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. The tendency of organisms to navigate up or down the flow is termed rheotaxis. Early theoretical studies predicted a positive rheotactic response for puller-type spherical Janus micromotors. However, recent experimental studies have focused on pusher-type Janus particles, finding that they exhibit cross-stream migration in externally applied flows. To study the response to the flow of swimmers with a qualitatively different flow pattern, we introduce Cu@SiO2 micromotors that swim toward their catalytic cap. On the basis of experimental observations, and supported by flow field calculations using a model for self-electrophoresis, we hypothesize that they behave effectively as a puller-type system. We investigate the effect of externally imposed flow on these spherically symmetrical Cu@SiO2 active Janus colloids, and we indeed observe a steady upstream directional response. Through a simple squirmer model for a puller, we recover the major experimental observations. Additionally, the model predicts a "jumping" behavior for puller-type micromotors at high flow speeds. Performing additional experiments at high flow speeds, we capture this phenomenon, in which the particles "roll" with their swimming axes aligned to the shear plane, in addition to being dragged downstream by the fluid flow.

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 H2O2 seems to strongly dominate the literature and although motion in H2O through water splitting is becoming more popular, so far only a few different reactions have been used for propulsion of photocatalytic microswimmers. Here, we investigate the possibility of extending the range of possible fuelling reactions to organic reactions with high significance in organic synthesis - the oxidation of amines to imines. Herein, motion of the microswimmers is analysed at different amine concentrations and light intensities. The findings thereof are correlated with the reaction products identified and quantified by gas chromatography (GC).

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. We find that these copper Janus particles display a chemotactic motion along the concentration gradient in both, positive and negative direction and we demonstrate the mechanical reaction of the particles to unbalanced drag forces, explaining this behaviour.

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. We study the details of motility and the influence of different reaction parameters to conclude that these galvanophoretic processes circumvent several steps that lose efficiency in catalytic micromotors. Furthermore, we investigate the chemical process, the charge, and flow conditions that lead to this highly efficient new type of active motility. Toward a better understanding of the underlying processes, we propose an electrokinetic model that we numerically solve via finite elements.

Light-powered active colloids from monodisperse and highly tunable microspheres with a thin TiO2 shell.

The emerging field of active matter, and its subset active colloid, is in great need of good model systems consisting of moving entities that are uniform and highly tunable. In this article, we address this challenge by introducing core-shell SiO2-TiO2 microspheres, prepared by chemically coating a thin layer of TiO2 on an inert core, that are highly monodisperse in size (polydispersity 4.1%) and regular in shape (circularity 0.93). Compared with similar samples prepared by the classic sol-gel method, Janus TiO2-Pt active colloids prepared with core-shell TiO2 spheres move faster and boast a much clearer Janus interface. Moreover, a unique feature of these core-shell TiO2 microspheres is their great tunability in the colloid size, shell thickness, and even the type of the core particle. These advantages are highlighted in two examples, one demonstrating a TiO2-Pt active colloid with a magnetic core that enables magnetic manipulation, and the other demonstrating the collective expansion and contraction of a uniform cluster of core-shell TiO2 colloids under UV light illumination. We believe that TiO2 microspheres produced by this core-shell technique compare favorably with many other types of active colloids being employed as model systems, and thus open up many research possibilities.

Synergistic Speed Enhancement of an Electric-Photochemical Hybrid Micromotor by Tilt Rectification.

A hybrid micromotor is an active colloid powered by more than one power source, often exhibiting expanded functionality and controllability than those of a singular energy source. However, these power sources are often applied orthogonally, leading to stacked propulsion that is just a sum of two independent mechanisms. Here, we report that TiO2-Pt Janus micromotors, when subject to both UV light and AC electric fields, move up to 90% faster than simply adding up the speed powered by either source. This unexpected synergy between light and electric fields, we propose, arises from the fact that an electrokinetically powered TiO2-Pt micromotor moves near a substrate with a tilted Janus interface that, upon the application of an electric field, becomes rectified to be vertical to the substrate. Control experiments with magnetic fields and three types of micromotors unambiguously and quantitatively show that the tilting angle of a micromotor correlates positively with its instantaneous speed, reaching maximum at a vertical Janus interface. Such "tilting-induced retardation" could affect a wide variety of chemically powered micromotors, and our findings are therefore helpful in understanding the dynamics of micromachines in confinement.

Bimetallic coatings synergistically enhance the speeds of photocatalytic TiO2 micromotors.

The design of powerful, more biocompatible microrobots calls for faster catalytic reactions. Here we demonstrate a two-fold increase in the speed of photocatalytic TiO2-metal Janus micromotors via a Au/Ag bi-layered coating. Electrochemical measurements show that such a bimetallic coating is a better photocatalyst than either metal alone. Similarly, an additional sputtered Ag layer could also significantly increase the speed of Pt-PS or TiO2-Pt micromotors, suggesting that applying bimetallic coatings is a generalizable strategy in the design of faster catalytic micromotors.

Confined 1D Propulsion of Metallodielectric Janus Micromotors on Microelectrodes under Alternating Current Electric Fields.

There is mounting interest in synthetic microswimmers ("micromotors") as microrobots as well as a model system for the study of active matters, and spatial navigation is critical for their success. Current navigational technologies mostly rely on magnetic steering or guiding with physical boundaries, yet limitations with these strategies are plenty. Inspired by an earlier work with magnetic domains on a garnet film as predefined tracks, we present an interdigitated microelectrodes (IDE) system where, upon the application of AC electric fields, metallodielectric (e.g., SiO2-Ti) Janus particles are hydrodynamically confined and electrokinetically propelled in one dimension along the electrode center lines with tunable speeds. In addition, comoving micromotors moved in single files, while those moving in opposite directions primarily reoriented and moved past each other. At high particle densities, turbulence-like aggregates formed as many-body interactions became complicated. Furthermore, a micromotor made U-turns when approaching an electrode closure, while it gradually slowed down at the electrode opening and was collected in large piles. Labyrinth patterns made of serpentine chains of Janus particles emerged by modifying the electrode configuration. Most of these observations can be qualitatively understood by a combination of electroosmotic flows pointing inward to the electrodes, and asymmetric electrical polarization of the Janus particles under an AC electric field. Emerging from these observations is a strategy that not only powers and confines micromotors on prefabricated tracks in a contactless, on-demand manner, but is also capable of concentrating active particles at predefined locations. These features could prove useful for designing tunable tracks that steer synthetic microrobots, as well as to enable the study of single file diffusion, active turbulence, and other collective behaviors of active matters.

A Review of Micromotors in Confinements: Pores, Channels, Grooves, Steps, Interfaces, Chains, and Swimming in the Bulk.

One of the recent frontiers of nanotechnology research involves machines that operate at nano- and microscales, also known as nano/micromotors. Their potential applications in biomedicine, environmental sciences and engineering, military and defense industries, self-assembly, and many other areas have fueled an intense interest in this topic over the last 15 years. Despite deepened understanding of their propulsion mechanisms, we are still in the early days of exploring the dynamics of micromotors in complex and more realistic environments. Confinements, as a typical example of complex environments, are extremely relevant to the applications of micromotors, which are expected to travel in mucus gels, blood vessels, reproductive and digestive tracts, microfluidic chips, and capillary tubes. In this review, we summarize and critically examine recent studies (mostly experimental ones) of micromotor dynamics in confinements in 3D (spheres and porous network, channels, grooves, steps, and obstacles), 2D (liquid-liquid, liquid-solid, and liquid-air interfaces), and 1D (chains). In addition, studies of micromotors moving in the bulk solution and the usefulness of acoustic levitation is discussed. At the end of this article, we summarize how confinements can affect micromotors and offer our insights on future research directions. This review article is relevant to readers who are interested in the interactions of materials with interfaces and structures at the microscale and helpful for the design of smart and multifunctional materials for various applications.