Our bimorph surface forces apparatus or MASIF (Measurement and Analysis of Surface Interactions and Forces) is one of the first prototypes produced by ASI/ANUTECH, Camberra, Australia. It is based on the design by J. Parker, who described it first in 1992 (see Parker, J. L. Langmuir 1992, 8, 551). We use it to measure the forces between two macroscopic surfaces of various geometries (sphere- sphere, sphere-flat)and materials (material A-material A, material A-material B). Direct force measurements can be performed in (dry) air, vapor, water or solutions, incl. adsorbing solutions (e.g. surfactants, polyelectrolytes, polymers, proteins).
The measurement chamber of the bimorph surface forces apparatus.
Schematic drawing of the bimorph surface forces apparatus, from Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv. Colloid Interface Sci. 1996, 67, 119.
One surface is mounted on a translation element (piezoelectric tube), the other on a spring (bimorph). When one surface is moved at a constant speed towards the other, the deflection of the spring is monitored at each distance. When is spring constant and the radii of curvature of the samples are determined, the deflection is transformed to a normalized force. The traveled distance relative to the position of contact at high forces ("hard wall contact") yields the separation of the samples. The normalized force, Force/Radius, is typically plotted against the separation as a force vs. distance function or "force curve":
Force (normalized by radius of curvature) vs. distance function of two polystyrene surfaces in surfactant solutions, at and below the cmc (0.020 M).