Techniques for Surface Force Measurements

The force resolution is generally of the order of 10 N. One of the main advantages of the SFA technique compared to the two others presented here is that it allows the measurement of both attractive and repulsive forces. 

Initially devised to measure interactions in single soap films (air/water/air) [8], the TFB technique has been progressively improved and its application has been broadened to emulsion films (oil/water/oil) [ 12] and asymmetric films (air/water/oil or air/water/solid) [13,14]. In a classical setup, a thin porous glass disk is fused on the side to a capillary tube and a small hole is drilled in the center of the disk. The liquid solution fills the disk, part of the capillary, and a thin horizontal film is formed across the hole. The disk is enclosed in a hermetically sealed box, with the capillary tube exposed to a constant reference pressure Pr. Under the effect of the pressure difference AP between the box and the reference, the 

In TFB technique, the thin film radius is typically of the order of 100 pm, far larger than the contact film radius likely to be formed when two micron-sized droplets approach. The magnetic chaining technique overcomes this limitation, allowing the direct measurement of force-distance profiles between liquid colloidal droplets. This technique exploits the properties of monodisperse ferrofluid 

The repulsive force, F), between the droplets must exactly balance the attractive force, Fm, between the dipoles induced by the applied magnetic field. Since the dipoles are aligned parallel to the field, this force can be calculated exactly and is given by [16]  

Rgure 2.4. Comparison of the free energy levels that are accessible using SFA, TFB, and MCT techniques. 

---