Wettability and Droplet Directional Spread Investigation of Crescent Array Surface Inspired by Slippery Zone of Nepenthes

Abstract

In this report, the surface topography of crescent array in the Nepenthes slippery zone is measured and bionic fabricated by two‐photon polymerization (TPP) technology. A subregion outside‐in scanning method (SOSM) is proposed to solve the photoresist converge and large area surface positioning problem existing in bionic crescent array surface (BCAS) fabrication. The geometric parameters of BCAS, including area fraction, height, offset, and outer–inner radius ratio, are systematically investigated to reveal their effects on the hydrophobic performance. Perfluorinated polyether oil and fluorocarbon solvent were used to increase the hydrophobicity and homogeneity of bionic surface. The contact angles of BCASs coated with and without the solution are measured. The maximum contact angle can reach to 152.1°. What's more, considering the anisotropy of BCAS, anisotropic wetting research is conducted. Offset and outer–inner radius ratio are found to have obvious effects on the contact angle differences between convexity and concavity directions. The maximum contact angle difference can reach to 5.1°. Spreading and pinning phenomenon caused by droplet anisotropic wetting are observed and explained. This work is expected to provide references for the BCAS to be applied in super‐hydrophobic and droplets motion fields.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

“… 1 The material systems that allow the spontaneous flow/penetration of liquid in only one direction are described as liquid diodes, which attract scientific and application interests in diverse fields such as liquid separation, water collection, digital microfluids 2 (enable fluidic functions at microscale for merging, splitting, transporting, mixing, and incubating, which makes them ideal for numerous biological and chemical platforms), energy, interface catalysis, and smart fabrics. 3 , 4 , 5 , 6 , 7 , 8 The anisotropic motion of droplets on a solid substrate was first observed by Greenspan in 1978. 9 The author pointed out that droplets tend to creep in a direction of greater adherence (lower contact angle) while retracting from weaker attachment regions (higher contact angle) because of the forces at the fluid/solid contact line.…”

A Review on Fabrication and Application of Tunable Hybrid Micro–Nano Array Surfaces

“…The periodic spindle structure of spider silk can move directionally to effectively collect submillimeter water droplets in fog; [ 3 ] the evenly distributed cone‐shaped spines and trichomes on cactus stems can directionally collect agglomerated droplets; [ 4 ] the ratchet‐like scales on butterfly wings have anisotropic superhydrophobicity, which can orient water droplets away their bodies; [ 5 ] desert beetles can collect fog or water vapor from the air using hydrophobic microgrooves and raised patterns of hydrophilic micrometers on their backs; [ 6 ] the periodic duckbill‐like microcavity structure of pitcher plants enables continuous water droplet transport. [ 7 ] Notably, the surfaces of these organisms tend to have hybrid periodic array structures ranging from micrometers to nanometers.…”

“…away their bodies; [5] desert beetles can collect fog or water vapor from the air using hydrophobic microgrooves and raised patterns of hydrophilic micrometers on their backs; [6] the periodic duckbill-like microcavity structure of pitcher plants enables continuous water droplet transport. [7] Notably, the surfaces of these organisms tend to have hybrid periodic array structures ranging from micrometers to nanometers.…”

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away their bodies; [5] desert beetles can collect fog or water vapor from the air using hydrophobic microgrooves and raised patterns of hydrophilic micrometers on their backs; [6] the periodic duckbill-like microcavity structure of pitcher plants enables continuous water droplet transport. [7] Notably, the surfaces of these organisms tend to have hybrid periodic array structures ranging from micrometers to nanometers.The droplet state on the solid surface depends on the surface tension between the solid-liquid-gas three-phase interface, which is determined by the chemical composition and surface pattern. [8] Organisms can adapt to their environment and respond to certain stimuli such as temperature and humidity, thereby regulating their surface wetting behavior.

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Wetting Effect on Patterned Substrates

“…Typical examples for the desired materials are self-cleaning, [32][33][34][35] anti-fouling, [36][37][38] antifogging [39,40] of solid substrates including glasses, metals, and ceramics. These functionalities may be achieved by changing the surface chemistry, [41] for example coating, [42,43] or by modifying the surface microstructures, [44][45][46][47] which are known to be chemically and mechanically patterned substrates, respectively. On these patterned substrates, we sometimes want the liquid to detach from the solid substrate with a relatively small roll-off angle and sometimes require a strong adhesion of the solid wall to the liquid.…”