  The passage of single air bubbles through the horizontal interface between
miscible viscoplastic and Newtonian fluids, considering various combinations of
densities and viscosities for the fluid layers, is studied computationally. The
primary focus is on the quantity of liquid transferred from the lower layer
(Viscoplastic fluid) to the upper layer (Newtonian fluid) as a result of the
bubble's ascent, a factor with significant implications for the turbidity of
methane-emitting lakes and water bodies. The results show that at $ Bo>1 $ and
moderate $ Ar $, prolate-shaped bubbles crossing the interface undergo
elongation in the direction of their poles. This elongation is further
accentuated when the viscosity of upper layer is less than the plastic
viscosity of the lower layer. The bubble is found to break up when leaving the
lower layer, of a critical capillary number, $ Ca_c \approx 5 $. The results
show a significant reduction in the volume of entrainment compared to the
Newtonian counterpart. This suggests disturbances caused by the rising bubble
at the interface dissipate over a smaller region. Four distinct entrainment
regimes are identified, mainly indicating the height to which the entrained
fluid can be transported away from the interface. In contrast to Newtonian
fluids, the volume of entrainment increases by decreasing the viscosity of the
upper layer. Interestingly, the heavy yield stress fluid that has been dragged
up into the the light Newtonian fluid does not recede down by time.
