Droplet interaction with a surface placed in an immiscible liquid surrounding is very important for liquid-liquid separation. This study investigates the influence of filter surface energy (14-43 mN/m), micro- and nanoscale surface roughness on wetting, mobility and coalescence of isooctane droplets (5-10 mu l) in immiscible water surrounding and explains their role in emulsion separation performance of nanoparticle coated filters. In water, coalescence among the oil droplets primarily takes place at filter surface. Despite different interfacial energies in air and water surrounding, the influence of substrate roughness on droplet mobility is same in both the surroundings. For effective separation of oil-in-water emulsion with a surface filter, it should be super hydrophilic, oriented vertically and have pore size smaller than the dispersed droplet size. In addition, the influent side of a surface filter should possess sufficient micro- and nanoscale surface roughness to provide high mobility, coalescence of the oil droplets and thereafter their easy detachment. For effective separation of oil from water a coalescing filter should be hydrophobic-oleophilic, oriented vertically with rough fibers surface to offer strong adhesion with the oil droplets and thereby high coalescence efficiency. (C) 2013 Elsevier B.V. All rights reserved.