A large transient surge of shear viscosity is observed in the course of spinodal demixing caused by off-critical quenching of a polymer solution. A characteristic length is derived from light scattering measurements along with a related relaxation time ${\mathrm{\tau }}_{\mathit{M}}$ whose value agrees with previous independent experiments. Viscosity peak values plotted versus S${\mathrm{\tau }}_{\mathit{M}}$ (where S is the shear rate) lie on a master curve of the type (S${\mathrm{\tau }}_{\mathit{M}}$${)}^{\mathrm{-}\mathit{p}}$, where p≊2. This scaling procedure helps bring into focus the mechanism responsible for the novel observation.

A free-electron laser operating in the gain-focusing regime is discussed. The variation of growth rate, radius of curvature of wave fronts, filling factor and efficiency with emittance, and energy spread is derived. The reults, which are based on the Vlasov-Maxwell system of equations, are obtained by minimizing a variational functional. When plotted as a function of emittance, the efficiency at maximum growth rate peaks at a nonzero value of emittance. For small values of energy spread, the efficiency at maximum growth rate increases with energy spread, in contrast to intuitive expectations.

We present the first experimental demonstration of phase locking between successive optical micropulses in a free-electron laser in which the electron pulses are separated by a fraction of the optical-cavity round-trip time. A Fox-Smith interferometer arrangement has been used to induce a high degree of interpulse phase coherence. Surprisingly, a substantial amount of coherence has also been observed without the phase-locking arrangement. A tentative explanation of this phenomenon is given.

We report on experimental observations of spatial modifications, induced by the Kerr effect, in the profile of a weak probe beam due to a cross-phase modulation from a strong pump beam. The measurements were performed in a self-defocusing nonlinear medium (semiconductor doped glass) where the observation of induced focusing and spatial modulation instability were made possible. The experimental results are in good agreement with numerical simulations.

We analyze line measurements of turbulent shear stress in a turbulent boundary layer in terms of isoshear contours. The statistics of the relative location of contour center lines points to a relation with coherent structures. The shape of these contours exhibits a remarkable scaling behavior that is reminiscent of that found in spatiotemporal chaos.

We propose a simple ansatz that relates the diffusion propagator for the molecules of a fluid confined in a porous medium to the pore-space structure factor. Theoretical arguments and numerical simulations show that it works well for both periodic and disordered geometries. The ansatz allows us to deconvolve structural data from momentum dependent pulsed field gradient spin-echo data.

We report on the first successful measurements of the spectral width of an x-ray laser line. The experiments observed the 206.38-Å laser in Ne-like Se and indicate an unamplified width of 50 mÅ, show gain narrowing to 10 mÅ in intermediate length amplifiers, and show no significant rebroadening in saturated amplifiers. The unamplified width is 1.4 times the expected Doppler width, and the lack of rebroadening leads to speculation that collisional effects may play a significant role in determining the line profiles.

A turbulent homogeneous perfect gas with no forcing is computed using the piecewise-parabolic method on a uniform grid of ${256}^3$ zones. The initial rms Mach number is one. The main results are (i) persistence of a quasisupersonic phase with strong density contrasts over several shock formation times; (ii) late appearance of a post-supersonic self-similarly decaying regime in which the total velocity spectrum varies with wave number as ${\mathit{k}}^{\mathrm{-}1}$; and (iii) both shock curvature and velocity gradients are important mechanisms of vorticity production in the first half of the supersonic phase.

An experimental study of the parametric instability of a liquid-vapor interface close to the critical point is presented. A cell containing carbon dioxide at the critical density is vertically vibrated sinusoidally; the acceleration amplitude ${\mathit{a}}_0$ and the instabiilty wavelength ${\lambda }_0$ at the instabiilty onset are measured with respect to the excitation frequency and the critical temperature difference ${\mathit{T}}_{\mathit{c}}$-T. These measurements provide a simple way to determine the surface tension and the density difference between the two phases. When ${\mathit{T}}_{\mathit{c}}$-T diminishes ${\lambda }_0$ decreases and reaches a finite value whereas ${\mathit{a}}_0$ diverges. The shape of the wave pattern appears to depend on ${\mathit{T}}_{\mathit{c}}$-T.

The slow upward migration, due to buoyancy, of a nonwetting fluid thorugh a saturated, three-dimensional porous medium has been studied experimentally, and by computer simulations. The height h of the invading fluid structure is proportional to the mass M of the invading fluid. The linear mass density S=M/h is found to scale with the dimensionless Bond number Bo (ratio of bouyancy and capillary forces) as S=A‖Bo${\mathrm{\Vert }}^{\mathrm{-}\mathrm{\alpha }}$, with α=0.72±0.06, consistent with theory and simulations. A noisy behavior in h(M) results from dynamical processes such as snapoffs and bursts.

We examine the derivation of eddy-diffusivity equations for transport of passive scalars in a turbulent velocity field. Our main contention is that, in the long-time–large-distance limit, the eddy-diffusivity equations can take very different forms according to the statistical properties of the subgrid velocity, and that these equations depend very sensitively on the interplay between spatial and temporal velocity fluctuations. Such crossovers can be represented in a ‘‘phase diagram’’ involving two relevant statistical parameters. Strikingly, the Kolmogorov-Obukhov statistical theory is shown to lie on a phase-transition boundary.

Amplification of spontaneous emission (ASE) at 23.6 nm has been studied in a Ge plasma heated by a 1 TW infrared laser pulse. The exponent of the axial gain reached 21 in a geometry with Fresnel number ≤1. Two plasma columns of combined length up to 36 mm were used with an extreme ultraviolet mirror giving double-pass amplification. Saturation of the ASE output was observed. The beam divergence was about 8× diffraction limited with a brightness estimated at ${10}^{14}$ W ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{sr}}^{\mathrm{-}1}$. The feedback from the mirror was significantly reduced probably by radiation damage from the plasma.

We present and test a method for simulating the dynamics of turbulence using a strongly reduced subset of Fourier modes with vector triadic interactions. The method allows a resolution of (10 000${)}^2$ in two dimensions and (3000${)}^3$ in three dimensions on present-day computers.

A family of scale-invariant, base-dependent, multiplier distributions is measured for the turbulence dissipation field in the atmospheric surface layer. The existence of these distributions implies the existence of the more traditional multifractal scaling functions, and we compute both positive and negative parts of the f(α) curve. The results support the conjecture of universality in the scaling properties of small-scale turbulence. A simple cascade model based on the measured multiplier distributions is shown to possess several advantages over previously considered models.

Measurements have been made of the probability density function (PDF) of velocity increments Δu(r) for a wide range of separation distances r. Stretched exponentials provide good working approximations to the tails of the PDF. The stretching exponent varies monotonically from 0.5 for r in the dissipation range to 2 for r in the integral scale range. Theoretical forms based on multifractal notions of turbulence agree well with the measured PDFs. When the largest scales in the velocity u are filtered out, the PDF of Δu(r) becomes symmetric and, for large r, close to exponential.

The temporally growing shear layer produced by tilting a tube filled with two immiscible fluids of different density is spatially forced via modulations of one of the lateral boundaries. The competition between the forcing wave number ${\mathit{k}}_{\mathit{f}}$ and the intrinsic wave number ${\mathit{k}}_{\mathit{n}}$ leads to a mode diagram that is compatible with corresponding observations of spatial mixing layers. As the detuning is gradually increased, the interface displays stationary phase solitons that are indicative of the existence of incommensurate states.

By means of a phase-conjugation experiment by nondegenerate six-wave mixing, we demonstrate a transient ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ grating induced by a nonzero ${\mathit{E}}^3$ time average in a solution of noncentrosymmetric dye molecules. The origin of the induced phase-matched second-harmonic generation is found in an orientational hole-burning effect. Its lifetime, limited by molecule rotation, is 210±10 psec and the ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ magnitude reaches 2.5×${10}^{\mathrm{-}3}$ pm/V for 1 GW ${\mathrm{cm}}^{\mathrm{-}2}$ pumps at 1064 nm.

We have studied the vibrations in a granular material and have found that they are qualitatively different from those found in solids or liquids. In response to an external oscillation, the vibrations experienced by a single particle in the material are exceedingly noisy with a power spectrum S(f)∝${\mathit{f}}^{2.2}$ over more than 5 decades in frequency. The frequency response depends on the detailed packing of the beads, reminiscent of conductance fluctuations in mesoscopic metals.

We report the observation of ordered capillary-wave states generated by three and four standing plane waves. The pattern produced by the four standing plane waves forms the first quasicrystal observed in a fluid dynamical system. The states are observed at aspect ratios above 45, and for amplitudes below the amplitude at which a square-symmetric state is formed. We also report the observation of a capillary-wave state generated by three radial waves.

Pressure fluctuations measured during slow drainage in a two-dimensional porous model exhibit sudden jumps that identify bursts where the invasion front proceeds abruptly. The pressure-jump size distribution is observed to be exponential. The nonscaling dynamics created fractal fronts and invaded regions described by invasion percolation. A new modified invasion percolation algorithm includes invasion dynamics. A capacitive volume associated with each interface throat results in a crossover from power-law behavior to an exponential pressure jump distribution consistent with observations.