Gm. Kepler et S. Fraden, VIDEO MICROSCOPY STUDY OF THE POTENTIAL-ENERGY OF A COLLOIDAL PARTICLE CONFINED BETWEEN 2 PLATES, Langmuir, 10(8), 1994, pp. 2501-2506
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.