2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/85934We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.5 pages, 4 figures. v2: bibliographic details, minor text correctionsOpticstext