High speed is a significant factor for the efficiency of technical processes like printing, pumping, varnishing, or foaming. But in many cases the substances and additives used tend to slow the process: At high speed they show unwanted surface phenomena, which lead to partially wetting, unwanted foaming or instability of produced foams or emulsions. The analysis of interfacial rheological characteristics with the new EDM/ODM module (Expanding Drop Module / Oscillating Drop Module) for the contact angle measuring system DSA100 helps to get the problems under control.

Different from pure liquids the surface tension of surfactant or polymer solutions varies due to the expansion of the surface, only to get back to a state of equilibrium after a while. This behavior is based on low mobility of large molecules. The change of surface tension depends on the degree of area change and on the speed of expansion. In interfacial rheology both phenomena are examined separately: the dependency on the degree of expansion is called interfacial elasticity, the dependency on the speed is called interfacial viscosity.

The new EDM/ODM module for the DSA100 is able to determine both parameters. By changing the volume the surface of a spherical drop oscillates. It is possible to  modulate the oscillation with constant frequency and amplitude to various undulations. The most important is the sinusoidal oscillation. The response of the system is measured in two ways: By analyzing the pressure signal for the change in volume and also by optical analysis of the changing drop image, which is monitored by a video camera. As surface rheological data one receives the loss modulus, which describes the viscous characteristic, and the storage modulus as the elastic parameter.

Furthermore the EDM measurement – “E” stands for “Expanding” – is implemented in the module. A drop is rapidly expanded or contracted with a constant size after that. The change of surface tension with time, i. e. the relaxation, is analyzed. These results provide information on mobility and adsorption behavior of dissolved molecules.

With the knowledge of the surface rheological properties it is possible to strongly modify the dynamic behavior of the system on the base of molecular characteristics.