(Institute of Physical Chemistry of the Polish Academy of Sciences) 5, 10, or maybe 15? How many nanometers should nanoparticles of a catalyst be to optimize the course of the reaction? Researchers usually look for the answer by laborious, repetitive tests. At the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw, a qualitatively new technique was developed to improve the process of such optimization in microfluidic systems. The size of the catalyst nanoparticles can now be changed interactively, during a continuous flow through the catalyst bed.