The Nanoscale Forces and Nanomechanics Facility consists of a Surface Forces Apparatus (SFA) and Atomic Force Microscope (AFM), which allow the direct measurement of surface interactions such as van der Waals, double layer, steric and bridging forces that exist in may colloidal and nanoparticle systems. We also use SFA and AFM to measurement of the mechanical properties of soft interfaces,such as polymer layers and membranes, on the nanoscale.
The Surface Forces Apparatus
The Atomic Force Microscope above: an experiment in progress
below: a schematic representation
The SFA gives a direct measure of sub-nanoscale surface interactions by monitoring the deflection of a sensitive cantilever spring in response to an attractive or repulsive interaction, as a function of surface separation. Surface separation is measured using an optical interferometric technique, Fringes of Equal Chromatic Order (FECO), that give high resolution, on the angstrom level. In our SFA, surface separation is controlled using a magnetic force transducer.
We use the SFA to measure forces, compressibility and elasticity/viscoelasticity on the molecular level in complex fluid systems such as polymers, polyelectrolytes, proteins and membranes.
Optical FECO fringes
Forces are detected in a similar way in AFM. The AFM tip or a colloidal probe are attached to a cantilever spring and are driven towards and away from a surface at a set frequency using piezoelectric devices. A laser diode is reflected from the back of the cantilever onto a position-sensitive split photodiode detector to give a signal that is a convolution of spring deflection (force) and surface separation. We use AFM to measure colloidal and intermolecular forces, as a nano-indenter to determine elastic and viscoelastic properties of biological cellular systems, and as a topographic imaging device.
