  We present a mechanistic model for a Newtonian fluid called fluid particle
dynamics. By analyzing the concept of ``fluid particle'' from the point of view
of a Voronoi tessellation of a molecular fluid, we propose an heuristic
derivation of a dissipative particle dynamics algorithm that incorporates shear
forces between dissipative particles. The inclusion of these non-central shear
forces requires the consideration of angular velocities of the dissipative
particles in order to comply with the conservation of angular momentum. It is
shown that the equilibrium statistical mechanics requirement that the linear
and angular velocity fields are Gaussian is sufficient to construct the random
thermal forces between dissipative particles. The proposed algorithm is very
similar in structure to the (isothermal) smoothed particle dynamics algorithm.
In this way, this work represents a generalization of smoothed particle
dynamics that incorporates consistently thermal fluctuations and exact angular
momentum conservation. It contains also the dissipative particle dynamics
algorithm as a special case. Finally, the kinetic theory of the dissipative
particles is derived and explicit expressions of the transport coefficients of
the fluid in terms of model parameters are obtained. This allows to discuss
resolution issues for the model.
