I

FIGURE 5.18 Maximum bubble pressure method of determining the surface tension of liquids.

Time

FIGURE 5.18 Maximum bubble pressure method of determining the surface tension of liquids.

growth beyond this point causes a decrease in the pressure. Eventually, the bubble formed breaks away from the tip and moves to the surface of the liquid; then another bubble begins to form and the whole process is repeated. The maximum bubble pressure can be related to the surface tension using suitable mathematical equations (Couper 1993).

The maximum bubble pressure method can be used to measure the static surface tension of pure liquids at any bubble frequency. It can also be used to monitor the dynamic surface tension of emulsifier solutions by varying the bubble frequency. The age of the gas-liquid interface is approximately half the time interval between the detachment of successive bubbles and can be varied by changing the flow rate of the gas. It is therefore possible to monitor adsorption kinetics of emulsifiers by monitoring the surface tension as a function of bubble frequency. The dynamic surface tension increases as the bubble frequency increases because there is less time for the emulsifier molecules to move to the surface of the droplets. The variation in the dynamic surface tension with bubble frequency therefore gives an indication of the speed at which emulsifier molecules are adsorbed to the surface. This information is important to food manufacturers because the adsorption rate of emulsifiers determines the size of the droplets produced during homogenization: the faster the rate, the smaller the droplet size (Chapter 6). Nevertheless, it should be pointed out that the maximum bubble pressure method can only be used to measure dynamic surface tensions down to about 50 ms, whereas many surfactants have faster adsorption rates than this. More rapid techniques are therefore needed to study these systems, such as the oscillating jet method (Section 5.10.7), the capillary wave method (Section 5.10.8), or the recently developed punctured membrane method (Stang et al. 1994).

One of the major advantages of the maximum bubble method is that it can be used to analyze optically opaque liquids because it is not necessary to visually observe the bubbles. In addition, it is not necessary to know the contact angle of the liquid because the maximum pressure occurs when the surface tension acts in a completely vertical direction.

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