Slide 10 of 12
Notes:
…was the addition surfactant of SDS. We find the expected trend from the model: the slope of the F(z) curve straight from the AFM decreases as the interfacial tension becomes weaker. But a more exciting finding is the ability to take dynamic interfacial tension measurements with resolution below 0.1 dyne/cm.
The slope of the curves shown for 10 mM SDS (above the CMC) are normalized to the first measurement. With each successive deflection of the drop, the interfacial tension decreases. As the interface is stretched, more SDS monomer adsorbs to restore chemical equilibrium; and when it is relaxed again, there is a finite and detectable time for the SDS to desorb and reform into micelles. In this case, the concentration gradients imposed are quite minute and need significant time to reach equilibrium. Each curve was collected within a few seconds of the previous one, and after several events the interfacial tension is reduced from its nominal value by about 1 dyne/cm.
This particular kind of experiment with a bubble may be useful for comparisons with dynamic surface tension measurements also being conducted in our labs with bubble tensiometry. But more importantly, dynamic interfacial tensions can now be measured with great sensitivity over a broad range.