Mechanically durable liquid-impregnated honeycomb surfaces

Liquid repellent surfaces typically work by keeping the fouling liquid in a metastable state, with trapped pockets of air between the substrate and the liquid. An alternative method with greater long-term stability utilizes liquid-impregnated surfaces, where the liquid being repelled slides over an immiscible liquid immobilized on a porous surface. Here, we report a method for creating honeycomb surfaces amenable to liquid-impregnation. Polystyrene dissolved in a water-immiscible, volatile solvent was deposited in a humid environment on a variety of substrates to achieve the necessary porosity. Evaporative cooling results in condensation of water in a breath figure array of droplets, forming a sacrificial template for the drying polymer film. These honeycomb surfaces were further functionalized with fluorosilane and dipped in the lubricating liquid to result in a durable, liquid-repellent surface. These surfaces were found to exhibit repellency towards water and oils with extremely low tilt angles due to the smooth liquid–liquid contact between the lubricating liquid and the liquid being repelled.

water. Evaporation of the solvent leads to evaporative cooling of the film surface, resulting in water condensation and the formation of a breath figure array. Droplet coalescence is limited either by the increasing viscosity of the drying polymer film or by precipitation of the polymer at the water-solvent interface 19 . This array of water droplets acts as a template for the drying polymer solution leading to the formation of the porous surface structure once the solvent and water have fully evaporated. Based on their appearance, such breath figure-templated surfaces are commonly called honeycomb surfaces.
Polystyrene is a common polymer used in the creation of honeycomb surfaces [20][21][22] . Here, polystyrene honeycomb films are created on glass and polymer substrates and further treated with UV irradiation and fluorosilane coupling to ensure the lubricating liquid will remain impregnated within the polymer structure and no preferential dewetting will occur when the liquid to be repelled is added to the surface. The chemically modified honeycomb surfaces were then dipped into the lubricating liquid. The repellency of these liquid-impregnated honeycomb surfaces was tested against water and hexadecane. The mechanical durability of these surfaces was investigated through the use of macrowear experiments. Previous work has demonstrated the ability of liquid-impregnated honeycomb surfaces for the repellency of various liquids 23 . However, no durability study was conducted. These liquid-repellent honeycomb surfaces will be of interest for a range of applications such as in packaging, where it is desirable for the product to be completely removed with little to no fouling of the walls of the container, reducing wastage and improving the recyclability of the container.

Methods
In order to achieve the porous structures required for creating liquid-impregnated surfaces, the surfaces described in this paper comprise polystyrene coatings cast in a volatile, water-immiscible solvent and allowed to dry in a humid environment, Fig. 1. The honeycomb surface is then activated using UV irradiation and treated with fluorosilane to better to ensure the impregnating liquid will preferably wet the surface. The surface is then dipped in the impregnating liquid to result in a liquid repellent surface.
Samples. Glass slides (Fisher Scientific) and polypropylene sheet (PP, ASTM D4101-0112, SPI) cut to dimensions of 15 by 15 mm were used as substrates. 0.2 g of polystyrene (Mw ~ 350,000, Sigma Aldrich) was dissolved in 10 mL chloroform (Mallinckrodt) at room temperature. Once the polymer was fully dissolved, a droplet of the solution was added to the glass surface at room temperature and ambient humidity (54% RH) and the surface was dried in air. To activate the polymer surface for silane attachment, samples were UV irradiated for 30 min (15 W, λ max = 254 nm). Samples were fluorinated via chemical vapor deposition of a silane (to lower surface energy), which was required in order to ensure preferential wetting by the lubricating liquid. One drop of trichloro(1H, 1H, 2H, 2H-perfluorooctyl) silane (fluorosilane, Sigma Aldrich) was deposited next to the samples which were covered and left for 2 h. The sample was then dipped into the impregnating liquid, in this instance a

Results and Discussion
Flat polystyrene (PS) is found to be slightly hydrophobic with water contact angles of 94 ± 1°, Table 1 and Fig. 2. In order to create the porous polymer surface, polystyrene was dissolved in a water immiscible, volatile solvent. A drop of the solution was cast onto a substrate and dried in a humid environment at room temperature. Evaporative cooling of the drying polymer film results in the condensation of water droplets and the formation of a breath figure. This array of water droplets acts as a sacrificial template for the drying polymer film and, once evaporation of the solvent and water is complete, results in a porous, honeycomb surface structure, Fig. 3. Fully dried, the polystyrene honeycomb surface was found to have a water contact angle of 107 ± 2° due to the increase in surface roughness.
The mechanical durability of the polystyrene honeycomb surface was investigated through the use of tribometer wear experiments and the resulting optical images, showing a portion of the wear track, are displayed in Fig. 4. The wear experiments were carried out with a load of 10 mN, with the tribometer put in reciprocating motion for 200 cycles. The images confirm that the polymer coating is not removed from the glass substrate. The density of the honeycomb structure appears to decrease in the wear location due to plastic deformation of the polymer. However, the porous structure is not completely destroyed, allowing for the impregnating liquid to remain in the wear region. It is believed that these surfaces can likely be more durable than many other examples of liquid-impregnated surfaces, which typically rely on poorly adhered wax coatings 15 or delicate surface structures 11 .
For the lubricating liquid to fully penetrate the porous surface, the chemistry of the honeycombs was altered, ensuring favorable wetting and no preferential dewetting when another liquid is added on top of the lubricating liquid layer. The polystyrene honeycomb coating was activated via UV irradiation to activate the surface for silane attachment. Following fluorosilane treatment, the polystyrene honeycomb surface displayed water contact angles  of 138 ± 2° and hexadecane contact angles of 107 ± 2°, Fig. 2. This altering of the surface energy is necessary to ensure that the lubricating liquid, in this case a lower surface tension fluorinated oil, will remain impregnated in the honeycomb structure and will not be preferentially replaced by the liquid to be repelled.
Finally, the honeycomb surface was dipped into the lubricating liquid. Following this, the liquid-impregnated surface exhibited water contact angles of 109 ± 2° and hexadecane contact angles of 70 ± 2°, Fig. 2. However, due to the presence of the lubricating liquid, the surface displays very low tilt angles of 2 ± 1° and 4 ± 2° for water and hexadecane respectively, Table 1. Because the low tilt angles are a product of the homogeneity of the liquid-liquid interface, the surface tension of the liquid being repelled has little effect on the repellency of the surface.
The low tilt angle means that liquid droplets placed on the surface are able to slide over the surface easily. In Fig. 5, droplets of hexadecane were added to fluorosilane-treated honeycomb and liquid-impregnated honeycomb surfaces. Hexadecane droplets on the fluorosilane-treated surface are not easily removed when the surface is titled due to high hysteresis and droplet pinning. In contrast, as the liquid-impregnated surface is tilted, the hexadecane droplet slides across the surface with very little resistance. The red dye present in the hexadecane droplet helps to confirm that the vacated area of the surface is not contaminated by the hexadecane. Further wear experiments carried out on honeycomb surfaces containing the lubricating liquid did not result in any change in the repellent properties of the surface, with droplets of hexadecane sliding over the wear location with no noticeable degradation in the repellency.
In certain applications, liquid-impregnated surfaces can exhibit greater long-term repellency than traditional liquid-repellent surfaces, the repellency of which is dependent on metastable states and trapped air. For instance, liquid-impregnated surface treatments could be better suited for applications where the contaminant liquid is in constant contact with the surface for extended periods of time or where the substrate is subject to vibration.

Conclusions
Liquid-repellent, slippery surfaces have been created on glass and polymer substrates via the formation of a honeycomb structure. Following UV activation and fluorosilane coupling to reduce the surface energy of the honeycombs, the substrate was dipped in a lubricating liquid, which became impregnated within the pores. This lubricating liquid layers repels other liquids placed on the surface through immiscible liquid-liquid contact. This results in very low tilt angles with droplets of both water and hexadecane sliding across the surface with no contamination. Such liquid-repellent surfaces will be more stable than repellent surfaces relying on the Cassie-Baxter state of wetting, where the liquid droplet being repelled is in a metastable state.