Volume 14, Issue 3, March 2002
Index of content:
14(2002); http://dx.doi.org/10.1063/1.1432696View Description Hide Description
Micron-resolution particle imagevelocimetry is used to measure the velocity profiles of water flowing through channels. The velocity profiles are measured to within 450 nm of the microchannel surface. When the surface is hydrophilic (uncoated glass), the measuredvelocity profiles are consistent with solutions of Stokes’ equation and the well-accepted no-slip boundary condition. However, when the microchannel surface is coated with a 2.3 nm thick monolayer of hydrophobic octadecyltrichlorosilane, an apparent velocity slip is measured just above the solid surface. This velocity is approximately 10% of the free-stream velocity and yields a slip length of approximately 1 μm. For this slip length, slip flow is negligible for length scales greater than 1 mm, but must be considered at the micro- and nano scales.
The effect of imperfections on the emergence of three-dimensionality in stationary vortex breakdown bubbles14(2002); http://dx.doi.org/10.1063/1.1448349View Description Hide Description
We conduct detailed simulations in order to investigate the structure of steady vortex breakdown bubbles. Careful examination of all experimental visualizations reveals a number of ubiquitous features, such as the existence of dye inside the bubble and asymmetries in its downstream end. We show numerically that such features can be attributed to imperfections in the setup of the experimental apparati, imperfections that may be minimized with careful manufacturing and setting up of the rig, but are never completely eliminated. We study such forced breaking of symmetry for imperfections of an order of magnitude typical to those of realistic experimental setups.