  Critical Casimir forces can play an important role for applications in
nano-science and nanotechnology, owing to their piconewton strength, nanometric
action range, fine tunability as a function of temperature, and exquisite
dependence on the surface properties of the involved objects. Here, we
investigate the effects of critical Casimir forces on the free dynamics of a
pair of colloidal particles dispersed in the bulk of a near-critical binary
liquid solvent, using blinking optical tweezers. In particular we measure the
time evolution of the distance between the two colloids to determine their
relative diffusion and drift velocity. Furthermore, we show how critical
Casimir forces change the dynamic properties of this two-colloid system by
studying the temperature dependence of the distribution of the so-called
first-passage time, i.e., of the time necessary for the particles to reach for
the first time a certain separation, starting from an initially assigned one.
These data are in good agreement with theoretical results obtained from Monte
Carlo simulations and Langevin dynamics.
