Showing papers on "Hele-Shaw flow published in 2019"

Dynamic microscale flow patterning using electrical modulation of zeta potential.

Abstract: The ability to move fluids at the microscale is at the core of many scientific and technological advancements. Despite its importance, microscale flow control remains highly limited by the use of discrete channels and mechanical valves, and relies on fixed geometries. Here we present an alternative mechanism that leverages localized field-effect electroosmosis to create dynamic flow patterns, allowing fluid manipulation without the use of physical walls. We control a set of gate electrodes embedded in the floor of a fluidic chamber using an ac voltage in sync with an external electric field, creating nonuniform electroosmotic flow distributions. These give rise to a pressure field that drives the flow throughout the chamber. We demonstrate a range of unique flow patterns that can be achieved, including regions of recirculating flow surrounded by quiescent fluid and volumes of complete stagnation within a moving fluid. We also demonstrate the interaction of multiple gate electrodes with an externally generated flow field, allowing spatial modulation of streamlines in real time. Furthermore, we provide a characterization of the system in terms of time response and dielectric breakdown, as well as engineering guidelines for its robust design and operation. We believe that the ability to create tailored microscale flow using solid-state actuation will open the door to entirely new on-chip functionalities.

Nonlinear stability results for the modified Mullins–Sekerka and the surface diffusion flow

Abstract: It is shown that any three-dimensional periodic configuration that is strictly stable for the area functional is exponentially stable for the surface diffusion flow and for the Mullins–Sekerka or Hele–Shaw flow. The same result holds for three-dimensional periodic configurations that are strictly stable with respect to the sharp-interface Ohta–Kawaski energy. In this case, they are exponentially stable for the so-called modified Mullins–Sekerka flow.

Impact of the gravity field on stability of premixed flames propagating between two closely spaced parallel plates

Abstract: The stability of a planar flame front propagating between two parallel adiabatic plates inclined at an arbitrary angle is investigated in the frame of narrow-channel approximation. It is demonstrated that buoyancy forces can suppress the hydrodynamic (Darrieus–Landau) and cellular (diffusive-thermal) instabilities for sufficiently large value of the gravity parameter for the case of downward-propagating flames. The stability analysis reveals that in the case of oscillatory diffusive-thermal instability, the flame front cannot be stabilized in the similar way. Finally, the stability results are compared satisfactorily with unsteady numerical simulations.

Predicting droplet velocity in a Hele-Shaw cell

Abstract: We study the motion of low viscous non-wetting droplet in a Hele-Shaw cell, while it is pushed by an external phase of imposed flow rate, at low capillary numbers. In this regime, the droplet's mobility, defined as the ratio between the droplet velocity and the external phase mean velocity, evolves non linearly with the capillary number, a signature of the different dissipation mechanisms at play. Experiments are performed with surfactant free air bubbles in fluorinated oil, and with surfactant laden fluorinated oil droplets in water. We propose a model based on a power balance which takes into account the dissipation in the thin wetting film trapped between the bubble (or the drop) and the channel wall. The full topography of this thin film is obtained theoretically for the bubble case. By contrast, the presence of surfactants in the drop case induces uncontrolled boundary conditions at the interface, thus imposing to use the experimental topography measured in the previous paper [Reichert et al., J. Fluid. Mech., 850 p.708 (2018)]. Remarkably, the model reproduces the experimental velocities and shows that the velocity can be strongly affected by a stagnant cap effect at the rear of the drop, even if localized in less than a few percents of the total film area.

Non-linear effects in a closed rotating radial Hele-Shaw cell

Abstract: This work reports high precise Computational Fluid Dynamics results for interface patterns for an incompressible binary fluid system in a rotating circular Hele-Shaw cell. In the initial set-up, the fluids with high and low density occupy, respectively, an inner circle and the remaining outer ring centered with the cell. Once the simulations take into account all non-linear terms in the equations of motion, one single three-dimensional model can be implemented to explore quite different flow regimes by an adequate choice of angular velocity and model parameters. Quantitative and qualitative results, obtained with the help of two grids differing only on the mesh length, are compared with those derived from experiments, linearized analytical expressions, and specific purposes numerical codes.

Boundary effects on the onset of miscible viscous fingering in a Hele-Shaw flow

Abstract: Linear stability analysis and nonlinear simulations confirm that the onset of viscous fingering (VF) instability is delayed in the miscible VF model with the absorbing boundary as compared to the reflective one. A threshold log mobility ratio is found for which such delayed onset is maximum.

Some Contradictions in the Multi-Layer Hele-Shaw Flow

Abstract: An important problem concerning the Hele-Shaw displacements is to minimize the Saffman - Taylor instability To this end, some constant viscosity fluid layers can be introduced in an intermediate region ( ) between the displacing fluids However, we prove that very small (positive) values of the growth rates can be obtained only for a very large (unrealistic) On the contrary, when the length is constrained by certain conditions (for instance, geological), then the maximum value of the growth constants can not fall below a certain value, not depending on the number of layers This maximum value is not so small

On the displacement of two immiscible Oldroyd-B fluids in a Hele-Shaw cell

Abstract: The Saffman–Taylor instability occurs when a Stokes fluid is displaced by a less viscous one in a Hele-Shaw cell. This model is useful to study the secondary oil recovery from a porous medium. Since 1960, polymer solutions were used as displacing fluids; moreover, the oil in a porous reservoir can often be considered a non-Newtonian fluid. Motivated by this fact, in this paper we study the linear instability of the displacement of two Oldroyd-B fluids in a rectilinear Hele-Shaw cell, even if the direct relevance for the flow through porous media is not so evident. We get an approximate formula of the growth rates of perturbations, which become very large when the Weissenberg numbers of the two fluids reach some critical values. This singularity is in agreement with numerical and experimental results already reported in several papers concerning the flow of complex fluids in Hele-Shaw cells.