Bubble pressure

The bubble pressure method

An easy-to-use method for determining the dynamic surface tension is the method of measuring the maximum bubble pressure.

In a bubble pressure tensiometer gas bubbles are produced in the sample liquid at an exactly defined bubble generation rate. The gas bubbles enter the liquid through a capillary whose radius is known. During this process the pressure passes through a maximum whose value is recorded by the instrument.

The following illustration shows the pressure curve during bubble formation plotted as a function of time:

1: The bubble is formed. Initially the pressure is below the maximum pressure; the radius of curvature of the air bubble is larger than the radius of the capillary.

2: The pressure curve passes through a maximum. At this point the air bubble radius is the same as that of the capillary; the air bubble forms an exact hemisphere. The following relationship exists between the maximum pressure $P_{max}$ , the hydro-static pressure in the capillary $P_0$ , the inner radius $r$ of the capillary and the surface tension $\sigma$ :

$\sigma = \frac {(P_{max} - P_0) \cdot r}{2}$

3: After the maximum the “dead time” of the measurement starts. The pressure decreases again, the radius of the air bubble becomes larger.

4: The bubble finally escapes from the capillary and rises. The cycle begins again with the formation of the next bubble.