Yacine Amarouchene

Droplet detachment and satellite bead formation in viscoelastic fluids

The presence of a very small amount of high molecular weight polymer significantly delays the pinch-off singularity of a drop of water falling from a faucet and leads to the formation of a long-lived cylindrical filament. In this Letter, we present experiments, numerical simulations, and theory which examines the pinch-off process in the presence of polymers. The numerical simulations are found to be in good agreement with experiment. As a test case, we establish the conditions under which a small bead remains on the filament; we find that the presence of a bead is due to the asymmetry induced by the self-similar pinch off of the droplet.

The Microcantilever: A Versatile Tool for Measuring the Rheological Properties of Complex Fluids

Silicon microcantilevers can be used to measure the rheological properties of complex fluids. In this paper two different methods will be presented. In the first method the microcantilever is used to measure the hydrodynamic force exerted by a confined fluid on a sphere that is attached to the microcantilever. In the second method the measurement of the microcantilever’s dynamic spectrum is used to extract the hydrodynamic force exerted by the surrounding fluid on the microcantilever. The originality of the proposed methods lies in the fact that not only may the viscosity of the fluid be measured but also the fluid’s viscoelasticity, i.e., both viscous and elastic properties, which are key parameters in the case of complex fluids. In both methods the use of analytical equations permits the fluid’s complex shear modulus to be extracted and expressed as a function of shear stress and/or frequency.

Turbulent drag reduction by polymers

The reduction of turbulent energy dissipation by addition of polymers is studied experimentally. We first address the question of where the action of the polymers is taking place. Subsequently, we show that there is a direct correlation of drag reduction with the elongational viscosity of the polymers. For this, the reduction of turbulent energy dissipation by addition of the biopolymer DNA is studied. These results open the way for a direct visualization study of the polymer conformation in a turbulent boundary layer.

Speed of sound from shock fronts in granular flows

We show, in two different experiments on stationary flow past an obstacle, that several features such as Mach cones and shock wave detachment usually observed in supersonic molecular fluids under extreme conditions are also observed for granular fluids. By pursuing this analogy, we measure the speed of sound in these experiments and find it in agreement with predictions from granular kinetic theories. Surprisingly, and in spite of this agreement, measured velocity distributions are far from being Gaussian and display algebraic tails.

The granular jump

When a fluid jet hits a solid surface, a hydraulic jumps occurs. This jump sharply delimits a thin film of liquid from a thicker film. We show here that a granular jet impinging on a solid surface also gives rise to several features reminiscent of the hydraulic jump and we refer to this situation as the granular jump. We describe, in detail, this phenomenon and show that if many of its features can be understood in analogy with the hydraulic jump, others are directly related to the granular nature of the medium and, in particular, the small-scale dynamics of the jump.

Reynolds number dependence of drag reduction by rodlike polymers

We present experimental and theoretical results addressing the Reynolds number (Re) dependence of drag reduction by sufficiently large concentrations of rodlike polymers in turbulent wall-bounded flows. It is shown that when Re is small the drag is enhanced. On the other hand, when Re increases, the drag is reduced and eventually, the maximal drag reduction asymptote is attained. The theory is shown to be in agreement with experiments, explaining the universal and rationalizing some of the the nonuniversal aspects of drag reduction by rodlike polymers.

Polymer conformations and hysteretic stresses in nonstationary flows of polymer solutions

The low Reynolds number flow of a polymer solution around a cylinder engenders a nonlinear drag force vs. the flow velocity. A velocity quench of such a flow gives rise to a long time relaxation and hysteresis of the stress due to history-dependent elastic effects. Our results suggest that such hysteretic behavior has its origin in the long time relaxation dynamics and hysteresis of the polymer conformations.

Intermittency of the velocity fluctuations in a granular surface flow

Velocity fluctuations in a granular surface flow are studied. These fluctuations are self-similar at small scales with an energy density spectrum showing a −5∕3 power law. The probability density functions of velocity increments are strongly non-Gaussian. The moments of velocity differences between two points separated by a distance r vary as a power law with the scale r; the exponents saturate at a value of 2∕3 showing very strong intermittency of the velocity fluctuations.

Sedimentation of granular columns in the viscous and weakly inertial regimes.

We investigate the dynamics of granular columns of point particles that interact via long-range hydrodynamic interactions and fall under the action of gravity. We investigate the influence of inertia using the Greens function for the Oseen equation. The initial conditions (density and aspect ratio) are systematically varied. Our results suggest that universal self-similar laws may be sufficient to characterize the temporal and structural evolution of the granular columns. A characteristic time above which an instability is triggered (which may enable the formation of clusters) is also retrieved and discussed.

Incompressible-compressible transition in falling granular jets

Through a systematic experimental investigation of the behavior of falling granular jets under the action of gravity for different particle sizes, funnel diameters and ambient air pressures, necessary conditions to obtain incompressible granular jets are identified. A transition from compressible (characterized by a significant density decrease along the propagation) to incompressible granular jets (characterized by a constant density) is observed. This transition depends solely on the aspect ratio between the diameter of the particles and the diameter of the funnel. Surprisingly, the incompressible liquid-like behavior observed here seems to find its origin in the balance between the heat flux and the dissipation in the funnel independently of the ambient fluid pressure. A simple granular hydrodynamic model provides a good description of the transition.