Axialsymmetric stress relaxation and surface dilation rheology of docosanic acid monolayers spread at the interface of pendant drops in the short time region

The pendent drop technique was used as a film balance to study insoluble docosanic acid monolayers spread at the drop surface at 20 °C and 30 °C. The drop profile was analysed by means of axialsymmetric drop shape analysis (ADSA) which yields the film pressure, the drop volume, and the surface area. The π/A isotherms of docosanic acid were constructed, and agree quite well in their shape with isotherms determined by the Langmuir film balance. By stepwise fast axialsymmetric compression or dilation of the surface area, the surface stress relaxation was monitored over a time scale of 0–300 s. Taking into account a series of relaxing elements, the stress relaxation was analysed for different monolayer states. The relaxation process was characterized by a main relaxation time and a relaxation distribution. The data treatment then leads to a measure of homogeneity of the relaxation process. Furthermore, effective surface dilation elasticities and viscosities were calculated. At 30 °C the stress relaxes faster than at 20 °C. In the liquid condensed region the elasticities of the monolayers are almost constant with narrow distribution of relaxation time, with a behavior that is almost elastic. In the solid region of monolayers the dilational elasticity strongly increases. The relaxation process, however, becomes inhomogeneous which is possibly owing to a combination of superimposed microscopic collapsing processes. © 1997 Elsevier Science S.A. All rights reserved.