Odd viscosity in a chiral active fluid*

We spin a system of colloidal magnets in an external magnetic field, forming a cohesive material that behaves like a liquid. Along a boundary of the fluid, we observe lively dynamics, including unidirectional surface waves that propagate due to an interplay of viscous stresses and surface tension. Through broken time-reversal and parity symmetries, this system allows the emergence of an anomalous transport coefficient known as odd (or Hall) viscosity. Unlike ordinary viscosity, this coefficient is dissipationless and absent in simple fluids. By reducing substrate drag, we are able to experimentally observe odd viscosity through the decay of free surface waves.