Towards Tomography-Based Real-Time Control of Multiphase Flows: A Proof of Concept in Inline Fluid Separation.

The performance of multiphase flow processes is often determined by the distribution of phases inside the equipment. However, controllers in the field are typically implemented based on flow variables, which are simpler to measure, but indirectly connected to performance (e.g., pressure). Tomography has been used in the study of the distribution of phases of multiphase flows for decades, but only recently, the temporal resolution of the technique was sufficient for real-time reconstructions of the flow. Due to the strong connection between the performance and distribution of phases, it is expected that the introduction of tomography to the real-time control of multiphase flows will lead to substantial improvements in the system performance in relation to the current controllers in the field. This paper uses a gas-liquid inline swirl separator to analyze the possibilities and limitations of tomography-based real-time control of multiphase flow processes. Experiments were performed in the separator using a wire-mesh sensor (WMS) and a high-speed camera to show that multiphase flows have two components in their dynamics: one intrinsic to its nonlinear physics, occurring independent of external process disturbances, and one due to process disturbances (e.g., changes in the flow rates of the installation). Moreover, it is shown that the intrinsic dynamics propagate from upstream to inside the separator and can be used in predictive and feedforward control strategies. In addition to the WMS experiments, a proportional-integral feedback controller based on electrical resistance tomography (ERT) was implemented in the separator, with successful results in relation to the control of the distribution of phases and impact on the performance of the process: the capture of gas was increased from 76% to 93% of the total gas with the tomography-based controller. The results obtained with the inline swirl separator are extended in the perspective of the tomography-based control of quasi-1D multiphase flows.

Air humidity effects on water-drop icing.

The canonical problem of the icing of a water drop lying on a cold substrate is revisited to take into account the effects of atmospheric conditions on the icing front kinetics and on the tip formation. Here, we demonstrate both experimentally and theoretically that the air humidity induces liquid-vapor phase change at the icing droplet interface and that the associated heat transfer has a strong influence on both the icing front kinetics and the iced drop shape. The experimental results obtained in this study, as well as results from literature, compare well to a modified Stefan model accounting for the effects of humidity, showing a good agreement with the experimental data of both the front kinetics and tip angle.

Deformation of a Liquid Near an AFM Tip: Molecular Dynamics Approach.

The interaction between an atomic force microscopy (AFM) probe and a thin film of water deposited over a flat substrate is studied using molecular dynamics (MD). The effects of the film thickness and the probe radius on both the deformation height of the liquid interface and the distance of the jump to contact at which the liquid comes in direct contact with the probe are investigated. The dynamics of the surface deformation and the role of interface fluctuations are studied in detail. The systems considered belong to the thin-film regime described in a semianalytical model previously established by Ledesma-Alonso et al. (Langmuir 2013, 29, 7749-7757). MD simulations predict that for shallow films, both the distance at which the jump to contact occurs and the surface maximal deformation height increase steadily with the layer thickness regardless of the probe radius, which is in agreement with the previously proposed theoretical model. The deformation of the surface was shown to be unstable because of the strong effect of thermal fluctuations. For each of the considered systems, the film thickness was such that interface fluctuations induced the jump to contact. The comparison of the deformation obtained in MD with the profiles predicted by the continuous model points out the complementarity between the two approaches. The results of the molecular approach not only are consistent with those of the continuous model but also provide more information on the description of nanoscale phenomena. In particular, MD results point out the importance of fluctuations when it comes to the description of the particular dynamics of nanosystems involving soft interfaces. This shows the need to improve continuous models by complementing them with a molecular approach for a better accuracy.

Lifetime of Surface Bubbles in Surfactant Solutions.

Despite the prevalence of surface bubbles in many natural phenomena and engineering applications, the effect of surfactants on their surface residence time is not clear. Numerous experimental studies and theoretical models exist but a clear understanding of the film drainage phenomena is still lacking. In particular, theoretical work predicting the drainage rate of the thin film between a bubble and the free surface in the presence and absence of surfactants usually makes use of the lubrication theory. On the other hand, in numerous natural situations and experimental works, the bubble approaches the free surface from a certain distance and forms a thin film at a later stage. This article attempts to bridge these two approaches. In particular, in this article, we review these works and compare them to our direct numerical simulations where we study the coupled influence of bubble deformation and surfactants on the rising and drainage process of a bubble beneath a free surface. In the present study, the level-set method is used to capture the air-liquid interfaces, and the transport equation of surfactants is solved in an Eulerian framework. The axisymmetric simulations capture the bubble acceleration, deformation, and rest (or drainage) phases from nondeformable to deformable bubbles, as measured by the Bond number (Bo), and from surfactant-free to surfactant-coated bubbles, as measured by the Langmuir number (La). The results show that the distance h between the bubble and the free surface decays exponentially for surfactant-free interfaces (La = 0), and this decay is faster for nondeformable bubbles (Bo ≪ 1) than for deformable ones (Bo ≫ 1). The presence of surfactants (La > 0) slows the decay of h, exponentially for large bubbles (Bo ≫ 1) and algebraically for small ones (Bo ≪ 1). For Bo ≈ 1, the lifetime is the longest and is associated with the (Marangoni) elasticity of the interfaces.

van der Waals interaction between a moving nano-cylinder and a liquid thin film.

We study the static and dynamic interaction between a horizontal cylindrical nano-probe and a thin liquid film. The effects of the physical and geometrical parameters, with a special focus on the film thickness, the probe speed, and the distance between the probe and the free surface are analyzed. Deformation profiles have been computed numerically from a Reynolds lubrication equation, coupled to a modified Young-Laplace equation, which takes into account the probe/liquid and the liquid/substrate non-retarded van der Waals interactions. We have found that the film thickness and the probe speed have a significant effect on the threshold separation distance below which the jump-to-contact instability is triggered. These results encourage the use of horizontal cylindrical nano-probes to scan thin liquid films, in order to determine either the physical or geometrical properties of the latter, through the measurement of interaction forces.

Dynamics of a thin liquid film interacting with an oscillating nano-probe.

The dynamic interaction between a local probe and a viscous liquid film, which provokes the deformation of the latter, has been studied. The pressure difference across the air-liquid interface is calculated with a modified Young-Laplace equation, which takes into account the effects of gravity, surface tension, and liquid film-substrate and probe-liquid attractive interaction potentials. This pressure difference is injected into the lubrication approximation equation, in order to depict the evolution of a viscous thin-film. Additionally, a simple periodic function is added to an average separation distance, in order to define the probe motion. The aforementioned coupled equations, which describe the liquid film dynamics, were analysed and numerically solved. The liquid surface undergoes a periodic motion: the approaching probe provides an input energy to the film, which is stored by the latter by increasing its surface deformation; afterwards, when the probe moves away, an energy dissipation process occurs as the surface attempts to recover its original flat shape. Asymptotic regimes of the film surface oscillation are discerned, for extreme probe oscillation frequencies, and several length, wavenumber and time scales are yielded from our analysis, which is based on the Hankel transform. For a given probe-liquid-substrate system, with well-known physical and geometric parameters, a periodic stationary regime and instantaneous and delayed probe wetting events are discerned from the numerical results, depending on the combination of oscillation parameters. Our results provide an interpretation of the probe-liquid film coupling phenomenon, which occurs whenever an AFM test is performed over a liquid sample.

Nanoscale deformation of a liquid surface.

We study the interaction between a solid particle and a liquid interface. A semianalytical solution of the nonlinear equation that describes the interface deformation points out the existence of a bifurcation behavior for the apex deformation as a function of the distance. We show that the apex curvature obeys a simple power-law dependency on the deformation. Relationships between physical parameters disclose the threshold distance at which the particle can approach the liquid before capillarity provokes a "jump to contact." A prediction of the interface original position before deformation takes place, as well as the attraction force measured by an approaching probe, are produced. The results of our analysis agree with the force curves obtained from atomic force microscopy experiments over a liquid puddle.

Lift, drag and added mass of a hemispherical bubble sliding and growing on a wall in a viscous linear shear flow.

The three-dimensional flow around a hemispherical bubble sliding and growing on a wall in a viscous linear shear flow is studied numerically by solving the full Navier-Stokes equations in a boundary-fitted domain. The main goal of the present study is to provide a complete description of the forces experienced by the bubble (drag, lift and added mass) over a wide range of sliding and shear Reynolds numbers (0.01