Dr. Astrid Drechsler
The interactions between solids on a nanoscopic scale determine many macroscopic properties as the stability of colloids or compounds, adhesion, wettability and bio-compatibility. They result from the interplay of different surface properties as charge, roughness and chemical composition and can be modified by thin adsorption layers.The interaction forces between a surface and a tip or colloidal particle on a nanoscopic scale can be measured directly. This measurement elucidates not only the interaction between surfaces and particles but gives also information about surface properties and the effect of adsorbed ions or molecules.
In our surface forces laboratory, we measure interaction forces in a distance range of nanometers up to some micrometers using a scanning force microscope in the presence of air, vapor and solutions. With standard AFM tips, imaging of the surface topography and spatially resolved force measurements can be done at the same time (force-volume mode). For a quantitative evaluation of interaction forces, the colloid probe technique is applied: The AFM tip is replaced by a spherical colloidal particle of a few micrometers size. The colloidal probe technique allows not only the elimination of the probe geometry but also the application of a variety of materials (e.g. polymers) as force probes and the measurement of weak interactions.
Forces between chemically modified surfaces are investigated by chemical force microscopy. With the AFM, not only vertical but also lateral (friction) forces can be determined.
Another instrument especially designed for the measurement of interaction forces between surfaces is the bimorph surface force apparatus (MASIF - Measurement and Analysis of Surface Interactions and Forces). It measures the forces between two spheres of about 1mm diameter. The spheres may consist of silica or polymers and may have a surface modification.
In several cooperations with industry partners, direct force measurements are carried out to reveal the interactions between solid surfaces and particles for special applications as e. g. textile laundry or microelectronics manufacturing. Two basic research projects supported by the DFG base mainly on direct force measurements:
- Electrokinetic and direct force measurements used to describe the influence of the surrounding medium on the crack propagation in polymers
- In situ investigation of the interaction forces between polyelectrolyte brushes