Quantitative Characterization of Surface Wettability and Interfacial Energy of Newtonian Fluids on PDMS with Engineered Roughness

This study systematically investigates the surface wettability and energy characteristics of four Newtonian fluids, distilled water, glycerol, PEG 300, and PEG 400, on polydimethylsiloxane (PDMS) substrates with tunable (engineered) surface roughness. Three PDMS surface conditions were fabricated: smooth (untreated), moderately rough (oxygen plasma etched), and highly rough (sandpaper imprinted during curing). Contact angles (static, advancing, and receding) were measured to quantify contact angle hysteresis under controlled temperature and humidity. Surface tension values were determined using the pendant drop analysis method. The Owens-Wendt model was used to decompose surface energy into polar and dispersive components, and the work of adhesion was computed using the Young-Dupré equation. Results reveal a strong dependence of wettability and adhesion on surface roughness. Increased roughness enhanced both the polar contributions to surface energy and the work of adhesion across all fluids, with water showing the most significant response due to its high surface tension. Contact angle hysteresis increased with roughness, indicating greater contact line pinning and metastable wetting states on textured surfaces. These findings establish a quantitative framework for engineering PDMS interfaces in microfluidic and biomedical applications where precise control of fluid-substrate interactions is critical.