Catalytic bimetallic nanomotors can swim at 100 body lengths per second as well as pick up, haul, and release micrometer-scale cargo. The electrokinetic locomotion of bimetallic nanomotors is driven by the electrocatalytic decomposition of hydrogen peroxide. The motors are typically fabricated by electrodeposition-based template synthesis techniques that result in heterogeneous samples and require specialized knowledge of electrochemistry, a three-electrode potentiostat setup, cyanide-based chemistry, and porous membranes. This paper presents a rapid and facile method for fabrication of spherical bimetallic motors that only requires access to metal deposition equipment and commercially available microspheres. The resulting spherical motors swim at speeds comparable to rod-shaped motors with the same dimensions and composition. The spherical motors' velocity increases with fuel concentration and decreasing diameter.

Spherical catalytic micromotors fabricated as described in Wheat et al. [Langmuir 26, 13052 (2010)] show fuel concentration dependent translational and rotational velocity. The motors possess short-time and long-time diffusivities that scale with the translational and rotational velocity with respect to fuel concentration. The short-time diffusivities are two to three orders of magnitude larger than the diffusivity of a Brownian sphere of the same size, increase linearly with concentration, and scale as v 2 /2ω. The measured long-time diffusivities are five times lower than the short-time diffusivities, scale as v 2 /{2D r [1 + (ω/D r ) 2 ]}, and exhibit a maximum as a function of concentration. Maximums of effective diffusivity can be achieved when the rotational velocity has a higher order of dependence on the controlling parameter(s), for example fuel concentration, than the translational velocity. A maximum in diffusivity suggests that motors can be separated or concentrated using gradients in fuel concentration. The decrease of diffusivity with time suggests that motors will have a high collision probability in confined spaces and over short times; but will not disperse over relatively long distances and times. The combination of concentration dependent diffusive time scales and nonmonotonic diffusivity of circle-swimming motors suggests that we can expect complex particle responses in confined geometries and in spatially dependent fuel concentration gradients.

A microfluidic method to rapidly measure the octanol-water partition coefficient in thousands of individual picoliter drops is described. A T-junction microfluidic chip is used to generate a segmented flow of monodisperse, fluorescein-laden water in octanol carrier fluid. The partitioning of individual drops reaches equilibrium in less than 2 s. Epifluorescence microscopy is used measure the partition coefficient of fluorescein as a function of pH. Results compare well with previous measurements using traditional shake-flask methods. The methods presented here are rapid, provide detailed statistics, and can be run in parallel, enabling the simultaneous partitioning of thousands of compounds for various applications such as drug development, environmental testing, and combinatorial chemistry. Microfluidic partitioning and extraction in picoliter drops may be useful for studying molecules and particles away from their equilibrium state and in cases with limited samples.

Many motile cells exhibit migratory behaviors, such as chemotaxis (motion up or down a chemical gradient) or chemokinesis (dependence of speed on chemical concentration), which enable them to carry out vital functions including immune response, egg fertilization, and predator evasion. These have inspired researchers to develop self-propelled colloidal analogues to biological microswimmers, known as active colloids, that perform similar feats. Here, we study the behavior of half-platinum half-gold (Pt/Au) self-propelled rods in antiparallel gradients of hydrogen peroxide fuel and salt, which tend to increase and decrease the rods’ speed, respectively. Brownian Dynamics simulations, a Fokker–Planck theoretical model, and experiments demonstrate that, at steady state, the rods accumulate in low-speed (salt-rich, peroxide-poor) regions not because of chemotaxis, but because of chemokinesis. Chemokinesis is distinct from chemotaxis in that no directional sensing or reorientation capabilities are required. The agreement between simulations, model, and experiments bolsters the role of chemokinesis in this system. This work suggests a novel strategy of exploiting chemokinesis to effect accumulation of motile colloids in desired areas.

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