Fabrication of super-hydrophobic channels

A new approach was developed in this work to create channels which had not only super-hydrophobic bottom surfaces but also super-hydrophobic sidewalls. Researchers have demonstrated that a flow experienced less drag forces and thus required smaller driving pressure in a channel of micro/nanostructure-formed top and bottom surfaces. The drag forces should be further reduced in a channel which has not only patterned top and bottom surfaces but also patterned sidewall surfaces. However, due to the limitation of the existing lithographic approaches, sidewalls could not be properly patterned. Therefore, a new approach was developed in this work to overcome this obstacle. Polydimethylsiloxane (PDMS) micropillars of aspect ratios 1.4, 2.0 and 2.7 were first generated on PDMS films using a molding method, and then transferred to the sidewalls and bottom surfaces of three 1 mm wide and 1 mm deep channels, respectively, applying a hot-embossing process. The corresponding deformation mechanism was considered. The widths of the PDMS films had a critical effect on the cross-section profiles of the generated channels. The radii of the channel corners and inclined degrees of the sidewalls increased with the film widths. Contact angles on the PDMS films before and after the deformations were measured and compared. The contact angles in the middle portions of the sidewalls, as well as at the bottoms of the generated channels, were little difference from those on the original PDMS films due to the small changes in the distances between the micropillars. However, the contact angles were increased and decreased, respectively, at the bottom and top corners of the generated channels since the PDMS films were compressed and stretched at these corners during the fabrication. The variation of the contact angle in each channel was further analyzed according to two existing theoretical formulas. These variations increased with the increasing aspect ratios of the PDMS micropillars. The super-hydrophobic channels fabricated could be potentially employed to reduce drag forces in microfluidic applications.