Imaging microscopic magnetohydrodynamic flows

Magnetohydrodynamic flows in nanoliter volumes of solution adjacent to the surface of a 25-mu m-radius Pt disk electrode have been imaged with a scann ing electrochemical microscope (SECM). Streamline and chaotic flows are ima ged with submicrometer resolution. SECM images demonstrate that magnetic fi eld-driven cyclotron flow within a similar to 50-mu m-radius torus results in spatial focusing of the electrogenerated molecules. SECM images also sug gest that natural convective now resulting from microscopic density gradien ts can be prevented by adjusting the external magnetic field strength such that magnetic and gravitational forces in the solution cancel one another.