Biomedical subjects

Paul E Lammert

Publications and source records attributed to Paul E Lammert.

3 recordsLinked to original sources

PubMed2006

Catalytically driven colloidal patterning and transport.

We recently reported the convection and pattern formation of tracers caused by a catalytically generated electric field. The electric field arises due to the heterogeneous electrochemical reduction and oxidation of hydrogen peroxide (H2O2) on silver (Ag) and gold (Au), respectively.1 Here we describe an electrokinetic model, developed in conjunction with experiments, that explains the details of the convection and pattern formation phenomenon. The model also enables the measurement of reaction kinetic parameters that are otherwise difficult to obtain. This quantitative model serves as a platform for the modeling of other catalytic redox systems and systems with broken symmetries.

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PubMed2006

Coarse-grained V representability.

The unsolved problem of determining which densities are ground state densities of an interacting electron system in some external potential is important to the foundations of density functional theory. A coarse-grained version of this ensemble V-representability problem is shown to be thoroughly tractable. Averaging the density of an interacting electron system over the cells of a regular partition of space produces a coarse-grained density. It is proved that every strictly positive coarse-grained density is coarse-grained ensemble V representable: there is a unique potential, constant over each cell of the partition, which has a ground state with the prescribed coarse-grained density. For a system confined to a box, the (coarse-grained) Lieb [Int. J. Quantum Chem. 24, 243 (1983)] functional is also shown to be Gâteaux differentiable. All results extend to open systems.

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PubMed2004

Catalytic nanomotors: autonomous movement of striped nanorods.

Rod-shaped particles, 370 nm in diameter and consisting of 1 microm long Pt and Au segments, move autonomously in aqueous hydrogen peroxide solutions by catalyzing the formation of oxygen at the Pt end. In 2-3% hydrogen peroxide solution, these rods move predominantly along their axis in the direction of the Pt end at speeds of up to 10 body lengths per second. The dimensions of the rods and their speeds are similar to those of multiflagellar bacteria. The force along the rod axis, which is on the order of 10(-14) N, is generated by the oxygen concentration gradient, which in turn produces an interfacial tension force that balances the drag force at steady state. By solving the convection-diffusion equation in the frame of the moving rod, it was found that the interfacial tension force scales approximately as SR(2)gamma/muDL, where S is the area-normalized oxygen evolution rate, gamma is the liquid-vapor interfacial tension, R is the rod radius, mu is the viscosity, D is the diffusion coefficient of oxygen, and L is the length of the rod. Experiments in ethanol-water solutions confirmed that the velocity depends linearly with the product Sgamma, and scaling experiments showed a strong dependence of the velocity on R and L. The direction of motion implies that the gold surface is hydrophobic under the conditions of the experiment. Tapping-mode AFM images of rods in air-saturated water show soft features that are not apparent in images acquired in air. These features are postulated to be nanobubbles, which if present in hydrogen peroxide solutions, would account for the observed direction of motion.

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