Entropic characterization of the coil-stretch transition of polymers in random flows.

Polymer molecules in a flow undergo a coil-stretch phase transition on an increase of the velocity gradients. Model-independent identification and characterization of the transition in a random flow has been lacking so far. Here we suggest to use the entropy of the extension statistics as a proper measure due to strong fluctuations around the transition. We measure experimentally the entropy as a function of the local Weisenberg number and show that it has a maximum, which identifies and quantifies the transition. We compare the new approach with the traditional one based on the theory using either linear Oldroyd-B or nonlinear finite extensible nonlinear elastic polymer models.

Dynamics of a vesicle in general flow.

An approach to quantitatively study vesicle dynamics as well as biologically-related micro-objects in a fluid flow, which is based on the combination of a dynamical trap and a control parameter, the ratio of the vorticity to the strain rate, is suggested. The flow is continuously varied between rotational, shearing, and elongational in a microfluidic 4-roll mill device, the dynamical trap, that allows scanning of the entire phase diagram of motions, i.e., tank-treading (TT), tumbling (TU), and trembling (TR), using a single vesicle even at lambda = eta(in)/eta(out) = 1, where eta(in) and eta(out) are the viscosities of the inner and outer fluids. This cannot be achieved in pure shear flow, where the transition between TT and either TU or TR is attained only at lambda>1. As a result, it is found that the vesicle dynamical states in a general are presented by the phase diagram in a space of only 2 dimensionless control parameters. The findings are in semiquantitative accord with the recent theory made for a quasi-spherical vesicle, although vesicles with large deviations from spherical shape were studied experimentally. The physics of TR is also uncovered.

Phase diagram of single vesicle dynamical states in shear flow.

We report the first experimental phase diagram of vesicle dynamical states in a shear flow presented in a space of two dimensionless parameters suggested recently by V. Lebedev et al. To reduce errors in the control parameters, 3D geometrical reconstruction and determination of the viscosity contrast of a vesicle in situ in a plane Couette flow device prior to the experiment are developed. Our results are in accord with the theory predicting three distinctly separating regions of vesicle dynamical states in the plane of just two self-similar parameters.

Shear instability in fluids with a density-dependent viscosity.

We present a shear instability, which can be triggered in compressible fluids with density-dependent viscosity at shear rates above critical. The instability mechanism is generic: It is based on density-dependent viscosity, compressibility, as well as flow two-(three-)dimensionality that provides coupling between streamwise and transversal velocity components and density variations. The only factor stabilizing the instability is fluid elasticity. The corresponding eigenvalue problem for a plane Couette flow is solved analytically in the limiting cases of large and small wave numbers.

Non-newtonian viscosity of complex-plasma fluids.

Investigations of shear flows in three-dimensional complex-plasma fluids produced in a dc discharge were carried out. The shear was induced either by an inhomogeneous gas flow or by a laser beam. The viscosity of complex plasmas was measured over a broad range of shear rates, up to the hydrodynamic limit when the discreteness becomes important. Analysis of the measurements reveals non-Newtonian behavior of complex plasmas accompanied by substantial shear thinning.

Mixing by polymers: experimental test of decay regime of mixing.

By using high molecular weight fluorescent passive tracers with different diffusion coefficients and by changing the fluid velocity we study the dependence of a characteristic mixing length on the Peclet number, Pe, which controls the mixing efficiency. The mixing length is found to be related to Pe by a power law, L(mix) proportional, variant Pe0.26+/-0.01, and increases faster than expected for an unbounded chaotic flow. The role of the boundaries in the mixing length abnormal growth is clarified. The experimental findings are in good quantitative agreement with recent theoretical predictions.

Vertical pairing of identical particles suspended in the plasma sheath.

It is shown experimentally that vertical pairing of two identical microspheres suspended in the sheath of a radio-frequency (rf) discharge at low gas pressures (a few Pa) appears at a well-defined instability threshold of the rf power. The transition is reversible, but with significant hysteresis on the second stage. A simple model which uses measured microsphere resonance frequencies and takes into account, in addition to the Coulomb interaction between negatively charged microspheres, their interaction with positive-ion-wake charges, seems to explain the instability threshold quite well.

Onset of wave drag due to generation of capillary-gravity waves by a moving object as a critical phenomenon.

The onset of the wave resistance, via generation of capillary-gravity waves of a small object moving with velocity V, is investigated experimentally. Because of the existence of a minimum phase velocity V(c) for surface waves, the problem is similar to the generation of rotons in superfluid helium near their minimum. In both cases waves or rotons are produced at V>V(c) due to Cherenkov radiation. We find that the transition to the wave drag state is continuous: in the vicinity of the bifurcation the wave resistance force is proportional to sqrt[V-V(c)] for various fluids.

Efficient mixing at low Reynolds numbers using polymer additives.

Mixing in fluids is a rapidly developing area in fluid mechanics, being an important industrial and environmental problem. The mixing of liquids at low Reynolds numbers is usually quite weak in simple flows, and it requires special devices to be efficient. Recently, the problem of mixing was solved analytically for a simple case of random flow, known as the Batchelor regime. Here we demonstrate experimentally that very viscous liquids containing a small amount of high-molecular-weight polymers can be mixed quite efficiently at very low Reynolds numbers, for a simple flow in a curved channel. A polymer concentration of only 0.001% suffices. The presence of the polymers leads to an elastic instability and to irregular flow, with velocity spectra corresponding to the Batchelor regime. Our detailed observations of the mixing in this regime enable us to confirm several important theoretical predictions: the probability distributions of the concentration exhibit exponential tails, moments of the distribution decay exponentially along the flow, and the spatial correlation function of concentration decays logarithmically.

Stretching of polymers in a random three-dimensional flow.

Behavior of a dilute polymer solution in a random three-dimensional flow with an average shear is studied experimentally. Polymer contribution to the shear stress is found to be more than 2 orders of magnitude higher than in a laminar shear flow. The results indicate that the polymer molecules are strongly stretched by the random motion of the fluid.

NonliNEAR vertical oscillations of a particle in a sheath of a rf discharge.

A new simple method to measure the spatial distribution of the electric field in the plasma sheath is proposed. The method is based on the experimental investigation of vertical oscillations of a single particle in the sheath of a low-pressure radio-frequency discharge. It is shown that the oscillations become strongly nonlinear as the amplitude increases. The theory of anharmonic oscillations provides a good quantitative description of the data and gives estimates for the first two anharmonic terms in an expansion of the sheath potential around the particle equilibrium.

Rigid and differential plasma crystal rotation induced by magnetic fields

Observations show that plasma crystals, suspended in the sheath of a radio-frequency discharge, rotate under the influence of a vertical magnetic field. Depending on the discharge conditions, two different cases are observed: a rigid-body rotation (all the particles move with a constant angular velocity) and sheared rotation (the angular velocity of particles has a radial distribution). When the discharge voltage is increased sufficiently, the particles may even reverse their direction of motion. A simple analytical model is used to explain qualitatively the mechanism of the observed particle motion and its dependence on the confining potential and discharge conditions. The model takes into account electrostatic, ion drag, neutral drag, and effective interparticle interaction forces. For the special case of rigid-body rotation, the confining potential is reconstructed. Using data for the radial dependence of particle rotation velocity, the shear stresses are estimated. The critical shear stress at which shear-induced melting occurs is used to roughly estimate the shear elastic modulus of the plasma crystal. The latter is also used to estimate the viscosity contribution due to elasticity in the plasma liquid. Further development is suggested in order to quantitatively implement these ideas.

Elastic turbulence in a polymer solution flow

Turbulence is a ubiquitous phenomenon that is not fully understood. It is known that the flow of a simple, newtonian fluid is likely to be turbulent when the Reynolds number is large (typically when the velocity is high, the viscosity is low and the size of the tank is large). In contrast, viscoelastic fluids such as solutions of flexible long-chain polymers have nonlinear mechanical properties and therefore may be expected to behave differently. Here we observe experimentally that the flow of a sufficiently elastic polymer solution can become irregular even at low velocity, high viscosity and in a small tank. The fluid motion is excited in a broad range of spatial and temporal scales, and we observe an increase in the flow resistance by a factor of about twenty. Although the Reynolds number may be arbitrarily low, the observed flow has all the main features of developed turbulence. A comparable state of turbulent flow for a newtonian fluid in a pipe would have a Reynolds number as high as 10(5) (refs 1, 2). The low Reynolds number or 'elastic' turbulence that we observe is accompanied by significant stretching of the polymer molecules, resulting in an increase in the elastic stresses of up to two orders of magnitude.

Health implications of nitrate and nitrite in drinking water: an update on methemoglobinemia occurrence and reproductive and developmental toxicity.

In 1987, an evaluation of the nitrate drinking water standard was performed with a primary focus on the effects of nitrate on methemoglobinemia and reproductive/developmental effects (Fan et al. (1987). Regul. Toxicol. Pharmacol. 7, 135-148). The present review presents an updated overview and evaluation of the available information on the same health effects of nitrate and nitrite with an emphasis on data not included in the previous review, which should be used as a compendium to this report. Recent epidemiologic data have suggested an association between developmental effects in offspring and the maternal ingestion of nitrate from drinking water, but a definite conclusion on the cause and effect relationship cannot be drawn. Animal experimental data have shown reproductive toxicity associated with high exposure levels to nitrate or nitrite, which are not likely to be encountered in drinking water. No teratogenic effects were observed in rats, mice, rabbits, and hamsters tested. Several cases of methemoglobinemia have been reported in infants in the United States using water containing nitrate at levels higher than the current maximum contaminant level (MCL) of 45 ppm (mg/liter) nitrate (NO3) or 10 ppm nitrate-nitrogen (nitrate-N), but none at or lower than the MCL. The uncertainties in the data base are discussed, noting that no uncertainty factor was applied in deriving the MCL in order to account for the uncertainties that exist in the data base.