Charles Rosenblatt

Microaerobic Conditions Are Required for Magnetite Formation Within Aquaspirillum magnetotacticum

Abstract The amount of magnetite (Fe3O4) within magnetosomes of the microaerophilic bacterium Aquaspirillum magnetotacticum varies with oxygen and nitrogen supply. The development of optical methods for directly measuring cell magnetism in culture samples has enabled us to quantitate bacterial Fe3O4 yields. We measured final cell yields, average cell magnetic moments, and magnetosome yields of growing cells. Cultures were grown with NO3‐, NH4 +, or both, in sealed, unshaken vials with initial headspace Po2 values ranging from 0 (trace) to 21 kPa. More than 50% of cells had detectable magnetosomes only when grown in the range of 0.5–5.0 kPa O2. Optimum cell magnetism (and Fe3O4 formation) occurred under microaerobic conditions (initial headspace Po2 of 0.5–1 kPa) regardless of the N source. At optimal conditions for Fe3O4 formation, denitrifying cultures produced more of this mineral than those growing with O2 as the sole terminal electron acceptor. This suggests that competition for O2 exists between proc...

Determination of the anisotropic potential at the nematic liquid crystal‐to‐wall interface

The integrated birefringence of a surfactant (C16H33NH2)‐aligned methoxybenzylidene butylaniline cell as a function of applied magnetic field (up to 100 kG) has been measured. The Oseen and Frank elastic continuum theory and a generalized torque balance equation at the interface have been utilized for the calculation to fit the measured data. A very good fit to the data was obtained using an anisotropic liquid crystal‐to‐wall interfacial potential of the form C cos2 θ+C4 cos4 θ for the calculation. It was found that C4/C=−0.38.

Continuous control of liquid crystal pretilt angle from homeotropic to planar

A mixture of two polyamic acids, one having an alkyl side chain and ordinarily used for vertical liquid crystal alignment and the other without a side chain and ordinarily used for planar alignment, is deposited on two substrates and baked at high temperatures. When the resulting cell is filled with the liquid crystal pentyl cyanobiphenyl, it is found that the pretilt angle θ0 is a function of the baking temperature, and can be controlled continuously over the range of 0°⩽θ0≲90°.

Nematic liquid-crystal polarization gratings by modification of surface alignment

The stylus of an atomic force microscope is used to scribe preferred directions for liquid-crystal alignment on a polyimide-coated substrate. The opposing substrate that comprises the liquid-crystal cell is rubbed unidirectionally, resulting in a twisted nematic structure associated with each micrometer-sized pixel. The polarization of light entering from the uniformly rubbed substrate rotates with the nematic director by a different amount in each pixel, and each of the two emerging polarization eigenmodes interferes separately. Two examples are discussed: a square grating that allows only odd-order diffraction peaks and a grating that combines rotation with optical retardation to simulate a blazed grating for circularly polarized light. The gratings can be electrically switched if used with semitransparent electrodes.

Orientation of lipid tubules by a magnetic field

Lipid tubules, which are straight hollow cylinders consisting of lipid bilayers, are shown to orient in strong magnetic fields. Birefringence measurements were made of dilute samples of tubules of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC23PC) in magnetic fields of up to 4 T. The tubules were found to orient with their long axes parallel to the field direction, with saturated orientation [P2 (cos theta] approximately greater than 0.95) found at approximately 2 T. From known distributions of lengths and the number of bilayers in the walls, a value delta chi = (-7 +/- 1) X 10(-9) erg cm-3 G-2 was calculated for the tubules, which compares well with some previously reported values for phosphatidylcholines. Magnetic alignment will permit more sophisticated structural studies of monomeric and polymeric tubules, and provide a method of orienting macromolecules in the tubule walls or interior.

Large, continuously controllable nematic pretilt from vertical orientation

A polyimide that is designed to produce homeotropic liquid-crystal orientation is baked at high temperature and rubbed with a cotton cloth. It is found that the resulting pretilt angle of the liquid-crystal pentylcyanobiphenyl is continuously controllable from 0° to nearly 40°.

The importance of fluctuations in fluid mixing

A ubiquitous example of fluid mixing is the Rayleigh–Taylor instability, in which a heavy fluid initially sits atop a light fluid in a gravitational field. The subsequent development of the unstable interface between the two fluids is marked by several stages. At first, each interface mode grows exponentially with time before transitioning to a nonlinear regime characterized by more complex hydrodynamic mixing. Unfortunately, traditional continuum modeling of this process has generally been in poor agreement with experiment. Here, we indicate that the natural, random fluctuations of the flow field present in any fluid, which are neglected in continuum models, can lead to qualitatively and quantitatively better agreement with experiment. We performed billion-particle atomistic simulations and magnetic levitation experiments with unprecedented control of initial interface conditions. A comparison between our simulations and experiments reveals good agreement in terms of the growth rate of the mixing front as well as the new observation of droplet breakup at later times. These results improve our understanding of many fluid processes, including interface phenomena that occur, for example, in supernovae, the detachment of droplets from a faucet, and ink jet printing. Such instabilities are also relevant to the possible energy source of inertial confinement fusion, in which a millimeter-sized capsule is imploded to initiate nuclear fusion reactions between deuterium and tritium. Our results suggest that the applicability of continuum models would be greatly enhanced by explicitly including the effects of random fluctuations.

Ultrahigh-resolution liquid crystal display with gray scale

An atomic force microscope was used to write planar alignment patterns on a polyimide-coated glass substrate. Paired with a substrate treated for homeotropic alignment, the resulting hybrid liquid crystal cell produced fixed gray scale images with pixel sizes of order 1 μm. The physics and efficacy of this architecture are compared to a cell having planar alignment at both substrates.

Birefringence Determination of Magnetic Moments of Magnetotactic Bacteria

A birefringence technique is used to determine the average magnetic moments of magnetotactic bacteria in culture. Differences in are noted between live and dead bacteria, as well as between normal density and high density samples of live bacteria.

Correlation between rub-induced grooves in a polyimide-treated substrate and microstructure of rubbing fiber: An atomic force microscopy study

Rubbed polyimide surfaces, which are used for liquid crystal alignment, generally exhibit microscopic grooves which lie parallel to the rubbing direction. Using atomic force microscopy we examined both the grooves and the fibers that create the grooves. We find that for a wide range of rubbing strengths, the microstructure of the grooves, as determined by their radii of curvature, correlates well with the microscopic topography of the fibers. This result indicates that the rubbing-induced topography depends on not only the characteristic rubbing strength, but on the structure of the rubbing fiber as well.