Guy Z. Ramon

Membrane-based production of salinity-gradient power

This perspective paper outlines the fundamental principles and state-of-the-art of membrane-based conversion of salinity-gradient energy, a renewable and environmentally benign energy source receiving increased attention in recent years. In particular, an attempt is made to identify the most important and promising directions for future research and technological innovation.

Capillary rise of a meniscus with phase change.

The Lucas-Washburn equation, describing the motion of a liquid body in a capillary tube, is extended to account for the effect of phase change - evaporation or condensation. The system is found to always possess a stable equilibrium state when the temperature jump across the interface is confined to a certain range. We show that phase change affects the equilibrium height of the meniscus, the transition threshold from monotonic to oscillatory dynamics, and the frequency of oscillations, when present. At higher mass transfer rates and/or large capillary radii, vapor recoil is found to be the dominant factor. Evaporation lowers the equilibrium height, increases the oscillation frequency and lowers the transition threshold to oscillations. For condensation, two regimes are identified: at high mass transfer rates similar trends to those of evaporation are observed, whereas the opposite is found for low mass transfer rates, resulting in an increased equilibrium height, lower oscillation frequencies and a shift of the transition threshold toward monotonic dynamics.

On the hydrodynamic interaction between a particle and a permeable surface

The motion and deposition of a particle translating perpendicular to a rigid, permeable surface is considered. The lubrication approximation is used to derive an equation for the pressure in the gap between the particle and the permeable surface, with a symmetric shape prescribed for the particle. The hydrodynamic force on a particle is, in general, a function of the particle size and shape, the distance from the surface and the surface permeability, and its sign depends on the relative motion of the particle and the background fluid permeating through the surface. As the particle becomes flatter, this force generally increases and is more sensitive to the surface permeability. In the case of a spherical particle, closed-form, approximate solutions are obtained using perturbation methods, in the limits of small permeability and close approach to contact. It is also shown that a sedimenting particle attains a finite velocity on close approach, which scales as k1/2 and k for a sphere and a disc, respectivel...

Theory of neutral and charged exciton scattering with electrons in semiconductor quantum wells

Electron scattering on both neutral (X) and charged (X - ) excitons in quantum wells is studied theoretically. A microscopic model is presented, taking into account both elastic and dissociating scattering. The model is based on calculating the exciton-electron direct and exchange interaction matrix elements, from which we derive the exciton scattering rates. Scattering by electrons is found to be an efficient process even for very low electron densities. In particular, the charged exciton linewidth due to electron scattering is larger than that of the neutral exciton, partially because of the larger contribution from the dissociating process. Calculated reflection spectra are then obtained by considering the three electronic excitations of the system, namely, the heavy-hole and light-hole 1S neutral excitons, and the heavy-hole IS charged exciton, with the appropriate oscillator strengths.

The effective flux through a thin-film composite membrane

Composite membrane structures, used extensively in separation processes, comprise an ultra-thin selective polymer film cast over a porous support, whose pores partially obstruct transport out of the top film. Here, we model the composite as a finite thickness slab with a periodic array of circular absorbing patches in an otherwise reflective surface and study the effective transport properties of the composite. We obtain an analytical approximation for the effective diffusive flux as a function of the geometrical parameters, namely the film thickness, the support porosity and the pore size. We find a good agreement with full numerical solutions, and that a good effective rate is achievable with a relatively small number of pores.

Elastic Relaxation of Fluid-Driven Cracks and the Resulting Backflow

Cracks filled with fluid propagation when the pressurized fluid is injected into the crack. Subsequently, when the fluid inlet is exposed to a lower pressure, the fluid flows backwards (backflow) and the crack closes due to the elastic relaxation of the solid. Here we study the dynamics of the crack closure during the backflow. We find that the crack radius remains constant and the fluid volume in the crack decreases with time in a power-law manner at late times. The balance between the viscous stresses in the fluid and elastic stresses in the fluid and the elastic stresses in the solid yields a scaling law that agrees with the experimental results for different fluid viscosities, Youngs moduli of the solid, and initial radii of the cracks. Furthermore, we visualize the time-dependent crack shapes, and the convergence to a universal dimensionless shape demonstrates the self-similarity of the crack shapes during the backflow process.

Direct observation of macromolecular deposition on a nanofiltration membrane

ABSTRACT The mechanistic aspects of membrane fouling have been studied extensively using direct observation, but limited to early stages of particulate deposition. Herein, we describe a versatile method extending direct observation to organic fouling, facilitating real-time monitoring of fouling formation and detachment. A transparent gel-like fouling layer is visualized using epi-fluorescent microscopy with the aid of labeled marker beads, trapped at various distances from the membrane, enabling monitoring of variations within the deposit. Fouling and cleaning experiments were conducted, examining alginate deposition and detachment, and illustrating the utility of the proposed method for studying the kinetics of fouling processes.

Magnetic field effect on the free electron–exciton scattering in GaAs/AlGaAs bare quantum wells and in microcavities

Abstract We report on a detailed study of the free electron scattering effects on the (e1:hh1)1S and (e1:lh1)1S excitons in a GaAs quantum well with a variable density two-dimensional electron gas, that is either bare or embedded in a GaAs/AlGaAs microcavity. These effects are studied by measuring the reflection line width of the bare excitons or cavity polaritons as a function of photoexcitation intensity, temperature (2 K ) and a perpendicularly applied magnetic field (0 T ). This field induces the formation of charged polaritons at temperatures higher than the range of charged exciton existence without a magnetic field. In order to interpret the line-width data observed at high temperatures, when no charged polaritons exist, we developed a theoretical model that is based on calculating the exciton–electron direct and exchange interaction matrix elements, from which we derive the scattering rates of bare excitons. The model accounts well for the polariton line-widths dependence on microcavity-photon detuning energy and the electron density.

Field-Induced Redistribution of Surfactants at the Oil/Water Interface Reduces Membrane Fouling on Electrically Conducting Carbon Nanotube UF Membranes

Membrane-based treatment of oily wastewater remains a significant challenge, particularly under high salinity conditions. The main difficulty associated with this separation process is membrane fouling, mostly caused by wetting and coalescence of emulsified oil droplets on the membrane surface. In this study, electrically conducting carbon nanotube-based ultrafiltration membranes were used to treat an emulsified oil suspension at ionic strengths as high as 100 mM. By tuning the electrical potential applied to the membrane surface, we demonstrate how fouling can be dramatically reduced, even under high salinity conditions. Permeate water quality is shown to improve upon application of a negative potential. Using optical microscopy, we observed dramatic changes in the shape of oil droplets at the membrane/water interface in response to the applied electric potential; this change is associated with a redistribution of charged surfactant molecules at the oil/water interface in response to the external electric field. Specifically, using the membrane as a cathode repels surfactant molecules away from the oil/membrane interface, while anodic conditions lead to increased surfactant concentrations. We speculate that this change in surfactant molecule distribution is responsible for changes in the surface tension of oil droplets at the membrane/water interface, which results in a decrease in oil coalescence and subsequent fouling. The membranes used in this study offer an attractive treatment option when separating emulsified oil from water under high salinity conditions.

Modeling of micro-scale thermoacoustics

Thermoacoustic phenomena, that is, onset of self-sustained oscillations or time-averaged fluxes in a sound wave, may be harnessed as efficient and robust heat transfer devices. Specifically, miniaturization of such devices holds great promise for cooling of electronics. At the required small dimensions, it is expected that non-negligible slip effects exist at the solid surface of the “stack”-a porous matrix, which is used for maintaining the correct temporal phasing of the heat transfer between the solid and oscillating gas. Here, we develop theoretical models for thermoacoustic engines and heat pumps that account for slip, within the standing-wave approximation. Stability curves for engines with both no-slip and slip boundary conditions were calculated; the slip boundary condition curve exhibits a lower temperature difference compared with the no slip curve for resonance frequencies that characterize micro-scale devices. Maximum achievable temperature differences across the stack of a heat pump were also...