Lance M. Baird

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.

Liquid-Filled Metal Microcapsules

A moisture-sensitive diisocyanate liquid is microencapsulated within a metal shell measuring less than 2 μm thick and 50 μm in diameter. This mild synthesis takes place through a series aqueous processing steps that occur at or near room temperature. Through a combination of emulsification, interfacial polymerization, and electroless plating, one can microencapsulate moisture- or air-sensitive chemicals within a metal seal. The liquid-filled metal microcapsules promise a number of advantages compared to conventional polymeric microencapsulation, including improved mechanical properties and improved barrier properties to gases and organic molecules.

Robust Composite-Shell Microcapsules via Pickering Emulsification

Microencapsulation technology has been increasingly applied toward the development of self-healing paints. Added to paint as a dry powder prior to spraying, the microcapsules store a liquid that can repair the protective barrier layer if released into a scratch. However, self-healing will not occur unless the microcapsules can withstand spray-painting, aggressive solvents in the paint, and long-term exposure to the elements. We have therefore developed a one-pot synthesis for the production of Pickering microcapsules with outstanding strength, solvent resistance, and barrier properties. Octadecyltrimethoxysilane-filled (OTS) microcapsules form via standard interfacial polycondensation, except that silica nanopowder (10-20 nm diameter) replaces the conventional surfactant or hydrocolloid emulsifier. Isophorone diisocyanate (IPDI) in the OTS core reacts with diethylenetriamine, polyethylenimine, and water to form a hard polymer shell along the interface. Compared to pure polyurea, the silica-polyurea composite improves the shelf life of the OTS by 10 times. The addition of SiO2 prevents leaching of OTS into xylenes and hexanes for up to 80 days, and the resulting microcapsules survive nebulization through a spray gun at 620 kPa in a 500 cSt fluid.

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.

Surfactant sculpting of biologically inspired hierarchical surfaces

We describe a method for fabricating biologically inspired hierarchical surfaces in a single step through surfactant self-assembly at an oil/water interface. The key to this system is the use of polydimethylsiloxane-diacrylate for the oil phase, which makes it possible to solidify these delicate structures with UV photocuring. Scanning electron microscopy (SEM) and 3-D optical profilometry reveals morphologies that capture the randomness, fractal geometry, and hierarchical organization of natural materials. The morphology is controlled by surfactant type, surfactant concentration, viscosity, film thickness, and time. The experimental evidence is consistent with a spontaneous increase in surface area driven by a transiently negative surface tension. Spontaneous emulsification generates distinct morphologies for a given surfactant and surfactant concentration in a manner reminiscent of phase behavior in a ternary phase diagram. When emulsification cannot keep pace with the increase in surface area, buckles form. These perturbations are then amplified at increasing length scales by dewetting and the Rayleigh–Taylor instability.

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