Difference between revisions of "Spatial cooperativity in soft glassy flows"

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==Keywords==
 
==Keywords==
spatial cooperativity, glass, velocity profile, shear stress, shear strain, Couette cell
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spatial cooperativity, glass, velocity profile, shear stress, shear strain, Couette cell, Herschel-Bulkley model
  
 
==Summary==
 
==Summary==
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A general feature of glassy materials is a strong nonlinear flow rule relating stress and strain.  This feature is no well-documented and poorly understood.  Many have tried to understand the glass transition by studying the dynamical heterogeneities in glass-forming materials, but how these heterogeneities affect flow remains unclear.  Using a local velocity measurement technique, the authors study the local flow of a film of confined glassy material.
 
A general feature of glassy materials is a strong nonlinear flow rule relating stress and strain.  This feature is no well-documented and poorly understood.  Many have tried to understand the glass transition by studying the dynamical heterogeneities in glass-forming materials, but how these heterogeneities affect flow remains unclear.  Using a local velocity measurement technique, the authors study the local flow of a film of confined glassy material.
  
The authors test flow in two main geometries: shear planar flow in a wide gap Couette cell, and pressure driven planar flow in a narrow microchannel (tens of hundreds micrometers in width).  The substance tested was an emulsion of silicone droplets (6.5um in diameter) in a glycerine-water mixture.  The local flow curves, which relate the local shear stress <math>\sigma</math> to the local shear rate <math>\dot{\gamma}</math>, are obtained from the measured velocity profiles of both geometries.  Figure 1 shows the results for the wide-gap Couette cell, and Figure 2 shows the results for the narrow microchannel.  Evidently, the flow curve is highly dependent on the geometry.  In the wide-gap case, the curve follows the Herschel-Bulkley model.
+
The authors test flow in two main geometries: shear planar flow in a wide gap Couette cell, and pressure driven planar flow in a narrow microchannel (tens of hundreds micrometers in width).  The substance tested was an emulsion of silicone droplets (6.5um in diameter) in a glycerine-water mixture.  The local flow curves, which relate the local shear stress <math>\sigma</math> to the local shear rate <math>\dot{\gamma}</math>, are obtained from the measured velocity profiles of both geometries.  Figure 1 shows the results for the wide-gap Couette cell, and Figure 2 shows the results for the narrow microchannel.  Evidently, the flow curve is highly dependent on the geometry.  In the wide-gap case, the curve follows the Herschel-Bulkley model.  However, in the microchannel setup, the data does not follow a single rheological curve.  The finite-size effect in the microchannel setup which did not appear in the wide-gap setup suggests that there are extended spatial correlations in the system.
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The authors checked that the change in rheology is not caused by a structural change in the emulsion, nor a change in density, nor boundary effects.  In order to explain this finite-size effect, they developed a model considering the plastic rearrangements that occur in concentrated emulsions.  Localized plastic events induce a non-local, long-range elastic relaxation of the stress over the system.

Revision as of 07:31, 6 December 2009

Reference

Goyon, J., Colin, A., Ovarlez, G., Ajdari, A., Bocquet, L., Nature 454 (2008).

Keywords

spatial cooperativity, glass, velocity profile, shear stress, shear strain, Couette cell, Herschel-Bulkley model

Summary

Figure 1.
Figure 2.

A general feature of glassy materials is a strong nonlinear flow rule relating stress and strain. This feature is no well-documented and poorly understood. Many have tried to understand the glass transition by studying the dynamical heterogeneities in glass-forming materials, but how these heterogeneities affect flow remains unclear. Using a local velocity measurement technique, the authors study the local flow of a film of confined glassy material.

The authors test flow in two main geometries: shear planar flow in a wide gap Couette cell, and pressure driven planar flow in a narrow microchannel (tens of hundreds micrometers in width). The substance tested was an emulsion of silicone droplets (6.5um in diameter) in a glycerine-water mixture. The local flow curves, which relate the local shear stress <math>\sigma</math> to the local shear rate <math>\dot{\gamma}</math>, are obtained from the measured velocity profiles of both geometries. Figure 1 shows the results for the wide-gap Couette cell, and Figure 2 shows the results for the narrow microchannel. Evidently, the flow curve is highly dependent on the geometry. In the wide-gap case, the curve follows the Herschel-Bulkley model. However, in the microchannel setup, the data does not follow a single rheological curve. The finite-size effect in the microchannel setup which did not appear in the wide-gap setup suggests that there are extended spatial correlations in the system.

The authors checked that the change in rheology is not caused by a structural change in the emulsion, nor a change in density, nor boundary effects. In order to explain this finite-size effect, they developed a model considering the plastic rearrangements that occur in concentrated emulsions. Localized plastic events induce a non-local, long-range elastic relaxation of the stress over the system.