Inside our cells, compartments also known as organelles enable to separate and regulate biomolecular processes : e.g. DNA replication in the nucleus or ATP production inside the mitochondria. Beyond these objects delimited by lipid bilayers it has been shown more recently that membrane-less structures known as biomolecular condensates can also segregate specific molecules. Such assemblies are formed by a demixing process called liquid-liquid phase separation (LLPS), in which molecular partners spontaneously enrich in a condensed phase usually forming droplets. Here at the Physics Laboratory of ENS de Lyon / CNRS, we propose a single molecule approach using cutting-edge technology to manipulate DNA molecules together with a protein involved in DNA maintenance, DciA,. This protein from Deinococcus radiodurans, has been shown to form LLPS with DNA molecules and is postulated to be able to recruit other repair factors. Using optical tweezers coupled with confocal microscopy and a microfluidic control, we will characterize the formation, the structure and the dynamics of these assemblies. Based on the ability of our system to exert and measure forces at the same time as they are visualized in fluorescence microscopy, our aim will be to understand the key molecular processes involved.