Catalytic Swimmers Research Articles

Power-law intermittency in the gradient-induced self-propulsion of colloidal swimmers.

Active colloidal microswimmers serve as archetypical active fluid systems, and as models for biological swimmers. Here, by studying in detail their velocity traces, we find robust power-law intermittency with system-dependent exponential cut off. We model the intermittent motion by an interplay of the field gradient-dependent active force, which depends on a fluid gradient and is reduced when the swimmer moves, and the locally fluctuating hydrodynamic drag, that is set by the wetting properties of the substrate. The model closely describes the velocity distributions of two disparate swimmer systems: AC field activated and catalytic swimmers. The generality is highlighted by the collapse of all data in a single master curve, suggesting the applicability to further systems, both synthetic and biological.

Accurate Solutions to Non-Linear PDEs Underlying a Propulsion of Catalytic Microswimmers

Catalytic swimmers self-propel in electrolyte solutions thanks to an inhomogeneous ion release from their surface. Here, we consider the experimentally relevant limit of thin electrostatic diffuse layers, where the method of matched asymptotic expansions can be employed. While the analytical solution for ion concentration and electric potential in the inner region is known, the electrostatic problem in the outer region was previously solved but only for a linear case. Additionally, only main geometries such as a sphere or cylinder have been favoured. Here, we derive a non-linear outer solution for the electric field and concentrations for swimmers of any shape with given ion surface fluxes that then allow us to find the velocity of particle self-propulsion. The power of our formalism is to include the complicated effects of the anisotropy and inhomogeneity of surface ion fluxes under relevant boundary conditions. This is demonstrated by exact solutions for electric potential profiles in some particular cases with the consequent calculations of self-propulsion velocities.

Slip Length Dependent Propulsion Speed of Catalytic Colloidal Swimmers near Walls.

Catalytic colloidal swimmers that propel due to self-generated fluid flows exhibit strong affinity for surfaces. Here, we report experimental measurements of a significant dependence of such microswimmers' speed on the nearby substrate material. We find that speeds scale with the solution contact angle θ on the substrate, which relates to the associated hydrodynamic substrate slip length, as V∝(cosθ+1)^{-3/2}. We show that such dependence can be attributed to osmotic coupling between swimmers and substrate. Our work points out that hydrodynamic slip at nearby walls, though often unconsidered, can significantly impact microswimmer self-propulsion.

Thermodynamic approach to the self-diffusiophoresis of colloidal Janus particles.

Most available theoretical predictions for the self-diffusiophoretic motion of colloidal particles are based on the hydrodynamic thin boundary layer approximation in combination with a solvent body force due to a self-generated local solute gradient. This gradient is enforced through specifying boundary conditions, typically without accounting for the thermodynamic cost to maintain the gradient. Here, we present an alternative thermodynamic approach that exploits a direct link between dynamics and entropy production: the local detailed balance condition. We study two cases: First, we revisit self-propulsion in a demixing binary solvent. At variance with a slip velocity, we find that propulsion is due to forces at the poles that are perpendicular to the particle surface. Second, for catalytic swimmers driven through liberating chemical free energy we recover previous expressions. In both cases we argue that propulsion is due to asymmetric dissipation and not simply due to an asymmetric concentration of molecular solutes.

Laser-Fabricated, Self-Forming Swimmers With Catalytic Propulsion and Magnetic Navigation

We have developed an inexpensive rapid process for creating soft, mm-scale self-forming catalytic swimmers for biomedical and environmental applications. The swimmers contain a catalyst such that when exposed to H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> they self-propel. With the additional incorporation of iron oxide nanoparticles (deposited as ferrofluid), we successfully control the navigation of the swimmers using a magnetic field. Unlike the previous reports of catalytic actuators, which use expensive materials (e.g., platinum) and hard substrates (e.g., titanium), our process makes an exclusive use of techniques, which are scalable, economical, and adaptable for large-volume manufacturing (e.g., roll-to-roll), along with materials that are non-toxic and eco-friendly. The devices are created using a planar design, but upon immersion in an aqueous environment, they fold up to create a 3-D architecture, specifically, a tapered cylinder. Tapering is achieved by a range of ablation levels during fabrication, thus creating a gradient of curvatures along the longitudinal axis of the cylinder for improved propulsion efficiency (compared with cylinders or rods). A maximum swimming speed of 7.2 mm/s was observed at 20% H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> .

Geometric tuning of self-propulsion for Janus catalytic particles

Catalytic swimmers have attracted much attention as alternatives to biological systems for examining collective microscopic dynamics and the response to physico-chemical signals. Yet, understanding and predicting even the most fundamental characteristics of their individual propulsion still raises important challenges. While chemical asymmetry is widely recognized as the cornerstone of catalytic propulsion, different experimental studies have reported that particles with identical chemical properties may propel in opposite directions. Here, we show that, beyond its chemical properties, the detailed shape of a catalytic swimmer plays an essential role in determining its direction of motion, demonstrating the compatibility of the classical theoretical framework with experimental observations.

Surface fouling as a mechanism for chemotaxis in isotropic catalytic swimmers.

We present microscopic models for surface fouling of an isotropic spherical catalytic microswimmer at and away from equilibrium and show how a foulant gradient can induce chemotactic behavior. Our simulations establish that the presence of foulant manifests itself in two ways: as a braking effect on propulsive particle motion, and as a drift term which probes the foulant concentration gradient. Our results suggest that, while foulant gradients are unlikely to be directly useful for chemotactically directed particles, they nevertheless exert a non-negligible influence on particle motion under a wide range of conditions.

Self-propelled screen-printable catalytic swimmers

A highly versatile 2D screen-printing fabrication of nature-inspired fish swimmers is described.

Diffusiophoresis of Polyelectrolytes in Nanodevices: Importance of Boundary

Considering the potential applications of diffusiophoresis conducted in nanodevices, a thorough theoretical analysis is performed for the first time on the diffusiophoresis of a polyelectrolyte (PE) in the presence of two representative types of boundaries: the direction of diffusiophoresis is either normal (type I) or parallel (type II) to a boundary, using two large parallel disks and a cylindrical pore as an example, respectively. It is interesting to observe that due to the effects of double-layer polarization, counterion condensation, polarization of condensed counterions, and diffusion of co-ions across a PE, its diffusiophoretic behavior can be influenced both quantitatively (magnitude of mobility) and qualitatively (direction of diffusiophoresis) by a boundary. In general, type I (II) boundary raises the diffusiophoretic mobility of a PE toward the high (low) salt concentration side. The results gathered provide necessary information for applications in, for example, designing catalytic swimmers and nanopore-based biosensor devices.

Direct Observation of the Direction of Motion for Spherical Catalytic Swimmers

Nonconductive Janus particle swimmers made by coating fluorescent polymer beads with hemispheres of platinum have been fully characterized using video microscopy to reveal that they undergo propulsion in hydrogen peroxide fuel away from the catalytic platinum patch. The platinum coating shadows the fluorescence signal from half of each swimmer to allow the orientation to be observed directly and correlated quantitatively with the resulting swimming direction. The observed swimmer direction is consistent with both the bubble release and diffusiophoretic propulsion mechanisms.