Bioinspired Edible Lubricant-Infused Surface with Liquid Residue Reduction Properties

Engineered Sustainable Omniphobic Coatings to Control Liquid Spreading on Food-Contact Materials

The adhesion of sticky liquid foods to a contacting surface can cause many technical challenges. The food manufacturing sector is confronted with many critical issues that can be overcome with long-lasting and highly nonwettable coatings. Nanoengineered biomimetic surfaces with distinct wettability and tunable interfaces have elicited increasing interest for their potential use in addressing a broad variety of scientific and technological applications, such as antifogging, anti-icing, antifouling, antiadhesion, and anticorrosion. Although a large number of nature-inspired surfaces have emerged, food-safe nonwetted surfaces are still in their infancy, and numerous structural design aspects remain unexplored. This Review summarizes the latest scientific research regarding the key principles, fabrication methods, and applications of three important categories of nonwettable surfaces: superhydrophobic, liquid-infused slippery, and re-entrant structured surfaces. The Review is particularly focused on new insights into the antiwetting mechanisms of these nanopatterned structures and discovering efficient platform methodologies to guide their rational design when in contact with food materials. A detailed description of the current opportunities, challenges, and future scale-up possibilities of these nanoengineered surfaces in the food industry is also provided.

A novel cultivation platform of duckweed (Lemna minor) via application of beeswax superhydrophobic coatings

Conventional establishment of laboratory cultures of duckweed Lemna minor are prepared in beakers, Erlenmeyer flasks or Schott bottles. These conventional cultivation methods limit the available surface area for growth which then causes layering of fronds that reduces the efficiency of plants in sunlight capturing. Here, acrylic sheets were spray-coated with a superhydrophobic (SHP) beeswax suspension and these coated acrylic sheets were used as a novel cultivation platform for L. minor. L. minor was grown for 7 days in conventional glass jar which acted as the control and were compared to SHP coated acrylic (SHPA) and SHP coated acrylic with aluminium mesh centrally placed (SHPAM) at similar duration and cultivation conditions. Addition of mesh was to entrap the plantlets and fixed the plantlets' position on the growing platform. The effects of cultivation platforms on growth rate and biochemical compositions of L. minor were monitored. The highest biomass growth was obtained from SHPA cultivation where the relative growth rate (RGR) was 0.0909 ± 0.014 day^-1 and the RGR was 2.17 times higher than the control. Moreover, L. minor harvested from SHPA displayed the highest values in total protein content, total carbohydrates content and crude lipid percentage. The values were 156.04 ± 12.13 mg/g, 94.75 ± 9.02 mg/g and 7.09 ± 1.14% respectively. However, the control showed the highest total chlorophyll content which was 0.7733 ± 0.042 mg/g FW. Although SHPA obtained a slightly lower chlorophyll content than the control, this growing platform is still promising as it displayed the highest growth rate as well as other biochemical composition. Hence, this study proved that the proposed method that applied superhydrophobic properties in cultivation of L. minor provided a larger surface area for L. minor to grow, which then resulted in a greater biomass production while simultaneously maintaining the quality of the biochemical compositions of duckweeds.

Whether and When Superhydrophobic/Superoleophobic Surfaces Are Fingerprint Repellent

Driven by the ever-increasing demand for fingerprint-resistant techniques in modern society, numerous researches have proposed to develop innovative antifingerprint coatings based on superhydrophobic/superoleophobic surface design. However, whether superhydrophobic/superoleophobic surfaces have favorable repellency to the microscopic fingerprint is in fact an open question. Here, we establish a reliable method that enables evaluating the antifingerprint capability of various surfaces in a quantitative way. We show that superhydrophobicity is irrelevant with fingerprint repellency. Regarding superoleophobic surfaces, two distinct wetting states of microscopic fingerprint residues, i.e., the “repellent” and the “collapsed” states, are revealed. Only in the “repellent” state, in which the fingerprint residues remain atop surface textures upon being pressed, superoleophobic surfaces can bring about favorable antifingerprint repellency, which correlates positively with their receding contact angles. A finger-deformation-dependent intrusion mechanism is proposed to account for the formation of different fingerprint wetting states. Our findings offer important insights into the mechanism of fingerprint repellency and will help the design of high-performance antifingerprint surfaces for diverse applications.

Fabrication of bioinspired edible liquid marble with phase transition and tunable water barrier property

Based on aphid wax-honeydew marble and the hydrophobic wax structure of lotus and its derived applications with superareophilic and superhydrophobic properties, edible carnauba wax and beeswax particles were mixed and utilized to mimic lotus wax and wrap liquid, thus forming liquid marbles (LMs). Through the utilization of continuous production system (CPS), wax as an interfacial surfactant, water and solid, air-phase or mixed-phase marble content was produced. The edible liquid marble (ELM) could encapsulate water and food droplets. Edible solid marble (ESM) and edible solid hollow marbles (ESHMs) could be fabricated by applying pectin or syrup. Moreover, through the heating of wax powders with different melting temperatures, stable tablets and hollow capsules could be produced. The wax powder as interfacial surfactant could firmly bind with pectin through hydrogen bonds on ESM. The edible LMs can therefore be applied for residue reduction, corrosion reduction, biohazard prevention and cleaning in the food industry. The other phase LMs could act as novel tools in the pharmaceutical and food industries with the above-mentioned water transport, preservation, sustained releasing and selective releasing abilities.

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The online version contains supplementary material available at 10.1007/s42242-021-00158-z.

Optimal Design of a Fog Collector: Unidirectional Water Transport on a System Integrated by Conical Copper Needles with Gradient Wettability and Hydrophilic Slippery Rough Surfaces

Inspired by a cactus spine and pitcher plant slippery surface, a strategy is proposed to design a superhydrophobic-hydrophilic conical copper needle (SHB-HL CCN) and hydrophilic slippery rough surface (SRS) integrative system. In this strategy, the SHB-HL CCN was inserted vertically on the hydrophilic SRS, and such a hydrophilic SRS + SHB-HL CCN system exhibited a high-efficiency cycle in droplet capture-coalescence (supply)-transport during the fog collection process. Even with a single SHB-HL CCN or hydrophilic SRS, the water collection rate is much higher than that of the usual materials (original copper needle, superhydrophobic substrate, hydrophobic SRS, etc.). It is demonstrated that a newly enhanced fog harvesting mechanism and higher fog collection rate can be realized due to the synergy between the Laplace pressure difference from the conical needle, wettability force of wettability difference in the conical copper needles, and released surface energy in droplet coalescence in addition to the attracting force from water bridges formed between needles and substrate. Compared with a single SHB-HL CCN and hydrophilic SRS, the water collection rate of the hydrophilic SRS + SHB-HL CCN system increased by approximately 328 and 152%, respectively. This fog collector provides direction to design water harvesting systems, which has important promotion significance for water collection application engineering in industry, aerospace, and other fields.

Condensation of Satellite Droplets on Lubricant-Cloaked Droplets

Condensation on lubricant-infused micro- or nanotextured superhydrophobic surfaces exhibits remarkable heat transfer performance owing to the high condensation nucleation density and efficient condensate droplet removal. When a low surface tension lubricant is used, it can spread on the condensed droplet and "cloak" it. Here, we describe a previously unobserved condensation phenomenon of satellite droplet formation on lubricant-cloaked water droplets using environmental scanning electron microscopy. The presence of satellite droplets confirms the cloaking behavior of common lubricants on water such as Krytox oils. More interestingly, we have observed satellite droplets on BMIm ionic liquid-infused surfaces, which is unexpected because BMIm was used in previous reports as a lubricant to eliminate cloaking during water condensation. Our studies reveal that the cloaking of BMIm on water droplets is theoretically favorable due to the fast timescale spreading during initial condensation when compared to the long timescale required for dissolution of the lubricant in water. We utilize a novel characterization approach based on Raman spectroscopy to confirm the existence of cloaking lubricant films on water droplets residing on lubricant-infused surfaces. The selected lubricants include Krytox oil, ionic liquid, and dodecane, which have drastically different surface tensions and polarities. In addition, spreading dynamics of cloaking and noncloaking lubricants on water droplets show that ionic liquid has the capability to mobilize water droplets spontaneously owing to cloaking and its relatively high surface tension. Our studies not only elucidate the physics governing cloaking and satellite droplet condensation phenomena at micro- and macroscales but also reveal a subset of previously unobserved lubricant-water interfacial interactions for a large variety of applications.

Effect of Ageing on the Structure and Properties of Model Liquid-Infused Surfaces

Liquid-infused surfaces (LISs) exhibit unique properties that make them ideal candidates for a wide range of applications, from antifouling and anti-icing coatings to self-healing surfaces and controlled wetting. However, when exposed to realistic environmental conditions, LISs tend to age and progressively lose their desirable properties, potentially compromising their application. The associated ageing mechanisms are still poorly understood, and results reflecting real-life applications are scarce. Here, we track the ageing of a model LIS composed of glass surfaces functionalized with hydrophobic nanoparticles and infused with silicone oil. The LISs are fully submerged in aqueous solutions and exposed to acoustic pressure waves for set time intervals. The ageing is monitored by periodic measurements of the LIS's wetting properties. We also track the changes to the LIS's nanoscale structure. We find that the LISs rapidly lose their slippery properties because of a combination of oil loss, smoothing of the nanoporous functional layer, and substrate degradation when directly exposed to the solution. The oil loss is consistent with water microdroplets entering the oil layer and displacing oil away from the surface. These mechanisms are general and could play a role in the ageing of most LISs.