Boundary layer in pastes—Displacement of a long object through a yield stress fluid

Abstract

When it moves through a yield stress fluid, a solid object continuously reaches and liquefies new solid regions, so that both flow in liquid regions and deformations in solid regions occur. In the present work, we focus on the displacement of a plate through simple yield stress fluids (non-thixotropic). Through force vs velocity and particle imaging velocimetry measurements with a detailed analysis of the deformation history, we are able to identify the solid and liquid regions and their respective role in the flow characteristics. It is shown that the displacement of a long object through a yield stress fluid gives rise to the formation of a liquid boundary layer (BL) of uniform thickness at short distance from the leading edge, while the rest of the material remains solid. The original result is that the thickness of this boundary layer, which is of the order of 10 mm, only slightly increases with velocity and does not tend to zero when the velocity tends to zero, in contrast with usual flows of yield s...