  In this thesis, we first devote a section on the history of the concept of
irreversibility; of the hydrodynamics, branch of physics in which the viscosity
appears; of the kinetic theory of gases establishing relationships between the
microscopic dynamics and macroscopic processes like viscosity; and, finally,
the interest brought in statistical mechanics of irreversible processes by the
theory of chaos, more precisely, the microscopic chaos. We propose a method
based on the Helfand moment in order to calculate the viscosity properties in
systems of particles with periodic boundary conditions. We apply this method to
the simplest system in which viscosity already exists: the two-hard-disk model.
The escape-rate formalism, establishing a direct relation between chaotic
quantities of the microscopic dynamics (e.g. Lyapunov exponents, fractal
dimensions, etc.), is applied in this system. The results are in excellent
agreement with those obtained by our Helfand-moment method. We extend the
calculation of the viscosity properties to systems with more than two hard
balls. Finally, we compute viscosity as well as thermal conductivity thanks to
our own method also based on the Helfand moment.
