The motion of membrane-bound objects is important in many aspects of biology and physical chemist... more The motion of membrane-bound objects is important in many aspects of biology and physical chemistry. A hydrodynamic model for this Fconfiguration was proposed by Saffman & Delbrück (1975) and here it is extended to study the translation of a disk-shaped object in a viscous surface film overlying a fluid of finite depth H. A solution to the flow problem is obtained in the form of a system of dual integral equations that are solved numerically. Results for the friction coefficient of the object are given for a complete range of the two dimensionless parameters that describe the system: the ratio of the sublayer (η) to membrane (ηm) viscosities, Λ=ηR/ηmh (where R and h are the object radius and thickness of the surface film, respectively), and the sublayer thickness ratio, H/R. Scaling arguments are presented that predict the variation of the friction coefficient based upon a comparison of the different length scales that appear in the problem: the geometric length scales H and R, the ...
The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in ... more The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in chemical equilibrium with a surface whose double-layer voltage is of order the thermal voltage, kBT /e = 25 mV. In nonlinear "induced-charge" electrokinetic phenomena, such as AC electroosmosis, several Volts ≈ 100kB T /e are applied to the double layer, so the theory breaks down and cannot explain many observed features. We argue that, under such a large voltage, counterions "condense" near the surface, even for dilute bulk solutions. Based on simple models, we predict that the double-layer capacitance decreases and the electro-osmotic mobility saturates at large voltages, due to steric repulsion and increased viscosity of the condensed layer, respectively. The former suffices to explain observed high frequency flow reversal in AC electro-osmosis; the latter leads to a salt concentration dependence of induced-charge flows comparable to experiments, although a complete theory is still lacking.
Using a micro particle imaging velocity technique, we resolve for the first time the three dimens... more Using a micro particle imaging velocity technique, we resolve for the first time the three dimensionnal structure of wormlike shear banding flows in straight microchannels. The study revealed two effects, which should be generic for shear banding flows: the first is a strong amplification of the confinement induced by the edge of the channel, the second is an instability of the interface between the shear bands. A detailed quantitative comparison of our experimental measurements with a theoretical study of the diffusive Johnson Segalman model leads to excellent agreement. Our study clarifies the nature of shear banding flow instabilities, and shows that, despite the challenging complexity of the situation and the uncertainty regarding their molecular structure, shear banding flows in confined geometries are amenable to quantitative modelling, a feature that opens pathways to their practical utilization.
Diffusio-phoresis is the movement of a colloid particle induced by a gradient of concentration of... more Diffusio-phoresis is the movement of a colloid particle induced by a gradient of concentration of a molecular solute. This form of transport originates at the colloid surface, within a nanometric layer where the interaction of the solute with the colloid surface differs from that of the solvent,{\ it eg} the electric Debye layer for a charged solute. The solute concentration gradient induces a pressure drop within the interfacial layer along the surface, hence propelling the particle in a direction which depends on the interaction potential between ...
Data Revues 16310748 V12i1 2 S1631074808001756, 2009
When immiscible fluids flow in microchannels, monodisperse droplets or jet are obtained depending... more When immiscible fluids flow in microchannels, monodisperse droplets or jet are obtained depending upon the flow rate of the aqueous phase and the oil phase. In this work, we present experimental data showing that the confinement and the shape of the geometry play a fundamental role. We analyze the stability of the jet in the framework of the lubrication at low Reynolds number. In cylindrical geometry, we relate the transition between the droplets regime and the jet regime to the absolute/convective transition of the Rayleigh Plateau instability and reach a remarkable agreement with the data. In rectangular geometry, we follow the same analysis and point out the existence of an absolutely stable bidimensional jet. To cite this article: P.
We consider two situations where hydrodynamic interactions are said to be "screened": hydrodynami... more We consider two situations where hydrodynamic interactions are said to be "screened": hydrodynamics in a gel or in a porous medium, and electrophoresis in an electrolyte. We focus on the corresponding Green functions, and show that the flow fields are similar in the two cases. Contrarily to statements often made, the fluid velocity decays algebraically with distance (1/r 3), i.e. not exponentially. We point out that the pressure fields are different in the two cases.
A basic yet essential element of every traffic flow control is the effect of a junction where the... more A basic yet essential element of every traffic flow control is the effect of a junction where the flow is separated into several streams. How do pedestrians, vehicles or blood cells divide when they reach a junction? How does the outcome depend on their density? Similar fundamental questions hold for much simpler systems: in this paper, we have studied the behaviour of periodic trains of water droplets flowing in oil through a channel as they reach a simple, locally symmetric, T junction. Depending on their dilution, we observe that the droplets are either alternately partitioned between both outlets or sorted exclusively into the shortest one. We show that this surprising behaviour results from the hydrodynamic feed-back of drops in the two outlets on the selection process occurring at the junction. Our results offer a first guide for the design and modelling of droplet traffic in complex branched networks, a necessary step towards parallelized droplet-based ``lab-on-chip'' devices.
The motion of membrane-bound objects is important in many aspects of biology and physical chemist... more The motion of membrane-bound objects is important in many aspects of biology and physical chemistry. A hydrodynamic model for this Fconfiguration was proposed by Saffman & Delbrück (1975) and here it is extended to study the translation of a disk-shaped object in a viscous surface film overlying a fluid of finite depth H. A solution to the flow problem is obtained in the form of a system of dual integral equations that are solved numerically. Results for the friction coefficient of the object are given for a complete range of the two dimensionless parameters that describe the system: the ratio of the sublayer (η) to membrane (ηm) viscosities, Λ=ηR/ηmh (where R and h are the object radius and thickness of the surface film, respectively), and the sublayer thickness ratio, H/R. Scaling arguments are presented that predict the variation of the friction coefficient based upon a comparison of the different length scales that appear in the problem: the geometric length scales H and R, the ...
The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in ... more The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in chemical equilibrium with a surface whose double-layer voltage is of order the thermal voltage, kBT /e = 25 mV. In nonlinear "induced-charge" electrokinetic phenomena, such as AC electroosmosis, several Volts ≈ 100kB T /e are applied to the double layer, so the theory breaks down and cannot explain many observed features. We argue that, under such a large voltage, counterions "condense" near the surface, even for dilute bulk solutions. Based on simple models, we predict that the double-layer capacitance decreases and the electro-osmotic mobility saturates at large voltages, due to steric repulsion and increased viscosity of the condensed layer, respectively. The former suffices to explain observed high frequency flow reversal in AC electro-osmosis; the latter leads to a salt concentration dependence of induced-charge flows comparable to experiments, although a complete theory is still lacking.
Using a micro particle imaging velocity technique, we resolve for the first time the three dimens... more Using a micro particle imaging velocity technique, we resolve for the first time the three dimensionnal structure of wormlike shear banding flows in straight microchannels. The study revealed two effects, which should be generic for shear banding flows: the first is a strong amplification of the confinement induced by the edge of the channel, the second is an instability of the interface between the shear bands. A detailed quantitative comparison of our experimental measurements with a theoretical study of the diffusive Johnson Segalman model leads to excellent agreement. Our study clarifies the nature of shear banding flow instabilities, and shows that, despite the challenging complexity of the situation and the uncertainty regarding their molecular structure, shear banding flows in confined geometries are amenable to quantitative modelling, a feature that opens pathways to their practical utilization.
Diffusio-phoresis is the movement of a colloid particle induced by a gradient of concentration of... more Diffusio-phoresis is the movement of a colloid particle induced by a gradient of concentration of a molecular solute. This form of transport originates at the colloid surface, within a nanometric layer where the interaction of the solute with the colloid surface differs from that of the solvent,{\ it eg} the electric Debye layer for a charged solute. The solute concentration gradient induces a pressure drop within the interfacial layer along the surface, hence propelling the particle in a direction which depends on the interaction potential between ...
Data Revues 16310748 V12i1 2 S1631074808001756, 2009
When immiscible fluids flow in microchannels, monodisperse droplets or jet are obtained depending... more When immiscible fluids flow in microchannels, monodisperse droplets or jet are obtained depending upon the flow rate of the aqueous phase and the oil phase. In this work, we present experimental data showing that the confinement and the shape of the geometry play a fundamental role. We analyze the stability of the jet in the framework of the lubrication at low Reynolds number. In cylindrical geometry, we relate the transition between the droplets regime and the jet regime to the absolute/convective transition of the Rayleigh Plateau instability and reach a remarkable agreement with the data. In rectangular geometry, we follow the same analysis and point out the existence of an absolutely stable bidimensional jet. To cite this article: P.
We consider two situations where hydrodynamic interactions are said to be "screened": hydrodynami... more We consider two situations where hydrodynamic interactions are said to be "screened": hydrodynamics in a gel or in a porous medium, and electrophoresis in an electrolyte. We focus on the corresponding Green functions, and show that the flow fields are similar in the two cases. Contrarily to statements often made, the fluid velocity decays algebraically with distance (1/r 3), i.e. not exponentially. We point out that the pressure fields are different in the two cases.
A basic yet essential element of every traffic flow control is the effect of a junction where the... more A basic yet essential element of every traffic flow control is the effect of a junction where the flow is separated into several streams. How do pedestrians, vehicles or blood cells divide when they reach a junction? How does the outcome depend on their density? Similar fundamental questions hold for much simpler systems: in this paper, we have studied the behaviour of periodic trains of water droplets flowing in oil through a channel as they reach a simple, locally symmetric, T junction. Depending on their dilution, we observe that the droplets are either alternately partitioned between both outlets or sorted exclusively into the shortest one. We show that this surprising behaviour results from the hydrodynamic feed-back of drops in the two outlets on the selection process occurring at the junction. Our results offer a first guide for the design and modelling of droplet traffic in complex branched networks, a necessary step towards parallelized droplet-based ``lab-on-chip'' devices.
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Papers by Armand Ajdari