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\nfluctuations of the absolute sphere-to-substrate distance are determined from changes of interference\nfringe pattern (Newtonian rings). Both the shape about its minimum and the absolute minimum equilibrium\ndistance of the interaction potential can be obtained by analyzing the distribution of distances\nin terms of a Boltzmann distribution. The timeautocorrelation function of distances yields the\nhydrodynamic friction. The method has been applied to the interaction of latex spheres with glass substrates\nin salt solutions of different ionic strength. The results correspond to classical electrostatic double layer\ntheory that leads to a characteristic dependence of the mean separation distance and the mean square\ndisplacement on the radius of the spheres. The hydrodynamic friction close to a wall exhibits the predicted\ninverse proportionality to the sphere-wall distance. It is demonstrated that the method can be applied\nto study the interaction between biologically relevant objects such as giant vesicles with bilayer covered\nsubstrates.