Diego M. Campana

Numerical prediction of the film thickening due to surfactants in the Landau–Levich problem

In this work numerical solutions of the dip coating problem in the presence of a soluble surfactant are shown. Predictions of film thickening as well as thickening factors are in very good agreement with published experimental data, showing that pure hydrodynamic modeling suffices to mimic the process. Our numerical solutions provide a wealth of information on the functioning of the dip coating system; they show the appearance of a second stagnation point located in the bulk phase near the dynamic meniscus and they give clues about how the flow patterns might change as the surfactant becomes less soluble.

Numerical analysis of the Rayleigh instability in capillary tubes: The influence of surfactant solubility

A two-dimensional (2D) free surface flow model already used to study the Rayleigh instability of thin films lining the interior of capillary tubes under the presence of insoluble surfactants [D. M. Campana, J. Di Paolo, and F. A. Saita, “A 2-D model of Rayleigh instability in capillary tubes. Surfactant effects,” Int. J. Multiphase Flow 30, 431 (2004)] is extended here to deal with soluble solutes. This new version that accounts for the mass transfer of surfactant in the bulk phase, as well as for its interfacial adsorption/desorption, is employed in this work to assess the influence of surfactant solubility on the unstable evolution. We confirm previously reported findings: surfactants do not affect the system stability but the growth rate of the instability [D. R. Otis, M. Johnson, T. J. Pedley, and R. D. Kamm, “The role of pulmonary surfactant in airway closure,” J. Appl. Physiol. 59, 1323 (1993)] and they do not change the successive shapes adopted by the liquid film as the instability develops [S. Kw...

A deeper insight into the dip coating process in the presence of insoluble surfactants: A numerical analysis

A numerical investigation is carried out to study the effects of an insoluble surfactant on the dip coating of a flat substrate. Predictions of both the film thickness and the concentration of surfactant in the film as a function of the capillary number compare well with the solutions of a simpler asymptotic model based on the lubrication approximation. Streamline patterns confirm the existence of a stagnation point located in the bulk phase in the region of the dynamic meniscus—a conjecture postulated forty years ago. The evolution of the flow patterns and the interfacial variables shows how the classical result of Landau and Levich is recovered as the coating speed is augmented. Finally, we show that the effect of inertia forces cannot be neglected when the viscosity of the coating liquid is low.

The influence of inertia and contact angle on the instability of partially wetting liquid strips: A numerical analysis study

The stability of a thread of fluid deposited on a flat solid substrate is studied numerically by means of the Finite Element Method in combination with an Arbitrary Lagrangian-Eulerian technique. A good agreement is observed when our results are compared with predictions of linear stability analysis obtained by other authors. Moreover, we also analysed the influence of inertia for different contact angles and found that inertia strongly affects the growth rate of the instability when contact angles are large. By contrast, the wave number of the fastest growing mode does not show important variations with inertia. The numerical technique allows us to follow the evolution of the free surface instability until comparatively late stages, where the filament begins to break into droplets. The rupture pattern observed for several cases shows that the number of principal droplets agrees reasonably well with an estimation based on the fastest growing modes.

Instability of a viscous liquid coating a cylindrical fibre

The instability of a liquid layer coating the surface of a thin cylindrical wire is studied experimentally and numerically with negligible gravity effects. The initial uniform film is obtained as the residual of a sliding drop, and the thickness measurements are performed with an anamorphic optical system that compresses the vertical scale (allowing to observe several wavelengths) and widens the horizontal one (to follow in detail the evolution of local minima and maxima). Experimental timelines showing the growth and position of the maxima and minima are compared with linear theory and fully nonlinear simulations. A primary mode grows in the early stages of the instability, and its wavelength λ 1 is not always in agreement with that corresponding to the maximum growth rate predicted by the linear theory λ m . In later stages, a secondary mode appears, whose wavelength is half that of the primary mode. The behaviour of the secondary mode allows us to classify the experimental results into two cases, depending on whether it is linearly stable (case I) or unstable (case II). In case I, the amplitude of the secondary mode remains small compared with that of the primary one, while in case II both amplitudes may become very similar at the end. Thus, the distance between the final drops may be quite different from that seen between initial protuberances. The analysis of the experiments allows us to define a simple criterion based on the comparison between λ 1 and λ m . Contrary to the predictions of widely used previous quasi-static theories, experiments show that the relation between maximum and minimum of the primary mode is better approximated by a kinematic model based on the assumption that primary maxima increase as fast as the minima decrease. Numerical simulations confirm this hypothesis.

Contraction of Surfactant-Laden Pores

The contraction of surfactant-laden pores at the microscale has implications for natural and technological processes ranging from the collapse of channels in lipid membranes to the stability of foams in the food processing industry. Despite their prevalence, our understanding of the mechanisms of pore contraction in the presence of surfactants remains unclear. These mechanisms have been challenging to study experimentally given the small length scale near the singularity and simulations capable of accurately characterizing the pore dynamics may help enhance our understanding of the process. Here, we use high-fidelity numerical simulations to gain insight into the fluid dynamics and interfacial phenomena underlying the contraction of viscous pores in the presence of an insoluble surfactant. Results show that surfactants accumulate on the advancing front of a collapsing pore due to the uneven deformation of the pore interface. Because of this accumulation, even a small amount of surfactant plays a major role in the way in which a collapsing pore approaches the singularity.

Computer Simulation of the Blood Flow in a Planar Configuration for a Pulsatile Ventricular Assist Device

Patients that suffer congestive cardiac insufficiency need to be assisted with assist devices (VAD) as a temporal solution when the cardiac transplantation is not possible. For that reason it is of great interest to know the performance of such devices, especially the potential blood damage that they can produce. In this work, a computational twodimensional simulation of a novel pulsatile VAD is performed. This new design of VAD has a double acting piston driven electromagnetically at 2.1 Hz. In addition, the VAD has four active valves. The flow velocities, the pressure and the shear stress developed in the blood are analyzed in the chambers and inflow and outflow conduits. The results suggest that the developed flow would not be dangerous for the blood.

Effects of gravity on the stability of the steady propagation of a liquid plug in a small conduit

In this work we numerically study the stability of the steady state displacement of a liquid plug in a capillary tube when gravity, inertia and surface forces are important. The methodology employed is based on the analysis of steady state solutions and has been presented in previous publications. Gravity is assumed to act only along the axis of the tube.