Maximum bubble pressure method

Theory: Dynamic Surface Tension

Bubble Pressure Method

One of the useful methods to determine the dynamic surface tension is measuring the maximum bubble pressure. In a bubble pressure tensiometer gas bubbles (ex. Air) 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. Through the attraction between the molecules of a liquid, air bubbles within a liquid are also subject to these forces i.e. a bubble formed within a liquid is being compressed by the surface tension. The resulting pressure rises with the decreasing bubble radius. This increased pressure, in comparison to the outside of the bubble, is used to measure surface tension. Air is pumped through a capillary into a liquid. The created bubble surface bulges and hence continuously decreases the bubble radius. During this process the pressure rises to a maximum pressure. Here the bubble has its smallest radius. This radius equals the radius of the capillary and forms a half sphere. After passing this point the bubble bursts and breaks away from the capillary. Now a new bubble can form at the capillary. During this process the pressure passes through a maximum whose value is recorded by the instrument. Following scheme shows each step of bubble forming and pressure change.

A, B: A bubble is formed and the pressure is below the maximum pressure; the radius of curvature of the air bubble is larger than the radius of the capillary.
C: 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 ρmax, the hydro-static pressure in the capillary ρ_o, the inner radius r of the capillary and the surface tension:

$\sigma ={\frac {(\rho _{max}-\rho _{o})*r}{2}}$

D: After the maximum the “dead time” of the measurement starts. The pressure decreases again, the radius of the air bubble becomes larger. The bubble finally escapes from the capillary and rises. The cycle begins again with the formation of the next bubble. Currently developed tensiometers monitors the pressure needed to form a bubble, the pressure difference between inside and outside the bubble, the radius of the bubble, and the surface tension of the sample are calculated. A data acquisition is carried out via PC control.