"The ability to apply forces to single molecules and bio-polymers has fundamentally changed the way we can interact with and understand biological systems. Yet, for many cellular mechanisms, it is rather the torque that is the relevant physical parameter. Excitingly, novel single-molecule techniques that utilize this parameter are now poised to contribute to novel discoveries. Here, I will study the angular dynamical behavior and response to external torque of biological systems at the molecular and cellular levels using the new optical torque wrench that I recently developed.In a first research line, I will unravel the angular dynamics of the e.coli flagellar motor, a complex and powerful rotary nano-motor that rotates the flagellum in order to propel the bacterium forwards. I will quantitatively study different aspects of torque generation of the motor, aiming to connect evolutionary, dynamical, and structural principles. In a second research line, I will develop an in-vivo manipulation technique based on the transfer of optical torque and force onto novel nano-fabricated particles. This new scanning method will allow me to map physical properties such as the local viscosity inside living cells and the spatial organization and topography of internal membranes, thereby expanding the capabilities of existing techniques towards in-vivo and ultra-low force scanning imaging.This project is founded on a multidisciplinary approach in which fundamental optics, novel nanoparticle fabrication, and molecular and cellular biology are integrated. It has the potential to answer biophysical questions that have challenged the field for over two decades and to impact fields ranging from single-molecule biophysics to scanning-probe microscopy and nanorheology, provided ERC funding is granted."