Box 10.1 — Total internal reflection microscopy Box 10.1 — Total internal reflection microscopy. Total internal reflection microscopy (TIRM) is a sensitive non-invasive technique for measuring the interaction potential between a colloidal particle and a wall with femtonewton resolution. The equilibrium distribution of the particle–wall separation distance z(t) is sampled by monitoring the intensity I(t) scattered by the Brownian particle under evanescent illumination. A sketch of a typical TIRM geometry with a single colloidal particle in front of a planar transparent wall is shown below (a). The particle is illuminated by the evanescent field created by the total internal reflection of a laser beam at the glass-fluid interface while undergoing Brownian motion. The scattered intensity I(t) is measured using a photomultiplier (PMT) (b). The forces acting on the particle are due to gravity Geff and electrostatic interactions Fel (arrows). Knowing the relationship between the scattered intensity and distance, i.e., I(z) (c), the intensity time series I(t) can be converted into the particle trajectory z(t) (d). More details on this technique can be found in Prieve (1999), Volpe et al. (2009) and Walz (1997).