AXIALSYMMETRIC STRESS-RELAXATION AND SURFACE DILATION RHEOLOGY OF DOCOSANIC ACID MONOLAYERS SPREAD AT THE INTERFACE OF PENDANT DROPS IN THESHORT-TIME REGION

The pendent drop technique was used as a film balance to study insolub le docosanic acid monolayers spread at the drop surface at 20 degrees C and 30 degrees C. The drop profile was analysed by means of axialsym metric drop shape analysis (ADSA) which yields the film pressure, the drop volume, and the surface area. The phi/A isotherms of docosanic ac id were constructed, and agree quite well in their shape with isotherm s determined by the Langmuir film balance. By stepwise fast axialsymme tric compression or dilation of the surface area, the surface stress r elaxation was monitored over a time scale of 0-300 s. Taking into acco unt a series of relaxing elements, the stress relaxation was analysed for different monolayer states. The relaxation process was characteriz ed by a main relaxation time and a relaxation distribution. The data t reatment then leads to a measure of homogeneity of the relaxation proc ess. Furthermore, effective surface dilation elasticities and viscosit ies were calculated. At 30 degrees C the stress relaxes faster than at 20 degrees 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 m onolayers the dilational elasticity strongly increases. The relaxation process, however, becomes inhomogeneous which is possibly owing to a combination of superimposed microscopic collapsing processes. (C) 1997 Elsevier Science S.A.