Study of superhydrophobic surfaces with hierarchical structures
Date Issued
2012
Date
2012
Author(s)
Cho, Kuan-Hung
Abstract
A series of pillar-like patterns silicon wafer with different pillar sizes and spacing are fabricated by photolithography and further modified by a self-assembled fluorosilaned monolayer. The regular pillar-like structure silica and Poly(ethylene terephthalate) (PET) surface is fabricated by embossing silica sol-gel precursor and PET precursor on glass substrates with an elastomeric mold. Both advancing and receding contact angles of water on these surfaces are carefully measured and found to be consistent with the theoretical predictions of the Cassie model and of the Wenzel model after surface modification. It is well understood that the superhydrophobicity could be achieved much more effectively by applying a hierarchical structure. The wetting behaviors of three different methods to construct hierarchical structures after surface modification will be discussed:
(1) The hierarchical structure silica surface of inlaying silica nanoparticles along a regular pillar-like pattern is fabricated by embossing silica sol-gel precursor mixed with silica nanoparticles on glass substrates with an elastomeric mold. The advancing/ receding contact angle measurements are performed to demonstrate that a water droplet on these surfaces under an appropriate roughness can exhibit a transition from the Wenzel state to the Cassie state due to the addition of silica nanoparticles to enhance its surface roughness. Thus, the hierarchical structure is followed to develop the PDMS mold from the substrate as the template, and then, the substrate with hierarchical structure of bumpy surfaces along the pillar-like pattern is fabricated by simply embossing the PDMS mold on the silica sol-gel precursor (with no nanoparticles) directly for forming patterns over large areas in a facile fabrication.
(2) Silica nanoparticles were spin-coated onto the flat/patterned (regular pillar-like) substrate to enhance the surface roughness. The advancing/receding contact angle and sliding angle measurements were performed to determine the wetting behavior of a water droplet on the surface. It is interesting to find out that a transition from the Wenzel surface to the sticky superhydrophobic surface is observed due to the spin-coating silica nanoparticles. The slippery superhydrophobic surface can be further obtained after secondary spin-coating silica nanoparticles to generate the multi-scale roughness structure. The prepared superhydrophobic substrates should be robust for practical applications. The adhesion between the substrate and nanoparticles is also examined and discussed.
(3) Dual-scale roughness superhydrophobic surfaces are prepared by rf-capacitively coupled plasma etching the regular pillar-like patterned poly(ethyleneterephthalate) (PET) surfaces. Different wetting behaviors (Wenzel state, Petal state and Cassie state) can be achieved on the prepared dual-scale roughness surfaces by simply controlling the plasma etching time. It is found that a slippery superhydrophobic surface of patterned PET structures can be obtained by plasma etching simply for 1 minute. The advancing/receding contact angles and the sliding angle are systematically performed and applied to distinguish the difference between the slippery and sticky superhydrophobic surfaces. The nanoscale texture created by plasma etching on top of the patterned surfaces is an important factor governing the sticky/slippery surface. As a consequence, slippery/sticky superhydrophobic surfaces could be achieved much more effectively by plasma etching process.
In addition, we prepare a series of pillar-like patterns PDMS with different pillar sizes and spacing, and we demonstrate that different wetting behavior of regular pillar-like patterned PDMS can be achieved by controlling the mixing ratio of polydimethylsiloxane’s (PDMS’s) prepolymer base and curing agent. The dynamic contact angles and sliding angles of water droplet on these patterned surfaces are carefully measured. It is found that the experimental results in PDMS substrates prepared at the mixing ratio of base to curing agent 5:1 and 10:1 are consistent with the theoretical predictions of the Wenzel model and Cassie model. However, for the PDMS substrates prepared at the mixing ratio of base to curing agent 20:1, the wetting behavior would change due to the deformation of the structure—collapse of pillars due to the softness. Besides, the dynamic phenomenon of the wetting transition process of water penetrating into the pillars is observed through the optical microscope when a water droplet is placed on the regular pillar-like patterned PDMS substrates. The pillars are pushed down by water one after one like dominoes at higher ratio of base to curing agent. The featured pattern of the impregnating process is discussed when the collapse is occured.
Subjects
superhydrophobic surface
soft embossing
hierarchical structure
advancing contact angle
SAM
Type
thesis
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