Rengaswamy Srinivasan

Dipolar assembly of ferromagnetic nanoparticles into magnetically driven artificial cilia

Taking inspiration from eukaryotic cilia, we report a method for growing dense arrays of magnetically actuated microscopic filaments. Fabricated from the bottom-up assembly of polymer-coated cobalt nanoparticles, each segmented filament measures approximately 5–15 µm in length and 23.5 nm in diameter, which was commensurate with the width of a single nanoparticle. A custom microscope stage actuates the filaments through orthogonal permanent and alternating magnetic fields. We implemented design of experiments (DOE) to efficiently screen the effects of cobalt nanoparticle concentration, crosslinker concentration, and surface chemistry. The results indicated that the formation of dense, cilia-mimetic arrays could be explained by physical, non-covalent interactions (i.e. dipolar association forces) rather than chemistry. The experiments also determined an optimal Co nanoparticle concentration of approximately 500 µg ml−1 for forming dense arrays near the ends of the permanent magnets, and a critical concentration of approximately 0.3 µg ml−1, below which particle assembly into chains was not observed.

Synergy between Galvanic Protection and Self-Healing Paints.

Painting is a cost-effective technique to delay the onset of corrosion in metals. However, the protection is only temporary, as corrosion begins once the coating becomes scratched. Thus, an increasingly common practice is to add microencapsulated chemical agents to paint in order to confer self-healing capabilities. The additives ability to protect the exposed surface from corrosion depends upon (i) how long the chemical agent takes to spread across the exposed metal; (ii) how long the agent takes to form an effective barrier layer; and (iii) what happens to the metal surface before the first two steps are complete. To understand this process, we first synthesized 23 ± 10 μm polyurea microcapsules filled with octadecyltrimethoxysilane (OTS), a liquid self-healing agent, and added them to a primer rich in zinc, a cathodic protection agent. In response to coating damage, the microcapsules release OTS into the scratch and initiate the self-healing process. By combining electrochemical impedance spectroscopy, chronoamperometry, and linear polarization techniques, we monitored the progress of self-healing. The results demonstrate how on-demand chemical passivation works synergistically with the cathodic protection: zinc preserves the surface long enough for self-healing by OTS to reach completion, and OTS prolongs the lifetime of cathodic protection.

Metal Nanotubes and their Applications in Micro Power Sources

Metal electrodes and separators used in the conventional manufacturing processes of batteries are 0.5to 2-mm thick; sheets of nickel metal grid with sintered nickel powder and polymer membranes are examples. Batteries with millimeter-thick electrodes include most aqueousand the organic-based lithium batteries [1, 2]. Among the so-called thin-film lithium batteries [3, 4], even the separator is 50-μm thick, and the electrodes are much thicker. In almost all cases, an electrode is a current collector, and their porous surface act as a support structure to house the reactive materials, such as Ni(OH)2 and metal hydrides. The porosity accounts for about half of the total volume of the electrode; the volume occupied by the energy-storing active material is only that fraction. The densities of the electrodes are also higher than that of the active material, usually by a factor two or more, contributing to the “dead” weight of the battery. This paper describes a thinner and lighter substitute for conventional metal electrodes; it is based on an emerging nanotechnology for populating porous polymers films with well ordered Metal Nanotubes (MNT). An important technological innovation that has enabled us to use porous polymer films as electrodes in batteries is the ability to grow MNTs inside the pores. Figure 1(a) show nickel nanotubes grown inside a 10-μmthick porous polycarbonate (PC) membrane; we currently have the ability to grow nanotubes of virtually any metal and alloy with nanometer precision and control of their wall thickness. The PC film matrix with MNTs is a thinner substitute for the conventional metal electrode. Figure 1 (b) shows the MNT filled with the active material NiOOH, making the polymer film the positive electrode in a nickel-metal hydride battery. Figure 2 shows the oxidation-reduction cycles of the NiOOH + e Ni(OH)2 reaction. We also made porous polymer matrices with nanotubes of a metal hydride that has the