Current environmental and health concerns associated with transport of colloids, bio-colloids, and nanoparticles in soils and aquifers increase the demand of predictive ability in colloid transport. Although colloid transport and colloid-facilitated transport of contaminants have received close attention in past years, gaps in knowledge remain. To improve fundamental understanding of colloid retention and transport processes, complex natural soil media are often simplified to model porous media. At present, the major physical mechanisms involving colloids in unsaturated porous media include retention on solid-water interface (SWI), air-water interface (AWI), and contact line (where all three phases meet). Additionally, physical straining of colloids in pore spaces has been discussed in the literature. However, some uncertainties in colloid transport exist, and many are related to colloid retention on air-water interface. While some researchers considered AWI as an important retention site others did not observe significant retention of colloids on AWI. The capillary structure of porous media suggests the importance of surface tension, and its experimental variation would provide valuable information about underlying retention mechanisms. In our experiments, we investigate the effect of surface tension on colloid behavior in model static and dynamic systems. The principle experimental design includes an open capillary channel observed with confocal microscope. Such design allows direct observation of colloid retention on AWI with different surface tensions. In addition to primary retention mechanisms studied in static system, dynamic experiments aid the estimation of the relative impact of colloid retention on AWI and surface tension values on the overall colloid transport.