Robust Oil-Fouling Resistance of Amorphous Cellulose Surface Underwater: A Wetting Study and Application

Reprogrammable, Sustainable, and 3D-Printable Cellulose Hydroplastic

Modern human societies are highly dependent on plastic materials, however, the bulk of them are non-renewable commodity plastics that cause pollution problems and consume large amounts of energy for their thermal processing activities. In this article, a sustainable cellulose hydroplastic material and its composites, that can be shaped repeatedly into various 2D/3D geometries using just water are introduced. In the wet state, their high flexibility and ductility make it conducive for the shaping to take place. In the ambient environment, the wet hydroplastic transits spontaneously into rigid materials with its intended shape in a short time of <30 min despite a thickness of hundreds of microns. They also possess humidity resistance and are structurally stable in highly humid environments. Given their excellent mechanical properties, geometry reprogrammability, bio-based, and biodegradable nature, cellulose hydroplastic poses as a sustainable alternative to traditional plastic materials and even "green" thermoplastics. This article also demonstrates the possibility of 3D-printing these hydroplastics and the potential of employing them in electronics applications. The demonstrated hydroshapable structural electronic components show capability in performing electronic functions, load-bearing ability and geometry versatility, which are attractive features for lightweight, customizable and geometry-unique electronic devices.

Liquid-infused interfacial floatable porous membrane as movable gate for ultrafast immiscible oil/water separation

Liquid separation methods are widely used in industrial and everyday applications, however, their applicability is often constrained by low efficiency, membrane fouling, and poor energy efficiency. Herein, a conceptually novel liquid-infused interfacial floatable porous membrane (LIIFPM) system for high-performance oil/water separation is proposed. The system functions by allowing a liquid to wet and fill a superamphiphilic porous membrane, thereby creating a stable liquid-infused interface that floats at the oil/water interface and prevents the passage of immiscible liquids. The lower-layer liquid can outflow directly, while the flow of the upper-layer liquid is stopped by the membrane. Remarkably, the efficiency of the LIIFPM system is independent of the membrane pore size, enabling ultrafast immiscible oil/water separation in an energy-saving and antifouling manner.

Presence, origins and effect of stable surface hydration on regenerated cellulose for underwater oil-repellent membranes

HYPOTHESIS

Underwater oil-repellency of polyelectrolyte brushes has been attributed mainly to electric double-layer repulsion forces based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Many non-polyelectrolyte materials also exhibit oil-repellent behaviour, but it is not clear if there exist similar electric double-layer repulsion and if it is the sole mechanism governing their underwater oil-repellency.

EXPERIMENTS/SIMULATIONS

In this article, the oil-repellency of highly amorphous cellulose exhibiting is investigated in detail, through experiments and molecular dynamics simulations (MDS).

FINDINGS

It was found that the stable surface hydration on regenerated cellulose was due to a combination of long-range electrostatic repulsions (DLVO theory) and short-range interfacial hydrogen bonding between cellulose and water molecules (as revealed by MDS). The presence of a stable water layer of about 200 nm thick (similar to that of polyelectrolyte brushes) was confirmed. Such stable surface hydration effectively separates cellulose surface from oil droplets, resulting in extremely low adhesion between them. As a demonstration of its practicality, regenerated cellulose membranes were fabricated via electrospinning, and they exhibit high oil/water separation efficiencies (including oil-in-water emulsions) as well as self-cleaning ability.

Structure-Activity Relationship between the Superhydrophilic Nanowire Structure and the Oil Dewetting Property

Materials require specific surface structures to achieve the best performance, but achieving an optimal structural design requires a systematic study of how structure affects performance. In this work, we comprehensively and systematically investigated the structure-activity relationship between the nanowire structure and the oil dewetting self-cleaning performance. It is easy for an oil droplet to enter this structure, but it is difficult for it to escape from the gaps between the structures even under the action of water. So, the oil dewetting ability is greatly reduced, showing that this "easy to enter and difficult to exit" mode is very disadvantageous for oil desorption. Moreover, if the structure is dissolved during the test, the oil dewetting ability will be restored. The desorption effect is affected by structural parameters and reaction conditions, which further verifies the negative effect of this structure. In contrast, copper(II) oxide nanowires completely lose their self-cleaning ability due to the enhancement of hydrophobicity and oleophilicity, and the larger-diameter copper(II) oxalate nanorods exhibit a "difficult to enter and difficult to exit" mode, leading to the partial recovery of the oil dewetting performance. This study helps us deeply understand the influence of the surface microstructure on the oil dewetting performance and lay a solid foundation for further appropriate structural design.

Fouling Resistance and Release Properties of Poly(sulfobetaine) Brushes with Varying Alkyl Chain Spacer Lengths and Molecular Weights

We examined the effects of alkyl carbon spacer length (CSL) and molecular weight on fouling resistance and release properties of zwitterionic poly(sulfobetaine methacrylate) brushes. Using surface-initiated atom transfer radical polymerization, we synthesized two series of brushes with CSL = 3 and 4 and molecular weight from 19 to 1500 kg ·mol^-1, corresponding to dry brush thickness from around 6 to 180 nm. The brush with CSL = 3 was nearly completely wet with water (independent of molecular weight), whereas the brush with CSL = 4 exhibited a strong increase in water contact angle with molecular weight. Though the two-brush series had distinct wetting properties, both series of brushes exhibited similarly great resistance against fouling by Staphylococcus epidermidis bacteria and Aspergillus niger fungi spores when submerged in water, indicating that neither molecular weight nor CSL strongly affected the antifouling behavior. We also compared the efficacy of brushes against fouling by fungi and silicon oil in air. Brushes grafted to filter paper were strongly fouled by fungi and silicon oil in air. Grafting the polymers to the filter paper, however, greatly enhanced removal of the foulant upon rinsing. The removal of fungi and silicon oil when rinsed with a salt solution was enhanced by 219 and 175%, respectively, as compared to a blank filter paper control. Thus, our results indicate that these zwitterionic brushes can promote foulant removal for dry applications in addition to their well-known fouling resistance in submerged conditions.

Enhancement of Meniscus Pump by Multiple Particles

We numerically investigate the behavior of a droplet spreading on a smooth substrate with multiple obstacles. As experimental works have indicated, the macroscopic contact line or the three-phase boundary line of a droplet exhibits significant deformation resulting in a local acceleration by successive interactions with an array of tiny obstacles settled on the substrate (Mu et al., Langmuir 2019, 35). We focus on the menisci formation and the resultant pressure and velocity fields inside a liquid film in a two-spherical-particle system to realize an optimal design for the effective liquid-transport phenomenon. Special attention is paid to the meniscus formation around the second particle, which influences the liquid supply related to the pressure difference around the first particle as a function of the distance between the two particles. We find that the meniscus around the first particle plays an additional role as the reservoir of the liquid supplied toward the second particle, which is found to enhance the total pumping effect.

Study of Oil Dewetting Ability of Superhydrophilic and Underwater Superoleophobic Surfaces from Air to Water for High-Effective Self-Cleaning Surface Designing

The superhydrophilic self-cleaning surface can perfectly deal with oil pollution, which cannot be realized by the superhydrophobic surface. This research is designed to study the mechanism of wetting behavior of superhydrophilic coating with different function groups and guide to design a stable self-cleaning surface. We prepare several hydrophilic coatings including nonionic, ionic, and zwitterionic coatings to investigate their self-cleaning performance underwater when they have been polluted by oil in the dry state. The chemical composition, surface roughness, static and dynamic wettability, underwater oil adhesive force, and swelling degree of the coatings are studied to explore their oil dewetting mechanism. The results indicate that the wettability of the coating to water and oil is the key factor to determine the self-cleaning performance. The smooth 3-sulfopropyl methacrylate potassium salt (SA) anionic coating shows the best self-cleaning performance even when polluted by heavy crude oil in the dry state in air. It is also found that in the dry state, the rough hydrophilic anionic surface will lock up the oil in the structures and then lose its self-cleaning ability underwater, whereas the oil droplet can detach from the smooth coating surface quickly. Meanwhile, the superhydrophilic and underwater superoleophobic SA anionic surfaces also exhibit excellent anti-fogging and oil-water separation performance.

3D-Printed Anti-Fouling Cellulose Mesh for Highly Efficient Oil/Water Separation Applications

The ability of additive manufacturing to print mesh structure was exploited to fabricate highly efficient filtration meshes for oil/water separation applications. Through Direct Ink Writing (DIW) technique, pure cellulose acetate with a mesh architecture can be created easily, using cellulose acetate/ethyl acetate solution as the ink and simply drying off the solvent in ambient conditions. Besides conventional mesh structures, more complex structures can be fabricated in order to manipulate the pore size and hence tune the separation properties of the mesh. The superhydrophilic 3D-printed cellulose meshes are able to achieve a high separation efficiency of >95% as long as the average pore size is smaller than 280 μm. More importantly, the mesh that possesses an unconventional complex structure boasts a separation efficiency of ∼99% while maintaining a high water flux of ∼160 000 Lm^2-h^-1. The 3D-printed cellulose meshes are also able to separate oil substances of a wide range of viscosity, from highly viscous PDMS (∼97 cP) to nonviscous cyclohexane (∼1 cP) and are chemically resistant to extreme acidic and alkaline conditions. Moreover, the 3D-printed cellulose meshes also possess antioil-fouling/self-cleaning ability, which makes its surfaces resilient to contamination. In addition, the 3D-printed meshes do not suffer from surface inhomogeneity and interfacial adhesion issues as compared to the usual coated meshes. Such a robust yet practical system is highly applicable for highly efficient oil-water separation applications.