Assembly Of Colloidal Particles Research Articles

Optical assembly of multi-particle arrays by opto-hydrodynamic binding of microparticles close to a one-dimensional chain of magnetic microparticles.

Two-dimensional materials possess a large number of interesting and important properties. Various methods have been developed to assemble two-dimensional aggregates. Assembly of colloidal particles can be achieved with laser-heating-induced thermal convective flow. In this paper, an opto-hydrodynamic binding method is proposed to assemble colloidal particles dispersed in a solution into multilayer structures. First, we use polystyrene (PS) microspheres to study the feasibility and characteristics of the assembly method. PS microspheres and monodispersed magnetic silica microspheres (SLEs) are dispersed in a solution to form a binary mixture system. Under the action of an external uniform magnetic field, SLEs in the solution form chains. An SLE chain is heated by a laser beam. Due to the photothermal effect, the SLE chain is heated to produce a thermal gradient, resulting in thermal convection. The thermal convection drives the PS beads to move toward the heated SLE chain and finally stably assemble into multilayer aggregates on both sides of the SLE chain. The laser power affects the speed and result of the assembly. When the laser power is constant, the degree of constraint of the PS microbeads in different layers is also different. At the same time, this method can also assemble the biological cells, and the spacing of different layers of cells can be changed by changing the electrolyte concentration of the solution. Our work provides an approach to assembling colloidal particles and cells, which has a potential application in the analysis of the collective dynamics of microparticles and microbes.

Percolation of nonequilibrium assemblies of colloidal particles in active chiral liquids.

The growing interest in the non-equilibrium assembly of colloidal particles in active liquids is driven by the motivation to create novel structures endowed with tunable properties unattainable within the confines of equilibrium systems. Here, we present an experimental investigation of the structural features of colloidal assemblies in active liquids of chiral E. coli. The colloidal particles form dynamic clusters due to the effective interaction mediated by active media. The activity and chirality of the swimmers strongly influence the dynamics and local ordering of colloidal particles, resulting in clusters with persistent rotation, whose structure differs significantly from those in equilibrium systems with attractive interactions, such as colloid-polymer mixtures. Our colloid-bacteria mixture displays several hallmark features of a percolation transition at a critical density, where the clusters span the system size. A closer examination of the critical exponents associated with cluster size distribution, the average cluster size, and the correlation length in the vicinity of the critical density shows deviations from the prediction of the standard continuum percolation model. Therefore, our experiments reveal a richer phase behavior of colloidal assemblies in active liquids.

Iridescent structural colors printing on cellulose fabrics with robust structural coloration

Structural coloration originating from colloidal particle assembly has drawn tremendous interest because of the brilliant dye-free colors in line with green and low-carbon development of the world. However, challenges like preparing structural colors with high saturation, durability, and time-efficiency in an unsmooth flexible surface such as fabrics are still remain. Here, a facile photopolymerization process is developed to construct high saturation and durable structural colors on cotton fabrics. The structural colors derive from the fast assembly of external magnetic field triggered Fe3O4@C particles. It is easy to regulate structural colors by adjusting magnetic particle sizes. In addition, by designing photo masks on transparent polyester films, the magnetic field induced assembly and photo polymerization strategy can conveniently print structural colored patterns on cotton fabric. This strategy may open a new gate in the textile printing field, as well as bring a new method by using fabrics to improve the mechanical properties of structural colors.

Stimuli‐Responsive Optical Switching Patterns in Millimetric Hydrogel Frames: Nanomaterial‐Free Anti‐Counterfeiting Technology

Stimuli‐responsive materials are employed in numerous on‐demand applications, such as anti‐counterfeiting. Although innovative hybrid spectral/graphical approaches have gained wide attention, they are limited by the need for user training and sophisticated technologies to decode the covert pattern which can be achieved through toxic organic dyes, pigments, up‐conversion crystals, nanoparticles, or self‐assembly of colloidal particles within a hydrogel framework. These approaches also involve a lengthy multistep fabrication that consumes several days. Although optical nanomaterials, which are responsive to external stimuli, such as magnetic field, can undergo a quicker self‐assembly, additional equipment is required to generate external fields and the use of nanomaterials alone limits this technology's wide applicability in consumer goods especially on food or pharmaceuticals. Herein, a nanomaterial‐free technology is demonstrated that utilizes various microscale motifs to display an optical shifting behavior without the need for passive or active assembly of colloidal particles, and any toxic dyes, pigments, or nanomaterials. The optical shifting property is purely induced by hydrogel porous microstructures and resulting light scattering. The slit channel lithography technique makes it easy for users to authenticate products using a simple smartphone device or inverted microscope by validating the optical switching behaviors of motifs embedded in hydrogel frames.

Nonadditivities of the Particle Sizes Hidden in Model Pair Potentials and Their Effects on Physical Adsorptions.

It is important to understand the mechanism of colloidal particle assembly near a substrate for development of drug delivery systems, micro-/nanorobots, batteries, heterogeneous catalysts, paints, and cosmetics. Understanding the mechanism is also important for crystallization of the colloidal particles and proteins. In this study, we calculated the physical adsorption of colloidal particles on a flat wall mainly using the integral equation theory, wherein small and large colloidal particles were employed. In the calculation system, like-charged electric double-layer potentials were used as pair potentials. In some cases, it was found that the small particles are more easily adsorbed. This result is unusual from the viewpoint of the Asakura-Oosawa theory, and we call it a "reversal phenomenon". Theoretical analysis revealed that the reversal phenomenon originates from the nonadditivities of the particle sizes. Using the knowledge obtained from this study, we invented a method to analyze the size nonadditivity hidden in model pair potentials. The method will be useful for confirmation of various simulation results regarding the adsorption and development of force fields for colloidal particles, proteins, and solutes.

Rapid template-assisted self-assembly: a practical route to the fast assembly of colloidal particles

Simple and quick techniques for assembling nanoparticles in topographically designed Poly(dimethylsiloxane) moulds of nanosized shapes have great potential in many spectroscopic and sensing tools. Close-packed particles pose rich plasmonic resonances, enabling the optical response to be tailored on both the nano- and macroscale. Template-assisted self-assembly (TASA) is a method that creates colloidal aggregates with controlled sizes formed by dewetting aqueous dispersions of NPs across surfaces. We present rapid TASA (rTASA), a modified version with an overall process time of under 10 min, improving speed and user-friendliness. Depending on the array pitch distance and average number of NPs per trap, the transmission through the template drops by between 20 and 80%, enabling them to be detected with even the simplest spectroscopic solutions. This rapid method is useful as a building block to generate self-assembled systems that exhibit exciting optical properties in crucial areas, particularly in building a fast test for size-selective NP detection.Graphical abstract

Directed colloidal assembly and banding via DC electrokinetics.

Manipulating the transport and assembly of colloidal particles to form segregated bands or ordered supracolloidal structures plays an important role in many aspects of science and technology, from understanding the origin of life to synthesizing new materials for next-generation manufacturing, electronics, and therapeutics. One commonly used method to direct colloidal transport and assembly is the application of electric fields, either AC or DC, due to its feasibility. However, as colloidal segregation and assembly both require active redistribution of colloidal particles across multiple length scales, it is not apparent at first sight how a DC electric field, either externally applied or internally induced, can lead to colloidal structuring. In this Perspective, we briefly review and highlight recent advances and standing challenges in colloidal transport and assembly enabled by DC electrokinetics.

Capillary Assembly of Anisotropic Particles at Cylindrical Fluid-Fluid Interfaces.

The unique behavior of colloids at liquid interfaces provides exciting opportunities for engineering the assembly of colloidal particles into functional materials. The deformable nature of fluid-fluid interfaces means that we can use the interfacial curvature, in addition to particle properties, to direct self-assembly. To this end, we use a finite element method (Surface Evolver) to study the self-assembly of rod-shaped particles adsorbed at a simple curved fluid-fluid interface formed by a sessile liquid drop with cylindrical geometry. Specifically, we study the self-assembly of single and multiple rods as a function of drop curvature and particle properties such as shape (ellipsoid, cylinder, and spherocylinder), contact angle, aspect ratio, and chemical heterogeneity (homogeneous and triblock patchy). We find that the curved interface allows us to effectively control the orientation of the rods, allowing us to achieve parallel, perpendicular, or novel obliquely orientations with respect to the cylindrical drop. In addition, by tuning particle properties to achieve parallel alignment of the rods, we show that the cylindrical drop geometry favors tip-to-tip assembly of the rods, not just for cylinders, but also for ellipsoids and triblock patchy rods. Finally, for triblock patchy rods with larger contact line undulations, we can achieve strong spatial confinement of the rods transverse to the cylindrical drop due to the capillary repulsion between the contact line undulations of the particle and the pinned contact lines of the sessile drop. Our capillary assembly method allows us to manipulate the configuration of single and multiple rod-like particles and therefore offers a facile strategy for organizing such particles into useful functional materials.

Continuum modeling of soft glassy materials under shear

Soft Glassy Materials (SGM) consist in dense amorphous assemblies of colloidal particles of multiple shapes, elasticity, and interactions, which confer upon them solid-like properties at rest. They are ubiquitously encountered in modern engineering, including additive manufacturing, semi-solid flow cells, dip coating, adhesive locomotion, where they are subjected to complex mechanical histories. Such processes often include a solid-to-liquid transition induced by large enough shear, which results in complex transient phenomena such as non-monotonic stress responses, i.e., stress overshoot, and spatially heterogeneous flows, e.g., shear banding or brittle failure. In the present article, we propose a pedagogical introduction to a continuum model based on a spatially resolved fluidity approach that we recently introduced to rationalize shear-induced yielding in SGMs. Our model, which relies upon non-local effects, quantitatively captures salient features associated with such complex flows, including the rate dependence of the stress overshoot, as well as transient shear-banded flows together with non-trivial scaling laws for fluidization times. This approach offers a versatile framework to account for subtle effects, such as avalanche-like phenomena, or the impact of boundary conditions, which we illustrate by including in our model the elasto-hydrodynamic slippage of soft particles compressed against solid surfaces.

Compression of colloidal monolayers at liquid interfaces: in situ vs. ex situ investigation.

The assembly of colloidal particles at liquid/liquid or air/liquid interfaces is a versatile procedure to create microstructured monolayers and study their behavior under compression. When combined with soft and deformable particles such as microgels, compression is used to tune not only the interparticle distance but also the underlying microstructure of the monolayer. So far, the great majority of studies on microgel-laden interfaces are conducted ex situ after transfer to solid substrates, for example, via Langmuir-Blodgett deposition. This type of analysis relies on the stringent assumption that the microstructure is conserved during transfer and subsequent drying. In this work, we couple a Langmuir trough to a custom-built small-angle light scattering setup to monitor colloidal monolayers in situ during compression. By comparing the results with ex situ and in situ microscopy measurements, we conclude that Langmuir-Blodgett deposition can alter the structural properties of the colloidal monolayers significantly.

Innovations in exploiting photo-controlled Marangoni flows for soft matter actuations.

The generation and control of microscale flows are crucial for fundamental as well as applied aspects of microfluidics. Commonly employed strategies for creating microflows are based on electric field, magnetic field, surface tension, temperature, pressure, light, etc. Among them, light as an external stimulus is gaining increased attention as it offers non-contact actuation, high spatial and temporal resolution, tunable wavelength and intensity, ease of miniaturization, and fast response. Optically tuning the surface tension is promising because a surface tension gradient of a few mN m-1 along the liquid surface is sufficient to create a strong Marangoni flow. This can be done by either optically heating (thermocapillary) or modulating the chemical composition (soluto-capillary) at the liquid surface, which could be exploited to realize the transport/assembly of colloidal particles, droplets, bubbles and liquid marbles. In this perspective article, we focus on the innovative approaches in modulating surface tension using light, in the context of soft matter and microfluidic applications. First, we discuss the photo-controlled Marangoni flow-driven patterning and assembly of colloidal particles in different microfluidic systems, such as liquid droplets and liquid films. Next, we review the various methods in which the photo-controlled Marangoni flows are being exploited for the actuation of liquid droplets and liquid marbles over solid as well as soft substrates. Finally, we highlight the recent trends in optically controlled flow-based reconfigurable optics and energy harvesting systems.

Continuous flow microfluidics for colloidal particle assembly on porous substrates.

Self-assembly of colloidal particles for 'bottom-up' fabrication of various patterns and structures is critical for a range of applications including, but not limited to, energy generation and storage, nanomaterial structures, biomimetics, and biosensing. Multiple self-assembly techniques, such as substrate templating-via topological or chemical patterning-and solvent evaporation were discussed in our previous papers and have been developed for the deposition of patterned self-assembled structures, such as bands of colloidal particles, on various substrates. While the templating techniques are limited in applications due to the requirements for pattern-specific prior substrate engineering to fabricate the desired structure, solvent evaporation requires longer assembly times and precise control over environmental conditions. In this paper, a template-free, continuous flow process, which is facilitated by continuous solvent drainage through porous substrates, is demonstrated for the self-assembly of colloidal particles into high-aspect ratio (>103, length to width) structures, such as linear arrays or grid structures. Colloidal particles were assembled both on polymeric and metallic porous membranes, with rapid assembly times.

Fabrication of three-dimensional opal nanolattices using template-directed colloidal assembly

Three-dimensional (3D) nanostructures play a crucial role in nanophotonics, lasers, and optical systems. This article reports on the fabrication of 3D nanostructures consisting of opal structures that are spatially aligned to an array of holes defined in the photoresist. The proposed method uses colloidal lithography to pattern a hexagonal array of holes, which are then used to direct the subsequent 3D assembly of colloidal particles. This approach allows the 3D opal structures to be aligned with the 2D array of holes, which can enhance spatial-phase coherence and reduce defects. The polymer patterns can be used as a sacrificial template for atomic layer deposition and create free-standing nanolattices. The final structure consists of a combination of nanolattice, upon which controlled deposition of opal structures is achieved. These structures result in nanostructured materials with high porosity, which is essential to create low-index materials for nanophotonics. A thick layer of titanium oxide with high refractive index is deposited over nanolattices to demonstrate the mechanical stability of underlying structures. These nanolattice structures with precisely controlled height can serve as a low-index layer and can find applications in Bragg reflectors, nanophotonics, and optical multilayers.

Bridging hollow carbon nanostructures to hierarchically pomegranate-like microspheres for efficient oil adsorption and catalysis applications

Hierarchical carbon superstructures that built from primary nanoscale objects and featured with a micro- or macroscopic architecture are highly desired, as credited by the synergistically integrated specialties. Here, we present a simple “microconfined-assembly-locking” approach to achieve pomegranate-like mesoporous carbon microspheres (PMCMs), in which the Pickering droplet-confined colloidal particle assembly and surfactant-directed interfacial locking are coupled. The formed PMCMs possess a unique micro-nano-hierarchical structure that consisting numerous mesoporous hollow carbon nanospheres within a permeable crust. Benefitting from this hierarchical arrangement, such a peculiar architecture not only preserves high surface area, large pore volume and good accessibility of the nano-building blocks, but also upgrades in mechanical strength and easy handling ability. In addition, this strategy allows the interior microstructures to be manipulated through combinatorial varying the assembled colloidal particles and interfacial chemical locking processes. Notably, the PMCMs show superior sorption capacity and cyclability for organics or oils from water in a fixed-bed system, which could also be applied to enhance the catalytic performance in hydrogenation of aromatic compounds after loading with Ru nanoparticles. This synthetic method provides a powerful bridge between nanoscale objects and microscale patterning, and paves a way to access other novel superstructures for expanding their potential applications.

Confinement and aggregation of colloidal particles in an ionic liquid microdroplet formed by optical tweezers

Currently, there is considerable interest in applying colloidal assemblies to photonic and plasmonic devices. Optical tweezing enables the preparation of such assemblies at desired positions, but the assembly process occurs only in areas irradiated by laser light. Here, we demonstrate the collection and assembly of colloidal particles in areas beyond the irradiation area. The particles are taken into a microdroplet formed by optical tweezing in a thermo-responsive ionic liquid (IL)/water mixture. The confined particles aggregate as the droplet shrinks. The mechanism of confinement and aggregation of colloidal particles are discussed in view of the surface charge of the particles.

Tools for the quantification of the dynamic assembly of colloidal chains of ellipsoidal particles

The assemblies of colloidal particles and their characterization are becoming more complex as new particle shapes, materials, and assembly methods are being introduced. Consequentially, there is a need for tools that can analyze videos and extract quantitative data from qualitative media to elucidate the dynamic processes of assembly formations. This contribution presents an algorithm that analyzes videos of the 2D assembly process of ellipsoidal particles. Particles are both classified using abstracted interparticle properties and tracked over time. Our algorithm is customized to quantiatively analyze the dynamic assembly of the ellipsoidal colloidal particles into chains, which break and reform over time. By analyzing the yielded data from our algorithm, we better elucidate the dynamic nature of particle chain formation. We foresee our algorithm being capable of analyzing videos in real time to study the dynamic assembly behavior of diverse matter such as biological samples and nanoparticles of various shapes.

Microbubble generation and large-scale assembly of colloidal particles by graphene oxide microstructures

The study of generation, growth and decay of microbubbles is interesting owing to its potential applications in imaging, trapping of colloidal particles and mass fluid flow. While there have been several reports on the generation of microbubbles using plasmonic nanostructure based substrates, they often are expensive and cumbersome to fabricate. In this study, we demonstrate a simple scheme for the generation of microbubbles using graphene oxide (GO) microstructures. Due to the excellent photothermal properties of this graphene-based 2D material, it is possible to generate and sustain microbubbles by laser illumination at low intensities of the order of few of μW μm−2. The size of the microbubbles can be tuned by changing the incident laser power. Furthermore, the generated microbubble acts as a concave wide-angle lens with variable focal length. We further utilised the fluid flow around the microbubble to generate large-scale assembly of silica beads and quantum dots using thermophoretic forces. This optically-assisted thermophoretic aggregation is rapid and reversible. Owing to the simple and cost-effective synthesis method of GO, this scheme is an excellent alternative to plasmonic heating based-methods for the generation of microbubbles .

Light‐Directed Assembly of Colloidal Matter

AbstractThe assembly of colloidal particles into 2D or 3D superstructures is significant as the colloidal assembly exhibits collective behavior beyond the sum of single particles. Technically, colloidal particles can either self‐assemble when thermodynamic equilibrium is reached, or directed into specific assembly under external stimulus, such as electric, magnetic, acoustic, or light field. Specifically, light can be focused locally and manipulated in a precise manner, providing the possibility to tailor the assembly kinetics in different degrees of freedom. The understanding of light–matter interaction during the assembly process is challenging but critical for the design and fabrication of diverse colloidal superstructures. In this review, these particles are treated as artificial atoms at colloidal scale to mimic the organization of matter. From this aspect, the light‐directed assembly process is discussed on the basis of the roles of light, including light‐directed nucleation, diffusion, interparticle bonding, and phase control. Beyond what has been observed at atomic scale, colloidal atoms exhibit diversity in size, shape, and composition, and their bonding force is irrelevant to the electronic state, which enriches the geometric complexity of colloidal matter. Finally, the authors summarize the emerging applications of these colloidal superstructures in nanophotonics, nanocatalysis, and nanomedicine, and outline the major challenge and future development.

Optically-assisted thermophoretic reversible assembly of colloidal particles and E. coli using graphene oxide microstructures

Optically-assisted large-scale assembly of nanoparticles have been of recent interest owing to their potential in applications to assemble and manipulate colloidal particles and biological entities. In the recent years, plasmonic heating has been the most popular mechanism to achieve temperature hotspots needed for extended assembly and aggregation. In this work, we present an alternative route to achieving strong thermal gradients that can lead to non-equilibrium transport and assembly of matter. We utilize the excellent photothermal properties of graphene oxide to form a large-scale assembly of silica beads. The formation of the assembly using this scheme is rapid and reversible. Our experiments show that it is possible to aggregate silica beads (average size 385 nm) by illuminating thin graphene oxide microplatelet by a 785 nm laser at low intensities of the order of 50–100 µW/µm2. We further extend the study to trapping and photoablation of E. coli bacteria using graphene oxide. We attribute this aggregation process to optically driven thermophoretic forces. This scheme of large-scale assembly is promising for the study of assembly of matter under non-equilibrium processes, rapid concentration tool for spectroscopic studies such as surface-enhanced Raman scattering and for biological applications.

The construction of pyramid structure of micro aluminum oxide

The assembly of colloidal particles to crystal structure on micrometer scale is attractive due to the unique and complex structure with stable physical and chemical properties, and even good anti-reflectivity. In this work, the pyramidal Al2O3 film was prepared via the designed process combining the sol-gel method and hydrothermal reaction. We employed Al(NO3)3 and H2NCONH2 as precursors and precipitant respectively on monocrystalline silicon wafer without etching. The bulk structure and components of the pyramidal Al2O3 film were investigated and analyzed by scanning electron microscope, energy dispersive X-ray spectrometers, X-ray diffraction, transmission electron microscope and X-ray photoelectron spectroscopy, etc. The fabricated Al2O3 films were composed of pyramidal structures, which mainly consisted of nanorods and nanospheres with both side length and thickness in micro scales. The surface reflectance and water contact angle results suggested that the surface of the film was hydrophilic and the average surface reflectance was 17.9%.