Direct measurement of critical Casimir forces

Similar to electromagnetic vacuum fluctuations which induce long- ranged interactions between uncharged, conducting surfaces, an analogue effect was predicted almost 30 years ago to occur in confined binary mixtures close to their critical point. This so-called critical Casimir effect has attracted considerable attention because it can strongly modify the interaction potential of colloidal particles immersed in a binary fluid. We present the first direct measurement of such critical Casimir forces between a colloidal particle and a flat surface in a water – 2,6-lutidine mixture. With total internal reflection microscopy (TIRM) which is capable to resolve forces down to 5femto Newton, we obtain distance resolved particle-wall interaction profiles. Upon approaching the critical point we observe long-ranged interactions which are attractive or repulsive depending on the specific boundary conditions of the walls. This behavior is in good agreement with recent theoretical predictions. We discuss how critical Casimir forces can be employed e.g. for directed growth of colloidal crystals or to generate active Brownian motion.