Figure 10.

Schematic image of the Dynamic Surface Tension (DST) measurement using the Micropipette Interfacial Area-expansion Method. (a) The micropipette is inserted into the surfactant solution under high positive applied pipette pressure such that the new interface meniscus (diameter ~22 µm) is close to the pipette tip. This is observed by initially positioning the micropipette tip in the field of view. To avoid convective flow inside the chamber, the microchamber is gently sealed with a plug of hexadecane after the pipette is inserted and the pipette moved to a new viewing position where a clean surface is expected to locate when a pre-set lower pipette pressure is applied. (b) Following a 15-fold decrease of the applied pipette pressure, ΔP, (from the high value of ~9 kPa, that held the interface close to the pipette tip, to 0.6 kPa), the surface area quickly expands to a new diameter of ~300 μm in 0.1 to 0.3 s. The pipette pressure is rapidly fixed to a constant reset pressure of 1.2 kPa, and the surface meniscus moves to the observed region (dotted box). The movement of the surface meniscus is tracked as surfactant adsorbs to the water-air surface and reduces the tension to the new equilibrium. With permission from Elsevier [7].