  We consider the process of convective dissolution in homogeneous and
isotropic porous media. The flow is unstable due to the presence of a solute
that induces a density difference responsible for driving the flow. The mixing
dynamics is thus driven by a Rayleigh-Taylor instability at the pore scale. We
investigate the flow at the scale of the pores using Hele-Shaw type experiment
with bead packs, 2D DNS and physical models. Experiments and simulations have
been specifically designed to mimic the same flow conditions, namely matching
porosities, high Schmidt numbers, and linear dependency of fluid density with
solute concentration. In addition, the solid obstacles of the medium are
impermeable to fluid and solute. We characterise the evolution of the flow via
the mixing length, which quantifies the extension of the mixing region and
grows linearly in time. The flow structure, analysed via the centre-line mean
wavelength, is observed to grow in agreement with theoretical predictions.
Finally, we analyse the dissolution dynamics of the system, quantified through
the mean scalar dissipation, and three mixing regimes are observed: (i) The
evolution is controlled by diffusion, which produces solute mixing across the
initial horizontal interface; (ii) when the interfacial diffusive layer is
sufficiently thick, it becomes unstable, forming finger-like structures and
driving the system into a convection-dominated phase; (iii) when the fingers
have grown sufficiently to touch the horizontal boundaries of the domain, the
mixing reduces dramatically due to the absence of fresh unmixed fluid. With
simple physical models, we explain the physics of the results obtained
numerically and experimentally. The solute evolution presents a self-similar
behaviour, and it is controlled by different length scales in each stage of the
mixing process, namely length scale of diffusion, pore size, and domain height.
