Polymeric Materials (Institute for Technical Chemistry and Polymer Chemistry)

FT-Rheology of emulsions

Beispiele für die Verwendung von Emulsionen
Figure 1: Examples for emulsions and polymer blends in daily use.
Emulsions and blends in general are a class of materials in which two or more constituents are blended together to create a new material able to tailor the properties of a “composite” material without resorting to expensive chemical synthesis. Their presence is ubiquitous in polymer industry, in food processing, in pharmacology, and in the cosmetics. Some examples are inks (colloidal pigment dispersion), detergents (surfactant), colored glasses (solid dispersion within a glass matrix of inorganic pigments), cosmetics (emulsions), and ointments (gels) (figure 1).The typical microstructure of a dilute emulsion at rest consists of spherical droplets immersed in a continuous matrix (figure 2).

Bruch der Trophen unter starker Scherung
Figure 2: Emulsions under rest (left),  under nonlinear shear (centre) and after nonlinear shearing, at rest again, droplet breakup occured (right).

The size and size distribution of these globular domains strongly affect both the processing and the mechanical properties of final products.
To this end, significant efforts were done in the last decades in order to quantitatively measure and control the morphology of blends through different non invasive measurement techniques e.g. with direct microscopical optical methods, light scattering, rheological experiments etc.
Within this regard a new methodology for the estimation of droplet size distribution was recently proposed, based on the so-called Fourier Transform Rheology (FTR), see also FT-Rheology at our homepage. FTR is an oscillatory shear experiment with large amplitude oscillatory shear (LAOS). The response is no longer linear but consists of an overlay of the excitation frequency and its higher harmonics. The intensity and the phase of the overtones include information about the droplet size distribution with known interfacial tension or vice versa.

Obertöne in Emulsionen
Figure 3: Commercial emulsion under nonlinear oscillatory shear LAOS. The record was the 147th overtone in the spectra.

The high sensitivity of LAOS measurements on highly filled emulsions (figure 3) will be used to calculate the desired quantities droplet size distribution respectively interfacial tension.