Formation of Gas Pockets in a Boundary Layer Under Turbulent Forcing

Our experiments examine the formation of gas pockets in a flat plate boundary
layer in water submitted to turbulent forcing. Air micro-bubbles are in-
jected from wall orifices over surfaces with hydrophilic, hydrophobic and
super-hydrophobic wetting properties. In this latter case, the surfaces
are coated with paints with water repellent characteristics, or covered
with vertically aligned carbon nanotubes (CNTs) that are chemically
tuned to produce super-hydrophobicity. The turbulence level of the in-
coming flow is adjusted through meshes and grids. Bubble injection is
operated at constant flow rate, and the mechanism of bubble growth
and detachment is investigated through high speed recordings. Super-
hydrophobicity is found to promote the attachment of discrete gas voids
to the wall, with CNTs favoring the formation of a gas layer. Another
finding is that the turbulence intensity affects the stability of microbub-
bles attached to the wall under super-hydrophobic conditions.