Self-propelled Motion Research Articles

Multifunctional superhydrophobic coatings for controllable self-propelled motion

Recently, multi-responsive actuators have attracted great attention, and it remains challenging to develop flexible and durable actuators with controllable motions on water surface. Here, a facile spray-coating method is proposed to fabricate superhydrophobic and multi-responsive acidified carbon nanotubes /Fe3O4/octadecylamine/polydimethylsiloxane (CFOP) coatings. The CFOP coating imparts the paper composite (CFOP@paper) with superhydrophobicity, self-cleaning performance, excellent photothermal conversion and magnetic properties. In addition, the strong interfacial adhesion between the coating and the paper ensures the surface stability and durability during cyclic bending and abrasion tests. The CFOP@paper can achieve controllable light-driven motion based on the Marangoni effect. The CFOP coating is also used for preparation of the superhydrophobic/superoleophilic sponge composite, and the controllable and fast motions of the sponge composite are realized by a contactless magnetic field, which can be used for high efficiency adsorption of oil on water surface. The multi-responsive coating exhibits promising applications in the field of flexible electronics, smart actuators, environmental protection, and so on.

Ficin-Cyclodextrin-Based Docking Nanoarchitectonics of Self-Propelled Nanomotors for Bacterial Biofilm Eradication.

Development of bioinspired nanomotors showing effective propulsion and cargo delivery capabilities has attracted much attention in the last few years due to their potential use in biomedical applications. However, implementation of this technology in realistic settings is still a barely explored field. Herein, we report the design and application of a multifunctional gated Janus platinum-mesoporous silica nanomotor constituted of a propelling element (platinum nanodendrites) and a drug-loaded nanocontainer (mesoporous silica nanoparticle) capped with ficin enzyme modified with β-cyclodextrins (β-CD). The engineered nanomotor is designed to effectively disrupt bacterial biofilms via H2O2-induced self-propelled motion, ficin hydrolysis of the extracellular polymeric matrix (EPS) of the biofilm, and controlled pH-triggered cargo (vancomycin) delivery. The effective synergic antimicrobial activity of the nanomotor is demonstrated in the elimination of Staphylococcus aureus biofilms. The nanomotor achieves 82% of EPS biomass disruption and a 96% reduction in cell viability, which contrasts with a remarkably lower reduction in biofilm elimination when the components of the nanomotors are used separately at the same concentrations. Such a large reduction in biofilm biomass in S. aureus has never been achieved previously by any conventional therapy. The strategy proposed suggests that engineered nanomotors have great potential for the elimination of biofilms.

Enhanced performance of a self-propelled flexible plate by a uniform shear flow and mechanism insight

The hydrodynamics of a two-dimensional self-propelled flexible plate in a uniform shear flow is explored using a penalty-immersed boundary method. The leading edge of the plate is enforced into a prescribed harmonic oscillation in the vertical direction but free to move in the horizontal direction. It is found that as the shear rate increases, the input power, the propulsive velocity, and the efficiency increase. This finding means that the plate enables to get substantial hydrodynamic benefits from the shear flow. Using the force decomposition method based on the weighted integral of the second invariant of the velocity gradient tensor, the hydrodynamic force exerted on the plate is decomposed into a body-acceleration force, a vortex-induced force, and forces due to viscous effects. The results show that the body-acceleration force is the main driving force of the self-propelled motion, and that it is almost invariant with the shear rate. The vortex-induced force offers a significant contribution to the drag, and it decreases with the shear rate. The viscous friction force provides a pure drag, and it increases with the propulsion velocity. Further investigation on the vortex evolution and the vortex-induced force shows that the incoming shear flow destroys the trailing-edge vortex that sheds during the downward half period and, therefore, reduces the vortex-induced drag, which is the reason for the enhanced propulsive performance in the shear flow. The result obtained in this study provides new insight into the self-propulsion mechanism in complex incoming flows.

Surface tension gradient driven autonomous fatty acid-tetrahydrofuran liquid moving drops: Spreading to pinning

In this work, we have studied the drop dynamics of binary tetrahydrofuran (THF) solutions containing fatty acids (stearic acid and myristic acid) or surfactants (N-Cetyl-N, N, N trimethyl ammonium bromide, and sodium dodecyl sulfate). Highly volatile non-aqueous pure THF drop shows dynamic wetting behaviour i.e., droplet expansion and contraction behaviour·THF spreads continuously on fabricated total wetting surface without pinning, however addition of fatty acids causes formation of drops (instead of thin films). Such drops show autonomous motility which can be tuned by maintaining the concentration of fatty acids. The addition of fatty acids provides evaporation driven surface tension gradient (Marangoni stress) which initialize the autonomous drop motion and reduces the innate contact line pinning. It is found that addition of fatty acids showed drop dynamics which was concentration dependent. At low concentrations drop moves, while contact line pinning and subsequent evaporation was noticed at higher concentrations. The study of drop area with respect to time conveyed about the slowdown of evaporation timing of THF. This work proclaims the novel combination of highly evaporating THF with solutes (fatty acids/surfactants) showing its potential applications in self-propelled motion of non-aqueous drops.

Dual-Modal Lateral Flow Test Strip Assisted by Near-Infrared-Powered Nanomotors for Direct Quantitative Detection of Circulating MicroRNA Biomarkers from Serum.

Quantification of microRNA (miRNA) has attracted intense interest owing to its importance as a biomarker for the early diagnosis of multiple diseases. However, the inefficient capture of microRNAs from complex biological samples due to the passive diffusion of detection probes essentially restricts their accurate quantification. Herein, we report near-infrared (NIR)-powered Janus nanomotors composed of Au nanorods and periodic mesoporous organo-silica microspheres (AuNR/PMO JNMs) as "swimming probes" to assist a lateral flow test strip (LFTS) for direct, amplification-free, and quantitative miRNA-21 detection in serum and cell medium. The AuNR/PMO JNMs were conjugated with designed hDNA as a recognition probe for miRNA-21. Under NIR irradiation, the exposed AuNRs can generate asymmetric thermal gradients around the JNMs to achieve vigorous self-propelled thermophoretic motion. The active movement significantly accelerated the recognition of miRNA-21 targets, which greatly improved the capture efficiency from 59.39 to 86.12% in the reaction buffer. The enhanced miRNA-21 capture enabled direct quantitative miRNA-21 detection on the LFTS assay with both visual and thermal signals. Under the assistance of AuNR/PMO JNMs, a limit-of-detection of 18 fmol/L for miRNA-21 was achieved, which was 12.22-fold compared to that of LFTS assay with static probes. The constructed LFTS assay was further successfully deployed to directly sense the miRNA-21 in spiked serum samples and MDA-MB-231 medium. Overall, the AuNR/PMO JNM-assisted LFTS system unveils a concrete point-of-care testing strategy for precise miRNA detection in real biological samples, which holds great potential for early diagnosis and treatment of miRNA-related diseases.

Going in circles: Slender body analysis of a self-propelling bent rod

We investigate the self-propelling motion of a bent rod. Our analysis reveals that the planar motion of the rod is always circular, and the radius of the circle and the speed of the particle can be tuned by changing the relative length of the two arms and the angle between them.

Self-propulsion of a calcium alginate surfer.

A droplet of sodium alginate dripped into calcium chloride solution results in plate or boat shaped hydrogels. Both exhibit several minute-long self-propelled motion on the liquid surface without any extra fuel added, offering a new method to making active materials. By changing the initial concentrations, we are able to tune the transient dynamic activities from translational to rotational or stop-and-run motion. Dynamics are governed by osmotic pressure induced Marangoni effect, depending on the density difference and initial concentrations.

Stability of schooling patterns of a fish pair swimming against a flow

Fish often swim in crystallized group formations (schooling) and orient themselves against the incoming flow (rheotaxis). At the intersection of these two phenomena, we investigate the emergence of unique schooling patterns through passive hydrodynamic mechanisms in a fish pair, the simplest subsystem of a school. First, we develop a fluid dynamics-based mathematical model for the positions and orientations of two fish swimming against a flow in an infinite channel, modelling the effect of the self-propelling motion of each fish as a point-dipole. The resulting system of equations is studied to gain an understanding of the properties of the dynamical system, its equilibria and their stability. The system is found to have five types of equilibria, out of which only upstream swimming in in-line and staggered formations can be stable. A stable near-wall configuration is observed only in limiting cases. It is shown that the stability of these equilibria depends on the flow curvature and streamwise interfish distance, below critical values of which, the system may not have a stable equilibrium. The study reveals that simply through passive fluid dynamics, in the absence of any other feedback/sensing, we can justify rheotaxis and the existence of stable in-line and staggered schooling configurations.

Energy Cost for Flocking of Active Spins: The Cusped Dissipation Maximum at the Flocking Transition.

We study the energy cost of flocking in the active Ising model (AIM) and show that, besides the energy cost for self-propelled motion, an additional energy dissipation is required to power the alignment of spins. We find that this additional alignment dissipation reaches its maximum at the flocking transition point in the form of a cusp with a discontinuous first derivative with respect to the control parameter. To understand this singular behavior, we analytically solve the two- and three-site AIM models and obtain the exact dependence of the alignment dissipation on the flocking order parameter and control parameter, which explains the cusped dissipation maximum at the flocking transition. Our results reveal a trade-off between the energy cost of the system and its performance measured by the flocking speed and sensitivity to external perturbations. This trade-off relationship provides a new perspective for understanding the dynamics of natural flocks and designing optimal artificial flocking systems.

Self-locomotive composites based on asymmetric micromotors and covalently attached nanosorbents for selective uranium recovery

Nanosized sorbents are widely used for uranium (VI) recovery due to their high specific surface area, but the research on the integration of micromotors with nanosorbents to establish self-locomotive composites is still worth exploring. Thus, self-locomotive composites PS@PDA@Ag/SiO2-AO (PPA/SA) based on asymmetric micromotors and covalently attached nanosorbents were designed for selective uranium recovery. Pickering emulsion, as an important micro-reactor, was firstly adopted to prepare Janus microspheres with chemically distinct structure (PS@PDA@Ag), on which one hemisphere was loaded with catalytic silver (Ag) nanoparticles. Then, amidoxime (AO) decorated SiO2 nanospheres were covalently attached onto another hemisphere of PS@PDA@Ag, anisotropic configuration of as-prepared PPA/SA avoided the overlap of catalytic and binding sites. The self-propelled motion of PPA/SA induced by asymmetrical bubble evolution by H2O2 was observed, and the maximum speed was about 25 μm s−1 in case of 5 % H2O2. The uranium adsorption onto PPA/SA achieved 92 % of the equilibrium amount (29.31 mg g−1) in the first 20 min, and this value increased to 38.71 mg g−1 under the condition of H2O2, suggesting that autonomous movement achieves rapid mixing and mass transportation. The maximum adsorption amount (Qm) is 114.0 mg g−1 at 298 K according to the Langmuir model. In addition, PPA/SA exhibited remarkable selectivity for uranium even in the presence of large amounts of Na+, Mg2+, K2+, Ca2+ and other ions in the simulated seawater solution.

Contraction of the rigor actomyosin complex drives bulk hemoglobin expulsion from hemolyzing erythrocytes

Erythrocyte ghost formation via hemolysis is a key event in the physiological clearance of senescent red blood cells (RBCs) in the spleen. The turnover rate of millions of RBCs per second necessitates a rapid efflux of hemoglobin (Hb) from RBCs by a not yet identified mechanism. Using high-speed video-microscopy of isolated RBCs, we show that electroporation-induced efflux of cytosolic ATP and other small solutes leads to transient cell shrinkage and echinocytosis, followed by osmotic swelling to the critical hemolytic volume. The onset of hemolysis coincided with a sudden self-propelled cell motion, accompanied by cell contraction and Hb-jet ejection. Our biomechanical model, which relates the Hb-jet-driven cell motion to the cytosolic pressure generation via elastic contraction of the RBC membrane, showed that the contributions of the bilayer and the bilayer-anchored spectrin cytoskeleton to the hemolytic cell motion are negligible. Consistent with the biomechanical analysis, our biochemical experiments, involving extracellular ATP and the myosin inhibitor blebbistatin, identify the low abundant non-muscle myosin 2A (NM2A) as the key contributor to the Hb-jet emission and fast hemolytic cell motion. Thus, our data reveal a rapid myosin-based mechanism of hemolysis, as opposed to a much slower diffusive Hb efflux.

Nonequilibrium dynamical structure factor of a dilute suspension of active particles in a viscoelastic fluid.

In this work we investigate the dynamics of the number-density fluctuations of a dilute suspension of active particles in a linear viscoelastic fluid. We propose a model for the frequency-dependent diffusion coefficient of the active particles which captures the effect of rotational diffusion on the persistence of their self-propelled motion and the viscoelasticity of the medium. Using fluctuating hydrodynamics, the linearized equationsfor the active suspension are derived, from which we calculate its dynamic structure factor and the corresponding intermediate scattering function. For a Maxwell-type rheological model, we find an intricate dependence of these functions on the parameters that characterize the viscoelasticity of the solvent and the activity of the particles, which can significantly deviate from those of an inert suspension of passive particles and of an active suspension in a Newtonian solvent. In particular, in some regions of the parameter space we uncover the emergence of oscillations in the intermediate scattering function at certain wave numbers which represent the hallmark of the nonequilibrium particle activity in the dynamical structure of the suspension and also encode the viscoelastic properties of the medium.

Carbon Quantum Dot-Decorated BiOBr/Bi2WO6 Photocatalytic Micromotor for Environmental Remediation and DFT Calculation

Efficient photocatalysts are crucial for degrading antibiotic pollutants in water and vital for environmental remediation. In this study, an efficient carbon quantum dot (CQD)-modified BiOBr/Bi2WO6 hybrid material was prepared by a mild hydrothermal method. Meanwhile, a visible-light-driven micromotor based on the photocatalyst was developed. The micromotor can achieve self-propelled motion powered by the photocatalytic degradation reaction. It exhibited remarkable photocatalytic activity toward common pollutants in water, such as sulfonamide, quinolone, and tetracycline antibiotics. Among them, 10CQD/BiOBr/Bi2WO6-4 (10CBBr-4) exhibited the highest photocatalytic activity, which was 2.8 and 5 times higher than those of BiOBr and Bi2WO6, respectively. The high activity is attributed to the decoration of CQDs, which promoted visible-light absorption. Additional contributions may be due to the specific Z-type electron transport mode and flower-like BiOBr inserted by two-dimensional (2D) Bi2WO6 nanosheets, which enriched the active reaction sites, facilitated the adsorption of antibiotic molecules, and promoted the final radical attack. Based on the theoretical calculation of molecular orbitals and the Fukui index, the possible intermediate products, degradation pathways, and photocatalytic mechanism were elucidated. This study suggests that structural engineering based on CQD modification effectively allows the design of self-propelled microrobots for antibiotic degradation.

Exploring the advantages of using artificial agents to investigate animacy perception in cats and dogs

Self-propelled motion cues elicit the perception of inanimate objects as animate. Studies usually rely on the looking behaviour of subjects towards stimuli displayed on a screen, but utilizing artificial unidentified moving objects (UMOs) provides a more natural, interactive context. Here, we investigated whether cats and dogs discriminate between UMOs showing animate vs inanimate motion, and how they react to the UMOs’ interactive behaviour. Subjects first observed, in turn, the motion of an animate and an inanimate UMO, and then they could move freely for 2 min while both UMOs were present (two-way choice phase). In the following specific motion phase, the animate UMO showed one of three interactive behaviours: pushing a ball, a luring motion, or moving towards the subject (between-subject design). Then, subjects could move freely for 2 min again while the UMO was motionless. At the end, subjects were free to move in the room while the UMO was moving semi-randomly in the room. We found that dogs approached and touched the UMO(s) sooner and more frequently than cats, regardless of the context. In the two-way choice phase, dogs looked at the animate UMO more often, and both species touched the animate UMO more frequently. However, whether the UMO showed playing, luring or assertive behaviour did not influence subjects’ behaviour. In summary, both species displayed distinctive behaviour towards the animate UMO, but in dogs, in addition to the physical contact this was also reflected by the looking behaviour. Overall, dogs were more keen to explore and interact with the UMO than cats, which might be due to the general increased stress of cats in novel environments. The findings indicate the importance of measuring multiple behaviours when assessing responses to animacy. The live demonstration using artificial agents provides a unique opportunity to study social perception in nonhuman species.

Synchronized motion of two camphor disks on a water droplet levitated under microgravity

Two camphor disks, as self-propelled objects, were placed on a water droplet levitated under microgravity to understand the nature of their self-propelled motion on a sphere field without a boundary. When a single camphor disk was placed on the droplet, unidirectional rotation with a constant speed was observed. When two camphor disks were placed on the droplet, synchronized motion occurred; that is, the two disks exhibited unidirectional motion while the distance between them was maintained. The characteristic features of the self-propelled motion are discussed in relation to the camphor molecules developed from the disk as the source of both the driving force of motion and the repulsive force between the disks. The present study suggests that self-propelled objects can exhibit cooperative motion in a field without boundary.

Self-Propelled Structural Color Cylindrical Micromotors for Heavy Metal Ions Adsorption and Screening.

Water contamination resulting from heavy metal ions (HMIs) poses a severe threat to public health and the ecosystem. Attempts are tending to develop functional materials to realize efficient and intelligent adsorption of HMIs. Herein, self-propelled structural color cylindrical micromotors (SCCMs) with reversible HMIs adsorption capacity and self-reporting property are presented. The SCCMs are fabricated by using platinum nanoparticles (Pt NPs) tagged responsive hydrogel of carboxymethyl chitosan (CMC) and polyacrylamide (PAM) to asymmetrically replicate tubular colloidal crystal templates (TCCTs). Owing to the self-propelled motion of the SCCMs, the enhancive ion-motor interactions bring significantly improved decontamination efficiency. Moreover, it is demonstrated that the SCCMs can realize quick and naked-eye-visible self-reporting during the adsorption/desorption process based on their responsive structure color variation and superior adsorption capacity. Thus, it is anticipated that such intelligent SCCMs can significantly facilitate the evolution of sensing assays and diverse environmental fields.

Design and evaluation of an MRI-ready, self-propelled needle for prostate interventions.

Prostate cancer diagnosis and focal laser ablation treatment both require the insertion of a needle for biopsy and optical fibre positioning. Needle insertion in soft tissues may cause tissue motion and deformation, which can, in turn, result in tissue damage and needle positioning errors. In this study, we present a prototype system making use of a wasp-inspired (bioinspired) self-propelled needle, which is able to move forward with zero external push force, thereby avoiding large tissue motion and deformation. Additionally, the actuation system solely consists of 3D printed parts and is therefore safe to use inside a magnetic resonance imaging (MRI) system. The needle consists of six parallel 0.25-mm diameter Nitinol rods driven by the actuation system. In the prototype, the self-propelled motion is achieved by advancing one needle segment while retracting the others. The advancing needle segment has to overcome a cutting and friction force while the retracting needle segments experience a friction force in the opposite direction. The needle self-propels through the tissue when the friction force of the five retracting needle segments overcomes the sum of the friction and cutting forces of the advancing needle segment. We tested the performance of the prototype in ex vivo human prostate tissue inside a preclinical MRI system in terms of the slip ratio of the needle with respect to the prostate tissue. The results showed that the needle was visible in MR images and that the needle was able to self-propel through the tissue with a slip ratio in the range of 0.78-0.95. The prototype is a step toward self-propelled needles for MRI-guided transperineal laser ablation as a method to treat prostate cancer.

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.

Visual Sensing System to Investigate Self-Propelled Motion and Internal Color of Multiple Aqueous Droplets.

This study proposes a visual sensing system to investigate the self-propelled motions of droplets. In the visual sensing of self-propelled droplets, large field-of-view and high-resolution images are both required to investigate the behaviors of multiple droplets as well as chemical reactions in the droplets. Therefore, we developed a view-expansive microscope system using a color camera head to investigate these chemical reactions; in the system, we implemented an image processing algorithm to detect the behaviors of droplets over a large field of view. We conducted motion tracking and color identification experiments on the self-propelled droplets to verify the effectiveness of the proposed system. The experimental results demonstrate that the proposed system is able to detect the location and color of each self-propelled droplet in a large-area image.

Synthetic Micro/Nanomotors for Drug Delivery

Synthetic micro/nanomotors (MNMs) are human-made machines characterized by their capacity for undergoing self-propelled motion as a result of the consumption of chemical energy obtained from specific chemical or biochemical reactions, or as a response to an external actuation driven by a physical stimulus. This has fostered the exploitation of MNMs for facing different biomedical challenges, including drug delivery. In fact, MNMs are superior systems for an efficient delivery of drugs, offering several advantages in relation to conventional carriers. For instance, the self-propulsion ability of micro/nanomotors makes possible an easier transport of drugs to specific targets in comparison to the conventional distribution by passive carriers circulating within the blood, which enhances the drug bioavailability in tissues. Despite the promising avenues opened by the use of synthetic micro/nanomotors in drug delivery applications, the development of systems for in vivo uses requires further studies to ensure a suitable biocompatibility and biodegradability of the fabricated engines. This is essential for guaranteeing the safety of synthetic MNMs and patient convenience. This review provides an updated perspective to the potential applications of synthetic micro/nanomotors in drug delivery. Moreover, the most fundamental aspects related to the performance of synthetic MNMs and their biosafety are also discussed.