Confined Catalytic Janus Swimmers in a Crowded Channel: Geometry-Driven Rectification Transients and Directional Locking

Selecting active matter according to motility in an acoustofluidic setup: self-propelled particles and sperm cells

Active systems - including sperm cells, living organisms like bacteria, fish, birds, or active soft matter systems like synthetic "microswimmers" - are characterized by motility, i.e., the ability to propel using their own "engine". Motility is the key feature that distinguishes active systems from passive or externally driven systems. In a large ensemble, motility of individual species can vary in a wide range. Selecting active species according to their motility represents an exciting and challenging problem. We propose a new method for selecting active species based on their motility using an acoustofluidic setup where highly motile species escape from the acoustic trap. This is demonstrated in simulations and in experiments with self-propelled Janus particles and human sperm. The immediate application of this method is selecting highly motile sperm for medically assisted reproduction (MAR). Due to the tunable acoustic trap, the proposed method is more flexible than the existing passive microfluidic methods. The proposed selection method based on motility can also be applied to other active systems that require selecting highly motile species or removing immotile species.

Steering Micromotors via Reprogrammable Optoelectronic Paths

Steering micromotors is important for using them in practical applications and as model systems for active matter. This functionality often requires magnetic materials in the micromotor, taxis behavior of the micromotor, or the use of specifically designed physical boundaries. Here, we develop an optoelectronic strategy that steers micromotors with programmable light patterns. In this strategy, light illumination turns hydrogenated amorphous silicon conductive, generating local electric field maxima at the edge of the light pattern that attracts micromotors via positive dielectrophoresis. As an example, metallo-dielectric Janus microspheres that self-propelled under alternating current electric fields were steered by static light patterns along customized paths and through complex microstructures. Their long-term directionality was also rectified by ratchet-shaped light patterns. Furthermore, dynamic light patterns that varied in space and time enabled more advanced motion controls such as multiple motion modes, parallel control of multiple micromotors, and the collection and transport of motor swarms. This optoelectronic steering strategy is highly versatile and compatible with a variety of micromotors, and thus it possesses the potential for their programmable control in complex environments.

Microdynamics of active particles in defect-rich colloidal crystals

HYPOTHESIS

Because they are self-propulsive, active colloidal particles can interact with their environment in ways that differ from passive, Brownian particles. Here, we explore how interactions in different microstructural regions may contribute to colloidal crystal annealing.

EXPERIMENTS

We investigate active particles propagating in a quasi-2D colloidal crystal monolayer produced by alternating current electric fields (active-to-passive particle ratio ∼ 1:720). The active particle is a platinum Janus sphere propelled by asymmetric decomposition of hydrogen peroxide. Crystals are characterized for changes in void properties. The mean-squared-displacement of Janus particles are measured to determine how active microdynamics depend on the local microstructure, which is comprised of void regions, void-adjacent regions (defined as within three particle diameters of a void), and interstitial regions.

FINDINGS

At active particle energy E_A = 2.55 k_BT, the average void size increases as much as three times and the average void anisotropy increases about 40% relative to the passive case. The average microdynamical enhancement, <δ(t)>, of Janus particles in the crystal relative to an equivalent passive Janus particle is reduced compared to that of a free, active particle (<δ(t) > is 1.88 ± 0.04 and 2.66 ± 0.08, respectively). The concentration of active particles is enriched in void and void-adjacent regions. Active particles exhibit the greatest change in dynamics relative to the passive control in void-adjacent regions (<δ(t)> = 2.58 ± 0.06). The results support the conjecture that active particle microdynamical enhancement in crystal lattices is affected by local defect structure.

Controlled propulsion of micro/nanomotors: operational mechanisms, motion manipulation and potential biomedical applications

Inspired by natural mobile microorganisms, researchers have developed micro/nanomotors (MNMs) that can autonomously move by transducing different kinds of energies into kinetic energy. The rapid development of MNMs has created tremendous opportunities for biomedical fields including diagnostics, therapeutics, and theranostics. Although the great progress has been made in MNM research, at a fundamental level, the accepted propulsion mechanisms are still a controversial matter. In practical applications such as precision nanomedicine, the precise control of the motion, including the speed and directionality, of MNMs is also important, which makes advanced motion manipulation desirable. Very recently, diverse MNMs with different propulsion strategies, morphologies, sizes, porosities and chemical structures have been fabricated and applied for various uses. Herein, we thoroughly summarize the physical principles behind propulsion strategies, as well as the recent advances in motion manipulation methods and relevant biomedical applications of these MNMs. The current challenges in MNM research are also discussed. We hope this review can provide a bird's eye overview of the MNM research and inspire researchers to create novel and more powerful MNMs.

Effect of anisotropy on the formation of active particle films

Active colloids belong to a class of nonequilibrium systems where energy uptake, conversion, and dissipation occur at the level of individual colloidal particles, which can lead to particles' self-propelled motion and surprising collective behavior. Examples include coexistence of vapor- and liquid-like steady states for active particles with repulsive interactions only, phenomena known as motility-induced phase transitions. Similarly to motile unicellular organisms, active colloids tend to accumulate at confining surfaces forming dense adsorbed films. In this work, we study the structure and dynamics of aggregates of self-propelled particles near confining solid surfaces, focusing on the effects of the particle anisotropic interactions. We performed Langevin dynamics simulations of two complementary models for active particles: ellipsoidal particles interacting through the Gay-Berne potential and rodlike particles composed of several repulsive Lennard-Jones beads. We observe a nonmonotonic behavior of the structure of clusters formed along the confining surface as a function of the particle aspect ratio, with a film spreading when particles are near-spherical, compact clusters with hedgehog-like particle orientation for more elongated active particles, and a complex dynamical behavior for an intermediate aspect ratio. The stabilization time of cluster formation along the confining surface also displays a nonmonotonic dependence on the aspect ratio, with a local minimum at intermediate values. Additionally, we demonstrate that the hedgehog-like aggregates formed by Gay-Berne ellipsoids exhibit higher structural stability as compared to the ones formed by purely repulsive active rods, which are stable due to the particle activity only.

Confined Motion: Motility of Active Microparticles in Cell-Sized Lipid Vesicles

Active materials can transduce external energy into kinetic energy at the nano and micron length scales. This unique feature has sparked much research, which ranges from achieving fundamental understanding of their motility to the assessment of potential applications. Traditionally, motility is studied as a function of internal features such as particle topology, while external parameters such as energy source are assessed mainly in bulk. However, in real-life applications, confinement plays a crucial role in determining the type of motion active particles can adapt. This feature has been however surprisingly underexplored experimentally. Here, we showcase a tunable experimental platform to gain an insight into the dynamics of active particles in environments with restricted 3D topology. Particularly, we examined the autonomous motion of coacervate micromotors confined in giant unilamellar vesicles (GUVs) spanning 10–50 μm in diameter and varied parameters including fuel and micromotor concentration. We observed anomalous diffusion upon confinement, leading to decreased motility, which was more pronounced in smaller compartments. The results indicate that the theoretically predicted hydrodynamic effect dominates the motion mechanism within this platform. Our study provides a versatile approach to understand the behavior of active matter under controlled, compartmentalized conditions.

Activity-induced interactions and cooperation of artificial microswimmers in one-dimensional environments

Cooperative motion in biological microswimmers is crucial for their survival as it facilitates adhesion to surfaces, formation of hierarchical colonies, efficient motion, and enhanced access to nutrients. Here, we confine synthetic, catalytic microswimmers along one-dimensional paths and demonstrate that they too show a variety of cooperative behaviours. We find that their speed increases with the number of swimmers, and that the activity induces a preferred distance between swimmers. Using a minimal model, we ascribe this behavior to an effective activity-induced potential that stems from a competition between chemical and hydrodynamic coupling. These interactions further induce active self-assembly into trains where swimmers move at a well-separated, stable distance with respect to each other, as well as compact chains that can elongate, break-up, become immobilized and remobilized. We identify the crucial role that environment morphology and swimmer directionality play on these highly dynamic chain behaviors. These activity-induced interactions open the door toward exploiting cooperation for increasing the efficiency of microswimmer motion, with temporal and spatial control, thereby enabling them to perform intricate tasks inside complex environments.

Biological microswimmers such as bacteria show collective motion that is made possible by an intricate interplay of sensing and signaling. Ketzetzi et al. reproduce this phenomenon in a catalytic system undergoing, for instance, cooperative speed-ups and dynamic reconfiguration of microswimmer chains.

Interaction of Active Janus Colloids with Tracers

Understanding the motion of artificial active swimmers in complex surroundings, such as a dense bath of passive particulate matter, is essential for their successful utilization as cargo (drug) carriers and sensors or for medical imaging, under microscopic domains. In this study, we experimentally investigated the motion of active SiO₂-Pt Janus particles (JPs) in a two-dimensional bath of smaller silica tracers dispersed with varying areal densities. Our observations indicate that when an active JP undergoes a collision with an isolated tracer, their interaction can have a significant impact on the swimmer's motion. However, the overall impact of tracers on the active JPs' motion (translation and rotation) depends on the frequency of collisions and also on the nature of the collision, which is marked by the time-duration for which the particles maintain contact during the collisions. Further, in the high-density tracer bath, our experiments reveal that the motion of the active JP results in a novel organizational behavior of the tracers on the trailing Pt (depletion of tracers) and the leading SiO₂ (accumulation of tracers) side. In laboratory frame the emergence and the subsequent vanishing of the depletion zone are discussed in detail.

Rotation reversal of a ratchet gear powered by active particles

Rotation of a gear powered by active particles is numerically investigated in a circular chamber. Due to the nonequilibrium properties of active particles, net gear rotation is achieved in a bath composed of self-propelling particles. Our setup can convert the random motion of active particles into the directional rotation of the ratchet gear. The direction of rotation is determined by the asymmetry of the gear and the persistence length (the ratio of the self-propulsion speed to the rotation diffusion coefficient) of active particles. Remarkably, the direction of rotation for large persistence length is opposite to the direction of rotation for small persistence length. Therefore, for a given asymmetric gear, we can observe the rotation reversal when tuning the system parameters (e.g., the self-propulsion speed, the rotation diffusion coefficient, and the packing fraction of active particles). Our findings are relevant to the experimental pursuit of rectifying random motion to directional motion in active matter.

Light- and magnetically actuated FePt microswimmers

Externally controlled microswimmers offer prospects for transport in biological research and medical applications. This requires biocompatibility of the swimmers and the possibility to tailor their propulsion mechanisms to the respective low Reynolds number environment. Here, we incorporate low amounts of the biocompatible alloy of iron and platinum (FePt) in its [Formula: see text] phase in microstructures by a versatile one-step physical vapor deposition process. We show that the hard magnetic properties of [Formula: see text] FePt are beneficial for the propulsion of helical micropropellers with rotating magnetic fields. Finally, we find that the FePt coatings are catalytically active and also make for Janus microswimmers that can be light-actuated and magnetically guided.

Brownian Sieving Effect for Boosting the Performance of Microcapillary Hydrodynamic Chromatography. Proof of Concept

Microcapillary hydrodynamic chromatography (MHDC) is a well-established technique for the size-based separation of suspensions and colloids, where the characteristic size of the dispersed phase ranges from tens of nanometers to micrometers. It is based on hindrance effects which prevent relatively large particles from experiencing the low velocity region near the walls of a pressure-driven laminar flow through an empty microchannel. An improved device design is here proposed, where the relative extent of the low velocity region is made tunable by exploiting a two-channel annular geometry. The geometry is designed so that the core and the annular channel are characterized by different average flow velocities when subject to one and the same pressure drop. The channels communicate through openings of assigned cut-off length, say A. As they move downstream the channel, particles of size bigger than A are confined to the core region, whereas smaller particles can diffuse through the openings and spread throughout the entire cross section, therein attaining a spatially uniform distribution. By using a classical excluded-volume approach for modeling particle transport, we perform Lagrangian-stochastic simulations of particle dynamics and compare the separation performance of the two-channel and the standard (single-channel) MHDC. Results suggest that a quantitative (up to thirtyfold) performance enhancement can be obtained at operating conditions and values of the transport parameters commonly encountered in practical implementations of MHDC. The separation principle can readily be extended to a multistage geometry when the efficient fractionation of an arbitrary size distribution of the suspension is sought.

Ultrafast Directional Janus Pt-Mesoporous Silica Nanomotors for Smart Drug Delivery

Development of bioinspired nanomachines with an efficient propulsion and cargo-towing has attracted much attention in the last years due to their potential biosensing, diagnostics, and therapeutics applications. In this context, self-propelled synthetic nanomotors are promising carriers for intelligent and controlled release of therapeutic payloads. However, the implementation of this technology in real biomedical applications is still facing several challenges. Herein, we report the design, synthesis, and characterization of innovative multifunctional gated platinum-mesoporous silica nanomotors constituted of a propelling element (platinum nanodendrite face), a drug-loaded nanocontainer (mesoporous silica nanoparticle face), and a disulfide-containing oligo(ethylene glycol) chain (S-S-PEG) as a gating system. These Janus-type nanomotors present an ultrafast self-propelled motion due to the catalytic decomposition of low concentrations of hydrogen peroxide. Likewise, nanomotors exhibit a directional movement, which drives the engines toward biological targets, THP-1 cancer cells, as demonstrated using a microchip device that mimics penetration from capillary to postcapillary vessels. This fast and directional displacement facilitates the rapid cellular internalization and the on-demand specific release of a cytotoxic drug into the cytosol, due to the reduction of the disulfide bonds of the capping ensemble by intracellular glutathione levels. In the microchip device and in the absence of fuel, nanomotors are neither able to move directionally nor reach cancer cells and deliver their cargo, revealing that the fuel is required to get into inaccessible areas and to enhance nanoparticle internalization and drug release. Our proposed nanosystem shows many of the suitable characteristics for ideal biomedical destined nanomotors, such as rapid autonomous motion, versatility, and stimuli-responsive controlled drug release.

Control over self-assembled Janus clusters by the strength of magnetic field in [Formula: see text]

Colloidal Janus microparticles can be propelled by controlled chemical reactions on their surfaces. Such microswimmers have been used as model systems for the behavior on the microscale and as carriers for cargo to well-defined positions in hard-to-access areas. Here we demonstrate the propagation motion of clusters of magnetic Janus particles driven by the catalytic decomposition of [Formula: see text] on their metallic caps. The magnetic moments of their caps lead to certain spatial arrangements of Janus particles, which can be influenced by external magnetic fields. We investigate how the arrangement of the particles and caps determines the driven motion of the particle clusters. In addition, we show the influence of confining walls on the cluster motion, which will be encountered in any real-life biological system.

Anisotropic Exclusion Effect between Photocatalytic Ag/AgCl Janus Particles and Passive Beads in a Dense Colloidal Matrix

Synthetic nano- and micromotors interact with each other and their surroundings in a complex manner. Here, we report on the anisotropy of active-passive particle interaction in a soft matter system containing an immobile yet photochemical Ag/AgCl-based Janus particle embedded in a dense matrix of passive beads in pure water. The asymmetry in the chemical gradient around the Janus particle, triggered upon visible light illumination, distorts the isotropy of the surrounding electric potential and results in the repulsion of adjacent passive beads to a certain distance away from the Janus particle. This exclusion effect is found to be anisotropic with larger distances to passive beads in front of the Ag/AgCl cap of the Janus particle. We provide insight into this phenomenon by performing the angular analysis of the radii of exclusion and tracking their time evolution at the level of a single bead. Our study provides a novel fundamental insight into the collective behavior of a complex mixture of active and passive particles and is relevant for various application scenarios, e.g., particle transport at micro- and nanoscale and local chemical sensing.

Enhanced motility in a binary mixture of active nano/microswimmers

It is often desirable to enhance the motility of active nano- or microscale swimmers such as, e.g., self-propelled Janus particles as agents of chemical reactions or weak sperm cells for better chances of successful fertilization. Here we tackle this problem based on the idea that motility can be transferred from a more active guest species to a less active host species. We performed numerical simulations of motility transfer in two typical cases, namely for interacting particles with a weak inertia effect, by analyzing their velocity distributions, and for interacting overdamped particles, by studying their effusion rate. In both cases, we detected motility transfer with a motility enhancement of the host species of up to a factor of four. This technique of motility enhancement can find applications in chemistry, biology and medicine.

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.

Transport of active particles induced by wedge-shaped barriers in straight channels with hard and soft walls

The transport of active particles in straight channels is numerically investigated. The periodic wedge-shaped barriers can produce the asymmetry of the system and induce the directed transport of the active particles. The direction of the transport is determined by the apex angle of the wedge-shaped barriers. By confining the particles in channels with hard and soft walls, the transport exhibits similar behaviors. The average velocity is a peaked function of the translational diffusion, while it decreases monotonously with the increase of the rotational diffusion. Moreover, the simulation results show that the transport is sensitive to the parameters of the confined structures, such as the pore width, the intensity of potential, and the channel period.

Visible Light Actuated Efficient Exclusion Between Plasmonic Ag/AgCl Micromotors and Passive Beads

Insight is provided into the collective behavior of visible-light photochemically driven plasmonic Ag/AgCl Janus particles surrounded by passive polystyrene (PS) beads. The active diffusion of single Janus particles and their clusters (small: consisting of two or three Janus particles and large: consisting of more than ten Janus particles), and their interaction with passive PS beads, are analyzed experimentally and in simulations. The diffusivity of active Janus particles, and thus the exclusive effect to passive PS beads, can be regulated by the number of single Janus particles in the cluster. On the simulation side, the Langevin equations of motion for self-propelled Janus particles and diffusing passive PS beads are numerically solved using Molecular-Dynamics simulations. The complex interactions of both subsystems, including elastic core-to-core interactions, short-range attraction, and effective repulsion due to light-induced chemical reactions are considered. This complex mixed system not only provides insight to the interactive effect between active visible light-driven self-propelled micromotors and passive beads, but also offers promise for implications in light-controlled propulsion transport and chemical sensing.

High-Motility Visible Light-Driven Ag/AgCl Janus Micromotors

Visible light-driven nano/micromotors are promising candidates for biomedical and environmental applications. This study demonstrates blue light-driven Ag/AgCl-based spherical Janus micromotors, which couple plasmonic light absorption with the photochemical decomposition of AgCl. These micromotors reveal high motility in pure water, i.e., mean squared displacements (MSD) reaching 800 µm² within 8 s, which is 100× higher compared to previous visible light-driven Janus micromotors and 7× higher than reported ultraviolet (UV) light-driven AgCl micromotors. In addition to providing design rules to realize efficient Janus micromotors, the complex dynamics revealed by individual and assemblies of Janus motors is investigated experimentally and in simulations. The effect of suppressed rotational diffusion is focused on, compared to UV light-driven AgCl micromotors, as a reason for this remarkable increase of the MSD. Moreover, this study demonstrates the potential of using visible light-driven plasmonic Ag/AgCl-based Janus micromotors in human saliva, phosphate-buffered saline solution, the most common isotonic buffer that mimics the environment of human body fluids, and Rhodamine B solution, which is a typical polluted dye for demonstrations of photocatalytic environmental remediation. This new knowledge is useful for designing visible light driven nano/micromotors based on the surface plasmon resonance effect and their applications in assays relevant for biomedical and ecological sciences.

Light-Activated, Multi-Semiconductor Hybrid Microswimmers

Using a dynamic fabrication process, hybrid, photoactivated microswimmers made from two different semiconductors, titanium dioxide (TiO₂ ) and cuprous oxide (Cu₂ O) are developed, where each material occupies a distinct portion of the multiconstituent particles. Structured light-activated microswimmers made from only TiO₂ or Cu₂ O are observed to be driven in hydrogen peroxide and water most vigorously under UV or blue light, respectively, whereas hybrid structures made from both of these materials exhibit wavelength-dependent modes of motion due to the disparate responses of each photocatalyst. It is also found that the hybrid particles are activated in water alone, a behavior which is not observed in those made from a single semiconductor, and thus, the system may open up a new class of fuel-free photoactive colloids that take advantage of semiconductor heterojunctions. The TiO₂ /Cu₂ O hybrid microswimmer presented here is but an example of a broader method for inducing different modes of motion in a single light-activated particle, which is not limited to the specific geometries and materials presented in this study.

Colloidal transport through trap arrays controlled by active microswimmers

We investigate the dynamics of a binary mixture consisting of active and passive colloidal particles diffusing in a 2D array of truncated harmonic wells, or traps. We explore the possibility of using a small fraction of active particles to manipulate a much larger fraction of passive particles, for instance, to confine them in or extract them from the traps. The results of our study have potential application in biology and medical sciences, for example, to remove dead cells or undesired contaminants from biological systems by means of self-propelled nano-robots.

Artificial living crystals in confined environment

Similar to the spontaneous formation of colonies of bacteria, flocks of birds, or schools of fish, "living crystals" can be formed by artificial self-propelled particles such as Janus colloids. Unlike usual solids, these "crystals" are far from thermodynamic equilibrium. They fluctuate in time forming a crystalline structure, breaking apart and re-forming again. We propose a method to stabilize living crystals by applying a weak confinement potential that does not suppress the ability of the particles to perform self-propelled motion, but it stabilizes the structure and shape of the dynamical clusters. This gives rise to such configurations of living crystals as "living shells" formed by Janus colloids. Moreover, the shape of the stable living clusters can be controlled by tuning the potential strength. Our proposal can be verified experimentally with either artificial microswimmers such as Janus colloids, or with living active matter.

Tailored magnetic carbon allotrope catalytic micromotors for 'on-chip' operations

Carbon allotrope micromotors are proposed as active components in lab-on-a-chip systems. Highly rough carbon black tubular engines are used for fluorescence detection operations. The potential of ultrafast lectin carbon nanonotube micromotors with an inner anti-biofouling layer for selective transport of sugar modified particles (as cell mimics) in human plasma is illustrated.

Communication: Cargo towing by artificial swimmers

An active swimmer can tow a passive cargo by binding it to form a self-propelling dimer. The orientation of the cargo relative to the axis of the active dimer's head is determined by the hydrodynamic interactions associated with the propulsion mechanism of the latter. We show how the tower-cargo angular configuration greatly influences the dimer's diffusivity and, therefore, the efficiency of the active swimmer as a micro-towing motor.