Surface forces apparatus experimental setup

At the times when DLVO theory was developed, the direct measurement of forces between colloidal particles and surfaces in solution was not possible, and the macroscopic observation of colloidal stability was the only experimental reference data. With increasing technological advancement, setups have been developed for the direct observation of such forces The surface force apparatus (SFA) allows for the measurements of forces between surfaces in solution [6], and with an atomic force microscope (AFM), forces on a colloidal particle can be detected [7]. It is a major success that DLVO theory predicts forces that agree nicely with the measured forces for large particle separations (more than 3-10 nm), but at the same time, it is obvious that in the regime of short particle separations, not aU effects are captured by DLVO. When the barrier for coagulation occurs at such low separations, the DLVO prediction for colloidal stability is not accurate (Fig. 2). 

Experimentally, electrostatic double-layer forces versus distance were first quantitatively measured in foam films [444—446]. Aqueous foam films with adsorbed charged surfactant at air-liquid interfaces are stabilized by double-layer forces, at least for some time. Voropaeva ef al. measured the height of the repulsive barrier between two platinum wires at different applied potentials and in different electrolyte solutions [447]. U sui et al. [448] observed that the coalescence of two mercury drops in aqueous electrolyte depends on the applied potential and the salt concentration. Accurate measurements between solid-liquid interfaces were first carried out between rubber and glass with a special setup [449]. In the late 1970s, DLVO force could be studied systematically with the surface forces apparatus [424,450,451]. With the introduction of the atomic force microscope, DLVO forces between dissimilar surfaces could be measured [198, 199, 452, 453]. 

Fig. 5.4 Schematic of the apparatus and procedures for measuring the wetting and adhesion interactions between water and various surfaces, (a) Experimental setup, (h) Wetting force measurement. (c) Adhesion force measurement (reproduced with permission from [62], copyright 2014 The American Chemical Society)...

---