Topographical Design and Thermal-Induced Organization of Interfacial Water Structure to Regulate the Wetting State of Surfaces

Abstract

Smart surfaces with superhydrophobic/superhydrophilic characteristics can be controlled by external stimuli, such as temperature. These transitions are attributed to the molecular-level conformation of the grafted polymer chains due to the varied interactions at the interface. Here, tunable surfaces were prepared by grafting two well-known thermo-responsive polymers, poly( N -isopropylacrylamide) (PNIPAM) and poly(oligoethylene glycol)methyl ether acrylate (POEGMA 188 ) onto micro-pollen particles of uniform morphology and roughness. Direct Raman spectra and thermodynamic analyses revealed that above the lower critical solution temperature, the bonded and free water at the interface partially transformed to intermediate water that disrupted the “water cage” surrounding the hydrophobic groups. The increased amounts of intermediate water produced hydrogen bonding networks that were less ordered around the polymer grafted microparticles, inducing a weaker binding interaction at the interface and a lower tendency to wet the surface. Combining the roughness factor, the bulk surface assembled by distinct polymer-grafted-pollen microparticles (PNIPAM or POEGMA 188 ) could undergo a different wettability transition for liquid under air, water, and oil. This work identifies new perspectives on the interfacial water structure variation at a multiple length scale, which contributed to the temperature-dependent surface wettability transition. It offers inspiration for the application of thermo-responsive surface to liquid-gated multiphase separation, water purification and harvesting, biomedical devices, and printing.

Boosting Solar-Driven Water Evaporation through Selective Water Gating Enabled by Thermo-Responsive Sporopollenin

“…At 20°C, the higher ratio suggested that the PN10-g-SEC surface had a larger amount of bound water, with the hydrophilic C=O and -NH 2 groups predominantly positioned at the air/PN-g-SEC/water interface. Conversely, the spectrum displayed a signi cant reduction in the 4-HBW/BW ratio at 40°C, indicating that the water underwent a transformation into a less ordered and weaker hydrogen-bonded structure, while the hydrophobic -CH 3 groups rearranged and exposed at the interface 7 (Figure 2e). Taking advantage of this characteristic together with the hierarchical structure on the outer surface of SEC, we obtained the thermo-responsive surfaces with switchable wettability transition as illustrated in our previous study 7 .…”

“…Conversely, the spectrum displayed a signi cant reduction in the 4-HBW/BW ratio at 40°C, indicating that the water underwent a transformation into a less ordered and weaker hydrogen-bonded structure, while the hydrophobic -CH 3 groups rearranged and exposed at the interface 7 (Figure 2e). Taking advantage of this characteristic together with the hierarchical structure on the outer surface of SEC, we obtained the thermo-responsive surfaces with switchable wettability transition as illustrated in our previous study 7 . By adjusting the polymer grafting density, we successfully obtained pollen microparticles with hydrophilic/hydrophobic transition states on the PN20-g-SEC surfaces (Figure S5) and the constructed PN10-g-SEC surfaces exhibited remarkable superhydrophobic/ superhydrophilic wettability transition characteristics.…”

“…Preparation of PNm-g-SEC microparticles Brie y, we prepared sun ower sporopollenin (SEC) via alkaline extraction method and modi ed its outer surface with the N-Isopropylacrylamide (NIPAM) monomer via free radical polymerization. 7 Following the previous study, we rst treated the defatted pollen with 10 wt% potassium hydroxide (KOH) at 80°C and stirred for 2 h to remove its internal cytoplasmic content. KOH-treated pollens were next subjected to acidolysis in 85% phosphoric acid (H 3 PO 4 ) and washed extensively with ethanol and water.…”

Surface Modification, Topographic Design and Applications of Superhydrophobic Systems

“…Additionally, some recent studies on the interfacial water structure indicated that the intrinsic wetting threshold for hydrophilic/hydrophobic was close to ∼65° rather than 90° [25] . This was associated with the water molecules forming denser structures at the solid/liquid interface, such as trihedral or tetrahedral conformation on hydrophilic surface, while the water molecules display weak interactions with hydrophobic surface, assembling into looser or non‐hydrogen‐bond water structures [26] …”

“…[25] This was associated with the water molecules forming denser structures at the solid/liquid interface, such as trihedral or tetrahedral conformation on hydrophilic surface, while the water molecules display weak interactions with hydrophobic surface, assembling into looser or non-hydrogen-bond water structures. [26] Young's equation quantitatively represents the relationship between CA, liquid surface tension γ Lv , and interfacial tension γ sL and solid surface tension γ sv for an ideal flat surface with homogeneous physical/chemical properties as described by Equation (1):…”