Benjamin Herzhaft | PubMed Health

PubMed2006

Aggregation behavior of two spheres falling through an aging fluid.

We study the behavior of two spheres that settle along their line of center in a yield stress fluid that "ages:" a Laponite suspension. In such a fluid, the fluid flow behind a falling single particle can either exhibit a negative wake, i.e., an upward motion, or not, according to the stress exerted by the particle on the fluid. We show that, if their initial separation distance is smaller than 15 radii, two identical particles cluster whatever the wake's structure. In addition, in the conditions within which a negative wake is observed, we evidence an unexpected lateral motion of the spheres.

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PubMed2002

Correlation between transient shear experiments and structure evolution of aqueous foams.

This experimental work deals with rheological properties of aqueous foams and slip phenomena. Rheological measurements are performed on a stable foam with a parallel plate rheometer. When a constant shear rate is applied to foam, two regimes can be identified in the recorded stress vs time curve: a transient regime where the structure evolves and where the recorded stress varies, followed by a steady state regime where the stress is stabilized. Measurements, modeling, and elimination of the slip velocity are performed. Experiments with grooved surfaces allow elimination of slip at the wall. From measurements at two gaps with smooth surfaces, we use two slip correction methods and check their validity by a direct comparison with actual measurements (with grooved surfaces). A foam rheological equation can be determined from the measurements. Finally, using an optical device coupled with image analysis software, foam texture is investigated on the basis of its evolution with shear rate and time. Evolution of the bubbles size and arrangement into the gap with time of shearing are shown. The transient regime is identified as a regime where the intimate structure of the foam evolves. Slip velocity is also evidenced and measured with the visualization device.

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