Rolling and ordering of micro rods in shear flow induced by rod wall interactions.

Dynamics of micro particles close to interfaces is a relevant topic in Soft Matter. Translational and rotational dynamics of particles possessing different shapes govern a broad range of interfacial phenomena from biofilm formation, drug delivery and particle active rolling motion. These dynamics usually occur in the presence of external fields such as shear flows, electric fields and gravity.

Autonomous engulfment of active colloids by giant lipid vesicles.

Our ability to design artificial micro/nanomachines able to perform sophisticated tasks crucially depends on our understanding of their interaction with biosystems and their compatibility with the biological environment. Here, we design Janus colloids fuelled only by glucose and light, which can autonomously interact with cell-like compartments and trigger endocytosis. We evidence the crucial role played by the far-field hydrodynamic interaction arising from the puller/pusher swimming mode and adhesion.

An Account on BiVO as Photocatalytic Active Matter.

Photocatalytic materials are gaining popularity and research investment for developing light-driven micromotors. While most of the early work used highly stable TiO as a material to construct micromotors, mostly in combination with noble metals, other semiconductors offer a wider range of properties, including independence from high-energy UV light. This review focuses on our work with BiVO which has shown promise due to its small band gap and resulting ability to absorb blue light.

Combining photocatalytic collection and degradation of microplastics using self-asymmetric Pac-Man TiO.

Microplastics are a significant environmental threat and the lack of efficient removal techniques further amplifies this crisis. Photocatalytic semiconducting nanoparticles have the potential to degrade micropollutants, among them microplastics. The hydrodynamic effects leading to the propulsion of micromotors can lead to the accumulation of microplastics in close vicinity of the micromotor.

Pair Interaction between Two Catalytically Active Colloids.

Due to the intrinsically complex non-equilibrium behavior of the constituents of active matter systems, a comprehensive understanding of their collective properties is a challenge that requires systematic bottom-up characterization of the individual components and their interactions. For self-propelled particles, intrinsic complexity stems from the fact that the polar nature of the colloids necessitates that the interactions depend on positions and orientations of the particles, leading to a 2d - 1 dimensional configuration space for each particle, in d dimensions. Moreover, the interactions between such non-equilibrium colloids are generically non-reciprocal, which makes the characterization even more complex.

Active BiVO Swimmers Propelled by Depletion Gradients Caused by Photodeposition.

Artificial active matter often self-propels by creating gradients of one or more species or quantities. For chemical swimmers, most frequently either O or H that are created in certain catalytic reactions are causing the interfacial flows which drive the self-propulsion. While the palette of reactions is extending constantly, especially toward more bio-compatible fuels, the depletion of species is often overlooked.

Determination of the swimming mechanism of Au@TiO active matter and implications on active-passive interactions.

Non-equilibrium dynamic assembly attracts considerable attention due to the possibility of forming diverse structures that can potentially lead to functional materials. Despite significant progress in understanding and modelling, the complexity of the system implies that different phases of the assembly formation are governed by different interactions. It is clear that both, hydrodynamic and chemical interactions stem from the activity of the particle, but correlation to specific chemical species remains not yet understood.

Colloidal Active Matter Mimics the Behavior of Biological Microorganisms-An Overview.

This article provides a review of the recent development of biomimicking behaviors in active colloids. While the behavior of biological microswimmers is undoubtedly influenced by physics, it is frequently guided and manipulated by active sensing processes. Understanding the respective influences of the surrounding environment can help to engineering the desired response also in artificial swimmers.

Beyond Janus Geometry: Characterization of Flow Fields around Nonspherical Photocatalytic Microswimmers.

Catalytic microswimmers that move by a phoretic mechanism in response to a self-induced chemical gradient are often obtained by the design of spherical janus microparticles, which suffer from multi-step fabrication and low yields. Approaches that circumvent laborious multi-step fabrication include the exploitation of the possibility of nonuniform catalytic activity along the surface of irregular particle shapes, local excitation or intrinsic asymmetry. Unfortunately, the effects on the generation of motion remain poorly understood.

Hydrodynamically Controlled Self-Organization in Mixtures of Active and Passive Colloids.

Active particles are known to exhibit collective behavior and induce structure in a variety of soft-matter systems. However, many naturally occurring complex fluids are mixtures of active and passive components. The authors examine how activity induces organization in such multi-component systems.

Microswimming by oxidation of dibenzylamine.

Chemiophoretic nano- and micromotors require a constant flow of product molecules to maintain a gradient that enables their propulsion. Apart from a smaller number of redox reactions that have been used, catalytic reactions are the main source of energy with the obvious benefit of making on-board fuel storage obsolete. However, the decomposition of HO seems to strongly dominate the literature and although motion in HO through water splitting is becoming more popular, so far only a few different reactions have been used for propulsion of photocatalytic microswimmers.

Upstream Rheotaxis of Catalytic Janus Spheres.

Fluid flow is ubiquitous in many environments that form habitats for microorganisms. Therefore, it is not surprising that both biological and artificial microswimmers show responses to flows that are determined by the interplay of chemical and physical factors. In particular, to deepen the understanding of how different systems respond to flows, it is crucial to comprehend the influence played by swimming pattern.

A Platform for Stop-Flow Gradient Generation to Investigate Chemotaxis.

The ability of artificial microswimmers to respond to external stimuli and the mechanistical details of their origins belong to the most disputed challenges in interdisciplinary science. Therein, the creation of chemical gradients is technically challenging, because they quickly level out due to diffusion. Inspired by pivotal stopped flow experiments in chemical kinetics, we show that microfluidics gradient generation combined with a pressure feedback loop for precisely controlling the stop of the flows, can enable us to study mechanistical details of chemotaxis of artificial Janus micromotors, based on a catalytic reaction.

Semiconductor-Based Microswimmers: Attention to Detail Matters.

Colloidal active matter is known for its sensitivity to external conditions; for example, the swimming speeds depend strongly on substrates, fuel concentration, and in the case of light-driven colloids, the illumination. While these points are regularly considered, the nanoscopic material properties of the motor bodies are often barely mentioned, but they are highly influential in the case of photocatalysts. In order to demonstrate the influence of subtle differences in chemical composition and interfacing between the different material compounds, we designed a system based on colloidal titania spheres asymmetrized by different nanoscale cobalt oxide species.

Highly Efficient Active Colloids Driven by Galvanic Exchange Reactions.

Micromotors are propelled by a variety of chemical reactions, with most of them being of catalytic nature. There are, however, systems based on redox reactions, which show clear benefits for efficiency. Here, we broaden the spectrum of suitable reactions to galvanic exchange processes, or an electrochemical replacement of a solid metal layer with dissolved ionic species of a more noble metal.

Microfluidics for Microswimmers: Engineering Novel Swimmers and Constructing Swimming Lanes on the Microscale, a Tutorial Review.

This paper provides an updated review of recent advances in microfluidics applied to artificial and biohybrid microswimmers. Sharing the common regime of low Reynolds number, the two fields have been brought together to take advantage of the fluid characteristics at the microscale, benefitting microswimmer research multifold. First, microfluidics offer simple and relatively low-cost devices for high-fidelity production of microswimmers made of organic and inorganic materials in a variety of shapes and sizes.

Impact of Segmented Magnetization on the Flagellar Propulsion of Sperm-Templated Microrobots.

Technical design features for improving the way a passive elastic filament produces propulsive thrust can be understood by analyzing the deformation of sperm-templated microrobots with segmented magnetization. Magnetic nanoparticles are electrostatically self-assembled on bovine sperm cells with nonuniform surface charge, producing different categories of sperm-templated microrobots. Depending on the amount and location of the nanoparticles on each cellular segment, magnetoelastic and viscous forces determine the wave pattern of each category during flagellar motion.

Active spheres induce Marangoni flows that drive collective dynamics.

For monolayers of chemically active particles at a fluid interface, collective dynamics is predicted to arise owing to activity-induced Marangoni flow even if the particles are not self-propelled. Here, we test this prediction by employing a monolayer of spherically symmetric active [Formula: see text] particles located at an oil-water interface with or without addition of a nonionic surfactant. Due to the spherical symmetry, an individual particle does not self-propel.

Study of Active Janus Particles in the Presence of an Engineered Oil-Water Interface.

We present a systematic study of motion of Pt@SiO Janus particles at a liquid-liquid interface. A special microfluidic trap is used for creating such an interface. The increased surface energy of the large surface results in partial wetting of the substrate, leaving patches of oil on the glass surface.

Apparent phototaxis enabled by Brownian motion.

Biomimetic behaviour in artificially created active matter that allows deterministic and controlled motility has become of growing interest in recent years. It is well known that phototrophic bacteria optimize their position with respect to light by phototaxis. Here, we describe how our fully artificial, magnetic and photocatalytic microswimmers undergo a specific type of behaviour that strongly resembles phototaxis: when crossing an illuminated stripe the particles repeatedly turn back towards the light once they reach the dark region, without any obvious reason for the particles to do so.

IRONSperm: Sperm-templated soft magnetic microrobots.

We develop biohybrid magnetic microrobots by electrostatic self-assembly of nonmotile sperm cells and magnetic nanoparticles. Incorporating a biological entity into microrobots entails many functional advantages beyond shape templating, such as the facile uptake of chemotherapeutic agents to achieve targeted drug delivery. We present a single-step electrostatic self-assembly technique to fabricate IRONSperms, soft magnetic microswimmers that emulate the motion of motile sperm cells.

Active Assembly of Spheroidal Photocatalytic BiVO Microswimmers.

We create single-component photocatalytic bismuth vanadate (BiVO) microswimmers with a spheroidal shape that move individually upon irradiation without any asymmetrization step. These particles form active assemblies which we investigate combining an experimental approach with numerical simulations and analytical calculations. We systematically explore the speed and assembly of the swimmers into clusters of up to four particles and find excellent agreement between experiment and theory, which helps us to understand motion patterns and speed trends.

Carbonate Micromotors for Treatment of Construction Effluents.

Concrete in construction has recently gained media coverage for its negative CO footprint, but this is not the only problem associated with its use. Due to its chemical composition, freshly poured concrete changes the pH of water coming in contact with the surface to very alkaline values, requiring neutralization treatment before disposal. Conventional methods include the use of mineral acid or CO pumps, causing high costs to building companies.

Sperm-Particle Interactions and Their Prospects for Charge Mapping.

In this article, a procedure to investigate sperm charge distribution by electrostatic sperm-particle interactions is demonstrated. Differently chargedparticles are fabricated and their attachment distribution on the bovine sperm membrane is investigated. The sperm-particle attachment sites are observed using bright field and cryo-scanning electron microscopy combined with energy-dispersive X-ray analysis.

Nano-and Micromotors Designed for Cancer Therapy.

Research on nano- and micromotors has evolved into a frequently cited research area with innovative technology envisioned for one of current humanities' most deadly problems: cancer. The development of cancer targeting drug delivery strategies involving nano-and micromotors has been a vibrant field of study over the past few years. This review aims at categorizing recent significant results, classifying them according to the employed propulsion mechanisms starting from chemically driven micromotors, to field driven and biohybrid approaches.