We analyze the power conversion efficiency of different classes of autonomous nano- and micromotors. For bimetallic catalytic motors that operate by a self-electrophoretic mechanism, there are four stages of energy loss, and together they result in a power conversion efficiency on the order of 10(-9). The results of finite element modeling agree well with experimental measurements of the efficiency of catalytic Pt-Au nanorod motors. Modifications of the composition and shape of bimetallic catalytic motors were predicted computationally and found experimentally to lead to higher efficiency. The efficiencies of bubble-propelled catalytic micromotors, magnetically driven flagellar motors, Janus micromotors driven by self-generated thermal gradients, and ultrasonically driven metallic micromotors are also analyzed and discussed.

Dead-end micro- and nanoscale channels are ubiquitous in nature and are found in geological and biological systems subject to frequent disruptions. Achieving fluid flows in them is not possible through conventional pressure-driven mechanisms. Here we show that chemically driven convective flows leading to transport in and out of dead-end pores can occur by the phenomenon of "transient diffusioosmosis". The advective velocity depends on the presence of an in situ-generated transient ion gradient and the intrinsic charge on the pore wall. The flows can reach speeds of 50 μm/s and cause extraction of otherwise-trapped materials. Our results illustrate that chemical energy, in the form of a transient salt gradient, can be transduced into mechanical motion with the pore wall acting as the pump. As discussed, the phenomena may underlie observed transport in many geological and biological systems involving tight or dead-end micro- and nanochannels.

Experimental data show that the speed of colloidal (catalytic) motors decreases as the size of the motor particles increases. However, previous electrokinetic models have shown that the colloidal motor speed for spheres is independent of size, at least for the case of infinitesimally thin double layers and reaction-limited catalysis. Although a size dependence of motor speed has been calculated for diffusion-limited catalysis, most motor experiments are done in the reaction-limited regime. This apparent contradiction led us to examine how motor speed (U) changes with distance (δ) from a wall, starting from the usual electrokinetic equations. A key finding is that interactions between a colloidal motor and a nearby wall produce a localized electroosmotic (LEO) flow field that can significantly alter the motor speed near the wall. Because large motor particles typically settle closer to the wall than small motors, LEO thus provides at least one explanation of the size dependence of motor speed. Furthermore, LEO provides a new method of creating flow fields in capillaries and microchannels.

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