Spatially and temporally reconfigurable assembly of colloidal crystals

Temporal Evolution of Interparticle Potentials of PMMA Colloids in CHB/Decalin

Colloidal dispersions composed of polymethylmetacrylate particles dispersed in a mixture of cyclohexyl bromide and decalin find widespread use as model systems in optical microscopy experiments. While the system allows simultaneous density and refractive index matching, preparing particles with hard potentials remains challenging, and strong variations in the physical parameters of samples prepared in the same manner are commonly observed. Here, we present data on the measurement of forces between individual pairs of particles in highly diluted dispersions over the course of tens of days using the blinking optical tweezers method. Our results show that the variations in the particle properties are indeed caused by a temporal evolution of the particles' charging. Additional measurements of the influence of the addition of tetrabutylammonium bromide (TBAB) to the dispersions show that already small concentrations of added TBAB salt drastically decrease the electrostatic forces between colloidal particles. However, small, non-negligible contact potentials remain even at the highest TBAB concentrations added.

Programmable Crowding and Tunable Phases in a Binary Mixture of Colloidal Particles under Light-Driven Thermal Convection

We employ photothermally driven self-assembly of colloidal particles to design microscopic structures with programmable size and tunable order. The experimental system is based on a binary mixture of "plasmonic heater" gold nanoparticles and "assembly building block" microparticles. Photothermal heating of the gold nanoparticles under visible light causes a natural convection flow that efficiently assembles the microscale building block particles (diameter 1-10 μm) into a monolayer. We identify the onset of active Brownian motion of colloidal particles under this convective flow by varying the conditions of light intensity, gold nanoparticle concentration, and sample height. We realize a crowded assembly of microparticles around the center of illumination and show that the size of the particle crowd can be programmed using patterned light illumination. In a binary mixture of gold nanoparticles and polystyrene microparticles, we demonstrate the formation of rapid and large-scale crystalline monolayers, covering an area of 0.88 mm2 within 10 min. We find that the structural order of the assembly can be tuned by varying the surface charge of the nanoparticles and the size of the microparticles, giving rise to the formation of different phases-colloidal crystals, crowds, and gels. Using Monte Carlo simulations, we explain how the phases emerge from the interplay between hydrodynamic and electrostatic interactions, as well as the assembly kinetics. Our study demonstrates the promise of self-assembly with programmable shapes and structural order under nonequilibrium conditions using an accessible setup comprising only binary mixtures and LED light.

Collective Behaviors of Isotropic Micromotors: From Assembly to Reconstruction and Motion Control under External Fields

Swarms of self-propelled micromotors can mimic the processes of natural systems and construct artificial intelligent materials to perform complex collective behaviors. Compared to self-propelled Janus micromotors, the isotropic colloid motors, also called micromotors or microswimmers, have advantages in self-assembly to form micromotor swarms, which are efficient in resistance to external disturbance and the delivery of large quantity of cargos. In this minireview, we summarize the fundamental principles and interactions for the assembly of isotropic active particles to generate micromotor swarms. Recent discoveries based on either catalytic or external physical field-stimulated micromotor swarms are also presented. Then, the strategy for the reconstruction and motion control of micromotor swarms in complex environments, including narrow channels, maze, raised obstacles, and high steps/low gaps, is summarized. Finally, we outline the future directions of micromotor swarms and the remaining challenges and opportunities.

Reconfigurable Grating Diffraction Structural Color in Self-Assembled Colloidal Crystals

Self-assembled colloidal crystals display structural colors due to light diffracted from their microscale, ordered structure. This color arises due to Bragg reflection (BR) or grating diffraction (GD); the latter mechanism is much less explored than the former. Here the design space for generating GD structural color is identified and its relative advantages are demonstrated. Electrophoretic deposition is used to self-assemble crystals with fine crystal grains from colloids of diameter 1.0 µm. The structural color in transmission is tunable across the full visible spectrum. The optimum optical response-represented by both color intensity and saturation-is observed at low layer number (≤5 layers). The spectral response is well predicted by Mie scattering of the crystals. Taken together, the experimental and theoretical results demonstrate that vivid grating colors with high color saturation can be produced from thin layers of micron-sized colloids. These colloidal crystals extend the potential of artificial structural color materials.

Electric and Magnetic Field-Driven Dynamic Structuring for Smart Functional Devices

The field of soft matter is rapidly growing and pushing the limits of conventional materials science and engineering. Soft matter refers to materials that are easily deformed by thermal fluctuations and external forces, allowing for better adaptation and interaction with the environment. This has opened up opportunities for applications such as stretchable electronics, soft robotics, and microfluidics. In particular, soft matter plays a crucial role in microfluidics, where viscous forces at the microscale pose a challenge to controlling dynamic material behavior and operating functional devices. Field-driven active colloidal systems are a promising model system for building smart functional devices, where dispersed colloidal particles can be activated and controlled by external fields such as magnetic and electric fields. This review focuses on building smart functional devices from field-driven collective patterns, specifically the dynamic structuring of hierarchically ordered structures. These structures self-organize from colloidal building blocks and exhibit reconfigurable collective patterns that can implement smart functions such as shape shifting and self-healing. The review clarifies the basic mechanisms of field-driven particle dynamic behaviors and how particle-particle interactions determine the collective patterns of dynamic structures. Finally, the review concludes by highlighting representative application areas and future directions.

Manipulating Interactions between Dielectric Particles with Electric Fields: A General Electrostatic Many-Body Framework

We derive a rigorous analytical formalism and propose a numerical method for the quantitative evaluation of the electrostatic interactions between dielectric particles in an external electric field. This formalism also allows for inhomogeneous charge distributions, and, in particular, for the presence of pointlike charges on the particle surface. The theory is based on a boundary integral equation framework and yields analytical expressions for the interaction energy and net forces that can be computed in linear scaling cost, with respect to the number of interacting particles. We include numerical results that validate the proposed method and show the limitations of the fixed dipole approximation at small separation between interacting particles. The proposed method is also applied to study the stability and melting of ionic colloidal crystals in an external electric field.

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.

Two-Dimensional Self-Assembly of Au@Ag Core-Shell Nanocubes with Different Permutations for Ultrasensitive SERS Measurements

Different self-assembly methods not only directly change the arrangement of noble metal particles on the substrate but also indirectly affect the local electromagnetic field distribution and intensity of the substrate under specific optical excitation conditions, which leads to distinguished different enhancement effects of the structure on molecular Raman signals. In this paper, first, the gold species growth method was used to prepare the silver-coated gold nanocubes (Au@Ag NCs) with regular morphology and uniform size, and then the two-phase and three-phase liquid-liquid self-assembly and evaporation-induced self-assembly methods were used to obtain the substrate structure with different NC arrangement patterns. The optimal arrangement of NCs was found by transverse comparison of Raman signal detection of probe molecules with the same concentration. Subsequently, surface-enhanced Raman scattering (SERS) measurements of Rhodamine (Rh6G) and aspartame (APM) were carried out. Furthermore, the finite element method (FEM) was employed to calculate the local electromagnetic fields of the substrates with different Au@Ag NC arrangements, and the calculated results were in agreement with the experimental results. The experimental results show that the SERS-active substrate was largely associated with the different arrangements of Au@Ag NCs, and the island membrane Au@Ag NCs array substrate obtained by evaporation-induced self-assembly can generate a strong local electromagnetic field due to the edge and corner bonding gap between the tightly arranged NCs; this endows the substrate with benign sensitivity and reproducibility and has great potential in molecular detection, biosensing, and food safety monitoring.

Programmable Dynamic Shapes with a Swarm of Light-Powered Colloidal Motors

We report robust control over the dynamic assembly, disassembly, and reconfiguration of light-activated molybdenum disulfide (MoS₂ ) colloidal motor swarms with features not possible in equilibrium systems. A photochemical reaction produces chemical gradients across the MoS₂ colloidal motors to drive them to move. Under illumination of a gradient light, these colloidal motors display a positive phototactic motion. Mesoscale simulations prove that the self-diffusiophoresis induced by the locally consumed oxygen gradient across MoS₂ colloidal motors dominates the phototactic process. By programming the structured illumination, the collective migration and well-defined shapes of colloidal motor swarms can be externally regulated. The successful realization of programmable swarm transformation of colloidal motors like the emergent behaviors of living systems in nature provides a direct proof-of-concept for active soft materials and systems, with adaptive and interactive functions.

Accelerated annealing of colloidal crystal monolayers by means of cyclically applied electric fields

External fields are commonly applied to accelerate colloidal crystallization; however, accelerated self-assembly kinetics can negatively impact the quality of crystal structures. We show that cyclically applied electric fields can produce high quality colloidal crystals by annealing local disorder. We find that the optimal off-duration for maximum annealing is approximately one-half of the characteristic melting half lifetime of the crystalline phase. Local six-fold bond orientational order grows more rapidly than global scattering peaks, indicating that local restructuring leads global annealing. Molecular dynamics simulations of cyclically activated systems show that the ratio of optimal off-duration for maximum annealing and crystal melting time is insensitive to particle interaction details. This research provides a quantitative relationship describing how the cyclic application of fields produces high quality colloidal crystals by cycling at the fundamental time scale for local defect rearrangements; such understanding of dynamics and kinetics can be applied for reconfigurable colloidal assembly.

Controlling colloidal crystals via morphing energy landscapes and reinforcement learning

We report a feedback control method to remove grain boundaries and produce circular shaped colloidal crystals using morphing energy landscapes and reinforcement learning-based policies. We demonstrate this approach in optical microscopy and computer simulation experiments for colloidal particles in ac electric fields. First, we discover how tunable energy landscape shapes and orientations enhance grain boundary motion and crystal morphology relaxation. Next, reinforcement learning is used to develop an optimized control policy to actuate morphing energy landscapes to produce defect-free crystals orders of magnitude faster than natural relaxation times. Morphing energy landscapes mechanistically enable rapid crystal repair via anisotropic stresses to control defect and shape relaxation without melting. This method is scalable for up to at least N = 10³ particles with mean process times scaling as N ^0.5 Further scalability is possible by controlling parallel local energy landscapes (e.g., periodic landscapes) to generate large-scale global defect-free hierarchical structures.

Influence of cap weight on the motion of a Janus particle very near a wall

The dynamics of anisotropic nano- to micro scale colloidal particles in confined environments, either near neighboring particles or boundaries, is relevant to a wide range of applications. We utilized Brownian dynamics simulations to predict the translational and rotational fluctuations of a Janus sphere with a cap of nonmatching density near a boundary. The presence of the cap significantly impacted the rotational dynamics of the particle as a consequence of gravitational torque at experimentally relevant conditions. Gravitational torque dominated stochastic torque for a particle >1 μm in diameter and with a 20-nm-thick gold cap. Janus particles at these conditions sampled mostly cap-down or "quenched" orientations. Although the results summarized herein showed that particles of smaller diameter (<1 μm) with a thin gold coating (<5 nm) behave similarly to an isotropic particle, small increases in either particle diameter or coating thickness quenched the polar rotation of the particle. Histogram landscapes of the separation distance from the boundary and orientation observations of particles with larger diameters or thicker gold coatings were mostly populated with quenched configurations. Finally, the histogram landscapes were inverted to obtain the potential energy landscapes, providing a road map for experimental data to be interpreted.

Light-Responsive Colloidal Crystals Engineered with DNA

A novel method for synthesizing and photopatterning colloidal crystals via light-responsive DNA is developed. These crystals are composed of 10-30 nm gold nanoparticles interconnected with azobenzene-modified DNA strands. The photoisomerization of the azobenzene molecules leads to reversible assembly and disassembly of the base-centered cubic (bcc) and face-centered cubic (fcc) crystalline nanoparticle lattices. In addition, UV light is used as a trigger to selectively remove nanoparticles on centimeter-scale thin films of colloidal crystals, allowing them to be photopatterned into preconceived shapes. The design of the azobenzene-modified linking DNA is critical and involves complementary strands, with azobenzene moieties deliberately staggered between the bases that define the complementary code. This results in a tunable wavelength-dependent melting temperature (Tm ) window (4.5-15 °C) and one suitable for affecting the desired transformations. In addition to the isomeric state of the azobenzene groups, the size of the particles can be used to modulate the Tm window over which these structures are light-responsive.

Anisotropy effects on the kinetics of colloidal crystallization and melting: comparison of spheres and ellipsoids

We use alternating current (AC) electric field assisted self-assembly to produce two-dimensional, millimeter scale arrays of ellipsoidal colloids and study the kinetics of their phase reconfiguration by means of confocal microscopy, light scattering, and computer simulation. We find that the kinetics of orientational and positional ordering can be manipulated by changing the shape of the colloids: ellipsoids with aspect ratio 2.0 melt into disordered structures 5.7 times faster compared to spheres. On the other hand, ellipsoids self-assemble into ordered crystals at a similar rate to spheres. Confocal microscopy is used to directly visualize defects in the self-assembled structures. Small-angle light scattering (SALS) quantifies the light diffraction response, which is sensitive to the kinetics of positional and orientational ordering in the self-assembled anisotropic structures. We find three different light diffraction patterns: a phase with high orientational order (with chain-like structure in real space), a phase with high positional and orientational order (characteristic of a close-packed structure), and a phase that is disordered in position but with intermediate orientational order. The large influence of aspect ratio on the kinetics of the positionally and orientationally ordered phase is explored through simulation; it is found that the number of particle degrees of freedom controls the difference between the melting rates of the ellipsoids and spheres. This research contributes to the understanding of reconfiguration kinetics and optical properties of colloidal crystals produced from anisotropic colloids.

Motile Micropump Based on Synthetic Micromotors for Dynamic Micropatterning

Micropump systems show great potential on the micropatterning process as a result of remarkable performance and functionality. However, existing micropumps cannot be employed as direct writing tools to perform the complex micropatterning process because of their lacking motility and controllability. Here, we propose a motile micropump system based on the combination of a water-driven ZnO/Ni/polystyrene Janus micromotor with a traditional immobilized micropump. This novel motile micropump system can translate the trajectory of Janus micromotors into predefined micropatterns by pumping away passive silica particles around the micromotor under the effect of diffusiophoresis. The resolution and efficiency of the micropatterning process can be regulated by controlling the diameters of Janus micromotors. Diverse surface micropatterns can be fabricated though remote magnetic control of the motile micropump system. Such ability to transform the versatile motile micropump into predetermined surface micropatterns creates new opportunities for mask-free micropatterning.

Photoswitchable Dissipative Two-Dimensional Colloidal Crystals

Control over particle interactions and organization at fluid interfaces is of great importance both for fundamental studies and practical applications. Rendering these systems stimulus-responsive is thus a desired challenge both for investigating dynamic phenomena and realizing reconfigurable materials. Here, we describe the first reversible photocontrol of two-dimensional colloidal crystallization at the air/water interface, where millimeter-sized assemblies of microparticles can be actuated through the dynamic adsorption/desorption behavior of a photosensitive surfactant added to the suspension. This allows us to dynamically switch the particle organization between a highly crystalline (under light) and a disordered (in the dark) phase with a fast response time (crystallization in ≈10 s, disassembly in ≈1 min). These results evidence a new kind of dissipative system where the crystalline state can be maintained only upon energy supply.

Tools and Functions of Reconfigurable Colloidal Assembly

We review work in reconfigurable colloidal assembly, a field in which rapid, back-and-forth transitions between the equilibrium states of colloidal self-assembly are accomplished by dynamic manipulation of the size, shape, and interaction potential of colloids, as well as the magnitude and direction of the fields applied to them. It is distinguished from the study of colloidal phase transitions by the centrality of thermodynamic variables and colloidal properties that are time switchable; by the applicability of these changes to generate transitions in assembled colloids that may be spatially localized; and by its incorporation of the effects of generalized potentials due to, for example, applied electric and magnetic fields. By drawing upon current progress in the field, we propose a matrix classification of reconfigurable colloidal systems based on the tool used and function performed by reconfiguration. The classification distinguishes between the multiple means by which reconfigurable assembly can be accomplished (i.e., the tools of reconfiguration) and the different kinds of structural transitions that can be achieved by it (i.e., the functions of reconfiguration). In the first case, the tools of reconfiguration can be broadly classed as (i) those that control the colloidal contribution to the system entropy-as through volumetric and/or shape changes of the particles; (ii) those that control the internal energy of the colloids-as through manipulation of colloidal interaction potentials; and (iii) those that control the spatially resolved potential energy that is imposed on the colloids-as through the introduction of field-induced phoretic mechanisms that yield colloidal displacement and accumulation. In the second case, the functions of reconfiguration include reversible: (i) transformation between different phases-including fluid, cluster, gel, and crystal structures; (ii) manipulation of the spacing between colloids in crystals and clusters; and (iii) translation, rotation, or shape-change of finite-size objects self-assembled from colloids. With this classification in hand, we correlate the current limits on the spatiotemporal scales for reconfigurable colloidal assembly and identify a set of future research challenges.

Painting with light-powered bacteria

Self-assembly is a promising route for micro- and nano-fabrication with potential to revolutionise many areas of technology, including personalised medicine. Here we demonstrate that external control of the swimming speed of microswimmers can be used to self assemble reconfigurable designer structures in situ. We implement such ‘smart templated active self assembly’ in a fluid environment by using spatially patterned light fields to control photon-powered strains of motile Escherichia coli bacteria. The physics and biology governing the sharpness and formation speed of patterns is investigated using a bespoke strain designed to respond quickly to changes in light intensity. Our protocol provides a distinct paradigm for self-assembly of structures on the 10 μm to mm scale.

The ability to generate microscale patterns and control microswimmers may be useful for engineering smart materials. Here Arlt et al. use genetically modified bacteria with fast response to changes in light intensity to produce light-induced patterns.

Light-Controlled Swarming and Assembly of Colloidal Particles

Swarms and assemblies are ubiquitous in nature and they can perform complex collective behaviors and cooperative functions that they cannot accomplish individually. In response to light, some colloidal particles (CPs), including light active and passive CPs, can mimic their counterparts in nature and organize into complex structures that exhibit collective functions with remote controllability and high temporospatial precision. In this review, we firstly analyze the structural characteristics of swarms and assemblies of CPs and point out that light-controlled swarming and assembly of CPs are generally achieved by constructing light-responsive interactions between CPs. Then, we summarize in detail the recent advances in light-controlled swarming and assembly of CPs based on the interactions arisen from optical forces, photochemical reactions, photothermal effects, and photoisomerizations, as well as their potential applications. In the end, we also envision some challenges and future prospects of light-controlled swarming and assembly of CPs. With the increasing innovations in mechanisms and control strategies with easy operation, low cost, and arbitrary applicability, light-controlled swarming and assembly of CPs may be employed to manufacture programmable materials and reconfigurable robots for cooperative grasping, collective cargo transportation, and micro- and nanoengineering.

Freeze the Moment: High Speed Capturing of Weakly Bonded Dynamic Nanoparticle Assemblies in Solution by Ag Ion Soldering

Despite the versatile forms of colloidal aggregates, these spontaneously formed structures are often hard to find a suitable application in nanotechnology and materials science. A determinate reason is the lack of a suitable method to capture the transiently formed and quickly evolving colloidal structures in solution. To address this challenge, a simple but highly efficient strategy is herein reported to capture the dynamic and metastable colloidal assemblies formed in an aqueous or nonaqueous solution. This process takes advantage of a recently developed Ag ion soldering reaction to realize a rapid fixation of as-formed metastable assemblies. This method works efficiently for both solid (3D) nanoparticle aggregates and weakly bonded fractal nanoparticle chains (1D). In both cases, very high capturing speed and close to 100% efficiency are achieved to fully retain a quickly growing structure. The soldered nanochains further enable a fabrication of discrete, uniform, and functionalizable nanoparticle clusters with enriched linear conformation by mechanical shearing, which would otherwise be difficult to make. The captured products are water dispersible and mechanically robust, favoring an exploration of their properties toward possible applications. The work paves a way to previously untouched aspects of colloidal science and thus would create new chances in nanotechnology.

Spatially controlled clustering of nucleotide-stabilized vesicles

A two-step hierarchical self-assembly process is presented relying on the GMP-induced formation of vesicles, which then cluster into large aggregates upon the addition of Ag⁺-ions.

Controllable Swarming and Assembly of Micro/Nanomachines

Motion is a common phenomenon in biological processes. Major advances have been made in designing various self-propelled micromachines that harvest different types of energies into mechanical movement to achieve biomedicine and biological applications. Inspired by fascinating self-organization motion of natural creatures, the swarming or assembly of synthetic micro/nanomachines (often referred to micro/nanoswimmers, micro/nanorobots, micro/nanomachines, or micro/nanomotors), are able to mimic these amazing natural systems to help humanity accomplishing complex biological tasks. This review described the fuel induced methods (enzyme, hydrogen peroxide, hydrazine, et al.) and fuel-free induced approaches (electric, ultrasound, light, and magnetic) that led to control the assembly and swarming of synthetic micro/nanomachines. Such behavior is of fundamental importance in improving our understanding of self-assembly processes that are occurring on molecular to macroscopic length scales.

Translational and rotational dynamics in dense suspensions of smooth and rough colloids

We demonstrate that colloidal particles with surface roughness exhibit hindered rotational diffusion in quiescent dense suspensions. This is accomplished by the use of confocal microscopy and particle tracking to follow the translational and rotational dynamics of smooth and rough colloids suspended in a refractive index and density matched organic solvent. Measurement of the three-dimensional rotational diffusion is enabled by the addition of inert Janus tracers made of native colloids coated with a thin layer of aluminum. These experiments show that the mean square displacement (MSD) is unaffected by particle roughness, while the mean square angular displacement (MSAD) decreases for rough colloids at high volume fractions. Our results quantify the slowdown in the rotational dynamics of rough colloids, which is evidently due to steric frustration caused by the surface topography of the particles.

Optics-Integrated Microfluidic Platforms for Biomolecular Analyses

Compared with conventional optical methods, optics implemented on microfluidic chips provide small, and often much cheaper ways to interrogate biological systems from the level of single molecules up to small model organisms. The optical probing of single molecules has been used to investigate the mechanical properties of individual biological molecules; however, multiplexing of these measurements through microfluidics and nanofluidics confers many analytical advantages. Optics-integrated microfluidic systems can significantly simplify sample processing and allow a more user-friendly experience; alignments of on-chip optical components are predetermined during fabrication and many purely optical techniques are passively controlled. Furthermore, sample loss from complicated preparation and fluid transfer steps can be virtually eliminated, a particularly important attribute for biological molecules at very low concentrations. Excellent fluid handling and high surface area/volume ratios also contribute to faster detection times for low abundance molecules in small sample volumes. Although integration of optical systems with classical microfluidic analysis techniques has been limited, microfluidics offers a ready platform for interrogation of biophysical properties. By exploiting the ease with which fluids and particles can be precisely and dynamically controlled in microfluidic devices, optical sensors capable of unique imaging modes, single molecule manipulation, and detection of minute changes in concentration of an analyte are possible.

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.

Rich complex behaviour of self-assembled nanoparticles far from equilibrium

A profoundly fundamental question at the interface between physics and biology remains open: what are the minimum requirements for emergence of complex behaviour from nonliving systems? Here, we address this question and report complex behaviour of tens to thousands of colloidal nanoparticles in a system designed to be as plain as possible: the system is driven far from equilibrium by ultrafast laser pulses that create spatiotemporal temperature gradients, inducing Marangoni flow that drags particles towards aggregation; strong Brownian motion, used as source of fluctuations, opposes aggregation. Nonlinear feedback mechanisms naturally arise between flow, aggregate and Brownian motion, allowing fast external control with minimal intervention. Consequently, complex behaviour, analogous to those seen in living organisms, emerges, whereby aggregates can self-sustain, self-regulate, self-replicate, self-heal and can be transferred from one location to another, all within seconds. Aggregates can comprise only one pattern or bifurcated patterns can coexist, compete, endure or perish.

Fuel-Free Synthetic Micro-/Nanomachines

Inspired by the swimming of natural microorganisms, synthetic micro-/nanomachines, which convert energy into movement, are able to mimic the function of these amazing natural systems and help humanity by completing environmental and biological tasks. While offering autonomous propulsion, conventional micro-/nanomachines usually rely on the decomposition of external chemical fuels (e.g., H₂ O₂ ), which greatly hinders their applications in biologically relevant media. Recent developments have resulted in various micro-/nanomotors that can be powered by biocompatible fuels. Fuel-free synthetic micro-/nanomotors, which can move without external chemical fuels, represent another attractive solution for practical applications owing to their biocompatibility and sustainability. Here, recent developments on fuel-free micro-/nanomotors (powered by various external stimuli such as light, magnetic, electric, or ultrasonic fields) are summarized, ranging from fabrication to propulsion mechanisms. The applications of these fuel-free micro-/nanomotors are also discussed, including nanopatterning, targeted drug/gene delivery, cell manipulation, and precision nanosurgery. With continuous innovation, future autonomous, intelligent and multifunctional fuel-free micro-/nanomachines are expected to have a profound impact upon diverse biomedical applications, providing unlimited opportunities beyond one's imagination.

Interrogating Cellular Functions with Designer Janus Particles

Janus particles have two distinct surfaces or compartments. This enables novel applications that are impossible with homogeneous particles, ranging from the engineering of active colloidal metastructures to creating multimodal therapeutic materials. Recent years have witnessed a rapid development of novel Janus structures and exploration of their applications, particularly in the biomedical arena. It, therefore, becomes crucial to understand how Janus particles with surface or structural anisotropy might interact with biological systems and how such interactions may be exploited to manipulate biological responses. This perspective highlights recent studies that have employed Janus particles as novel toolsets to manipulate, measure, and understand cellular functions. Janus particles have been shown to have biological interactions different from uniform particles. Their surface anisotropy has been used to control the cell entry of synthetic particles, to spatially organize stimuli for the activation of immune cells, and to enable direct visualization and measurement of rotational dynamics of particles in living systems. The work included in this perspective showcases the significance of understanding the biological interactions of Janus particles and the tremendous potential of harnessing such interactions to advance the development of Janus structure-based biomaterials.

Optimal Feedback Controlled Assembly of Perfect Crystals

Perfectly ordered states are targets in diverse molecular to microscale systems involving, for example, atomic clusters, protein folding, protein crystallization, nanoparticle superlattices, and colloidal crystals. However, there is no obvious approach to control the assembly of perfectly ordered global free energy minimum structures; near-equilibrium assembly is impractically slow, and faster out-of-equilibrium processes generally terminate in defective states. Here, we demonstrate the rapid and robust assembly of perfect crystals by navigating kinetic bottlenecks using closed-loop control of electric field mediated crystallization of colloidal particles. An optimal policy is computed with dynamic programming using a reaction coordinate based dynamic model. By tracking real-time stochastic particle configurations and adjusting applied fields via feedback, the evolution of unassembled particles is guided through polycrystalline states into single domain crystals. This approach to controlling the assembly of a target structure is based on general principles that make it applicable to a broad range of processes from nano- to microscales (where tuning a global thermodynamic variable yields temporal control over thermal sampling of different states via their relative free energies).

Reconfigurable optical assembly of nanostructures

Arrangements of nanostructures in well-defined patterns are the basis of photonic crystals, metamaterials and holograms. Furthermore, rewritable optical materials can be achieved by dynamically manipulating nanoassemblies. Here we demonstrate a mechanism to configure plasmonic nanoparticles (NPs) in polymer media using nanosecond laser pulses. The mechanism relies on optical forces produced by the interference of laser beams, which allow NPs to migrate to lower-energy configurations. The resulting NP arrangements are stable without any external energy source, but erasable and rewritable by additional recording pulses. We demonstrate reconfigurable optical elements including multilayer Bragg diffraction gratings, volumetric photonic crystals and lenses, as well as dynamic holograms of three-dimensional virtual objects. We aim to expand the applications of optical forces, which have been mostly restricted to optical tweezers. Holographic assemblies of nanoparticles will allow a new generation of programmable composites for tunable metamaterials, data storage devices, sensors and displays.

Reconfigurable materials are of interest for many photonic applications. Here, Montelongo et al. demonstrate optical elements such as Bragg diffraction gratings, volumetric photonic crystals, lenses, and holograms in a composite with dispersed nanoparticles which can be recorded and erased.

Janus particles for biological imaging and sensing

Janus particles, named after the two-faced Roman god Janus, have different surface makeups, structures or compartments on two sides. This review highlights recent advances in employing Janus particles as novel analytical tools for live cell imaging and biosensing. Unlike conventional particles used in analytical science, two-faced Janus particles provide asymmetry and directionality, and can combine different or even incompatible properties within a single particle. The broken symmetry enables imaging and quantification of rotational dynamics, revealing information beyond what traditional measurements offer. The spatial segregation of molecules on the surface of a single particle also allows analytical functions that would otherwise interfere with each other to be decoupled, opening up opportunities for novel multimodal analytical methods. We summarize here the development of Janus particles, a few general methods for their fabrication and, more importantly, the emerging and novel applications of Janus particles as multi-functional imaging probes and sensors.

In situ microscopy of the self-assembly of branched nanocrystals in solution

Solution-phase self-assembly of nanocrystals into mesoscale structures is a promising strategy for constructing functional materials from nanoscale components. Liquid environments are key to self-assembly since they allow suspended nanocrystals to diffuse and interact freely, but they also complicate experiments. Real-time observations with single-particle resolution could have transformative impact on our understanding of nanocrystal self-assembly. Here we use real-time in situ imaging by liquid-cell electron microscopy to elucidate the nucleation and growth mechanism and properties of linear chains of octapod-shaped nanocrystals in their native solution environment. Statistical mechanics modelling based on these observations and using the measured chain-length distribution clarifies the relative importance of dipolar and entropic forces in the assembly process and gives direct access to the interparticle interaction. Our results suggest that monomer-resolved in situ imaging combined with modelling can provide unprecedented quantitative insight into the microscopic processes and interactions that govern nanocrystal self-assembly in solution.

Understanding the structure and transformation of colloidal matter requires probing configurations from monomers to extended assemblies. Here, the authors use liquid-cell electron microscopy to elucidate the nucleation and growth properties of linear chains of branched nanocrystals in solution.

Reconfigurable multi-scale colloidal assembly on excluded volume patterns

The ability to create multi-scale, periodic colloidal assemblies with unique properties is important to emerging applications. Dynamically manipulating colloidal structures via tunable kT-scale attraction can provide the opportunity to create particle-based nano- and microstructured materials that are reconfigurable. Here, we report a novel tactic to obtain reconfigurable, multi-scale, periodic colloidal assemblies by combining thermoresponsive depletant particles and patterned topographical features that, together, reversibly mediate local kT-scale depletion interactions. This method is demonstrated in optical microscopy experiments to produce colloidal microstructures that reconfigure between well-defined ordered structures and disordered fluid states as a function of temperature and pattern feature depth. These results are well described by Monte Carlo simulations using theoretical depletion potentials that include patterned excluded volume. Ultimately, the approach reported here can be extended to control the size, shape, orientation, and microstructure of colloidal assemblies on multiple lengths scales and on arbitrary pre-defined pattern templates.

Color and texture morphing with colloids on multilayered surfaces

Dynamic morphing of marine species to match with environment changes in color and texture is an advanced means for surviving, self-defense, and reproduction. Here we use colloids that are placed inside a multilayered structure to demonstrate color and texture morphing. The multilayer is composed of a thermal insulating base layer, a light absorbing mid layer, and a liquid top layer. When external light of moderate intensity (∼0.2 W cm(-2)) strikes the structure, colloids inside the liquid layer will be assembled to locations with an optimal absorption. When this system is exposed to continuous laser pulses, more than 18,000 times of reversible responses are recorded, where the system requests 20 ms to start the response and another 160 ms to complete. The flexibility of our concept further allows the system to be built on a variety of light-absorbing substrates, including dyed paper, gold thin film, and amorphous silicon, with the top layer even a solid.

Kinetics of colloidal deposition, assembly, and crystallization in steady electric fields

We quantify and model the deposition and crystallization kinetics of initially dilute colloidal spheres due to application of a steady, direct current electric field in the thin gap between parallel electrodes. The system studied is poly(12-hydroxystearic acid) (PHSA)-stabilized poly(methyl methacrylate) (PMMA) spheres dispersed in a mixture of cyclohexylbromide (CHB), decalin, and a low concentration of the partially disassociating salt tetrabutylammonium chloride (TBAC). The temporal and spatial evolution of the colloidal volume fraction in the ∼1 mm gap between the electrodes is quantified under conditions of both deposition and relaxation by confocal laser scanning microscopy (CLSM). During deposition assembly, the spatial dependence of the colloid volume fraction approaches steady state at times between hundreds of minutes at the lowest electric field strength (as characterized by a Peclet number, Pe) and at tens of minutes at higher field strengths. During disassembly, the volume fraction relaxes nearly exponentially. The kinetics are modeled by adapting a treatment for sedimentation (Davis and Russel, Phys. Fluids A, 1989, 1, 82) to the case of steady electric fields. The model's predictions show good agreement with the measured kinetics at low Pe; however, agreement progressively deteriorates with increasing Pe. At low Pe the deposits are initially disordered. After an initial delay, 1D crystal growth propagates from the electrode surface at rates of several hundred nm min(-1). The sharp crystal boundary propagates as a characteristic of constant colloidal volume fraction, consistent with an equilibrium crystalline phase transition. The results inform operational ranges for devices that produce active colloidal matter by reversible assembly.

Surface charge and interactions of 20-nm nanocolloids in a nematic liquid crystal

We studied real-time motion of individual 20-nm silica nanoparticles in a thin layer of a nematic liquid crystal using a dark-field optical videomicroscopy. By tracking the positions of individual nanoparticles we observed that particle pair interactions are not only mediated by strong thermal fluctuations of the nematic liquid crystal, but also with a repulsive force of electric origin. We determined the total electric charge of silanated silica particles in the nematic liquid crystal 5CB by observing the electric-force-driven drift. Surprisingly, the surface electric charge density depends on colloidal size and is ∼4.5×10(-3)C/m(2) for 20-nm nanocolloids, and two orders of magnitude lower, i.e., ∼2.3×10(-5)C/m(2), for 1-μm colloids. We conclude that electrostatic repulsion between like-charged particles prevents the formation of permanent colloidal assemblies of nanometer size. We also observed strong attraction of 20-nm silica nanoparticles to confining polyimide surfaces and larger clusters, which gradually results in complete expulsion of nanoparticles from the nematic liquid crystal to the surfaces of the confining cell.

Electrical conductivity equations derived with the rate process theory and free volume concept

Inspired by the Marcus theory of electron transfer, electrical conductivity equations without reference to any specific materials are derived on the basis of Eyring’s rate process theory and the free volume concept.

Directing colloidal assembly and a metal-insulator transition using a quench-disordered porous rod template

Replica and effective-medium theory methods are employed to elucidate how to massively reconfigure a colloidal assembly to achieve globally homogeneous, strongly clustered, and percolated equilibrium states of high electrical conductivity at low physical volume fractions. A key idea is to employ a quench-disordered, large-mesh rigid-rod network as a templating internal field. By exploiting bulk phase separation frustration and the tunable competing processes of colloid adsorption on the low-dimensional network and fluctuation-driven colloid clustering in the pore spaces, two distinct spatial organizations of greatly enhanced particle contacts can be achieved. As a result, a continuous, but very abrupt, transition from an insulating to metallic-like state can be realized via a small change of either the colloid-template or colloid-colloid attraction strength. The approach is generalizable to more complicated template or colloidal architectures.

Direct current electric field assembly of colloidal crystals displaying reversible structural color

We report the application of low-voltage direct current (dc) electric fields to self-assemble close-packed colloidal crystals in nonaqueous solvents from colloidal spheres that vary in size from as large as 1.2 μm to as small as 0.1 μm. The assemblies are created rapidly (∼2 min) from an initially low volume fraction colloidal particle suspension using a simple capacitor-like electric field device that applies a steady dc electric voltage. Confocal microscopy is used to observe the ordering that is produced by the assembly method. This spatial evidence for ordering is consistent with the 6-fold diffraction patterns identified by light scattering. Red, green, and blue structural color is observed for the ordered assemblies of colloids with diameters of 0.50, 0.40, and 0.29 μm, respectively, consistent with spectroscopic measurements of reflectance. The diffraction and spectrophotometry results were found to be consistent with the theoretical Bragg's scattering expected for closed-packed crystals. By switching the dc electric field from on to off, we demonstrate reversibility of the structural color response on times scales ∼60 s. The dc electric field assembly method therefore represents a simple method to produce reversible structural color in colloidal soft matter.