This work discusses certain fundamental aspects in the mixing of particle clouds plunging into water. In particular, the momentum transfer in the isothermal interaction is considered here, as part of a broader investigation involving very high-temperature particles and thus heat transfer and phase change, as related to the premixing phase of steam explosions. The focus is on the multifield behaviour, and the investigation is carried out experimentally in the MAGICO-2000 facility: we observe front advancements, mixing region evolution and water displacements. The effect of particle volume fraction, density, inlet velocity and pool geometry are analyzed. Measurements are taken by means of high-speed videos and X-ray radiography: the latter allows us to map the interior of the mixing region, highlighting interesting features of the interaction. All results are compared to the numerical simulations obtained with the PM-ALPHA code, and the exercise reveals good predictive capabilities for it.