Interfacial wall-slip and its consequences on Lissajous plots

Emulsions and foams are generally stabilized by low molecular weight surfactants, proteins, or by particles. The high adsorption energy of particle systems results in ultra-stable foams used in various industrial applications. In this study, hydrophobic silica and clay particles are compared due to their completely different particle network properties: Silica particles build strong networks in contrast to clay particles which have weak network structure. To measure the interfacial rheological properties of these networks a biconical geometry was used and the results are analyzed in stressstrain Lissojous plots. The rheological measurements were combined with microscopy and optical coherence tomography (OCT) to visualize the flow behavior of the interfacial layer. It turned out that non-linear Lissajous plots showed a parallelogram like shape. In addition, the rheological response of silica particle interfaces showed a characteristic oscillation in the horizontal part of the Lissajous plots. The optical measurements revealed that the particular shape of the Lissojous figures is caused by interfacial wall-slip at the biconus for the silica and at the cup wall for the clay particles. We propose that the truncation of the Lissajous plots can be correlated with the wall-slip. The results emphasize in combination with morphological information, Lissajous plots can help to understand complex nonlinear rheological behavior.