Measuring particle interactions with total internal reflection microscopy

Since its introduction in 1987, the optical technique of total internal reflection microscopy (TIRM) has evolved into a powerful tool for studying the interactions between a single, colloidal particle and a planar surface. In addition to being a highly sensitive method for quantifying the potential energy profile between a particle and plate (sensitivity of order 0.1 kT), the technique has also proven capable of measuring the hindered diffusion coefficient and mobility of a particle when the particle-plate gap width is much less than the particle radius. One of the most useful improvements in TIRM has been the incorporation of optical radiation pressure, which allows micromanipulating a single particle in three spatial dimensions. A similar method, termed evanescent wave light scattering (EWS), offers the possibility of measuring colloidal interactions using nanometer-sized particles.