuously diminishing toward the coating/substrate interface, the FGC is demonstrated to exhibit simultaneously high surface hardness, stiffness, and wear-resistance, as well as superb interfacial durability that outperforms the homogeneous counterparts over an order of magnitude. The concept of FGC represents a mechanically optimized strategy in achieving maximal performances with minimal use and site-specific distribution of the reinforcements, in accordance with the design principles of many load-bearing biological materials. The presented manufacturing technique for gradient nanocomposites can be extended to develop various bioinspired heterogeneous materials with desired mechanical performances.

Electric Field Assembly of Colloidal Superstructures

The assembly of materials from building blocks has been in the core of a wide range of applications from catalysis to photonics and electronics. External electric fields enable the interactions between building blocks to be controlled via induced dipoles. Dipolar interactions were used so far to obtain one-dimensional chains or three-dimensional non-close-packed lattices. However, complex colloidal assemblies and clusters of simple spherical particles are rare. Here we demonstrate a novel self-assembly approach enabling the formation of regular axially symmetric clusters, an array of colloidal assemblies as per design of posts, and hierarchical complex assemblies by using posts and dipolar interactions or combining them. Regulating the polarization of the particles from positive to negative allows us to control the interparticle interactions from attractive to repulsive at the poles or equator of the particles. Therefore, such particle-particle interactions enable switching between Saturn ring-like and candle-flame-like axially symmetric assemblies, which could potentially be exploited for display applications.

Capillary assembly as a tool for the heterogeneous integration of micro- and nanoscale objects

During the past decade, capillary assembly in topographical templates has evolved into an efficient method for the heterogeneous integration of micro- and nano-scale objects on a variety of surfaces. This assembly route has been applied to a large spectrum of materials of micrometer to nanometer dimensions, supplied in the form of aqueous colloidal suspensions. Using systems produced via bulk synthesis affords a huge flexibility in the choice of materials, holding promise for the realization of novel superior devices in the fields of optics, electronics and health, if they can be integrated into surface structures in a fast, simple, and reliable way. In this review, the working principles of capillary assembly and its fundamental process parameters are first presented and discussed. We then examine the latest developments in template design and tool optimization to perform capillary assembly in more robust and efficient ways. This is followed by a focus on the broad range of functional materials that have been realized using capillary assembly, from single components to large-scale heterogeneous multi-component assemblies. We then review current applications of capillary assembly, especially in optics, electronics, and in biomaterials. We conclude with a short summary and an outlook for future developments.

Colloidal shuttles for programmable cargo transport

The active transport of cargo molecules within cells is essential for life. Developing synthetic strategies for cargo control in living or inanimate thermal systems could lead to powerful tools to manipulate chemical gradients at the microscale and thus drive processes out of equilibrium to realize work. Here we demonstrate a colloidal analog of the complex biological shuttles responsible for molecular trafficking in cells. Our colloidal shuttles consist of magneto-dielectric particles that are loaded with cargo particles or living cells through size-selective dielectrophoretic trapping using electrical fields. The loaded colloidal shuttle can be transported with magnetic field gradients before cargo is released at the target location by switching off the electrical field. Such spatiotemporal control over the distribution of chemically active cargo in a reversible fashion can be potentially exploited for fundamental biological research or for the development of novel technologies for advanced cell culturing, drug discovery and medical diagnosis.

Biological shuttles efficiently traffic molecules in cells, inspiring the development of synthetic analogs. Here, the authors create colloidal shuttles that collect, transport, and deliver cargo particles and cells under the control of external electrical and magnetic fields.

Highly Anisotropic Conductors

Composite materials with ordered microstructures often lead to enhanced functionalities that a single material can hardly achieve. Many biomaterials with unusual microstructures can be found in nature; among them, many possess anisotropic and even directional physical and chemical properties. With inspiration from nature, artificial composite materials can be rationally designed to achieve this anisotropic behavior with desired properties. Here, a metallic wood with metal continuously filling the wood vessels is developed, which demonstrates excellent anisotropic electrical, thermal, and mechanical properties. The well-aligned metal rods are confined and separated by the wood vessels, which deliver directional electron transport parallel to the alignment direction. Thus, the novel metallic wood composite boasts an extraordinary anisotropic electrical conductivity (σ_|| /σ_⊥ ) in the order of 10¹¹ , and anisotropic thermal conductivity (κ_|| /κ_⊥ ) of 18. These values exceed the highest reported values in existing anisotropic composite materials. The anisotropic functionality of the metallic wood enables it to be used for thermal management applications, such as thermal insulation and thermal dissipation. The highly anisotropic metallic wood serves as an example for further anisotropic materials design; other composite materials with different biotemplates/hosts and fillers can achieve even higher anisotropic ratios, allowing them to be implemented in a variety of applications.

Colloidal assembly and 3D shaping by dielectrophoretic confinement

For decades, scientists and engineers have strived to design means of assembling colloids into ordered structures. By now, the literature is quite peppered with reports of colloidal assemblies. However, the available methods can assemble only a narrow range of structures or are applicable to specific types of colloids. There are still only few generic methods that would lead to arbitrary colloidal arrays or would shape colloidal assemblies into predesigned structures. Here, we first discuss in detail how to spatially control the assembly and crystallization of colloids through the balance of dielectrophoretic and dipolar forces. Furthermore, we demonstrate how to flexibly program and shape arrays of 3D microstructures that can be permanently affixed by in situ UV polymerization and calcination by using two facing similar or distinct micro-fabricated electrodes.

Multiscale directed self-assembly of composite microgels in complex electric fields

This study explored the application of localized electric fields for reversible directed self-assembly of colloidal particles in 3 dimensions. Electric field microgradients, arising from the use of micro-patterned electrodes, were utilized to direct the localization and self-assembly of polarizable (charged) particles resulting from a combination of dielectrophoretic and multipolar forces. Deionized dispersions of spherical and ellipsoidal core-shell microgels were employed for investigating their assembly under an external alternating electric field. We demonstrated that the frequency of the field allowed for an exquisite control over the localization of the particles and their self-assembled structures near the electrodes. We extended this approach to concentrated binary dispersions consisting of polarizable and less polarizable composite microgels. Furthermore, we utilized the thermosensitivity of the microgels to adjust the effective volume fraction and the dynamics of the system, which provided the possibility to dynamically "solidify" the assembly of the field-responsive particles by a temperature quench from their initial fluid state into an arrested crystalline state. Reversible solidification enables us to re-write/reconstruct various 3 dimensional assemblies by varying the applied field frequency.

Contact-Free Templating of 3-D Colloidal Structures Using Spatially Nonuniform AC Electric Fields

The formation of ordered and regularly shaped structures of colloidal particles with the aid of spatially nonuniform electric fields is a modern research area of great interest. This work illustrates how alternating current (AC) electrokinetic effects (dielectrophoresis, electroosmosis) can serve as contact-free templates, inside which colloidal microspheres can assemble into a variety of shapes and sizes. We show how three-dimensional colloidal structures of square, circular, and diamond shape of many tens of micrometers in size can be reproducibly formed with a single set of quadrupolar microelectrodes. Numerical simulations performed help to explain the role of AC electroosmosis and AC dielectrophoresis on the shaping of these structures as a function of applied voltage and frequency. We also demonstrate how the templating repertoire is further enhanced with the simultaneous application of a second, individually controlled AC electric field, which enables a variety of asymmetric colloidal structures to be produced using the same set of quadrupolar microelectrodes. As the preservation of shape and size of such electric-field templated structures after medium evaporation still remains a big challenge, here we also report on a novel method that permits the stabilization and isolation of these particle assemblies through medium gelation and subsequent hydrogel removal with a UV/ozone treatment.