On the measurement of weak repulsive and frictional colloidal forces by reflection interference contrast microscopy

terference contrast (RIC) microscopy in combination with real-time
image processing. The temporal fluctuations of the absolute
sphere-to-substrate distance are determined from changes of
interference fringe pattern (Newtonian rings). Both the shape about
its minimum and the absolute minimum equilibrium distance of the
interaction potential can be obtained by analyzing the distribution
of distances in terms of a Boltzmann distribution. The
timeautocorrelation function of distances yields the hydrodynamic
friction. The method has been applied to the interaction of latex
spheres with glass substrates in salt solutions of different ionic
strength. The results correspond to classical electrostatic double
layer theory that leads to a characteristic dependence of the mean
separation distance and the mean square displacement on the radius
of the spheres. The hydrodynamic friction close to a wall exhibits
the predicted inverse proportionality to the sphere-wall distance.
It is demonstrated that the method can be applied to study the
interaction between biologically relevant objects such as giant
vesicles with bilayer covered substrates.