In recent years we have focused on the development of a new AFM technique that promises to bring clear advantages in the analysis of soft matter systems. It's called Force Feedback Microscope (FFM). If conventional AFMs work akin to spring scales, where the weight of an object is measured by observing the deflection of a spring (the tip sample forces are measured by observing the deflection of the cantilever), FFMs are like balance scales, where a counterweight balances the movement of the scale's arm. In FFM operation the AFM tip always remains in the same average position relative to the laboratory, due to a "counterweight" that balances exactly the tip-sample forces. This counterweight is exerted by a simple feedback mechanism that can be easily retrofitted to all commercial AFM with the inclusion of an extra piezoelectric actuator in the cantilever holder. This simple methodology has deep implications in FFM operation. First, the jump to contact mechanism, that is always present in conventional AFMs when soft cantilevers are employed, and that prevents the study of tip sample attractive interactions. This means that the force gradient and dissipation can be directly calculated through the cantilever amplitude and phase of oscillation. FFM can thus be thought as a linear AFM. Secondly, since the tip position never changes throughout any FFM experiment, the observables of the so called "force-distance" experiments are directly modelled by Hertz's Contact Theory, and allow for the measurement of a sample's elasticity. On AFMs, complex numerical calculations are necessary to apply Hertz's theory to the experiment, which yields much more inaccurate results. Thirdly, by changing the detector from the normal laser beam deflection methodology to a fiber optic based interferometer, one has access to the dynamical behaviour of the probe at frequencies not linked to its modes of oscillation, allowing for true spectroscopy measurements of properties of soft samples. There are already several publications demonstrating the FFM methodology. We have developed several working prototypes and retrofitted the FFM capabilities to a normal conventional AFM (see Equipment).
Schematics of the FFM functioning: the microscope is based on the application of a counterweight that balances the tip-sample force (left). This is achieved with a piezoelectric actuator, together with an optical fiber to detect the motion of the cantilever (right).