I develop scientific principles for understanding complex structure and dynamics in multi-body interacting systems based on experiments involving dispersions of microscale to nanoscale particles, droplets, and polymers. Thermal motion of probe particles in polymer solutions and other complex fluids can be used to measure frequency dependent mechanical response through passive microrheology (Mason & Weitz, Phys. Rev. Lett. 1995; review Squires & Mason, Ann. Rev. Fluid Mech. 2010). Roughness controlled depletion attractions (Zhao & Mason, Phys. Rev. Lett Read more.