Micro/nanomachines capable of propulsion through fluidic environments provide diverse opportunities in important biomedical applications. In this paper, we present a theoretical study on micromotors steered through liquid by an external rotating magnetic field. A purely geometric tight upper bound on the propulsion speed normalized with field frequency, known as propulsion efficiency, d, for an arbitrarily shaped object is derived. Using this bound, we estimate the maximum propulsion efficiency of previously reported random magnetic aggregates. We introduce a complementary definition of the propulsion efficiency, d*, that ranks propellers according to their maximal speed in body lengths per unit time and that appears to be preferable over the standard definition in a search for fastest machines. Using a bead-based hydrodynamic model combined with genetic algorithms, we determine that d*-optimal propeller deviates strongly from the bioinspired slim helix and has a surprising chubby skew-symmetric shape. It is also shown that optimized propellers with preprogrammed shape are substantially more efficient than random magnetic aggregates. We anticipate that the results of the present study will provide guidance toward prospective experimental design of more efficient magnetic micro/nanomachines.

Recently, one has been observing abundant studies on the application of surface acoustic waves (SAWs) in solid substrates for manipulating liquids and particulates in micron-to-nanometer thick films and channels and in porous media. At these length scales, contributions of SAWs to the electrical double layer (EDL) of ions and of the latter to particulates and flow may become appreciable. However, the nature of the interplay between SAWs and EDLs is unknown. We demonstrate the contribution of a SAW to the near-equilibrium electrical and ion-concentration fields in an EDL near inert and piezoelectric substrates. In particular, we concentrate on the leakage of transient and steady components of electrical potential through the excited EDL. Far from the solid, the leakage may be interpreted by different models of the EDL to give information about the EDL characteristic relaxation time, ζ-potential, and the Stern layer therein. In addition, the analysis given here may explain observed SAW-induced electrochemical effects on piezoelectric substrates.

A previous experiment showed that the rate of the electropolishing of a copper anode may be increased by twofold when generating a 60 KHz to 1.7 MHz frequency vibration in the anode. In this work we use theory to elucidate the mechanisms by which the vibration may enhance the transport of ions in the electrolyte solution and support the formation of dents in the anode, which was observed in experiment. We find that in the limit of weak ion convection the transport of ions mainly supports the formation of dents in the anode. However, in the limit of prominent ion convection we find an appreciable contribution of the vibration to the efficiency of the electropolishing process, in accordance with the previous experimental findings. The contribution of the vibration to ion transport is given by 2 √ PeDkC s /π √ π, in which the Peclét number, Pe, quantifies the ratio between the convective and diffusive fluxes of ions, and D, k, and C s are the diffusion coefficient of the ions, the wavenumber of the vibration, and the solubility limit of the ions in the electrolyte. Electropolishing is a well-known technique used to polish metal surfaces by employing electrochemistry.1 The most basic electropolishing system is a classic electrochemical cell, comprising an anode and a cathode in an electrolyte solution, as depicted in Fig. 1. The application of an electrical potential difference between the electrodes will decompose the surface of the anode -the metallic object to be polished -to metal ions that dissolve in the electrolyte solution. Sharp corners on the metal anode dissolve to ions faster than flat areas. Hence, the electropolishing process renders the surface of the anode smooth. In certain systems, the chemical complexation of metal ions with the electrolyte and water molecules will support phase separation and the formation of a viscous liquid phase (viscous layer) adjacent to the anode.2-8 The layer of viscous liquid serves as a barrier to the diffusion of ions from the surface of the anode and vice-versa, thereby reducing the efficiency of the electropolishing process. The density of the viscous liquid is assumed to be similar to the density of the electrolyte solution.It was previously found that exciting a high frequency (60-1700 KHz) mechanical vibration in the anode may enhance the efficiency of the electropolishing process by two-fold. 9,10 However, it appears that the vibration further supports the formation of dents on the surface of the anode. The experimental system investigated previously is depicted in Fig. 1. It is an electrochemical cell, which is comprised of copper electrodes, a power supply, and an electrolyte solution containing water, concentrated phosphoric acid, and isopropyl alcohol. A piezoelectric transducer generates vibration in the anode. Upon the application of an electrical potential difference between the electrodes, which initiates the electropolishing process, a viscous layer of a thickness of about 1-2 mm forms adjacent to the electrode, as depicted in Fig. 2. The viscous layer is...

No abstract

The leading-edge of a substrate undergoing convective mass deposition is a region of significant local deposition rate compared to the mass deposition at the downstream Leveque concentration boundary layer. The local increase in mass deposition is due to an intrinsic topological transition at the leading edge, a transition which is usually in the chemistry or geometry of the target surface for deposition. We study two leading-edge cases for model convective electrodeposition: a flat and a corner/step transitions between the inert wall and active cathode. We find that mass deposition at the leading-edge is faster than at the boundary layer and is connected to the Pe\'clet number differently. Its rate is correlated with the transition length and decays downstream to match the deposition rate at the boundary layer.