Effect of undulations on surface energy: A quantitative assessment

Contact angle measurements have been widely used to estimate the surface en ergy of various materials. Such measurements are severely limited with subs trate surfaces that exhibit surface restructuring, are contaminated, and/or are porous. Although the captive bubble/drop method addresses the capillar ity problem, surface undulations have not previously been accounted for in a quantitative way. We do so here with a series of 8 different pore size sy nthetic polymer membranes, all fabricated from poly(ether sulfone), as mode l rough porous surfaces of the same surface chemistry. Also, 3 of the 8 dif ferent pore size membranes were rendered hydrophilic through photoinduced g raft polymerization producing 17 different modified membranes that are simi larly tested. By incorporation of roughness parameters obtained from AFM me asurements, corrections to the captive bubble/drop constant angle measureme nts were successfully made using a simple model of the surface. The predict ed average value for the sessile drop contact angle of poly(ether sulfone) accounting for undulations (44.5 +/- 1.3 degrees) was, within error, equal to that value estimated for a smooth relatively nonporous PES nanofiltratio n membrane (42.9 +/- 2.5 degrees).