VIDEO MICROSCOPY STUDY OF THE POTENTIAL-ENERGY OF A COLLOIDAL PARTICLE CONFINED BETWEEN 2 PLATES

Using video microscopy techniques, we have measured the position distr ibution of polystyrene spheres [P(z)] suspended in deionized water and confined between two glass plates. The sphere-plate interaction poten tial [U(z)] was deduced using equilibrium statistical mechanics: P(z) = C exp[-U(z)/k(B)T]. Measurements are presented for five different va lues of the plate spacing, and the results show that the rms displacem ent of the spheres, perpendicular to the plates, increases exponential ly as the gap increases. The average position of the spheres is not al ways located at the midplane of the gap, indicating that there may be variations in the surface charge of the two glass plates. A comparison is made with the theory of Derjaguin, Landau, Verwey, and Overbeek (D LVO) for the interaction of colloidal particles with plane surfaces, a nd the experimental results are shown to be consistent with a harmonic approximation to the theory.