Buoyancy effects on micro-annulus formation: Density unstable Newtonian–Bingham fluid displacements in vertical channels

•Density unstable displacement of Bingham fluid by a Newtonian fluid.•Many different instabilities observed (m  <  1 tends to be of Kelvin-Helmholz type; m  >  1 for inverse bamboo and mushroom.•Increasing yield stress tends to destabilize.•Find partially static residual layers: static only closest to the wall.•Static wall layers, even for BN <  6, unlike the density stable situation.