Chemically Active Particles: From One to Few on the Way to Many

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.

Light-Induced Material Motion Fingerprint - A Tool Toward Selective Interfacial Sensitive Fractioning of Microparticles via Microfluidic Methods

In this article, a novel strategy is presented to selectively separate a mixture of equally sized microparticles but differences in material composition and surface properties. The principle relies on a photosensitive surfactant, which makes particles under light illumination phoretically active. The latter hovers microparticles from a planar interface and together with a superimposed fluid flow, particles experience a drift motion characteristic to its interfacial properties. The drift motion is investigated as a function of applied wavelength, demonstrating that particles composed of different material show a unique spectrally resolved light-induced motion profile. Differences in those motion profile allow a selective fractioning of a desired particle from a complex particle mixture made out of more than two equally sized different particle types. Besides that, the influence of applied wavelength is systematically studied, and discussed the origin of the spectrally resolved chemical activity of microparticles from measured photo-isomerization rates.

Chemical Logic Gates on Active Colloids

Recent studies have shown that active colloidal motors using enzymatic reactions for propulsion hold special promise for applications in fields ranging from biology to material science. It will be desirable to have active colloids with capability of computation so that they can act autonomously to sense their surroundings and alter their own dynamics. It is shown how small chemical networks that make use of enzymatic chemical reactions on the colloid surface can be used to construct motor-based chemical logic gates. The basic features of coupled enzymatic reactions that are responsible for propulsion and underlie the construction and function of chemical gates are described using continuum theory and molecular simulation. Examples are given that show how colloids with specific chemical logic gates, can perform simple sensing tasks. Due to the diverse functions of different enzyme gates, operating alone or in circuits, the work presented here supports the suggestion that synthetic motors using such gates could be designed to operate in an autonomous way in order to complete complicated tasks.

Non-reciprocity induces resonances in a two-field Cahn-Hilliard model

We consider a non-reciprocally coupled two-field Cahn-Hilliard system that has been shown to allow for oscillatory behaviour and suppression of coarsening. After introducing the model, we first review the linear stability of steady uniform states and show that all instability thresholds are identical to the ones for a corresponding two-species reaction-diffusion system. Next, we consider a specific interaction of linear modes-a 'Hopf-Turing' resonance-and derive the corresponding amplitude equations using a weakly nonlinear approach. We discuss the weakly nonlinear results and finally compare them with fully nonlinear simulations for a specific conserved amended FitzHugh-Nagumo system. We conclude with a discussion of the limitations of the employed weakly nonlinear approach. This article is part of the theme issue 'New trends in pattern formation and nonlinear dynamics of extended systems'.

Assembly and manipulation of responsive and flexible colloidal structures by magnetic and capillary interactions

The long-ranged interactions induced by magnetic fields and capillary forces in multiphasic fluid-particle systems facilitate the assembly of a rich variety of colloidal structures and materials. We review here the diverse structures assembled from isotropic and anisotropic particles by independently or jointly using magnetic and capillary interactions. The use of magnetic fields is one of the most efficient means of assembling and manipulating paramagnetic particles. By tuning the field strength and configuration or by changing the particle characteristics, the magnetic interactions, dynamics, and responsiveness of the assemblies can be precisely controlled. Concurrently, the capillary forces originating at the fluid-fluid interfaces can serve as means of reconfigurable binding in soft matter systems, such as Pickering emulsions, novel responsive capillary gels, and composites for 3D printing. We further discuss how magnetic forces can be used as an auxiliary parameter along with the capillary forces to assemble particles at fluid interfaces or in the bulk. Finally, we present examples how these interactions can be used jointly in magnetically responsive foams, gels, and pastes for 3D printing. The multiphasic particle gels for 3D printing open new opportunities for making of magnetically reconfigurable and "active" structures.

The interplay between chemo-phoretic interactions and crowding in active colloids

Many motile microorganisms communicate with each other and their environments via chemical signaling which leads to long-range interactions mediated by self-generated chemical gradients. However, consequences of the interplay between crowding and chemotactic interactions on their collective behavior remain poorly understood. In this work, we use Brownian dynamics simulations to investigate the effect of packing fraction on the formation of non-equilibrium structures in a monolayer of diffusiophoretic self-propelled colloids as a model for chemically active particles. Focusing on the case when a chemical field induces attractive positional and repulsive orientational interactions, we explore dynamical steady-states of active colloids of varying packing fractions and degrees of motility. In addition to collapsed, active gas, and dynamical clustering steady-states reported earlier for low packing fractions, a new phase-separated state emerges. The phase separation results from a competition between long-range diffusiophoretic interactions and motility and is observed at moderate activities and a wide range of packing fractions. Our analysis suggests that the fraction of particles in the largest cluster is a suitable order parameter for capturing the transition from an active gas and dynamical clustering states to a phase-separated state.

Active colloidal molecules in activity gradients

We consider a rigid assembly of two active Brownian particles, forming an active colloidal dimer, in a gradient of activity. We show analytically that depending on the relative orientation of the two particles the active dimer accumulates in regions of either high or low activity, corresponding to, respectively, chemotaxis and antichemotaxis. Certain active dimers show both chemotactic and antichemotactic behavior, depending on the strength of the activity. Our coarse-grained Fokker-Planck approach yields an effective potential, which we use to construct a nonequilibrium phase diagram that classifies the dimers according to their tactic behavior. Moreover, we show that for certain dimers a higher persistence of the motion is achieved similar to the effect of a steering wheel in macroscopic devices. This work could be useful for designing autonomous active colloidal structures which adjust their motion depending on the local activity gradients.

High shear rate propulsion of acoustic microrobots in complex biological fluids

Untethered microrobots offer a great promise for localized targeted therapy in hard-to-access spaces in our body. Despite recent advancements, most microrobot propulsion capabilities have been limited to homogenous Newtonian fluids. However, the biological fluids present in our body are heterogeneous and have shear rate-dependent rheological properties, which limit the propulsion of microrobots using conventional designs and actuation methods. We propose an acoustically powered microrobotic system, consisting of a three-dimensionally printed 30-micrometer-diameter hollow body with an oscillatory microbubble, to generate high shear rate fluidic flow for propulsion in complex biofluids. The acoustically induced microstreaming flow leads to distinct surface-slipping and puller-type propulsion modes in Newtonian and non-Newtonian fluids, respectively. We demonstrate efficient propulsion of the microrobots in diverse biological fluids, including in vitro navigation through mucus layers on biologically relevant three-dimensional surfaces. The microrobot design and high shear rate propulsion mechanism discussed herein could open new possibilities to deploy microrobots in complex biofluids toward minimally invasive targeted therapy.

Investigating Time-Dependent Active Motion of Janus Micromotors using Dynamic Light Scattering

Advances in fabrication methods have positioned Janus micromotors (JMs) as candidates for use as autonomous devices in applications across diverse fields, spanning drug delivery to environmental remediation. While the design of most micromotors is straightforward, the non-steady state active motion exhibited by these systems is complex and difficult to characterize. Traditionally, JM active motion is characterized using optical microscopy single particle tracking for systems confined in 2D. Dynamic light scattering (DLS) offers an alternative high-throughput method for characterizing the 3D active motion in bulk JM dispersions with additional capabilities to quantify time-dependent behavior for a broader range of JM sizes. Here, the active motion of spherical JMs is examined by DLS and it is demonstrated that the method enables decoupling of the translational and rotational diffusion. Systematic studies quantifying the time-dependent diffusive properties as a function of fuel concentration, JM concentration, and time after fuel addition are presented. The analyses presented in this work position DLS to facilitate future advances of JM systems by serving as a fast-screening characterization method for active motion.

Modeling of chemically active particles at an air-liquid interface

The collective motion of chemically active particles at an air-liquid interface is studied theoretically as a dynamic self-organization problem. Based on a physical consideration, we propose a minimal model for self-propelled particles by combining hydrodynamic interaction, capillary interaction, driving force by Marangoni effect, and Marangoni flow. Our model has successfully captured the features of chemically active particles, that represent dynamic self-organized states such as crystalline, chain, liquid-like and spreading states.

Collective hydrodynamic transport of magnetic microrollers

We investigate the collective transport properties of microscopic magnetic rollers that propel close to a surface due to a circularly polarized, rotating magnetic field. The applied field exerts a torque to the particles, which induces a net rolling motion close to a surface. The collective dynamics of the particles result from the balance between magnetic dipolar interactions and hydrodynamic ones. We show that, when hydrodynamics dominate, i.e. for high particle spinning, the collective mean velocity linearly increases with the particle density. In this regime we analyse the clustering kinetics, and find that hydrodynamic interactions between the anisotropic, elongated particles, induce preferential cluster growth along a direction perpendicular to the driving one, leading to dynamic clusters that easily break and reform during propulsion.

Asymmetric assembly of Lennard-Jones Janus dimers

Self-assembly of Janus (or "patchy") particles is dependent on the precise interaction between neighboring particles. Here, the orientations of two amphiphilic Janus spheres within a dimer in an explicit fluid are studied with high geometric resolution. Molecular dynamics simulations and semianalytical energy calculations are used with hard- and soft-sphere Lennard-Jones potentials, and temperature and hydrophobicity are varied. The most probable center-center-pole angles are in the range of 40^{∘}-55^{∘} with pole-to-pole alignment not observed due to orientational entropy. Angles near 90^{∘} are energetically unfavored due to solvent exclusion, and the relative azimuthal angle between the spheres is affected by solvent ordering. Relatively large polar angles become more favored as the hydrophobic surface area (i.e., Janus balance) is increased.

Self-Organization of Active Droplets into Vortex-like Structures

Natural or artificial active objects can demonstrate mirror asymmetry of collective motion when they are moving coherently in a vortex. The majority of known cases related to the emergence of collective dynamical chirality are referred to as active objects with individual structure chirality and/or dynamical chirality. Here, we demonstrate that dynamically and structurally achiral active droplets can self-organize into vortex-like structures. Octane droplets dispersed in the aqueous solution of an anionic surfactant are activated with ammonia addition. The motion of droplets is due to the Marangoni flow emerging at the interfaces of the droplets. We found out that different modes of vortex motion of droplets in the emulsion can arise depending on the size of the region that confines the motion of the droplets and their number density and velocity.

Interconnectivity between Surface Reactivity and Self-Assembly of Kemp Elimination Catalyzing Nanorods

Understanding the fundamental facts behind dynamicity of catalytic processes has been a longstanding quest across disciplines. Herein, we report self-assembly of catalytically active gold nanorods that can be regulated by tuning its reactivity towards a proton transfer reaction at different pH. Unlike substrate-induced templating and co-operativity, the enhanced aggregation rate is due to alteration of catalytic surface charge only during reactivity as negatively charged transition state of reactant (5-nitrobenzisoxazole) is formed on positively charged nanorod while undergoing a concerted E2-pathway. Herein, enhanced diffusivity during catalytic processes might also act as an additional contributing factor. Furthermore, we have also shown that nanosized hydrophobic cavities of clustered nanorods can also efficiently accelerate the rate of an aromatic nucleophilic substitution reaction, which also demonstrates a catalytic phenomenon that can lead to cascading of other reactions where substrates and products of the starting reactions are not directly involved.

Crystallization of Active Emulsion

Active matter contains self-propelled units able to convert stored or ambient free energy into motion. Such systems demonstrate amazing features related to the phenomenon of self-organization and phase transitions and can be used for the development of artificial materials and machines that operate away from equilibrium. Significant advances in the fabrication of active matter were achieved when studying low-density gas and small crystallites. However, the technique of preparation of active matter, where one can observe the formation of stable crystals, is extremely challenging. Here, we describe the novel method to obtain a stable 2D crystal in the active octane-in-water emulsion during the process of heterogeneous crystallization. Active motion is driven by the Marangoni flow emerging at the interface of the droplet. It is established that the crystal volume increases linearly in time in the process of crystallization. Moreover, the dependence of the crystal growth rate on the average velocity of droplets motion in the emulsion has a maximum. The kinetics of crystal growth is defined by a competition between the processes of attachment and detachment of droplets from the crystal surface. Crystallization proceeds via condensation of droplets from the gas phase through the formation of liquid as an intermediate phase, which covers the crystal surface with a thin layer. Inside the liquid layer the bond-orientational order of droplets decreases from the crystal surface toward the gas phase. We anticipate our study to be a starting point for the development of new materials and technologies on the basis of nonequilibrium droplet systems.

Stability of amphiphilic Janus dimers in shear flow: a molecular dynamics study

Amphiphilic Janus particles in a flow are thought to experience a torque due to the asymmetry in slip at their surfaces. This effect has the potential to destabilise self-assembled Janus structures in flows due to the forces and torques applied to individual Janus nanoparticles. In this work, we investigate the stability of amphiphilic Janus dimers and homogeneous hydrophobic dimers in shear flow using molecular dynamics, and study possible break-up mechanisms. In particular, we consider the influence of the activation enthalpy and entropy on the thermal break-up rate of these dimers. Janus dimers are less stable than hydrophobic dimers, and increasing the applied shear rate has a greater effect on break-up for Janus dimers. Two mechanisms leading to increased break-up in shear flow are studied, namely the rotational speed of the dimers and the orientation of individual spheres in the dimers, and we propose a descriptive equation for calculation of the break-up rate. Overall, the results indicate that the stability of dimers in shear flow depends on the slip length at the spheres' surfaces, and that the slip length difference on Janus dimers could contribute to destabilisation.