When someone says "Everything flows -- nothing stays the same," he seldom refers to toothpaste, ointment or paint. But manufacturers are immensely interested in the way such products "flow" in daily use. For example, wall paint should be thin in consistency, easy to work with and still adhere to walls after application.
The amount of thixotropic agent added to viscous or paste-like substances greatly influences the speed at which fluid particles move under mechanical stress: the smaller the particle size of the silicic acid and silica gels used, the greater the likelihood they will lump together, which in turn increases the viscosity of the substance.
Manufacturers need to know the size and distribution of particles, and their behavior must be monitored during production. But optical methods of measuring frequently prove unsuccessful due to insufficient transparency of the substances.
Diluting them, on the other hand, alters their flow properties. One solution for measuring, without resorting to costly dilution procedures, is 3D cross-correlation spectroscopy.
Dr. Lisa Aberle of the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research IFAM in Bremen, Germany, explains why this measuring technique works even with undiluted samples:
"When a laser beam gets through a sample of relatively low transparency, the light is scattered manyfold. In extreme cases, the scattered light pattern will not provide enough information on the size of the particles.
"This is where our apparatus differs: Two laser beams set at angles penetrate the sample and cross within the substance. The light from each beam is scattered and generates two patterns of light spots on the monitors behind the sample.
"The spatial and temporal variations in these patterns of light are not identical. Through electronic comparison or correlation, we are subsequently able to calculate and filter out the proportion attributable to multiple scattering."
This procedure even works when solid particles constitute half the weight of a sample of silica gel. The system is capable of measuring particles from just a few nanometers up to several micrometers in size.
Through repeated measurements, researchers are able to observe how the tiny particles change, lump together, or form sediment. Questions of product stability concern many sectors of the pharmaceutical, food and cosmetics industries and the field of life science.
The prototype measuring device is ready to go into production and Dr. Aberle is seeking partners in industry for its marketing.
(Reference: Research News 6-2001 of the Fraunhofer-Gesellschaft.)
[Contact: Dr. Lisa Aberle]
26-Jun-2001
