Contact Angle Hysteresis on Textured Surfaces with Nanowire Clusters

Abstract

Nanowire arrays with various agglomeration patterns were synthesized by adjusting the solvent evaporation rates. Nanowires with 200 nm diameter and 2-25 microm in length were fabricated from an anodic aluminum oxide (AAO) porous template. Various drying treatments were applied to develop nanostructured surfaces with topological differences. Due to surface tension forces, copper nanowires after thermal and evaporative drying treatments agglomerated into clusters, while supercritical drying technique provided excellent bundled-free and vertically-standing nanowire arrays. Although all dried surfaces exhibited hydrophobic nature, the contact angle hysteresis, or the difference between advancing and receding angles, was found to be larger on those surfaces with bundled nanowire clusters. To explain the difference, the wetted solid fraction on each surface was calculated using the Cassie-Baxter model to show that the hysteresis was contributed by liquid/solid contact area on the textured surfaces.

Poly(3-hexylthiophene) Nanotube Array Surfaces with Tunable Wetting and Contact Thermal Energy Transport

“…38 We found that the surface fraction of the PNT array decreases as etch time and ridge formation increases, in contrast to earlier studies that have found an increase in surface fraction with increasing Cu and Si nanowire bundling. 18,27,28 For etch times of 6 min and longer, there is no measurable change in the width of the PNT ridges, while the spacing between the ridges increases considerably (Figure 3). Here, we use a SEM contrasting method 27 to determine the surface fraction, f, as a function of etch time where heavily contrasted images were used to attain pixel counts for the ridged regions only.…”

“…The surface morphologies are dictated by the length of nanotubes freed in a template etching process, with longer etch times resulting in larger exposed lengths, larger spacing between the nanotube surface ridges, and smaller surface fractions, in contrast to prior reports of increased surface fractions with capillary bundling of vertical nanowires. 18,27,28 The self-assembled poly(3hexylthiophene) nanotube array surfaces roughly follow CassieÀBaxter wetting trends and are superhydrophobic with static contact angles greater than 150°a nd contact angle hysteresis less than or equal to 10°w hen the surface fraction is below 0.15. The total thermal resistance of an interface assembled with the poly(3-hexylthiophene) nanotube arrays increases from a minimum of 21 ( 2 mm 2 3 K/W to a maximum of 38 ( 3 mm 2 3 K/W as the etch time increased from 6 to 12 min, which is likely caused by the reduction in surface fraction and contact area at the interface.…”

“…Nanotube aggregation can dramatically alter the surface morphology and the corresponding interactions between the PNT array and a contacting liquid or substrate. For example, capillary effects can result in a surface with microstructures comprising smaller nanostructures, achieving a hierarchical arrangement similar to surfaces often found in nature and that are known to display special wettability. ,, Although nanotube array surface morphology has been proven to influence surface wetting properties, ,,,− we find no prior reports that correlate the degree of aggregation to the contact thermal energy transport of PNT array interfaces. Understanding this relationship is critical because, despite their high intrinsic thermal conductivities, thermal contact resistance (TCR) has been a major limitation of high aspect ratio structures such as carbon nanotube (CNT) array TIMs. − In addition, the favorable mechanical properties and strong adhesion of soft materials, as compared to CNTs, further supports the hypothesis that PNTs could potentially reduce the high contact resistance associated with traditional nanotube interfaces.…”

“…These traits make solution-processed polymer nanotubes interesting for many potential commercial applications. For example, polythiophene nanotube array thermal interface materials (TIMs) and polyethylene nanowire arrays , that exhibit dramatically improved anisotropic thermal conductivity were recently reported, and there have been multiple reports on the use of vertically aligned, high-aspect-ratio polymer nanotubes to create surfaces with superhydrophobic and tunable wetting properties. − This study investigates the thermal transport and wetting properties of the polythiophene derivative regioregular poly(3-hexylthiophene) (rr-P3HT) PNT arrays. We choose this polymer because rr-P3HT is stable at high temperatures and can undergo solvation in organic solvents (solution processing) at room temperature, but is stable in aqueous environments .…”

“…Understanding this relationship is critical because, despite their high intrinsic thermal conductivities, thermal contact resistance (TCR) has been a major limitation of high aspect ratio structures such as carbon nanotube (CNT) array TIMs. − In addition, the favorable mechanical properties and strong adhesion of soft materials, as compared to CNTs, further supports the hypothesis that PNTs could potentially reduce the high contact resistance associated with traditional nanotube interfaces. However, prior methods to fabricate high aspect ratio nanostructures with tunable surface morphology, and thus surface properties, are still relatively complex, ,,,− often relying on lithographic processes, special drying conditions, or varying structure dimensions, so there is a need to develop simple approaches.…”