  The dynamics of the diffusion flow of holes photoinjected into a mesoscopic
GaAs channel of variable width, where they, together with background electrons,
form a hydrodynamic electron-hole fluid, is studied using time-resolved
microphotoluminescence. It is found that the rate of recombination of
photoinjected holes, which is proportional to the rate of their flow, decreases
when holes pass through the expanded sections of the channel. In fact, this is
the Venturi effect, which consists in a decrease in the velocity of the fluid
in the expanded sections of the pipe. Moreover, a non-uniform diffusion
velocity profile is observed, similar to the parabolic Hagen-Poiseuille
velocity profile, which indicates a viscous hydrodynamic flow. It is shown that
in argeement with a theory, the magnetic field strongly suppresses the
viscosity of the electron-hole fluid. Additional evidence of the viscous nature
of the studied electron-hole fluid is the observed increase in the
recombination rate with increasing temperature, which is similar to the
decrease in the electrical resistance of viscous electrons with temperature.
