Drag on a sphere moving through an aging systemH. Tabuteau1, F. K. Oppong1, J. R. de Bruyn1 and P. Coussot1, 2
1 Department of Physics and Astronomy, University of Western Ontario - London, ON N6A 3K7, Canada
2 Institut Navier - Paris, France
received 4 January 2007; accepted in final form 7 May 2007; published June 2007
published online 4 June 2007
We have investigated the drag on a sphere falling through a clay suspension that has a yield stress and exhibits rheological aging. The drag force increases with both speed and the rest time between the preparation of the system and the start of the experiment, but there exists a non-zero minimum speed below which steady motion is not possible. We find that only a very thin layer of material around the sphere is fluidized when it moves, while the rest of the suspension is deformed elastically. This is in marked contrast to what is found for yield-stress fluids that do not age.
82.70.-y - Disperse systems; complex fluids.
83.60.La - Viscoplasticity; yield stress.
64.70.Pf - Glass transitions.
© Europhysics Letters Association 2007