Tuning disorder in structurally colored bioinspired photonic glasses

Colloidal crystals, such as opals, display bright and iridescent colors when assembled from submicron particles. While the brightness and purity of iridescent colors are well suited for ornaments, signaling, and anticounterfeiting, their angle dependence limits the range of their applications. In contrast, colloidal glasses display angle-independent structural color that is tunable by the size and local arrangement of particles. However, the angle-independent color of colloidal photonic glasses usually yields pastel colors that are not vivid due to the disorder in the particle assembly. Here, we report an electrophoretic assembly platform for tuning the level of disorder in the particle system from a colloidal crystal to a colloidal glass. Altering the electric field in our electrophoretic platform allows for deliberate control of the assembly kinetics and thus the level of order in the particle assembly. With the help of microscopy, X-ray scattering, and optical characterization, we show that the photonic properties of the assembled films can be tuned with the applied electric field. Our analyses reveal that angle-independent color with optimum color brightness can be achieved in typical colloidal suspensions when the range of order is at ∼3.2 particle diameters, which is expected at a moderate electric field of ∼15 V mm-1.

Light-Based 3D Printing of Complex-Shaped Photonic Colloidal Glasses

Colloidal glasses display angle-independent structural color that is tunable by the size and local arrangement of sub-micrometer particles. While films, droplets, and microcapsules with isotropic structural color have been demonstrated, the shaping of colloidal glasses in three dimensions remains an open manufacturing challenge. Here, a light-based printing platform for the shaping of colloidal glasses into 3D objects featuring complex geometries and vivid structural color after thermal treatment is reported. Rheology, photopolymerization, and calcination experiments are performed to design the photoreactive resins leading to printable colloidal glasses. With the help of microscopy, scattering, and optical characterization, it is shown that the photonic properties of the printed objects reflect the locally ordered microstructure of the glass. The capability of the platform in creating 3D objects with isotropic structural color is illustrated by printing lattices and miniaturized sculpture replicas with unique shapes and multimaterial designs.

Kirigami Makes a Soft Magnetic Sheet Crawl

Limbless crawling on land requires breaking symmetry of the friction with the ground and exploiting an actuation mechanism to generate propulsive forces. Here, kirigami cuts are introduced into a soft magnetic sheet that allow to achieve effective crawling of untethered soft robots upon application of a rotating magnetic field. Bidirectional locomotion is achieved under clockwise and counterclockwise rotating magnetic fields with distinct locomotion patterns and crawling speed in forward and backward propulsions. The crawling and deformation profiles of the robot are experimentally characterized and combined with detailed multiphysics numerical simulations to extract locomotion mechanisms in both directions. It is shown that by changing the shape of the cuts and orientation of the magnet the robot can be steered, and if combined with translational motion of the magnet, complex crawling paths are programed. The proposed magnetic kirigami robot offers a simple approach to developing untethered soft robots with programmable motion.

Amphibious Transport of Fluids and Solids by Soft Magnetic Carpets

One of the major challenges in modern robotics is controlling micromanipulation by active and adaptive materials. In the respiratory system, such actuation enables pathogen clearance by means of motile cilia. While various types of artificial cilia have been engineered recently, they often involve complex manufacturing protocols and focus on transporting liquids only. Here, soft magnetic carpets are created via an easy self-assembly route based on the Rosensweig instability. These carpets can transport not only liquids but also solid objects that are larger and heavier than the artificial cilia, using a crowd-surfing effect.This amphibious transportation is locally and reconfigurably tunable by simple micromagnets or advanced programmable magnetic fields with a high degree of spatial resolution. Two surprising cargo reversal effects are identified and modeled due to collective ciliary motion and nontrivial elastohydrodynamics. While the active carpets are generally applicable to integrated control systems for transport, mixing, and sorting, these effects can also be exploited for microfluidic viscosimetry and elastometry.

Magnetic propulsion of colloidal microrollers controlled by electrically modulated friction

Precise control over the motion of magnetically responsive particles in fluidic chambers is important for probing and manipulating tasks in prospective microrobotic and bio-analytical platforms. We have previously exploited such colloids as shuttles for the microscale manipulation of objects. Here, we study the rolling motion of magnetically driven Janus colloids on solid substrates under the influence of an orthogonal external electric field. Electrically induced attractive interactions were used to tune the load on the Janus colloid and thereby the friction with the underlying substrate, leading to control over the forward velocity of the particle. Our experimental data suggest that the frictional coupling required to achieve translation, transitions from a hydrodynamic regime to one of mixed contact coupling with increasing load force. Based on this insight, we show that our colloidal microrobots can probe the local friction coefficient of various solid surfaces, which makes them potentially useful as tribological microsensors. Lastly, we precisely manipulate porous cargos using our colloidal rollers, a feat that holds promise for bio-analytical applications.

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.

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.

Active cargo transport with Janus colloidal shuttles using electric and magnetic fields

Active colloids show non-equilibrium behavior that departs from classical Brownian motion, thus providing a platform for novel fundamental phenomena and for enticing possible applications ranging from water treatment to medicine and microrobotics. Although the physics, motion mechanisms and guidance have been extensively investigated, active colloids are rarely exploited to simultaneously guide and transport micron-sized objects in a controllable and reversible manner. Here, we use autonomous active Janus particles as colloidal shuttles to controllably transport cargo at the microscale using external electric and magnetic fields. The active motion arises from the metallodielectric characteristics of the Janus particles, which allows them to also trap, transport and release cargo particles through dielectrophoretic interactions induced by an AC electric field. The ferromagnetic nature of the nickel layer that forms the metallic hemisphere of the Janus colloids provides an additional mechanism to direct the motion of the shuttle using an external magnetic field. With this highly programmable colloidal system, we are able to harness active colloid motion and use it to transport cargo particles to specific destinations through a pre-defined route. A simple analytical model is derived to successfully describe the motion of the shuttle-cargo assembly in response to the applied electrical field. The high level of control on cargo pick-up, transport and release leads to a powerful delivery tool, which could eventually be used in microactuators, microfluidics or for controlled delivery within organ-on-a-chip devices.

Emulsions Stabilized by Chitosan-Modified Silica Nanoparticles: pH Control of Structure-Property Relations

In food-grade emulsions, particles with an appropriate surface modification can be used to replace surfactants and potentially enhance the stability of emulsions. During the life cycle of products based on such emulsions, they can be exposed to a broad range of pH conditions and hence it is crucial to understand how pH changes affect stability of emulsions stabilized by particles. Here, we report on a comprehensive study of the stability, microstructure, and macroscopic behavior of pH-controlled oil-in-water emulsions containing silica nanoparticles modified with chitosan, a food-grade polycation. We found that the modified colloidal particles used as stabilizers behave differently depending on the pH, resulting in unique emulsion structures at multiple length scales. Our findings are rationalized in terms of the different emulsion stabilization mechanisms involved, which are determined by the pH-dependent charges and interactions between the colloidal building blocks of the system. At pH 4, the silica particles are partially hydrophobized through chitosan modification, favoring their adsorption at the oil-water interface and the formation of Pickering emulsions. At pH 5.5, the particles become attractive and the emulsion is stabilized by a network of agglomerated particles formed between the droplets. Finally, chitosan aggregates form at pH 9 and these act as the emulsion stabilizers under alkaline conditions. These insights have important implications for the processing and use of particle-stabilized emulsions. On one hand, changes in pH can lead to undesired macroscopic phase separation or coalescence of oil droplets. On the other hand, the pH effect on emulsion behavior can be harnessed in industrial processing, either to tune their flow response by altering the pH between processing stages or to produce pH-responsive emulsions that enhance the functionality of the emulsified end products.

One-Step Bulk Fabrication of Polymer-Based Microcapsules with Hard-Soft Bilayer Thick Shells

Microcapsules are important for the protection, transport, and delivery of cargo in a variety of fields but are often too weak to withstand the high mechanical stresses that arise during the preparation and formulation of products. Although thick-shell strong capsules have been developed to circumvent this issue, the microfluidic or multistep methods utilized thus far limit the ease of fabrication and encapsulation throughput. Here, we exploit the phase separation of ternary liquid mixtures to achieve a high-throughput fabrication of strong bilayer microcapsules using a one-step bulk emulsification process. Phase separation is induced by the diffusion of water from the continuous phase into droplets that initially contain a mixture of monomers, cross-linkers, an initiator, and cosolvent γ-butyrolactone. The double emulsions generated via such a phase separation are converted into microcapsules through a polymerization reaction triggered by UV illumination. Surprisingly, the shells of the consolidated capsules exhibit a hard-soft bilayer structure that can be designed to show a resilient eggshell-like fracture behavior. Our method allows for the production of large volumes of microcapsules with such a strong bilayer shell within a time scale of only a few minutes, thus offering an enticing pathway toward the high-throughput fabrication of mechanically robust encapsulation systems.

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.

Magnetofluidic Tweezing of Nonmagnetic Colloids

Magnetofluidic tweezing based on negative magnetophoresis and microfabricated core-shell magnetic microtips allows controlled on-demand assembly of colloids and microparticles into various static and dynamic structures such as colloidal crystals (as shown for 3.2 μm silica particles).

Robust Microcompartments with Hydrophobically Gated Shells

We report on robust synthetic microcompartments with hydrophobically gated shells that can reversibly swell and contract multiple times upon external stimuli. The gating mechanism relies on a hydrophilic-hydrophobic transition of a polymer layer that is grafted on inorganic colloidosomes using atom-transfer radical polymerization. As a result of such a transition, the initially tight hydrophobic shell becomes permeable to the diffusion of hydrophilic solutes across the microcompartment walls. Surprisingly, the microcompartments are strong enough to retain their spherical shape during several swelling and contraction cycles. This provides a powerful alternative platform for the creation of synthetic microreactors and protocells that interact with the surrounding media through a simple gating mechanism and are sufficiently robust for further engineering of increasingly complex compartmentalized structures.

Colloidal assembly directed by virtual magnetic moulds

Interest in assemblies of colloidal particles has long been motivated by their applications in photonics, electronics, sensors and microlenses. Existing assembly schemes can position colloids of one type relatively flexibly into a range of desired structures, but it remains challenging to produce multicomponent lattices, clusters with precisely controlled symmetries and three-dimensional assemblies. A few schemes can efficiently produce complex colloidal structures, but they require system-specific procedures. Here we show that magnetic field microgradients established in a paramagnetic fluid can serve as 'virtual moulds' to act as templates for the assembly of large numbers (∼10(8)) of both non-magnetic and magnetic colloidal particles with micrometre precision and typical yields of 80 to 90 per cent. We illustrate the versatility of this approach by producing single-component and multicomponent colloidal arrays, complex three-dimensional structures and a variety of colloidal molecules from polymeric particles, silica particles and live bacteria and by showing that all of these structures can be made permanent. In addition, although our magnetic moulds currently resemble optical traps in that they are limited to the manipulation of micrometre-sized objects, they are massively parallel and can manipulate non-magnetic and magnetic objects simultaneously in two and three dimensions.