A double-slit model developed can predict the frictional two-phase pressure drop external liquid holdup, pellet-scale external wetting efficiency, and gas - liquid interfacial area in cocurrent downflow trickle-bed reactors operated under partially wetted conditions in the trickle flow regime. The model, an extension of the Holub et al. (1992, 1993) mechanistic pol e-scale phenomenological approach, was designed to mimic the actual bed void by two inclined and interconnected slits: wet and dry slit. The external wetting efficiency, is linked to both the pressure drop and external liquid holdup. The model also predicts gas - liquid interfacial areas in partially wetted conditions. An extensive trickle-flow regime database including over 1, 200 measurements of two-phase pressure drop, liquid holdup, gas - liquid interfacial area and wetting efficiency, published in 1974-1998 on the partial-wetted conditions, was wed to validate the modeling approach. Two new improved slip-factor functions were also developed rising dimensional analysis and artificial neural networks. High-pressure and -temperature wetting efficiency, liquid holdup, pressure drop, and gas-liquid interfacial area data from the literature on the trickle-flow regime rising conventional monosized beds and catalyst bed-dilution conditions were successfully forecasted by, the model.