■ Tuned to dynamics - Interfacial rheology with the DSA100
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.
■ THE OSCILLATING DROP METHOD (ODM)
Two versions of the oscillating drop method are used in this module: a) Oscillating drop method based on conventional drop shape analysis (DSA). The frequency range is increased to 10 Hz, which gives wider possibilities for data interpretation. b) Oscillating drop method based on measurement of the pressure. This new approach provides direct information for the capillary pressure. The net surface stress is calculated by taking into account the role of viscous friction in the bulk. The new method is applied for smaller drops, especially with spherical surfaces, which ensures pure dilatation during surface expansion and contraction. The method is also applicable to liquids with hardly different densities (e.g. water and silicone oil)
ODM experiment (Brij 58)
. Adsorption of Brij 58 is slow and the elastic modulus determined with both types of measurements coincides
■ THE EXPANDING DROP METHOD (EDM)
The novel expanding drop method, now provides possibilities to expand or contract small spherical drops with a specified deformation and rate of deformation, precisely controlled by the computer. After the expansion (contraction) the drop area stops changing and the relaxation of the interfacial tension is measured. Expanding drop method is based on measurement of the pressure. The corrections of the pressure due to the bulk viscosity of liquids account for obtaining the correct value of the capillary pressure.
EDM experiment (Brij 58)
■ The main advantages of this procedure are
a) The deformation and the rate of deformation are uniform at the drop surface because the drop is smal l and spherical - pure dilatation is realized and the true dilatational surface viscosity can be calculated from experimental data.
b) The precise control of the constant rate of deformation allows simple procedures for determining the surface viscous term according to the rheological law. The solution of the respective diffusion equations is simpler and known for a spherical geometry.
c) The subsequent (after drop deformation) relaxation measurements provide additional information for the rheological law.
d) The method is applicable for highly viscous fluids like silicone oils, crude oils, etc.; and for liquid/liquid systems with the same density.
Interfacial dynamics of surfactants by:
Expanding Drop Module (EDM)Oscillating Drop Module (ODM) Determination of the adsorption / relaxation time of surfactants Determination of surface dilatational elasticity and viscosity Determination by capillary pressure measurement and drop shape analysis Applicable also for liquids with hardly different densities and highly viscous fluids Simple calibration with one liquid Fully compatible with the modular structure of the Drop Shape Analysis System DSA100
■ Spezifications of the DSA100 with EDM/ODM-Module
Dimensions: 100 x 100 x 50 mm
Weight: 420 g Temperature
range: 10 to 50 °C
Volume change driver
Max. volume change: 20 mm 3 , depends on frequency