2.5.1. Amplitude Sweep

The barriers were moved to the position where interface area was 140 cm², the oil solution containing 1% (w/w) surfactant was added. The barriers were held for 60 min until the surfactant adsorbed completely to the oil-water interface. The linear viscoelastic region (LVR) was determined by the amplitude sweep, amplitude was varied from 1% to 28% of the interface area. The oscillation frequency was set to 10 mHz as Rühs [25] described, and the amplitude sweep was executed from small to large amplitudes, each amplitude ran for 10 sinusoidal oscillation periods, and between two oscillations followed by a time corresponding to 30 cycles without any oscillation [21], making sure the interfacial film have enough time to recover. The amplitude sweep data were analyzed by calculating the interface dilatational modulus and drawing Lissajous curves.

The interface dilatational modulus was calculated basing on the assumption that the sinusoidal strain would produce a pure sinusoidal stress response. If the strain exceeds the linear viscoelastic range, the stress response will not be a perfect sinusoidal curve, at this point, dilatational modulus might with deviation. Lissajous curve is redrawn according to the relation between the interfacial tension (δπ) and relative strain (δa/a₀), where δπ is the difference between the interfacial tension and the initial interfacial tension, δa is the difference between the interfacial area and the initial interfacial area a₀, and a₀ is the initial interfacial area. Lissajous curve [18,26,27] can be used to quantify the non-sinusoidal strain response and to analyze the interfacial dilatation rheological properties of the interfacial film outside the linear viscoelastic region.