The Evolution of Active Particles: Toward Externally Powered Self-Propelling and Self-Reconfiguring Particle Systems

Self-Organized Patterns in Non-Reciprocal Active Droplet Systems

Non-equilibrium patterns are widespread in nature and often arise from the self-organization of constituents through nonreciprocal chemotactic interactions. In this study, we demonstrate how active oil-in-water droplet mixtures with predator-prey interactions can result in a variety of self-organized patterns. By manipulating physical parameters, the droplet diameter ratio and number ratio, we identify distinct classes of patterns within a binary droplet system, rationalize the pattern formation, and quantify motilities. Experimental results are recapitulated in numerical simulations using a minimal computational model that solely incorporates chemotactic interactions and steric repulsion among the constituents. The time evolution of the patterns is investigated and chemically explained. We also investigate how patterns vary with differing interaction strength by altering surfactant composition. Leveraging insights from the binary droplet system, the framework is extended to a ternary droplet mixture composed of multiple chasing droplet pairs to create chemically directed hierarchical organization. Our findings demonstrate how rationalizable, self-organized patterns can be programmed in a chemically minimal system and provide the basis for exploration of emergent organization and higher order complexity in active colloids.

In-situ isomerization and reversible self-assembly of photoresponsive polymeric colloidal molecules enabled by ON and OFF light control

Photocatalytic colloids enable light-triggered nonequilibrium interactions and are emerging as key components for the self-assembly of colloidal molecules (CMs) out of equilibrium. However, the material choices have largely been limited to inorganic substances and the potential for reconfiguring structures through dynamic light control remains underexplored, despite light being a convenient handle for tuning nonequilibrium interactions. Here, we introduce photoresponsive N,O-containing covalent organic polymer (NOCOP) colloids, which display multi-wavelength triggered fluorescence and switchable diffusiophoretic interactions with the addition of triethanolamine. Our system can form various flexible structures, including ABn-type molecules and linear chains. By varying the relative sizes of active to passive colloids, we significantly increase the structural diversity of A2B2-type molecules. Most importantly, we demonstrate in-situ transitions between different isomeric configurations and the reversible assembly of various structures, enabled by on-demand light ON and OFF control of diffusiophoretic interactions. Our work introduces a new photoresponsive colloidal system and a novel strategy for constructing and reconfiguring colloidal assemblies, with promising applications in microrobotics, optical devices, and smart materials.

Dynamics of Aggregation in Systems of Self-Propelled Rods

We highlight camphene-camphor-polypropylene plastic as a useful material for self-propelled objects that show aggregation while floating on a water surface. We consider self-propelled rods as an example of aggregation of objects characterized by non-trivial individual shapes with low-symmetry interactions between them. The motion of rods made of the camphene-camphor-polypropylene plastic is supported by dissipation of the surface-active molecules. The physical processes leading to aggregation and the mathematical model of the process are discussed. We analyze experimental data of aggregate formation dynamics and relate them to the system's properties. We speculate that the aggregate structure can be represented as a string of symbols, which opens the potential applicability of the phenomenon for information processing if objects floating on a water surface are regarded as reservoir computers.

Unveiling Hidden Hyperuniformity: Radial Turing Pattern Formation of Marangoni-Driven SiO2 Nanoparticles on Liquid Metal Surface

Mastering the self-organization of nanoparticle morphologies is pivotal in soft matter physics and film growth. Silicon dioxide (SiO2) nanoparticles are an archetypical model of nanomotor in soft matter. Here, the emphasis is on the self-organizing behavior of SiO2 nanoparticles under extreme conditions. It is unveiled that manipulating the states of the metal substrate profoundly dictates the motion characteristics of SiO2 nanoparticles. This manipulation triggers the emergence of intricate morphologies and distinctive patterns. Employing a reaction-diffusion model, the fundamental roles played by Brownian motion and Marangoni-driven motion in shaping fractal structures and radial Turing patterns are demonstrated, respectively. Notably, these radial Turing patterns showcase hyperuniform order, challenging conventional notions of film morphology. These discoveries pave the way for crafting non-equilibrium morphological materials, poised with the potential for self-healing, adaptability, and innovative applications.

Light-Triggered Inflation of Microdroplets

Driven systems composed largely of droplets and fuel make up a significant portion of microbiological function. At the micrometer scale, fully synthetic systems that perform an array of tasks within a uniform bulk are much more rare. In this work, we introduce an innovative design for solid-in-oil composite microdroplets. These microdroplets are engineered to nucleate an internal phase, undergo inflation, and eventually burst, all powered by a steady and uniform energy input. We show that by altering the background input, volumetric change and burst time can be tuned. When the inflated droplets release the inner contents, colloidal particles are shown to transiently attract to the release point. Lastly, we show that the system has the ability to perform multiple inflation-burst cycles. We anticipate that our conceptual design of internally powered microdroplets will catalyze further research into autonomous systems capable of intricate communication as well as inspire the development of advanced, responsive materials.

A Survey of Recent Developments in Magnetic Microrobots for Micro-/Nano-Manipulation

Magnetically actuated microrobots have become a research hotspot in recent years due to their tiny size, untethered control, and rapid response capability. Moreover, an increasing number of researchers are applying them for micro-/nano-manipulation in the biomedical field. This survey provides a comprehensive overview of the recent developments in magnetic microrobots, focusing on materials, propulsion mechanisms, design strategies, fabrication techniques, and diverse micro-/nano-manipulation applications. The exploration of magnetic materials, biosafety considerations, and propulsion methods serves as a foundation for the diverse designs discussed in this review. The paper delves into the design categories, encompassing helical, surface, ciliary, scaffold, and biohybrid microrobots, with each demonstrating unique capabilities. Furthermore, various fabrication techniques, including direct laser writing, glancing angle deposition, biotemplating synthesis, template-assisted electrochemical deposition, and magnetic self-assembly, are examined owing to their contributions to the realization of magnetic microrobots. The potential impact of magnetic microrobots across multidisciplinary domains is presented through various application areas, such as drug delivery, minimally invasive surgery, cell manipulation, and environmental remediation. This review highlights a comprehensive summary of the current challenges, hurdles to overcome, and future directions in magnetic microrobot research across different fields.

Symmetrically pulsating bubbles swim in an anisotropic fluid by nematodynamics

Swimming in low-Reynolds-number fluids requires the breaking of time-reversal symmetry and centrosymmetry. Microswimmers, often with asymmetric shapes, exhibit nonreciprocal motions or exploit nonequilibrium processes to propel. The role of the surrounding fluid has also attracted attention because viscoelastic, non-Newtonian, and anisotropic properties of fluids matter in propulsion efficiency and navigation. Here, we experimentally demonstrate that anisotropic fluids, nematic liquid crystals (NLC), can make a pulsating spherical bubble swim despite its centrosymmetric shape and time-symmetric motion. The NLC breaks the centrosymmetry by a deformed nematic director field with a topological defect accompanying the bubble. The nematodynamics renders the nonreciprocity in the pulsation-induced fluid flow. We also report speed enhancement by confinement and the propulsion of another symmetry-broken bubble dressed by a bent disclination. Our experiments and theory propose another possible mechanism of moving bodies in complex fluids by spatiotemporal symmetry breaking.

Multimode microdimer robot for crossing tissue morphological barrier

Swimming microrobot energized by magnetic fields exhibits remotely propulsion and modulation in complex biological experiment with high precision. However, achieving high environment adaptability and multiple tasking capability in one configuration is still challenging. Here, we present a strategy that use oriented magnetized Janus spheres to assemble the microdimer robots with two magnetic distribution configurations of head-to-side configuration (HTS-config) and head-to-head configuration (HTH-config), achieving performance of multiple tasks through multimode transformation and locomotion. Modulating the magnetic frequency enables multimode motion transformation between tumbling, rolling, and swing motion with different velocities. The dual-asynchronization mechanisms of HTS-config and HTH-config robot dependent on magnetic dipole-dipole angle are investigated by molecular dynamic simulation. In addition, the microdimer robot can transport cell crossing morphological rugae or complete drug delivery on tissues by switching motion modes. This microdimer robot can provide versatile motion modes to address environmental variations or multitasking requirements.

Bubble-Based Microrobots with Rapid Circular Motions for Epithelial Pinning and Drug Delivery

Remotely powered microrobots are proposed as next-generation vehicles for drug delivery. However, most microrobots swim with linear trajectories and lack the capacity to robustly adhere to soft tissues. This limits their ability to navigate complex biological environments and sustainably release drugs at target sites. In this work, bubble-based microrobots with complex geometries are shown to efficiently swim with non-linear trajectories in a mouse bladder, robustly pin to the epithelium, and slowly release therapeutic drugs. The asymmetric fins on the exterior bodies of the microrobots induce a rapid rotational component to their swimming motions of up to ≈150 body lengths per second. Due to their fast speeds and sharp fins, the microrobots can mechanically pin themselves to the bladder epithelium and endure shear stresses commensurate with urination. Dexamethasone, a small molecule drug used for inflammatory diseases, is encapsulated within the polymeric bodies of the microrobots. The sustained release of the drug is shown to temper inflammation in a manner that surpasses the performance of free drug controls. This system provides a potential strategy to use microrobots to efficiently navigate large volumes, pin at soft tissue boundaries, and release drugs over several days for a range of diseases.

Inverted Sedimentation of Active Particles in Unbiased ac Fields

Gaining control over the motion of active particles is crucial for applications ranging from targeted cargo delivery to nanomedicine. While much progress has been made recently to control active motion based on external forces, flows, or gradients in concentration or light intensity, which all have a well-defined direction or bias, little is known about how to steer active particles in situations where no permanent bias can be realized. Here, we show that ac fields with a vanishing time average provide an alternative route to steering active particles. We exemplify this route for inertial active particles in a gravitational field, observing that a substantial fraction of them persistently travels in the upward direction upon switching on the ac field, resulting in an inverted sedimentation profile at the top wall of a confining container. Our results offer a generic control principle that could be used in the future to steer active motion, direct collective behaviors, and purify mixtures.

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.

Rotary Structural Color Spindles from Droplet Confined Magnetic Self-Assembly

Structural colors materials are profoundly explored owing to their fantastic optical properties and widespread applications. Development of structural color materials bearing flexible morphologies and versatile functionalities is highly anticipated. Here, a droplet-confined, magnetic-induced self-assembly strategy for generating rotary structural color spindles (SCSPs) by fast solvent extraction is proposed. The as-prepared SCSPs exhibit an orderly close-packed lattice structure, thus appearing brilliant structural colors that serve as encoding tags for multiplexed bioassays. Besides, benefitting from the abundant specific surface area, biomarkers can be labeled on the SCSPs with high efficiency for specific detection of analytes in clinical samples. Moreover, the directional magnetic moment arrangement enables contactless magnetic manipulation of the SCSPs, and the resultant rotary motions of the SCSPs generates turbulence in the detection solution, thus significantly improving the detection efficiency and shortening the detection time. Based on these merits, a portable point-of-care-testing strip integrating the rotary SCSPs is further constructed and the capability and advantages of this platform for multiplexed detection of tumor-related exosomes in clinical samples are demonstrated. This study offers a new way for the control of bottom-up self-assembly and extends the configuration and application values of colloidal crystal structural colors materials.

Printing on Particles: Combining Two-Photon Nanolithography and Capillary Assembly to Fabricate Multimaterial Microstructures

Additive manufacturing at the micro- and nanoscale has seen a recent upsurge to suit an increasing demand for more elaborate structures. However, the integration of multiple distinct materials at small scales remains challenging. To this end, capillarity-assisted particle assembly (CAPA) and two-photon polymerization direct laser writing (2PP-DLW) are combined to realize a new class of multimaterial microstructures. 2PP-DLW and CAPA both are used to fabricate 3D templates to guide the CAPA of soft- and hard colloids, and to link well-defined arrangements of functional microparticle arrays produced by CAPA, a process that is termed "printing on particles." The printing process uses automated particle recognition algorithms to connect colloids into 1D, 2D, and 3D tailored structures, via rigid, soft, or responsive polymer links. Once printed and developed, the structures can be easily re-dispersed in water. Particle clusters and lattices of varying symmetry and composition are reported, together with thermoresponsive microactuators, and magnetically driven "micromachines", which can efficiently move, capture, and release DNA-coated particles in solution. The flexibility of this method allows the combination of a wide range of functional materials into complex structures, which will boost the realization of new systems and devices for numerous fields, including microrobotics, micromanipulation, and metamaterials.

Light-Programmed Bistate Colloidal Actuation Based on Photothermal Active Plasmonic Substrate

Active particles have been regarded as the key models to mimic and understand the complex systems of nature. Although chemical and field-powered active particles have received wide attentions, light-programmed actuation with long-range interaction and high throughput remains elusive. Here, we utilize photothermal active plasmonic substrate made of porous anodic aluminum oxide filled with Au nanoparticles and poly( N -isopropylacrylamide) (PNIPAM) to optically oscillate silica beads with robust reversibility. The thermal gradient generated by the laser beam incurs the phase change of PNIPAM, producing gradient of surface forces and large volume changes within the complex system. The dynamic evolution of phase change and water diffusion in PNIPAM films result in bistate locomotion of silica beads, which can be programmed by modulating the laser beam. This light-programmed bistate colloidal actuation provides promising opportunity to control and mimic the natural complex systems.

Symmetry-breaking in patch formation on triangular gold nanoparticles by asymmetric polymer grafting

Synthesizing patchy particles with predictive control over patch size, shape, placement and number has been highly sought-after for nanoparticle assembly research, but is fraught with challenges. Here we show that polymers can be designed to selectively adsorb onto nanoparticle surfaces already partially coated by other chains to drive the formation of patchy nanoparticles with broken symmetry. In our model system of triangular gold nanoparticles and polystyrene-b-polyacrylic acid patch, single- and double-patch nanoparticles are produced at high yield. These asymmetric single-patch nanoparticles are shown to assemble into self-limited patch‒patch connected bowties exhibiting intriguing plasmonic properties. To unveil the mechanism of symmetry-breaking patch formation, we develop a theory that accurately predicts our experimental observations at all scales—from patch patterning on nanoparticles, to the size/shape of the patches, to the particle assemblies driven by patch‒patch interactions. Both the experimental strategy and theoretical prediction extend to nanoparticles of other shapes such as octahedra and bipyramids. Our work provides an approach to leverage polymer interactions with nanoscale curved surfaces for asymmetric grafting in nanomaterials engineering.

Twin-Engine Janus Supramolecular Nanomotors with Counterbalanced Motion

Supramolecular nanomotors were created with two types of propelling forces that were able to counterbalance each other. The particles were based on bowl-shaped polymer vesicles, or stomatocytes, assembled from the amphiphilic block copolymer poly(ethylene glycol)-block-polystyrene. The first method of propulsion was installed by loading the nanocavity of the stomatocytes with the enzyme catalase, which enabled the decomposition of hydrogen peroxide into water and oxygen, leading to a chemically induced motion. The second method of propulsion was attained by applying a hemispherical gold coating on the stomatocytes, on the opposite side of the opening, making the particles susceptible to near-infrared laser light. By exposing these Janus-type twin engine nanomotors to both hydrogen peroxide (H₂O₂) and near-infrared light, two competing driving forces were synchronously generated, resulting in a counterbalanced, "seesaw effect" motion. By precisely manipulating the incident laser power and concentration of H₂O₂, the supramolecular nanomotors could be halted in a standby mode. Furthermore, the fact that these Janus stomatocytes were equipped with opposing motile forces also provided a proof of the direction of motion of the enzyme-activated stomatocytes. Finally, the modulation of the "seesaw effect", by tuning the net outcome of the two coexisting driving forces, was used to attain switchable control of the motile behavior of the twin-engine nanomotors. Supramolecular nanomotors that can be steered by two orthogonal propulsion mechanisms hold considerable potential for being used in complex tasks, including active transportation and environmental remediation.

Field-Induced Assembly and Propulsion of Colloids

Electric and magnetic fields have enabled both technological applications and fundamental discoveries in the areas of bottom-up material synthesis, dynamic phase transitions, and biophysics of living matter. Electric and magnetic fields are versatile external sources of energy that power the assembly and self-propulsion of colloidal particles. In this Invited Feature Article, we classify the mechanisms by which external fields impact the structure and dynamics in colloidal dispersions and augment their nonequilibrium behavior. The paper is purposely intended to highlight the similarities between electrically and magnetically actuated phenomena, providing a brief treatment of the origin of the two fields to understand the intrinsic analogies and differences. We survey the progress made in the static and dynamic assembly of colloids and the self-propulsion of active particles. Recent reports of assembly-driven propulsion and propulsion-driven assembly have blurred the conceptual boundaries and suggest an evolution in the research of nonequilibrium colloidal materials. We highlight the emergence of colloids powered by external fields as model systems to understand living matter and provide a perspective on future challenges in the area of field-induced colloidal phenomena.

Small-Angle X-ray Scattering Analysis of Colloidal Crystals and Replica Materials Made from l-Arginine-Stabilized Silica Nanoparticles

Colloidal crystals made from sub-100 nm silica nanoparticles have provided a versatile platform for the template-assisted synthesis of three-dimensionally interconnected semiconducting, metallic, and magnetic replicas. However, the detailed structure of these materials has not yet been characterized. In this study, we investigated the structures of colloidal crystalline films and germanium replicas by scanning electron microscopy and small angle X-ray scattering. The structures of colloidal crystals made by evaporative assembly depends on the size of l-arginine-capped silica nanoparticles. Particles smaller than ∼31 nm diameter assemble into non-close-packed arrangements (bcc) whereas particles larger than 31 nm assemble into random close-packed structures with disordered hexagonal phase. Polycrystalline films of these materials retain their structures and long-range order upon infiltration at high temperature and pressure, and the structure is preserved in Ge replicas. The shear force during deposition and dispersity of silica nanoparticles contributes to the size-based variation in the structure and to the size of crystal domains in the colloidal crystal films.

Design, fabrication and application of magnetically actuated micro/nanorobots: a review

Magnetically actuated micro/nanorobots are typical micro- and nanoscale artificial devices with favorable attributes of quick response, remote and contactless control, harmless human-machine interaction and high economic efficiency. Under external magnetic actuation strategies, they are capable of achieving elaborate manipulation and navigation in extreme biomedical environments. This review focuses on state-of-the-art progresses in design strategies, fabrication techniques and applications of magnetically actuated micro/nanorobots. Firstly, recent advances of various robot designs, including helical robots, surface walkers, ciliary robots, scaffold robots and biohybrid robots, are discussed separately. Secondly, the main progresses of common fabrication techniques are respectively introduced, and application achievements on these robots in targeted drug delivery, minimally invasive surgery and cell manipulation are also presented. Finally, a short summary is made, and the current challenges and future work for magnetically actuated micro/nanorobots are discussed.

Collective Behaviors of Magnetic Active Matter: Recent Progress toward Reconfigurable, Adaptive, and Multifunctional Swarming Micro/Nanorobots

Active matter refers to the nonequilibrium system composed of interacting units that continually dissipate energy at a single-unit level and transduce it into mechanical force or motion. Such systems are ubiquitous in nature and span most of the biological scales, ranging from cytoskeleton protein polymers at the molecular level to bacterial colonies at the cellular level to swarms of insects, flocks of birds, schools of fish, and even crowds of humans on the organismal scale. The consumption of energy within systems tends to induce the self-organization of active matter as well as the spontaneous emergence of dynamic, complex, and collective states with extraordinary properties, such as adaptability, reconfigurability, taxis, and so on. The research into active matter is expected to deepen the understanding of the underlying mechanisms of how the units in living systems interact with each other and regulate the flow of energy to improve the survival efficiency, which in turn can provide valuable insights into the engineering of artificial active systems with novel and practical collective functionalities.Because of the striking similarity in collective states, a colloidal system is an emerging approach to understanding the guiding principles of the coordinated activities in living systems. Thanks to the capabilities in batch fabrication, size control, and the modulation of interactions (e.g., dipole-dipole interactions, capillary forces, electrostatic interactions, and so on), various complex collective states have been reproduced and programmed in colloidal suspension through the elaborate design of compositions and unit-unit interactions. Among the developed colloidal systems, magnetic colloids energized by alternating magnetic fields demonstrate several unique advantages, including the high-degree-of-freedom and simple modulation of the magnetic field parameters as well as the excellent compatibility of the magnetic field with many application scenarios. Therefore, magnetic active matter not only constitutes a useful platform that leads to a discovery of fascinating emergent collective behaviors but also promises enormous potential in a variety of engineering fields.In this Account, we summarize and highlight the key efforts carried out by our group and others on the investigation of the collective behavior of magnetic active matter in the past 5 years. First, we elucidate the generation mechanisms of the emergent coordinated behaviors, which are classified according to the dominating interactions among agents, that is, the magnetic dipole-dipole interaction, hydrodynamic interaction, and weak interaction. Then we illustrate the construction of magnetic active matter with a higher level of collective effects and functionalities (e.g., reconfigurability, environmental adaptability, 3D swarming, cooperative multifunctionality, and so on) via the synergistic effects between magnetic fields and other fields. Next, potential applications of magnetic active matter are discussed, which mainly focus on the exploration in revolutionizing traditional biomedical fields. Finally, an outlook of future opportunities is presented to promote the development of magnetic active matter, which facilitates a better understanding of living counterparts and the further realization of practical applications.

Dimeric colloidal inclusion in a chiral active bath: Effective interactions and chirality-induced torque

Colloidal inclusions suspended in a bath of smaller particles experience an effective bath-mediated attraction at small intersurface separations, which is known as the depletion interaction. In an active bath of nonchiral self-propelled particles, the effective force changes from attraction to repulsion, an effect that is suppressed when the active bath particles are chiral. Using Brownian dynamics simulations, we study the effects of channel confinement and bath chirality on the effective forces and torques that are mediated between two inclusions that may be fixed within the channel or may be allowed to rotate freely as a rigid dimer around its center of mass. We show that the confinement has a strong effect on the effective interactions, depending on the orientation of the dimer relative to the channel walls. The active particle chirality leads to a force imbalance and, hence, a net torque on the inclusion dimer, which we investigate as a function of the bath chirality strength and the channel height.

Self-Propelling Hybrid Gels Incorporating an Active Self-Assembled, Low-Molecular-Weight Gelator

Hybrid gel beads based on combining a low-molecular-weight gelator (LMWG) with a polymer gelator (PG) demonstrate an enhanced ability to self-propel in water, with the LMWG playing an active role. Hybrid gel beads were loaded with ethanol and shown to move in water owing to the Marangoni effect changes in surface tension caused by the expulsion of ethanol - smaller beads move farther and faster than larger beads. Flat shapes of the hybrid gel were cut using a "stamp" - circles moved the furthest, whereas stars showed more rotation on their own axes. Comparing hybrid LMWG/PG gel beads with PG-only beads demonstrated that the LMWG speeds up the beads, enhancing the rate of self-propulsion. Self-assembly of the LMWG into a "solid-like" network prevents its leaching from the gel. The LMWG also retains its own unique function - specifically, remediating methylene blue pollutant dye from basic water as a result of noncovalent interactions. The mobile hybrid beads accumulate this dye more effectively than PG-only beads. Self-propelling gel beads have potential applications in removal/delivery of active agents in environmental or biological settings. The ability of self-assembling LMWGs to enhance mobility and control removal/delivery suggests that adding them to self-propelling systems can add significant value.

Controllable Positive/Negative Phototaxis of Millimeter-Sized Objects with Sensing Function

Phototaxis, which is the directional motion toward or away from light, is common in nature and inspires development of artificial light-steered active objects. Most of the light-steered objects developed so far exhibit either positive or negative phototaxis, and there are few examples of research on objects that exhibit both positive and negative phototaxis. Herein, small objects showing both positive and negative phototaxis on the water surface upon near-infrared (NIR) light irradiation, with the direction controlled by the position of light irradiation, are reported. The millimeter-sized tetrahedral liquid marble containing gelled water coated by one polymer plate with light-to-heat photothermal characteristic, which adsorbs onto the bottom of the liquid marble, and three polymer plates with highly transparent characteristic, which adsorb onto the upper part of the liquid marble, is utilized as a model small object. Light irradiation on the front side of the object induces negative phototaxis and that on the other side induces positive phototaxis, and the motion can be controlled to 360° arbitrary direction by precise control of the light irradiation position. Thermographic studies confirm that the motions are realized through Marangoni flow generated around the liquid marble, which is induced by position-selective NIR light irradiation. The object can move centimeter distances, and numerical analysis indicates that average velocity and acceleration are approximately 12 mm/s and 71 mm/s², respectively, which are independent of the direction of motions. The generated force is estimated to be approximately 0.4 μN based on Newton's equation. Furthermore, functional cargo can be loaded into the inner phase of the small objects, which can be delivered and released on demand and endows them with environmental sensing ability.

Reconfigurable artificial microswimmers with internal feedback

Self-propelling microparticles are often proposed as synthetic models for biological microswimmers, yet they lack the internally regulated adaptation of their biological counterparts. Conversely, adaptation can be encoded in larger-scale soft-robotic devices but remains elusive to transfer to the colloidal scale. Here, we create responsive microswimmers, powered by electro-hydrodynamic flows, which can adapt their motility via internal reconfiguration. Using sequential capillary assembly, we fabricate deterministic colloidal clusters comprising soft thermo-responsive microgels and light-absorbing particles. Light absorption induces preferential local heating and triggers the volume phase transition of the microgels, leading to an adaptation of the clusters' motility, which is orthogonal to their propulsion scheme. We rationalize this response via the coupling between self-propulsion and variations of particle shape and dielectric properties upon heating. Harnessing such coupling allows for strategies to achieve local dynamical control with simple illumination patterns, revealing exciting opportunities for developing tactic active materials.

High-order simulation scheme for active particles driven by stress boundary conditions

We study the dynamics and interactions of elliptic active particles in a two dimensional solvent. The particles are self-propelled through prescribing a fluid stress at one half of the fluid-particle boundary. The fluid is treated explicitly solving the Stokes equation through a discontinuous Galerkin scheme, which allows to simulate strictly incompressible fluids. We present numerical results for a single particle and give an outlook on how to treat suspensions of interacting active particles.

Fabrication and Electric Field-Driven Active Propulsion of Patchy Microellipsoids

Active colloids are a synthetic analogue of biological microorganisms that consume external energy to swim through viscous fluids. Such motion requires breaking the symmetry of the fluid flow in the vicinity of a particle; however, it is challenging to understand how surface and shape anisotropies of the colloid lead to a particular trajectory. Here, we attempt to deconvolute the effects of particle shape and surface anisotropy on the propulsion of model ellipsoids in alternating current (AC) electric fields. We first introduce a simple process for depositing metal patches of various shapes on the surfaces of ellipsoidal particles. We show that the shape of the metal patch is governed by the assembled structure of the ellipsoids on the substrate used for physical vapor deposition. Under high-frequency AC electric field, ellipsoids dispersed in water show linear, circular, and helical trajectories which depend on the shapes of the surface patches. We demonstrate that features of the helical trajectories such as the pitch and diameter can be tuned by varying the degree of patch asymmetry along the two primary axes of the ellipsoids, namely longitudinal and transverse. Our study reveals the role of patch shape on the trajectory of ellipsoidal particles propelled by induced charge electrophoresis. We develop heuristics based on patch asymmetries that can be used to design patchy particles with specified nonlinear trajectories.

Programmable Phototaxis of Metal-Phenolic Particle Microswimmers

Light-driven directional motion is common in nature but remains a challenge for synthetic microparticles, particularly regarding collective motion on a macroscopic scale. Successfully engineering microparticles with light-driven collective motion could lead to breakthroughs in drug delivery, contaminant sensing, environmental remediation, and artificial life. Herein, metal-phenolic particle microswimmers capable of autonomously sensing and swimming toward an external light source are reported, with the speed regulated by the wavelength and intensity of illumination. These microswimmers can travel macroscopic distances (centimeters) and can remain illuminated for hours without degradation of motility. Experimental and theoretical analyses demonstrate that motion is generated through chemical transformations of the organic component of the metal-phenolic complex. Furthermore, cargos with specific spectral absorption profiles can be loaded into the particles and endow the particle microswimmers with activated motion corresponding to these spectral characteristics. The programmable nature of the light navigation, tunable size of the particles, and versatility of cargo loading demonstrate the versatility of these metal-phenolic particle microswimmers.

Magnetically Driven Micro and Nanorobots

Manipulation and navigation of micro and nanoswimmers in different fluid environments can be achieved by chemicals, external fields, or even motile cells. Many researchers have selected magnetic fields as the active external actuation source based on the advantageous features of this actuation strategy such as remote and spatiotemporal control, fuel-free, high degree of reconfigurability, programmability, recyclability, and versatility. This review introduces fundamental concepts and advantages of magnetic micro/nanorobots (termed here as “MagRobots”) as well as basic knowledge of magnetic fields and magnetic materials, setups for magnetic manipulation, magnetic field configurations, and symmetry-breaking strategies for effective movement. These concepts are discussed to describe the interactions between micro/nanorobots and magnetic fields. Actuation mechanisms of flagella-inspired MagRobots (i.e., corkscrew-like motion and traveling-wave locomotion/ciliary stroke motion) and surface walkers (i.e., surface-assisted motion), applications of magnetic fields in other propulsion approaches, and magnetic stimulation of micro/nanorobots beyond motion are provided followed by fabrication techniques for (quasi-)spherical, helical, flexible, wire-like, and biohybrid MagRobots. Applications of MagRobots in targeted drug/gene delivery, cell manipulation, minimally invasive surgery, biopsy, biofilm disruption/eradication, imaging-guided delivery/therapy/surgery, pollution removal for environmental remediation, and (bio)sensing are also reviewed. Finally, current challenges and future perspectives for the development of magnetically powered miniaturized motors are discussed.

Fully Steerable Symmetric Thermoplasmonic Microswimmers

A cornerstone of the directed motion of microscopic self-propelling particles is an asymmetric particle structure defining a polarity axis along which these tiny machines move. This structural asymmetry ties the orientational Brownian motion to the microswimmers directional motion, limiting their persistence and making the long time motion effectively diffusive. Here, we demonstrate a completely symmetric thermoplasmonic microswimmer, which is propelled by laser-induced self-thermophoresis. The propulsion direction is imprinted externally to the particle by the heating laser position. The orientational Brownian motion, thus, becomes irrelevant for the propulsion, allowing enhanced control over the particles dynamics with almost arbitrary steering capability. We characterize the particle motion in experiments and simulations and also theoretically. The analysis reveals additional noise appearing in these systems, which is conjectured to be relevant for biological systems. Our experimental results show that even very small particles can be precisely controlled, enabling more advanced applications of these micromachines.

Structurally Anisotropic Janus Particles with Tunable Amphiphilicity via Polymerization of Dynamic Complex Emulsions

A facile one-step approach for the synthesis of physically and chemically anisotropic polymer particles with tunable size, shape, composition, wettability, and functionality is reported. Specifically, dynamically reconfigurable oil-in-water Janus emulsions containing photocurable hydrocarbon or fluorocarbon acrylate monomers as one of the droplet phases are used as structural templates to polymerize them into precision Janus particles with highly uniform anomalous morphologies including (hemi-) spheres, lenses, and bowls. During polymerization, each interface is exposed to a different chemical environment, yielding particles with an intrinsic Janus character that can be amplified via side-selective postfunctionalization. The fabrication method allows to start with various common emulsification techniques, thus generating particles in the range of 200 nm –150 μm, also at a technical scale. The anisotropic shape combined with the asymmetric wettability profile of the produced particles promotes their directed self-assembly into colloidal clusters as well as their directional alignment at fluid interfaces. We foresee the application of such Janus particles in technical emulsions or oil recovery, for the manufacturing of programmed self-assembled architectures, and for the engineering of microstructured interfaces.

Asymmetry controlled dynamic behavior of autonomous chemiluminescent Janus microswimmers

Asymmetrically modified Janus microparticles are presented as autonomous light emitting swimmers. The localized dissolution of hybrid magnesium/polymer objects allows combining chemiluminescence with the spontaneous production of H₂ bubbles, and thus generating directed motion. These light-emitting microswimmers are synthesized by using a straightforward methodology based on bipolar electromilling, followed by indirect bipolar electrodeposition of an electrophoretic paint. An optimization of the experimental parameters enables in the first step the formation of well-defined isotropic or anisotropic Mg microparticles. Subsequently, they are asymmetrically modified by wireless deposition of an anodic paint. The degree of asymmetry of the resulting Janus particles can be fine-tuned, leading to a controlled directional motion due to anisotropic gas formation. This autonomous motion is coupled with the emission of bright orange light when Ru(bpy)₃²⁺ and S₂O₈²⁻ are present in the solution as chemiluminescent reagents. The light emission is based on an original process of interfacial redox-induced chemiluminescence, thus allowing an easy visualization of the swimmer trajectories.

Asymmetrically modified Janus microparticles are presented as autonomous light emitting swimmers with shape-controlled trajectories.

Clustering and Phase Separation in Mixtures of Dipolar and Active Particles in an External Field

Directing the assembly of colloidal particles through the use of external electric or magnetic fields shows promise for the creation of reconfigurable materials. Self-propelled particles can also be used to dynamically drive colloidal systems to nonequilibrium steady states. We investigate colloidal systems that combine both of these methods of directed assembly, simulating mixtures of passive dipolar colloids and active soft spheres in an external magnetic field using Brownian dynamics in two dimensions. In these systems, the dipolar particles align in the direction of the external field, but the active particles are unaffected by the field. The phase behaviors exhibited included a percolated dipolar network, dipolar string-fluid, isotropic fluid, and phase-separated state. We find that the external field allows the dipolar particles to form a percolated network more easily compared to when no external field is present. Additionally, the mixture phase separates at lower active particle velocity in an external field than when no field is present. Our results suggest that combining multiple methods of directing colloidal assembly could lead to new pathways to fabricate reconfigurable materials.

Active colloidal molecules assembled via selective and directional bonds

The assembly of active and self-propelled particles is an emerging strategy to create dynamic materials otherwise impossible. However, control of the complex particle interactions remains challenging. Here, we show that various dynamic interactions of active patchy particles can be orchestrated by tuning the particle size, shape, composition, etc. This capability is manifested in establishing dynamic colloidal bonds that are highly selective and directional, which greatly expands the spectrum of colloidal structures and dynamics by assembly. For example, we demonstrate the formation of colloidal molecules with tunable bond angles and orientations. They exhibit controllable propulsion, steering, reconfiguration as well as other dynamic behaviors that collectively reflect the bond properties. The working principle is further extended to the co-assembly of synthetic particles with biological entities including living cells, giving rise to hybrid colloidal molecules of various types, for example, a colloidal carrousel structure. Our strategy should enable active systems to perform sophisticated tasks in future such as selective cell treatment.

The assembly of active and self-propelled particles is an emerging strategy to create dynamic materials otherwise impossible. Here, the authors show the assembly of active colloidal molecules with a wide spectrum of new structures and dynamics, conferred to them by highly selective and directional interactions.

A practical guide to active colloids: choosing synthetic model systems for soft matter physics research

Synthetic active colloids that harvest energy stored in the environment and swim autonomously are a popular model system for active matter. This emerging field of research sits at the intersection of materials chemistry, soft matter physics, and engineering, and thus cross-talk among researchers from different backgrounds becomes critical yet difficult. To facilitate this interdisciplinary communication, and to help soft matter physicists with choosing the best model system for their research, we here present a tutorial review article that describes, in appropriate detail, six experimental systems of active colloids commonly found in the physics literature. For each type, we introduce their background, material synthesis and operating mechanisms and notable studies from the soft matter community, and comment on their respective advantages and limitations. In addition, the main features of each type of active colloid are summarized into two useful tables. As materials chemists and engineers, we intend for this article to serve as a practical guide, so those who are not familiar with the experimental aspects of active colloids can make more informed decisions and maximize their creativity.

Frequency Response of Induced-Charge Electrophoretic Metallic Janus Particles

The ability to manipulate and control active microparticles is essential for designing microrobots for applications. This paper describes the use of electric and magnetic fields to control the direction and speed of induced-charge electrophoresis (ICEP) driven metallic Janus microrobots. A direct current (DC) magnetic field applied in the direction perpendicular to the electric field maintains the linear movement of particles in a 2D plane. Phoretic force spectroscopy (PFS), a phase-sensitive detection method to detect the motions of phoretic particles, is used to characterize the frequency-dependent phoretic mobility and drag coefficient of the phoretic force. When the electric field is scanned over a frequency range of 1 kHz-1 MHz, the Janus particles exhibit an ICEP direction reversal at a crossover frequency at ~30 kH., Below this crossover frequency, the particle moves in a direction towards the dielectric side of the particle, and above this frequency, the particle moves towards the metallic side. The ICEP phoretic drag coefficient measured by PFS is found to be similar to that of the Stokes drag. Further investigation is required to study microscopic interpretations of the frequency at which ICEP mobility switched signs and the reason why the magnitudes of the forward and reversed modes of ICEP are so different.

Active Patchy Colloids with Shape-Tunable Dynamics

Controlling the complex dynamics of active colloids-the autonomous locomotion of colloidal particles and their spontaneous assembly-is challenging yet crucial for creating functional, out-of-equilibrium colloidal systems potentially useful for nano- and micromachines. Herein, by introducing the synthesis of active "patchy" colloids of various low-symmetry shapes, we demonstrate that the dynamics of such systems can be precisely tuned. The low-symmetry patchy colloids are made in bulk via a cluster-encapsulation-dewetting method. They carry essential information encoded in their shapes (particle geometry, number, size, and configurations of surface patches, etc.) that programs their locomotive and assembling behaviors. Under AC electric field, we show that the velocity of particle propulsion and the ability to brake and steer can be modulated by having two asymmetrical patches with various bending angles. The assembly of monopatch particles leads to the formation of dynamic and reconfigurable structures such as spinners and "cooperative swimmers" depending on the particle's aspect ratios. A particle with two patches of different sizes allows for "directional bonding", a concept popular in static assemblies but rare in dynamic ones. With the capability to make tunable and complex shapes, we anticipate the discovery of a diverse range of new dynamics and structures when other external stimuli (e.g., magnetic, optical, chemical, etc.) are employed and spark synergy with shapes.

Tackling the Short-Lived Marangoni Motion Using a Supramolecular Strategy

Inspired by the intriguing capability of beetles to quickly slide on water, scientists have long translated this surface-tension-gradient–dominated Marangoni motion into various applications, for example, self-propulsion. However, this classical spontaneous motion is limited by a short lifetime due to the loss of the surface tension gradient. Indeed, the propellant of amphiphilic surfactants can rapidly reach an adsorption equilibrium and an excessive aggregation state at the air/liquid interface. Here, we demonstrate a supramolecular host–guest chemistry strategy that allows the breaking of the physical limit of the adsorption equilibrium and the simultaneous removal of surfactant molecules from the interface. By balancing the competitive kinetics between the two processes, we have prolonged the lifetime of the motion 40-fold. Our work presents an important advance in the query of long-lived self-propulsion transport through flexible interference at the molecular level and holds promise in electricity generation applications .

Programmable Locomotion Mechanisms of Nanowires with Semihard Magnetic Properties Near a Surface Boundary

We report on the simplest magnetic nanowire-based surface walker that is able to change its propulsion mechanism near a surface boundary as a function of the applied rotating magnetic field frequency. The nanowires are made of CoPt alloy with semihard magnetic properties synthesized by means of template-assisted galvanostatic electrodeposition. The semihard magnetic behavior of the nanowires allows for programming their alignment with an applied magnetic field as they can retain their magnetization direction after premagnetizing them. By engineering the macroscopic magnetization, the nanowires' speed and locomotion mechanism are set to tumbling, precession, or rolling depending on the frequency of an applied rotating magnetic field. Also, we present a mathematical analysis that predicts the translational speed of the nanowire near the surface, showing a very good agreement with experimental results. Interestingly, the maximal speed is obtained at an optimal frequency (∼10 Hz), which is far below the theoretical step-out frequency (∼345 Hz). Finally, vortices are found by tracking polystyrene microbeads, trapped around the CoPt nanowire, when they are propelled by precession and rolling motion.

Shape-Dependent Motion of Structured Photoactive Microswimmers

We investigate the dynamics of structured photoactive microswimmers and show that morphology sensitively determines the swimming behavior. Particular to this study, a major portion of the light-activated particles' underlying structure is built from a photocatalytic material, made possible by dynamic physical vapor deposition (DPVD). We find that swimmers of this type exhibit unique shape-dependent autonomous swimming that is distinct from what is seen in systems with similar structural morphology but not fabricated directly from the catalyst. Notably, the direction of motion is a function of these parameters. Because the swimming behavior is strongly correlated with particle shape and material composition, DPVD allows for engineering small-scale propulsion by adjusting the fabrication parameters to match the desired performance.

Direct Visualization of Planar Assembly of Plasmonic Nanoparticles Adjacent to Electrodes in Oscillatory Electric Fields

Electric field-directed assembly of colloidal nanoparticles (NPs) has been widely adopted for fabricating functional thin films and nanostructured surfaces. While first-order electrokinetic effects on NPs are well-understood in terms of classical models, effects of second-order electrokinetics that involve induced surface charge are still poorly understood. Induced charge electroosmotic phenomena, such as electrohydrodynamic (EHD) flow, have long been implicated in electric field-directed NP assembly with little experimental basis. Here, we use in situ dark-field optical microscopy and plasmonic NPs to directly observe the dynamics of planar assembly of colloidal NPs adjacent to a planar electrode in low-frequency (<1 kHz) oscillatory electric fields. We exploit the change in plasmonic NP color resulting from interparticle plasmonic coupling to visualize the assembly dynamics and assembly structure of silver NPs. Planar assembly of NPs is unexpected because of strong electrostatic repulsion between NPs and indicates that there are strong attractive interparticle forces oriented perpendicular to the electric field direction. A parametric investigation of the voltage- and frequency-dependent phase behavior reveals that planar NP assembly occurs over a narrow frequency range below which irreversible ballistic deposition occurs. Two key experimental observations are consistent with EHD flow-induced NP assembly: (1) NPs remain mobile during assembly and (2) electron microscopy observations reveal randomly close-packed planar assemblies, consistent with strong interparticle attraction. We interpret planar assembly in terms of EHD fluid flow and develop a scaling model that qualitatively agrees with the measured phase regions. Our results are the first direct in situ observations of EHD flow-induced NP assembly and shed light on long-standing unresolved questions concerning the formation of NP superlattices during electric field-induced NP deposition.

Reconfigurable engineered motile semiconductor microparticles

Locally energized particles form the basis for emerging classes of active matter. The design of active particles has led to their controlled locomotion and assembly. The next generation of particles should demonstrate robust control over their active assembly, disassembly, and reconfiguration. Here we introduce a class of semiconductor microparticles that can be comprehensively designed (in size, shape, electric polarizability, and patterned coatings) using standard microfabrication tools. These custom silicon particles draw energy from external electric fields to actively propel, while interacting hydrodynamically, and sequentially assemble and disassemble on demand. We show that a number of electrokinetic effects, such as dielectrophoresis, induced charge electrophoresis, and diode propulsion, can selectively power the microparticle motions and interactions. The ability to achieve on-demand locomotion, tractable fluid flows, synchronized motility, and reversible assembly using engineered silicon microparticles may enable advanced applications that include remotely powered microsensors, artificial muscles, reconfigurable neural networks and computational systems.

Active particles that demonstrate life-like behavior may find use in bio-inspired technologies, but achieving on-demand reconfiguration remains challenging. Here, the authors demonstrate controllable, collective behavior in silicon microparticles, which are fabricated via conventional semiconductor methods.