COLLOIDAL PARTICLE SCATTERING - A NEW METHOD TO MEASURE SURFACE FORCES

A new method for determining the forces between colloidal particles is presented, based on observing the changes in two-particle collision t rajectories in a linear shear flow and inverting the trajectory equati ons describing such collisions. In the absence of colloidal forces and under low Reynolds number conditions, collisions are symmetric and re versible. When colloidal forces are acting between the particles, this symmetry is broken, and the degree of asymmetry is a measure of the m agnitude of colloidal forces. From a sufficiently large number of expe rimentally observed collision trajectories we can determine the colloi dal forces by a minimization method, assuming some relationship betwee n the interaction force and interparticle distance. This relationship can either be taken from theory, e.g., classical DLVO theory, or be re presented by a general function of interparticle distance with adjusta ble parameters which can be determined from the best fit between theor y and experiment. From Monte Carlo simulations it has been found that the number of collisions required for a reliable determination of the colloidal force-distance relationship is about 25. Some experiments ha ve been done with a ''surface collision apparatus'', which we describe in detail. The results for latex particles in mixtures of glycerol-wa ter and D2O-water show that the method is capable of detecting forces that are 3-4 orders of magnitude smaller than those measured by a conv entional surface force apparatus or by atomic force microscopy. A mini mization analysis of data obtained previously with the traveling micro tube apparatus is also presented.