presence of Pluronic. However, in the presence of 0.1% Pluronic more liquid was present
in the upward jet and Trinh et al. (1994) reasoned that this may have masked the
occurrence of increased cell concentrations near the bubble surface. The higher volume
also resulted in the number of cells in the jet being quite comparable to that for media
without Pluronic. Because the viability of these cells was identical to that in the bulk, the
main mechanism of protection does not seem to be the exclusion of cells from the danger
zone. More likely protection is mainly caused by a reduction of the magnitude of the
hydrodynamic forces or an increase in the strength of the cells or both. This is in
accordance with the previously discussed protection offered by PVP and PEG, which
cause an increased cell-bubble attachment but still offer protection.
Wu and Goosen (1994) developed a falling-film flow device in which they exposed
cells to well-defined moving air-liquid interfaces in the presence of different additives.
They showed that cell death was associated with the falling of the film into the bulk
liquid and not with the interaction of cells with the air-liquid interface. Using this device,
they showed that surface-active compounds like Pluronic F68 and methyl-cellulose had a
protective effect, while additives that only enhanced the viscosity showed no clear effect.
This suggests that protection is at least in part offered through interaction of the
compounds with the cell membranes. The situation created in this device is, however,
quite different from the situation in a bursting bubble. The film thickness (200–400 μm)
is, for instance, significantly larger than the bubble film thickness (5–20 μm), which also
results in larger amounts of liquid being involved. Furthermore, in the device the falling
of the film is caused by gravity, while at bubble break-up interfacial tension is the main
force. Finally, the liquid velocities (0.5–2 m·s−^1 ) are somewhat lower than the retracting-
rim velocity and the velocities around the bubble cavity (3–20 m·s−^1 ).
In conclusion, numerical simulations of the cavity collapse show that for bubbles
smaller then 6 mm hydrodynamic stresses occur around the bubble cavity that can
potentially damage cells. This agrees with the fact that in the absence of protective
additives all cells in the upward jet are dead. The hydrodynamic stresses associated with
cavity collapse increase exponentially with a decrease in bubble size. For bubbles of 6
mm and larger no upward jet is formed anymore. However, cell death has not been
studied for these large bubbles. To draw firm conclusions from the numerical simulations
with respect to the mechanism of cell death, the interaction of the calculated flows with
the cells as well as the strength of the cells should be included. Protection offered by
additives seems most likely due to decreasing the severity of the hydrodynamic forces
and strengthening of the cell. In addition, reducing the amount of cells in the danger zone
may also contribute.
Cell Death in a FoamChalmers and Bavarian (1991) showed that cells may be pushed into the foam. The
situation with the presence of a foam layer differs essentially from the situation of bubble
burst in the absence of a foam layer. In the foam layer only the retracting toroidal rim is
present and not the flow of liquid into the cavity with the associated upward and
downward jet. Cell death in a foam layer may be caused by liquid draining from the
foam, breaking of films at foam rupture and nutrient exhaustion in the foam.
Furthermore, foaming may cause cells to be physically lost from the bulk liquid. As
Lethal effects of bubbles in animal-cell culture 479