Flow of Complex Fluids in Confined Geometry
22 octobre 2009
We use microfluidic devices to perform rheological measurements. The main advantages of such devices are the following three points :
1. They allow the study of complex fluids in complex flows (e.g. extensional or Poiseuille flow) which is not possible with a classical rheometer ;
2. They allow us to confine the fluid at the length scale of its mesoscopic entities ;
3. They allow us to study in great details the role of the nature of the surface that bound the flow.
We point out the role of the nature of the surface in the flow of complex fluids. Surfaces change the slip velocity but also rule the shear rate at the wall. For the same applied shear stress at the wall, various shear rates may be obtained as a function of the nature of the surface ;
In the jammed state, there is no universal relationship between the local shear stress and the local shear rate.
The flow curve of wormike micellar solutions is non local.
- Velocity profiles at various deltaP ; solid lines correspond to the theoretical profiles according to the Dhont model with the same fitting parameters for all of them and a standard deviation 83 mm.s-1. The curves are fiited using a non local model.
- In c are shown the local flow curves extracted from the velocity profiles measured within a wide gap Couette geometry. Inset : dimensionless velocity profiles V(R)/V(Ri) as a function of the radial coordinate R. From bottom to top the rotation velocities are 5, 10, 20, 50 and 100 rpm. In d are shown local flow curves, extracted from the velocity profiles measured in a 250 micrometers thick microchannel with rough surfaces, for various pressure drops as a function of the reduced coordinate z/w (see inset). No overlap of the local flow curves is observed. Dashed lines are predictions for the local flow curves at the given deltaP, as obtained from the non-local rheological model with a flow cooperativity length Ksi = 22.3 µm. From Spatial Cooperativity in Soft Glassy Flows, J. Goyon, A. Colin, G. Ovarlez, A. Ajdari, and L. Bocquet. Nature 454, (2008) 84-87