Caseinate sodium gels

Our collaboration with G. H. McKinley focuses on the rheological response of caseinate sodium gels under an imposed oscillatory stress (Lyon) and strain (Cambridge). The local displacement analysis of the gel provides local Lissajous-Bowditch curves highlighting ... a homogeneous response !

Lissajous-Bowditch curves (local Lissajous-Bowditch, thin coloured lines obtained using LORE) of a 6% caseinate gel under an oscillatory stress of 180 Pa.
Lissajous-Bowditch  curves (local curves obtained through echographic analysis, thin coloured line) of a 6% sodium caseinate gel under an oscillatory stress = 180 Pa and a typical strain of 60 %.

The Lissajous-Bowditch curve analysis yields interesting information on the nature (strain-hardening or softening) of these gels. We are currently looking for ways to predict gel failure from the weakly non-linear response of the gels. A publication about this is under way !

LORE and local concentration measurements

We developed two original measures based on ultrasonic echography in suspensions. We add tracers in small quantities in Newtonian fluids to image them.  These tracers will backscatter ultrasonic pulses emitted at regular time intervals. The recorded pressure signal called speckle is then recorded for further use.

Our ultrasonic transducer images in the vertical plane crossing the center of the Couette cell. The speckle phase and its intensity provide information on the local particle concentration and velocity. Our technique can be used both at constant shear rate and in oscillatory experiments, in which case it is synchronised to standard rheological measurements (a technique called LORE).

Local displacement analysis in a water-UCON  fluid under large amplitude oscillatory stress applied at 0.1 Hz. The shift between shear stress and shear strain is typical of Newtonian fluids.

Resuspension in non-Newtonian flows

We use our local particle concentration measurement to determine the migration and resuspension of particles in non-Newtonian suspending fluids. Our first results aimed to verify the viscous resuspension results obtained by A. Acrivos concerning the height of the sediment. We then studied settling under shear in Newtonian and non-Newtonian fluids  (water-guar gum, water-Flopaam). Elastic effects are visible below the elastic instability threshold. The sediment exhibits one height « bump ». Additional echography measurements show a pronounced slip of the sediment correlated to the displacement of the bump.

Flopaam Sediment under shear (100 1/s).
Sédiment de Flopaam sous cisaillement ( = 100 1/s).