Preparation of graded materials by laterally controlled template synthesis

A novel scheme is presented for the synthesis of graded materials by electrodeposition in porous insulating templates. Lateral control of copper electrodeposition in nanoporous alumina membranes is achieved by application of a lateral potential gradient on a thin Au film evaporated on the membrane, used as the cathode. Formation of metal gradients in the membranes is shown to occur under conditions where essentially no gradient is formed on similar bare electrodes. This is attributed to the permanent resistivity of the thin Au film between the pores, which does not disappear upon Cu deposition, allowing a potential gradient to be maintained. Formation of a copper gradient in porous alumina membranes by uniform deposition followed by gradient dissolution is also demonstrated. These results establish the feasibility of controlled electrodeposition and gradient formation in nanoporous insulating templates.     

Light Irradiation of Mouse Spermatozoa: Stimulation of In Vitro Fertilization and Calcium Signals

Irradiation of mouse spermatozoa by 630 nm He-Ne laser was found to enhance the intracellular calcium levels and fertilizing potential of these cells. The effect of light on calcium transport and on fertilization rate was abrogated in the absence of Ca²⁺during the irradiation time, indicating that the effect of light is Ca²⁺dependent. The stimulatory effect of light on Ca²⁺uptake was abolished in the presence of a voltage-dependent Ca²⁺-channel inhibitor nifedipine, indicating the involvement of a plasma membrane voltage-dependent Ca²⁺channel. Furthermore, the stimulatory effect of light was completely inhibited by the mitochondrial uncoupler FCCP, indicating that laser irradiation might affect the mitochondrial Ca²⁺transport mechanisms. A causal association between laser irradiation, reactive oxygen species (ROS) generation and sperm function was indicated by studies with ROS scavengers, superoxide dismutase (SOD) and catalase, and exogenous hydrogen peroxide. The SOD treatment, which enhanced H₂O₂ production, resulted in increased Ca²⁺uptake and enhanced fertilization rate. On the other hand, catalase, which decomposes H₂O₂, impaired the light-induced stimulation in Ca²⁺uptake and the fertilization rate. Taken together, the data suggest that H₂O₂ might be involved in the irradiation effects, and indeed laser irradiation enhances the production of H₂O₂, by spermatozoa. These results indicate that the effect of 630 nm He-Ne laser irradiation is mediated through the generation of H₂O₂ by the spermatozoa and that this effect plays a significant role in the augmentation of the sperm cells' capability to fertilize metaphase H-arrested eggs in vitro.     

Optimal coverage of convex regions

The problem of the coverage of convex regions with polygons and quadratic configurations of minimal volume is considered. The regions are presented as inequality constraints of a linear or nonlinear programming problem. It is shown that the problem of the optimal coverage with an arbitrary polygon can be reduced to a convex one of coverage with a multidimensional rectangle. If, however, rotation of the coordinate system is allowed, an additional nonconvex problem must be solved. It is also shown that, to find the minimal covering hypersphere or hyperellipsoid, one has to solve two convex programming problems. Algorithms and examples illustrating the feasibility of the proposed methods are presented.This work was supported by funds for the promotion of research at the Technion under Contract No. 190-515.The authors would like to express their indebtedness to Prof. Aharon Ben-Tal from Technion and to Prof. L. C. W. Dixon of the Hatfield Polytechnic for several valuable suggestions on an earlier draft of this paper.     

Dynamics of strongly entangled polymer systems: activated reptation

The effect of excluded-volume interactions on the reptation dynamics of long polymer chains is considered theoretically. It is shown that interactions give rise to an exponential increase of the reptation time, , if polymer chains are long enough: , where is the number of monomers per entanglement. We propose a novel dynamical mechanism of activated reptation implying that neighboring chains exchange conformations of their terminal fragments. It is shown that the exchange mechanism is compatible with the equilibrium polymer chain statistics and that it provides a bridge between the previous theories. Received: 25 July 1997 / Accepted: 8 October 1997     

Molecular visualization of conformation-triggered flow instability

A new type of flow fingering instability was observed in monolayer-thick polymer films as they spread on a solid substrate. Tracing the movement of individual molecules by atomic force microscopy enabled us to follow the development of the flow instability on the molecular level and to understand the underlying physical mechanism. The fingering instability was observed to be triggered by conformational changes of brushlike macromolecules in response to the pressure gradient driving the flow.     

Adsorption of polyelectrolytes at an oppositely charged surface

We develop a scaling theory of polyelectrolyte adsorption at an oppositely charged surface. At low surface charge densities, the thickness of the adsorbed layer is determined by the balance between electrostatic attraction to the charged surface and chain entropy. At high surface charge densities, it is determined by the balance between electrostatic attraction and short-range monomer-monomer repulsion. These different stabilizing mechanisms result in the nonmonotonic dependence of the layer thickness on the surface charge density.     

Some properties of two analytic functions associated with complex polynomials

Two basic analytic functions α(z) and β(z) defined in domains depending on the location of the zeros of a complex polynomial P(z) are given by $\text{P′}\text{P}\text{=}\text{n}\text{(z ? α)}$ and P = (z ? β)ⁿ. These functions are studied with respect to their growth and their Laurent expansion coefficients. Applications to the location of zeros of complex polynomials are indicated.     

Photoproduction of πΔ(1236) from protons below 1.1 GeV

A model for the annihilation of electron-positron pairs into hadrons for q² = s > 4 GeV² and up to the highest measured energies is developed. The main ingredients are point-like couplings between three resonances and photons and the high level of opening thresholds that follows from an effective Lagrangian formalism. The model gives an almost constant annihilation cross section, soft pions everywhere and at all energies, high multiplicities and a large absolute cross section. The details of all exclusive channels are also predicted. Other consequences of this point of view concern the discovery of hitherto ignored couplings in low-energy photon initiated reactions. We argue that the energy regime explored by SPEAR is not the asymptopia and we discuss possible further constancy of the cross section in a range of s.     

Bounds to frequencies of shafts in torsional vibration with restraints and attached masses

In this paper upper and lower bounds are given to eigenvalues of torsional vibration of the following uniform structures: free shaft elastically restrained at a point; fixed shaft elastically restrained at a point; free shaft with masses elastically attached at points; fixed shaft with a mass elastically attached over an interval. Upper bounds were obtained by using the well-known Rayleigh-Ritz procedure. The more difficult problem of obtaining lower bounds was resolved by using recently developed methods of Bazley and Fox. The various problems discussed in this paper illustrate the use of these lower bound methods and their effectiveness in producing excellent bounds.