Juan Fernandez

Density-driven unstable flows of miscible fluids in a Hele-Shaw cell

Density-driven instabilities between miscible fluids in a vertical Hele-Shaw cell are investigated by means of experimental measurements, as well as two- and three-dimensional numerical simulations. The experiments focus on the early stages of the instability growth, and they provide detailed information regarding the growth rates and most amplified wavenumbers as a function of the governing Rayleigh number Ra . They identify two clearly distinct parameter regimes: a low- Ra , ‘Hele-Shaw’ regime in which the dominant wavelength scales as Ra −1 , and a high- Ra ‘gap’ regime in which the length scale of the instability is 5±1 times the gap width. The experiments are compared to a recent linear stability analysis based on the Brinkman equation. The analytical dispersion relationship for a step-like density profile reproduces the experimentally observed trend across the entire Ra range. Nonlinear simulations based on the two- and three-dimensional Stokes equations indicate that the high- Ra regime is characterized by an instability across the gap, wheras in the low- Ra regime a spanwise Hele-Shaw mode dominates.

Viscous fingering with chemical reaction: effect of in-situ production of surfactants

Viscous fingering experiments are performed in a radial Hele-Shaw cell for a liquid-liquid system in the presence of a well-characterized interfacial reaction capable of changing the surface tension on the time scale of the experiments. The reaction is a neutralization of a fatty acid by an alkaline material to form a surfactant, which exhibits first-order kinetics for the surface tension as a function of time. The experiments are carried out for capillary numbers, Ca, high enough for the fingering to always be in the fractal regime, and for a wide range of Damkohler numbers, Da. The fingers are typically wider in the presence of the chemical reaction than the non-reactive case. We observe two different-behaviours of the reactive fingering patterns

Wavelength selection of fingering instability inside Hele–Shaw cells

Fingering instabilities involving fluids confined between two plates sometimes give rise to a typical wavelength λ proportional to the gap h. This unexplained behavior is investigated for the case of the Rayleigh–Taylor instability between two liquids of the same viscosity. Using qualitative scaling arguments and linear stability analysis for a simplified model of hydrodynamics, we show that, in the miscible case, h becomes a natural cut-off when diffusion is negligible, i.e., when the Peclet number Pe=h3Δρg/(ηD) is large (η viscosity, g gravitational acceleration, D diffusivity, Δρ density difference). The same result holds in the immiscible case for large capillary number Ca=h2Δρg/(12γ) (γ surface tension). In this saturation regime, the dominant wavelength is given by λ≈2.3h, while in the opposite limit (low Pe or low Ca) λ scales, respectively, as h/Pe or h/Ca1/2. These results are in agreement with a recent experimental study.

Coarsening of monodisperse wet microfoams

We report experiments on the aging of monodisperse wet microfoams. We use the flow focusing technique to produce perfectly monodisperse microbubbles from 50 to 80 μm in diameter. This results in a foam of spherical bubbles of the same size ordered in random ordered lattices like “crystal grains” above the foam/liquid interface. We observe two different behaviors while the foam drains without any interference. At early times, the foam bubble size is almost constant from 10 to 20 min depending on the initial diameter of the bubbles. For longer times, the foam reaches a scaling state where the bubbles mean diameter growth agrees with the theoretical prediction t1/3.

Chemical reaction-driven tip-streaming phenomena in a pendant drop

We describe a chemical oscillatory phenomenon which occurs as follows: a pendant drop of water is immersed in a viscous oil, both phases containing reagents which react to produce a surfactant at the interface. As the drop falls away from the tip of a small needle, the remaining interface spontaneously elongates into a sharp cone and ejects small droplets from the pointed tip. The tip then either contracts and re-elongates periodically, or remains steadily elongated. Small droplets continue to be ejected in either case, and the phenomenon persists over a period of tens of minutes. Quantitative measurements connect these chemically driven phenomena with studies in four-roll mills. A mechanism is proposed by which these phenomena are sustained by Marangoni stresses.

Acoustic characterization of monodisperse lipid-coated microbubbles: Relationship between size and shell viscoelastic properties

The acoustic attenuation spectrum of lipid-coated microbubble suspensions was measured in order to characterize the linear acoustic behavior of ultrasound contrast agents. For that purpose, microbubbles samples were generated with a very narrow size distribution by using microfluidics techniques. A performance as good as optical characterization techniques of single microbubbles was achieved using this method. Compared to polydispersions (i.e., contrast agents used clinically), monodisperse contrast agents have a narrower attenuation spectrum, which presents a maximum peak at a frequency value corresponding to the average single bubble resonance frequency. The low polydispersity index of the samples made the estimation of the lipid viscoelastic properties more accurate since, as previously reported, the shell linear parameters may change with the equilibrium bubble radius. The results showed the great advantage of dealing with monodisperse populations rather than polydisperse populations for the acoustic characterization of ultrasound contrast agents.

Experimental study of a surfactant-driven fingering phenomenon in a Hele-Shaw cell

We present an experimental study of a new surfactant-driven fingering phenomenon in a Hele-Shaw cell. First, the threshold of instability is examined and compared with the linear theory. Second, the nonlinear evolution of fingering is quantified, and steady and unsteady patterns are distinguished. A wide range of dynamical behaviour is observed from drifting and merging fingers to cusp formation between fingers and subsequent ejection of air bubbles. All experiments are performed with a pure surfactant – sodium dodecyl sulphate – thus allowing us to obtain a well-defined bifurcation map for the specific kinetic and material properties of this surface-active substance. The measurements are conducted in a Hele-Shaw cell with smooth and roughened walls. A basic physical model is proposed to obtain further insight into the influence of the surfactant properties on the dip-coating process and, as a result, on the critical phenomena. The study allows us to resolve the discrepancies between previous experimental results and linear theory.

A message transmission system with lightweight encryption as a project in a Master subject

Master subjects should ideally be very practical, to allow students to apply the knowledge they have acquired to the solving of specific problems. This paper proposes the design of a secure communications system using an SPI bus as a Master subject. The system designed uses a stream cipher to encrypt and decrypt data and allows transmission of random length messages. It also uses CRCs to check message integrity.