DOI: 10.1140/epje/i2002-10110-5
Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering
J.-B. Salmon, L. Bécu, S. Manneville and A. ColinCentre de Recherche Paul Pascal, Avenue Schweitzer, 33600 Pessac, France salmon@crpp.u-bordeaux.fr
(Received 1 November 2002 and Received in final form 8 January 2003 / Published online: 1 April 2003)
Abstract
We present local velocity measurements in emulsions under shear using heterodyne
Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume
fraction
% and a concentrated system with
%. Velocity profiles
in both systems clearly show the presence of wall slip. We investigate the evolution
of slip velocities as a function of shear
stress and discuss the validity of the corrections for wall slip
classically used in rheology. Focussing on the bulk flow, we show that
the dilute system is Newtonian and that the concentrated
emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles
is compatible with a shear-thinning exponent of 0.4 consistent with global rheological
data. However, even if individual
profiles can be accounted for by a power law fluid (with or without a
yield stress), we could not find a fixed set of parameters
that would fit the whole range of applied shear rates.
Our data, thus, raise the question of the definition of a global flow
curve for such a concentrated system. These results show
that local measurements are a crucial
complement to standard rheological tools. They are discussed in the light of
recent works on soft glassy materials.
83.80.Iz - Emulsions and foams.
83.50.Rp - Wall slip and apparent slip.
83.60.La - Viscoplasticity; yield stress.
83.85.Ei - Optical methods; rheo-optics.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2003
