The evolution of a magnetic field in a chaotic web is studied. The model flow possessing the web is closely related to the nearly integrable ABC flow with A=B and C≪1. The magnetic diffusivity is taken to be zero and the field is followed using the Cauchy solution. It is found that the flow folds the magnetic field constructively, in the sense that the average magnetic field in a chaotic region grows exponentially in time. This is suggestive of fast dynamo action, although the effect of diffusion of the strong streamwise magnetic field remains to be assessed.

We formulate the statistical mechanics of a two-dimensional inviscid incompressible fluid in a manner which, for the first time, respects all conservation laws. For a special case, we demonstrate that a mean-field theory is exact. A consequence of our arguments is that, in an inviscid fluid evolving from initial conditions to statistical equilibrium, only the energy and certain one-body integrals appear to be conserved. Our methods may be applied to a variety of Hamiltonian systems possessing an infinite number of conservation laws.

We report the observation of saturation with chain length of the second hyperpolarizability of a series of long, well-defined, conjugated oligothiophenes. This saturation occurs at the same length as that of the band gap and of the polarizability, and is attributed to a saturation of the effective conjugation length. Solvatochromic and thermochromic measurements indicate that conformational effects play a role.

The temporal instabilities of a linear array of vortices are studied experimentally; a short-wavelength instability is observed. The results are compared to a one-dimensional model of coupled nonlinear oscillators, leading to excellent agreement.

A one-dimensional theory of dynamical hysteresis in periodically switched bistable systems is formulated. The predictions of the theory are tested and verified by experiments on a bistable semiconductor laser. We find that the increase of the area of the hysteresis loop scales as the two-thirds power of the switching frequency. From this, we deduce a law for the increase in input power necessary to maintain repetitive switching of the bistable system as the driving frequency is increased.

Inhibition of the radiative decay of dye molecules embedded in an ordered aqueous suspension of polystyrene spheres has been measured using picosecond pump-probe techniques. The inhibition is interpreted in terms of excluded vacuum modes and compared with results based on the Wigner-Weisskopf formalism and a three-dimensional scattering model for the fcc colloidal structures.

Trapping of femtosecond optical solitons has been observed for the first time in an erbium-doped fiber amplifier operating at 1.55 μm. The pulse width of the trapped soliton is adiabatically narrowed by the moderate optical gain. By further increasing the gain, the trap is broken and the carrier wavelength of the soliton starts to move toward longer wavelengths by the soliton self-frequency shift.

We use the pressure generated by a small-amplitude oscillatory flow to probe the dynamics of moving contact lines. By measuring the Fourier amplitude at harmonics of the applied frequency as a function of the steady-state velocity, we are able to determine the velocity dependence of the excess dissipation caused by the moving contact line. We find that the pressure drop associated with this dissipation scales as a power of the contact-line velocity V as ${\mathit{P}}_{\mathrm{cap}}$∝${\mathit{V}}^{\mathit{x}}$, with x=0.40±0.05. We discuss these results in terms of recent theoretical models.

We have observed antiphase dynamical states in an N-mode laser with an intracavity doubling crystal. We have experimentally observed such states for N=3, while numerical simulations indicate their persistance up to (at least) N=5. Analogous dynamical states have been predicted for Josephson-junction arrays; in both cases, the multiplicity of coexisting attractors can be traced to the global nature of the coupling between degrees of freedom.

We report experiments on weakly nonlinear traveling-wave convection in an annular cell. The evolution of small-amplitude waves consists of the repetitive formation and sudden collapse of spatially localized pulses. This leads to continuously erratic dynamics with no stable saturated state, even near onset, and even when convection begins with a unidirectional, nearly spatially uniform state. Such behavior is reminiscent of simulations of the complex Ginzburg-Landau equation in the limit of strong nonlinear dispersion.

Analytical solutions are derived for the quasisteady shape and speed of a single wormhole resulting from the coupled processes of Darcian fluid motion and chemical dissolution in a soluble permeable material. For an initially unsaturated medium, two-dimensional solutions are obtained by addressing an inverted free-boundary problem in which the spatial coordinates are treated as dependent variables on the plane of a complex potential. For initially saturated materials, solutions are obtained by analogy to Ivantsov’s problem of dendrite growth.

The baroeffect is analyzed including a third nondiffusing component (gas 3). Gas 3 reduces the pressure drop in all cases, whether it raises or lowers the mean density. Four distinct regimes are identified for diffusive slip: positive slip directed from heavy to light gas; negative slip directed from light to heavy gas; a no-slip regime; and a fourth regime characterized by a maximum in diffusive slip and (contrary to the other regimes) an increase in positive slip upon addition of low-molecular-weight gas 3. Unlike binary gases alone, gas 3 leads to nonzero diffusive slip even in the limit of very disparate masses for the diffusing gases. Implications for controlling concentration creep in crystal growth are discussed.

The modulation instability of a two-dimensional field with a saturable self-focusing nonlinearity is investigated numerically. We show that approximate recurrence to an initial homogeneous field occurs, but oscillation around the exact recurrence is observed. This pseudorecurrence arises only for a restricted range of spatial modulation frequencies.

We present fluorescence spectra showing that the spontaneous-emission spectrum of a molecule in front of a mirror is not invariant on propagation. We also calculate the cross-spectral density of this light source and show that it does not obey Wolf’s scaling law [Phys. Rev. Lett. 56, 1370 (1986)].

The transition from traveling-wave to stationary convection in a binary-fluid mixture is studied as a function of Rayleigh number. The experiments are conducted in an annular container, which provides periodic boundary conditions in the direction of the wave motion. In agreement with the predictions of recent theories, the transition is shown to be continuous with no measurable hysteresis. The functional dependence of the wave phase velocity on Rayleigh number is in agreement with the predictions over the entire traveling-wave branch.

At equilibrium, Bloch walls are chiral interfaces between domains with different magnetization. Far from equilibrium, a set of forced oscillators can exhibit walls between states with different phases. In this Letter, we show that when these walls become chiral, they move with a velocity simply related to their chirality. This surprising behavior is a straightforward consequence of nonvariational effects, which are typical of nonequilibrium systems.

The Kolmogorov argument for the existence of an inertial range is reexamined in situations for which neither Fourier modes nor homogeneity and local isotropy are natural assumptions. Scaling arguments are shown which are still valid, and generalizations to the -5/3 law are given for the eigenvalue spectrum of the two-point velocity-correlation matrix. Results from several different numerical simulations are presented. Data derived from simulations of channel and convection flows show that a sensible inertial range appears at very modest Reynolds numbers.

The spectral density of temperature fluctuations in an anisotropic heated turbulent channel flow is studied experimentally using a new optical technique. The spectrum in the direction transverse to the flow shows an equilibrium subrange behavior characteristic of isotropic flows with a wavenumber dependence of ${\mathrm{\Phi }}_{\mathrm{\theta }}$(${\mathrm{\kappa }}_{\mathit{y}}$)∼${\mathrm{\kappa }}_{\mathit{y}}^{\mathrm{-}11/3}$, whereas in the flow direction, ${\mathrm{\Phi }}_{\mathrm{\theta }}$(${\mathrm{\kappa }}_{\mathit{x}}$)∼${\mathrm{\kappa }}_{\mathit{x}}^{\mathrm{-}9/3}$. This difference is shown to result from the dominance of the mean strain rate over the turbulent strain rate field at low wave numbers, and the observed slopes are explained on dimensional grounds.

We have employed an ultrasharp tip, prepared by field-ion techniques, as a coherent point source of low-energy electrons. The source, which constitutes the origin of a spherical electron wave, is positioned close to small carbon fibers, at which some of the electrons are elastically scattered. At macroscopic distances a hologram is observed, which arises from the interference of the scattered electrons with the reference wave. The high magnification of the interference pattern is brought about purely by geometric projection. No electron lenses are emloyed and, thus, the high-resolution microscopic information gained by this novel technique is not affected by aberrations.

We consider a spherical system composed of N concentric fluid shells having perturbations of amplitude ${\mathrm{\eta }}_{\mathit{i}}$ at interface i, i=1,2,...,N-1. For arbitrary implosion-explosion histories ${\mathit{R}}_{\mathit{i}}$(t), we present the set of N-1 second-order differential equations describing the time evolution of the ${\mathrm{\eta }}_{\mathit{i}}$ which are coupled to the two adjacent ${\mathrm{\eta }}_{\mathit{i}\pm 1}$. We report analytical solutions for the N=2 and N=3 cases. We also present a model to describe the evolution of a turbulent mixing layer in spherical geometry when the interface between two fluids undergoes a constant acceleration or a shock.