Governing mechanisms in immobilization and mobilization of colloids in the unsaturated zone are not well understood. Recent findings show that colloids attach to the air-water-solid (AWS) interface. The interaction of the various forces and the flow field on colloids near the AWS interface is complex. In this presentation, we will determine the interaction force conditions for attachment of colloids to the AWS for flow fields consisting of flow around solid grains in the presence of and air-water system of fluids. The flow field is determined using COMSOL-MP to numerically solve the Navier-Stokes equations. The viscous drag force for the moving fluid on the colloids is then quantified from this numerical solution and the balance of this force with other field forces resulting from interfaces (solid-liquid and liquid-air) is computed to determine the potential for attachment of colloids to the AWS interface. For the present study spherical solid grains are considered, while in future studies we will consider solids grains of more realistic shape.