No abstract

We show that electrochemical formation of long probes with nanosharp tips can be controlled by choosing an appropriate thermodynamic pathway of metal to metal oxide and hydroxide transformation. Currently, convection-limited electropolishing (CLE) is extensively used. Nanosharp probes are produced by electrochemically etching a wire until it breaks into two pieces. This process is difficult to control because of the complexity of the associated hydrodynamic flows. We introduce transport-limited electropolishing (TLE), where the electrochemical reaction results in the formation of metal oxides and hydroxides which form a porous surface layer hindering the flow of electrolyte. The developed TLE method enables one to make long tapered needles. The taper can spread over more than 6 mm while the radius of tip curvature can be decreased down to 30 nm. These needles are strong and were successfully applied for piercing single smooth vascular muscle cells.

Electropolishing of metals often leads to supersaturation conditions resulting in precipitation of complex compounds. The solubility diagrams and Gibbs adsorption isotherms of the electropolishing products are thus very important to understand the thermodynamic mechanism of precipitation of reaction products. Electropolishing of tungsten wires in aqueous solutions of potassium hydroxide is used as an example illustrating the different thermodynamic scenarios of electropolishing. Electropolishing products are able to form highly viscous films immiscible with the surrounding electrolyte or porous shells adhered to the wire surface. Using X-ray nanotomography, we discovered a gel-like phase formed at the tungsten surface during electropolishing. The results of these studies suggest that the electropolishing products can form a rich library of compounds. The surface tension of the electrolyte depends on the metal oxide ions and alkali-metal complexes.

We developed a special electrochemical cell enabling quantitative analysis and in situ X-ray nanotomography of metal/electrolyte interfaces subject to corrosion. Using this cell and applying the nodoid model to describe menisci formed on tungsten wires during anodization, the evolution of the electrolyte surface tension, the concentration of reaction products, and the meniscus contact angle were studied. In contrast to the electrowetting effect, where the applied electric field decreases the contact angle of electrolytes, anodization of the tungsten wires increases the contact angle of the meniscus. Hence, an electric field favors dewetting rather than wetting of the newly formed surface. The discovered effect opens up new opportunities for the control of wetting phenomena and calls for the revision of existing theories of electrowetting.