Rädler, Joachim O.; Sackmann, E.
On the measurement of weak repulsive and frictional colloidal forces by reflection interference contrast microscopy.
In: Langmuir, No. 3: pp. 848-853
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