Three-dimensional optical trapping and evanescent wave light scattering for direct measurement of long range forces between a colloidal particle and a surface

A novel technique is described to measure weaker long range forces between a single colloidal particle and a flat surface as a function of separation distance. The technique uses a three-dimensional optical trap as a force transducer and evanescent wave light scattering to measure the particle-surface separation distance. The three-dimensional optical trap allows direct manipulation of the particle near the surface, and the measured displacement of the particle from the center of the optical trap determines the force of interaction between the particle and the surface. The particle position is determined by measuring the scattered light from the particle in an evanescent wave created by total internal reflection of a laser beam on the surface. This combination of optical techniques allows forces to be directly measured for smaller particles (similar to 0.25-5.0 mu m) and over a range of forces (similar to 10(-14)-10(-11) N) than has been previously reported for similar force measurement techniques such as atomic force microscopy and total internal reflection microscopy. The experimental results have been compared with DLVO theory predictions for 1.5 mu m silica microspheres interacting with glass in NaCl solutions of known ionic strength. (C) 1999 American Institute of Physics. [S0034-6748(99)00306-8].