Direct measurement of static and dynamic forces between a colloidal particle and a flat surface using a single-beam gradient optical trap and evanescent wave light scattering

A new measurement technique is described for the simultaneous measurement of static and dynamic interactions between a micron-sized colloidal particle and a flat surface. The technique uses a single-beam gradient optical trap as a sensitive force transducer and evanescent wave light scattering to precisely measure the particle position within the trap. The static force is determined from the deflection of the particle position from the trap center, and the viscous force is measured from the relaxation time of the particle fluctuations near the equilibrium position. Each force contribution is measured as a function of the particle-surface separation distance by scanning the particle toward the surface. Absolute separation distances are determined by curve fitting the viscous force data to hydrodynamic theory in regions where the static force is negligible. The static force data were found to agree well with Derjaguin-Landau-Verwey-Overbeek theory over the entire range of separation distances using 1.0 and 1.5 mum silica spheres in solutions of NaCl. The viscous force data obeyed hydrodynamic theory well until there was an appreciable overlap of the double layers at close separations. This departure from theory is likely due to electroviscous phenomena that enhance the effective drag coefficient of the particle as it moves normal to the flat plate. We also observed light interference effects as the trap focus was placed near the solid-liquid interface. A simple method was found to reduce this effect sufficiently to yield accurate force-distance profiles.