Multiphase Bioreactor Design

higher than one meter, while in the presence of these additives the adsorption of cells to
bubbles is minimal and the height dependence will be even less important. Finally, the
rising of the bubble itself is not likely to result in cell death for either attached or non-
attached cells. However, this is difficult to prove, since cell death during bubble rise will
not be distinguishable from death at the surface.

Cell Death at the Surface

There is ample experimental evidence that cell death occurs during bubble burst at the
air-liquid interface. The bubble rupture process is schematically shown in Figure 15.5. As
the bubble approaches the surface, liquid starts to drain from the top of the bubble due to
gravity forces. A thin liquid cap is formed, from which liquid continues to drain due to
gravity and the pressure differences caused by the negative curvatures at the film
boundaries. At a given thickness a hole may be formed in the thinning film. Driven by
surface tension the film retracts forming a growing toroidal rim, where the liquid is
collected. Depending on variations in surface tension, thickness irregularities, and the
escaping air, this rim may break up into small droplets, which are ejected from the film
cap. The retraction process is very rapid (less then 1 ms) and cells in the bubble film
essentially do not experience the presence of the toroidal rim until it reaches them, upon
which the cells are rapidly accelerated to the speed of the rim. As the rim reaches the
edge of the bubble cavity, it creates a small ripple that moves down the cavity during its
collapse. The much slower process of cavity collapse (order of 5 ms) starts with liquid in
a thin layer surrounding the bubble cavity flowing to the bottom of the cavity due to the
surface tension and cavity curvature. The convergence of these liquid flows at the bottom
of the cavity results in the formation of an upward and downward jet. The upward jet into
the air breaks up into several drops depending amongst others on the original bubble
diameter and surface properties. Maclntyre (1972) showed that the liquid in the upward
jet originates from a small boundary layer around the bubble cavity as shown in Figure
15.5.
For describing cell death at bubble break-up, two regions or events may be discerned;
the bubble cap including the thinning, breakage and retraction process and the bubble
cavity and its collapse.

Bubble cap

Chalmers and Bavarian (1991) and Cherry and Hulle (1992) have visually observed the
presence of cells in the bubble cap film. Trinh et al. (1994) captured the film cap and the
cells therein by contacting the film cap with a haemacytometer before rupture. In the
absence of surfactants about 300 cells were present in the film cap for a 3.5-mm bubble at
a bulk cell concentration of about 10^6 cells/ml. By bursting bubbles using a pipette with
the same cell suspension Trinh et al. (1994) found that 90% of the cells were killed upon

Lethal effects of bubbles in animal-cell culture 473