Effect of Undulations on Surface Energy:  A Quantitative Assessment

Abstract

Contact angle measurements have been widely used to estimate the surface energy of various materials. Such measurements are severely limited with substrate surfaces that exhibit surface restructuring, are contaminated, and/or are porous. Although the captive bubble/drop method addresses the capillarity 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 synthetic polymer membranes, all fabricated from poly(ether sulfone), as model rough porous surfaces of the same surface chemistry. Also, 3 of the 8 different pore size membranes were rendered hydrophilic through photoinduced graft polymerization producing 17 different modified membranes that are similarly tested. By incorporation of roughness parameters obtained from AFM measurements, corrections to the captive bubble/drop constant angle measurements were successfully made using a simple model of the surface. The predicted average value for the sessile drop contact angle of poly(ether sulfone) accounting for undulations (44.5 ± 1.3°) was, within error, equal to that value estimated for a smooth relatively nonporous PES nanofiltration membrane (42.9 ± 2.5°).

Spreading of Liquid Drops over Dry Porous Layers: Complete Wetting Case

“…(6), the solution of Eq. [7] can be obtained using "outer" and "inner" solutions. The outer solution can be deduced in the following way: the left-hand side of Eq.…”

“…However, the vast majority of solid surfaces are rough to a different degree and in many cases surfaces are either porous or covered with a thin porous sublayer. The presence of roughness and/or a porous sublayer changes the wettability of the substrate (7) and, hence, the spreading conditions (8)(9)(10). The theoretical description of spreading over real surfaces is usually based on an ad hoc empirical "slippage condition" (11)(12)(13)(14)(15).…”

Surfactant Fouling of Nanofiltration Membranes: Measurements and Mechanisms

“…To check whether the difference between the measured contact angles is significant, a student t-test was employed with a critical t-parameter of 2.63 (significance level of 0.01 with 98 degrees of freedom). Although the contact-angle technique is strictly only valid for smooth membranes, literature studies [27,28] on ultrafiltration membranes reveal that the effect of roughness is significantly larger for membranes with larger pore size. For NFPES10, the difference between the experimentally determined and corrected contact angles was only 38.…”

“…For NFPES10, the difference between the experimentally determined and corrected contact angles was only 38. [27] For the other nanofiltration membranes, an even smaller influence of roughness is assumed due to the smaller pore size and is therefore not taken into account below.…”

Surface characterization of hollow fiber membranes used in artificial kidney

“…Despite the popularity and maturity of these techniques, they were not all successfully applied to the inside and outside surfaces of polysulfone dialysis hollow fibers. 6 This was because of difficulties introduced by the surface curvature, small size, and low transparency of polysulfone hollow fibers. The hollow fibers studied in this work were melt-spun polysulfone hollow fiber membranes with an average inside diameter of 200 m and an average wall thickness of 10 m. The hollow fiber membranes were manufactured from a synthetic high performance polymer via the thermally induced phase separation (TIPS) process.…”