(429a) Theoretical and Experimental Investigation of the Effects of Ordered Polymeric Nanofibers On Wetting Regimes

Investigating the wetting phenomena on parallel fibers is one of the classical problems in the area of drop-surface interaction. In fact this is the basis for Cassie-Baxter model for wetting of rough surfaces. However, difficulties associated with manufacturing of highly aligned nano-fibers have hindered comprehensive experimental examination of interactions between parallel nano-fibers and drops. In this work, the effect of highly-aligned engineered polymeric nano-fibers on wetting regimes of hydrophobic and hydrophilic surfaces are investigated theoretically and experimentally. The two possible wetting regimes are defined as: (i) Air trapping (AT) in which liquid drop sits on the fibers with air trapped underneath, and (ii) Fully wetting (FW) regimes in which the liquid drop penetrates in between the fibers and wets the underlying substrate. Using the concept of Gibbs energy minimization, an analytical model was developed to identify the stable wetting regimes and the related apparent contact angles of a constant volume axisymmetric drop on the fibers. The theoretical analyses were performed for the most general scenario in which the fibers and the underlying substrates were made of different material. The apparent contact angle values were found to be equal to the values predicted by the general Cassie model. To validate the theoretical results, engineered micro/nano-scale surface features in form of parallel polymeric fibers are generated using the pseudo-dry spinning method, STEP. Using this technique, aligned polystyrene (PS) nano-fibers were deposited on Silicon (Si), PS, polydimethylsiloxane (PDMS) and air substrates (suspended fibers). The experimental stable wetting regime and advancing contact angels for water drops with triple-phase contact line (TCL) perpendicular and parallel to the fibers were measured and compared to the analytical predictions. In the direction parallel to the fibers, the TCL behavior is in close agreement with the theoretical values predicted for the stable regime. Initial findings suggest that in the perpendicular direction, the water drop preferentially sits on the PS fiber irrespective of the substrate material. Investigation of the contact angle anisotropy shows that the drop would be completely axisymmetric when AT regime is predicted as the theoretical stable regime and the geometrical anisotropy will not results in contact angle anisotropy.