The multi-phase Lattice Boltzmann Method (LBM) is used to explore some unprecedented aspects of laminar forced convection in a bottom heated rectangular microchannel. Important physical parameters, such as forces exerted on fluid parcels as well as on the dispersed nanoparticle phase are studied, in an attempt to elucidate the mechanism that results in establishment of a relative velocity between nanoparticles and the continuous fluid phase (slip velocity). The significance of the external forces, such as the gravitational, thermophoresis and Brownian forces is investigated. A recently established expression for the estimation of thermophoresis force in nanofluids is employed to study the true effect of thermophoresis, as other studies either neglect this effect, or are parametric or employ expressions that overestimate this effect. The results indicate that in laminar forced convection, the Brownian force has a significant effect on flow and heat transfer characteristics for low Re number flows (Re∼1-10), but thermophoresis may be safely neglected for all flow conditions. At low Re number flows, the nanofluid flow is heterogeneous, and heat transfer characteristics of nanofluid compared to the base fluid, such as Nu and convection heat transfer coefficient, significantly increase, while at higher Re numbers, such as Re = 100, flow behaves homogeneously and therefore the application of a nanofluid is not justified.