Abstract
This communication reports liquid wetting properties of DI-water on one-dimensional nano-patterned photoresist lines atop a silicon substrate as the pattern period is varied from 0.3- to 1.0-µm. Both constant photoresist height and constant width/height ratios are investigated. The line/period ratio was fixed at 0.3 (0.4) for different measurement sequences. The surface of the photoresist was treated with a short CHF3 reactive ion etch to ensure consistent hydrophobic photoresist: water surface energies. Average parallel contact angle (θ||), average perpendicular contact angle (θ⊥), drop width (W), and drop length (L) at constant volume were measured on nano-patterned surfaces fabricated with interferometric lithography. Both θ|| and θ⊥ contact angles increase as the period (0.3- to 1-μm) increases; the θ|| spreading rate is faster than θ⊥ due to pinning on the grooves resulting in an elongated drop shape. The traditional Wenzel and Cassie-Baxter models of drop contact angles were developed for isotropic random 2D roughness and do not account for the anisotropy induced by the 1D line patterns. The observed angular variations with period are not consistent with either model. Understanding liquid wetting properties and hydrophobicity on 1D silicon surfaces has many applications in lab-on-a-chip, micro/nano-fluidic devices, roll-to-roll nano-imprint fabrication, self-cleaning surfaces, and micro-reactors.
Highlights
The interaction of liquid drops with patterned surfaces has both intrinsic scientific interest as a result of the complex three-phase interface, which is impacted by both chemical and structural variations of the surface, and technological interest as it impacts diverse contemporary topics such as lab-on-a-chip biosensors, nanoimprint lithography, self-cleaning surfaces and water shedding
The contact angles increase with period when the spec cycle (SC) (x/h) is fixed at 26%
Note that as a result of experimental limitations; the duty cycle (DC) for the smallest 0.3 μm period with h and x/h fixed is somewhat larger than the desired 30% which probably accounts for the lower contact angles for h fixed at this period
Summary
The interaction of liquid drops with patterned surfaces has both intrinsic scientific interest as a result of the complex three-phase interface, which is impacted by both chemical and structural variations of the surface, and technological interest as it impacts diverse contemporary topics such as lab-on-a-chip biosensors, nanoimprint lithography, self-cleaning surfaces and water shedding. The measurement and analysis of contact angles of water on randomly rough 2D surfaces has a long history Both chemically[3,4] and structurally[5,6,7,8,9] inhomogeneous surfaces have been investigated. Direction as it would be on a uniform surface Both the Cassie-Baxter and Wenzel Models were formulated for randomly textured 2D surfaces and are not immediately applicable to anisotropic 1D periodic patterns[20]. Most often these models have been applied to the contact angle perpendicular to the 1D lines (θ⊥). As a result of computational limitations, the simulations are limited to very small drops covering only a small number nm-scale line/space pairs[25,26,27,28,29]
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