Design of Robust Lubricant-Infused Surfaces for Anti-Corrosion

Material Design for Durable Lubricant-Infused Surfaces That Can Reduce Liquid and Solid Fouling

Liquid and solid fouling is a pervasive problem in numerous natural and industrial settings, significantly impacting energy efficiency, greenhouse emissions, operational costs, equipment lifespan, and human health. Inspired by pitcher plants, recently developed lubricant-infused surfaces (LISs) demonstrate resistance to both liquid and solid accretion under diverse environmental conditions, offering a potential solution to combat various foulants such as ice, bacteria, and mineral deposits. However, the commercial viability for most fouling-resistant LISs has thus far been compromised due to the challenges associated with maintaining a stable lubricant layer during operation. This review aims to address this important concern by providing systematic material design guidelines for fabricating durable LISs. We discuss fundamental design principles, methods for evaluating fouling resistance, and strategies to prevent lubricant loss. By presenting a comprehensive design methodology for this important class of materials, this review aims to aid future advancements in the field of antifouling surfaces, potentially impacting a variety of industries ranging from marine engineering to medical device manufacturing.

Unraveling the anti-biofouling mechanisms of slippery liquid-infused porous surface from molecular interaction perspective

Newly developed slippery liquid-infused porous surfaces (SLIPS) exhibit highly effective anti-fouling performance without harming organisms, making them a promising solution for both environmental and material protection. However, previous studies have primarily understood the anti-fouling effects of SLIPS from a mechanical perspective, neglecting the atomic interactions involved in the anti-fouling process. In this study, we combined microbiological experiments with multi-scale simulations to elucidate the microscopic mechanisms behind the unique anti-biofouling effects of SLIPS. After developing SLIPS with robust liquid-repellency, we characterized its physical and chemical properties and demonstrated its superior effectiveness in preventing Pseudomonas aeruginosa attachment. To probe the initial contact during bacterial attachment, all-atom molecular dynamics (MD) simulations were conducted, revealing that the liquid-liquid interface suppresses the effective pilin adhesion on SLIPS. Further analysis through steered MD, ab initio MD, and density functional theory calculations revealed that the flexible siloxane backbone and the non-polar nature of silicone oil molecules enhance the diffusivity of interfacial water and lead to the continuous nanoscale fluctuation of liquid-liquid interface, thus inhibiting the role of protein dynamics in promoting bio-adhesion. These novel insights into the characteristics of liquid-liquid and nano-bio interface during the anti-biofouling process of SLIPS may promote the future development of bio-inspired functional surfaces.

Multi-Dimensional Design of Slippery Liquid-Infused Coatings Empowering Long-Term Corrosion Protection for Sintered Nd-Fe-B Magnets in Harsh Environments

In spite of superior magnetic properties, neodymium-iron-boron (Nd-Fe-B) applications in harsh environments have been greatly hindered by their susceptibility to corrosion, humidity, mechanical, and temperature attacks. Herein, a construction strategy of robust slippery liquid-infused porous surfaces (SLIPS) coatings is proposed via the multi-dimensional design of surface, bulk coating, and interface. The manipulation of colloidal states of silica particles by chemical hydrophobization allows for building a dense polymer network, enhancing interfacial adhesion and locking lubricant film effectively. The resulting coating is demonstrated to be extremely stable, endowing the magnet with excellent liquid repellency, anti-corrosion, and anti-icing properties. Strikingly, no trace of corrosion is detected after 136-day immersion in 3.5 wt.% NaCl solution and the impedance modulus at 0.1 Hz can be maintained as the initial of 3.31 × 108 Ω·cm2 even after 132-day immersion, which is far superior to that of commercial Ni-Cu-Ni, Zn, and EP coatings. Additionally, anti-icing performance at low temperatures is evidenced by the delayed icing time and decreased ice adhesion strength. Importantly, the self-healing property offers the surface intensified durability even after mechanical damage. This work demonstrates a construction strategy of robust SLIPS coatings with a multi-dimensional design, enabling the practical applications of Nd-Fe-B magnets in extreme environments like offshore wind turbines.

Shear Evolution and Slippage of the Liquid-Liquid Interface over a Liquid-Infused Surface: A Many-Body Dissipative Particle Dynamics Study

The liquid-liquid interface (LLI), which is the key to cause flow slippage and thus promote drag reduction of liquid-infused surfaces (LISs), does suffer from the action of flow shear. In the current study, the transverse many-body dissipative dynamics simulation method is applied to explore the shear evolution of LLI and the corresponding slippage over a periodically grooved LIS. Results show that a relatively small shear rate only induces a slight deformation of LLI and the corresponding effective slippage is dependent on the shear rate. With a further increase of the shear rate, LLI deforms apparently and then the downstream three phase contact line depins to move once the balance between the capillary force and the shear force is broken, which results in an apparent increase of the slippage, specifically for a convex LLI. Compared with a convex LLI or a concave LLI, a flat LLI remains relatively stable under the same shear action, and an increase of the viscosity ratio and a decrease of the LLI fraction can both strengthen the shear resistance of an LLI, while they are less effective to promote the slippage. Consequently, the current results not only indicate that the slippage is related to the interface deflection and the shear rate but also suggest that both the shear resistance and the slippage of LLI should be considered when designing an effective LIS.

Ice Adhesion Properties on Micropillared Superhydrophobic Surfaces

In this work, we investigate the freezing behavior and ice adhesion properties of sessile drops on micropillared superhydrophobic surfaces (SHSs) with various sizes, which are of practical importance for anti/deicing. First of all, it is demonstrated that the recalescence is related only to the supercooling degree of drops but not to the geometrical parameters of micropillars. The freezing time of sessile drops first increases and then decreases with the area fraction of the SHSs, which demonstrates the nonmonotonic dependence of the icing time on the area fraction. Moreover, the influence of the geometrical parameters of the micropillars on the ice adhesion is discussed. With the decrease of the substrate temperature, the wetting state of the adhesive ice can be transformed from the Cassie ice to the Wenzel ice. For the Cassie ice, the adhesive force is proportional to the area fraction of the SHSs. Interestingly, experimental results show that there exist two interfacial debonding modes of the Wenzel ice: translational debonding and rotational debonding. Furthermore, it is found that the rotational debonding mode contributes to a much lower adhesive force between the ice and the micropillared surface compared to that of the translational debonding mode. By analyzing the critical interfacial energy release rate of the two modes, we deduce the threshold between the two modes, which is quantified as the geometrical parameters of the micropillars. In addition, quantitative relations between the geometrical parameters and the adhesion strengths of the two modes are also obtained. We envision that this work would shed new light on the design optimization of anti/deicing materials.

Advances in the Fabrication of Nanoporous Anodic Aluminum Oxide and Its Applications to Sensors: A Review

Nanoporous anodic aluminum oxide (AAO) is an important template for 1D nanomaterial synthesis. It is used as an etching template for nanopattern transfer in a variety of contexts, including nanostructured material synthesis, electrical sensors, optical sensors, photonic and electronic devices, photocatalysis, and hardness and anticorrosion improvement. In this review, we focus on various fabrication methods, pore geometry modification, and recent advances of AAO, as well as sensor applications linked to our environment, daily life, and safety. Pore geometry is concerned with the material composition, applied voltage mold, electrolyte type, temperature, and anodizing time during the fabrication of AAOs and for adjusting their pore size and profile. The applied voltage can be divided into four types: direct current anodization (DCA), reverse pulse anodization, pulse anodization (PA), and hybrid pulse anodization (HPA). Conventional AAOs are fabricated using DCA and mild anodization (MA) at a relatively low temperature (-5~15 °C) to reduce the Joule heating effect. Moreover, the issues of costly high-purity aluminum and a long processing time can be improved using HPA to diminish the Joule heating effect at relatively high temperatures of 20-30 °C with cheap low-purity (≤99%) aluminum. The AAO-based sensors discussed here are primarily divided into electrical sensors and optical sensors; the performance of both sensors is affected by the sensing material and pore geometry. The electrical sensor is usually used for humidity or gas measurement applications and has a thin metal film on the surface as an electrode. On the contrary, the AAO optical sensor is a well-known sensor for detecting various substances with four kinds of mechanisms: interference, photoluminescence, surface plasma resonance, and surface-enhanced Raman scattering (SERS). Especially for SERS mechanisms, AAO can be used either as a solid support for coating metal nanoparticles or a template for depositing the metal content through the nanopores to form the nanodots or nanowires for detecting substances. High-performance sensors will play a crucial role in our living environments and promote our quality of life in the future.

Silicone-Based Lubricant-Infused Slippery Coating Covalently Bound to Aluminum Substrates for Underwater Applications

Wetting of solid surfaces is crucial for biological and industrial processes but is also associated with several harmful phenomena such as biofouling and corrosion that limit the effectiveness of various technologies in aquatic environments. Despite extensive research, these challenges remain critical today. Recently, we have developed a facile UV-grafting technique to covalently attach silicone-based coatings to solid substrates. In this study, the grafting process was evaluated as a function of UV exposure time on aluminum substrates. While short-time exposure to UV light results in the formation of lubricant-infused slippery surfaces (LISS), a flat, nonporous variant of slippery liquid-infused porous surfaces, longer exposure leads to the formation of semi-rigid cross-linked polydimethylsiloxane (PDMS) coatings, both covalently bound to the substrate. These coatings were exposed to aquatic media to evaluate their resistance to corrosion and biofouling. While the UV-grafted cross-linked PDMS coating effectively inhibits aluminum corrosion in aquatic environments and allows organisms to grow on the surface, the LISS coating demonstrates improved corrosion resistance but inhibits biofilm adhesion. The synergy between facile and low-cost fabrication, rapid binding kinetics, eco-friendliness, and nontoxicity of the applied materials to aquatic life combined with excellent wetting-repellent characteristics make this technology applicable for implementation in aquatic environments.

Dropwise Condensation in Ambient on a Depleted Lubricant-Infused Surface

Durability of a lubricant-infused surface (LIS) is critical for heat transfer, especially in condensation-based applications. Although LIS promotes dropwise condensation, each departing droplet condensate acts as a lubricant-depleting agent due to the formation of wetting ridge and cloaking layer around the condensate, thus gradually leading to drop pinning on the underlying rough topography. Condensation heat transfer further deteriorates in the presence of non-condensable gases (NCGs) requiring special experimental arrangements to eliminate NCGs due to a decrease in the availability of nucleation sites. To address these issues while simultaneously improving heat-transfer performance of LIS in condensation-based systems, we report fabrication of both fresh LIS and a lubricant-depleted LIS using silicon porous nanochannel wicks as an underlying substrate. Strong capillarity in the nanochannels helps retain silicone oil (polydimethylsiloxane) on the surface even after it is severely depleted under tap water. The effect of oil viscosity was investigated for drop mobility and condensation heat transfer under ambient conditions, i.e., in the presence of NCGs. While fresh LIS prepared using 5 cSt silicone oil exhibited a low roll-off angle (∼1°) and excellent water drop (5 μL) sliding velocity ∼66 mm s^-1, it underwent rapid depletion as compared to higher viscosity oils. Condensation performed on depleted nanochannel LIS with higher viscosity oil (50 cSt) resulted in a heat-transfer coefficient (HTC) of ∼2.33 kW m^-2 K^-1, which is a ∼162% improvement over flat Si-LIS (50 cSt). Such LIS promote fast drop shedding as is evident from the little change in the fraction of drops with diameter <500 μm from ∼98% to only ∼93% after 4 h of condensation. Improvement in HTC was also seen in condensation experiments conducted for 3 days where a steady HTC of ∼1.46 kW m^-2 K^-1 was achieved over the last 2 days. The ability of reported LIS to maintain long-term hydrophobicity and dropwise condensation will aid in designing condensation-based systems with improved heat-transfer performance.

Multifunctional Edible Oil-Impregnated Nanoporous Oxide Layer on AISI 304 Stainless Steel

Slippery liquid-infused porous surface (SLIPS) realized on commercial materials provides various functionalities, such as corrosion resistance, condensation heat transfer, anti-fouling, de/anti-icing, and self-cleaning. In particular, perfluorinated lubricants infused in fluorocarbon-coated porous structures have showed exceptional performances with durability; however, they caused several issues in safety, due to their difficulty in degradation and bio-accumulation. Here, we introduce a new approach to create the multifunctional lubricant-impregnated surface with edible oils and fatty acid, which are also safe to human body and degradable in nature. The edible oil-impregnated anodized nanoporous stainless steel surface shows a significantly low contact angle hysteresis and sliding angle, which is similar with general surface of fluorocarbon lubricant-infused systems. The edible oil impregnated in the hydrophobic nanoporous oxide surface also inhibits the direct contact of external aqueous solution to a solid surface structure. Due to such de-wetting property caused by a lubricating effect of edible oils, the edible oil-impregnated stainless steel surface shows enhanced corrosion resistance, anti-biofouling and condensation heat transfer with reduced ice adhesion.

Key Factors Affecting Durable Anti-Icing of Slippery Surfaces: Pore Size and Porosity

Slippery liquid-infused porous surfaces (SLIPSs) are widely used as an effective passive approach to reduce icing disasters. However, various porous structures make SLIPSs exhibit different droplet mobility and lubricant stability. Undoubtedly, the substrate surface has a great impact on the durable anti-icing of SLIPSs. Herein, surfaces with different pore sizes and porosities were prepared to study their effects on the performance of SLIPS. The results show that small pores and high porosity are beneficial for the preparation of durable anti-icing SLIPS. The small pore size (about 100 nm) has a strong capillary pressure on the lubricant, and the surface with high porosity (66%) possesses a large lubricant-liquid contact ratio. These two can greatly improve the lubricant stability of SLIPS and achieve rapid self-healing. The SLIPS prepared by a suitable porous surface shows excellent anti-icing performance in the simulated glaze ice and durable anti-icing ability in the long-term icing/deicing cycles. In detail, the prepared SLIPS experiences more than 140 icing/deicing cycles through four effective self-healing while maintaining extremely low ice adhesion (<20 kPa). This work proposes a certain improved SLIPS with small pores and high porosity to achieve excellent durable anti-icing performance, broadening the practical applications of SLIPS.

Control of the Nanopore Architecture of Anodic Alumina via Stepwise Anodization with Voltage Modulation and Pore Widening

Control of the morphology and hierarchy of the nanopore structures of anodic alumina is investigated by employing stepwise anodizing processes, alternating the two different anodizing modes, including mild anodization (MA) and hard anodization (HA), which are further mediated by a pore-widening (PW) step in between. For the experiment, the MA and HA are applied at the anodizing voltages of 40 and 100 V, respectively, in 0.3 M oxalic acid, at 1 °C, for fixed durations (30 min for MA and 0.5 min for HA), while the intermediate PW is applied in 0.1 M phosphoric acid at 30 °C for different durations. In particular, to examine the effects of the anodizing sequence and the PW time on the morphology and hierarchy of the nanopore structures formed, the stepwise anodization is conducted in two different ways: one with no PW step, such as MA→HA and HA→MA, and the other with the timed PW in between, such as MA→PW→MA, MA→PW→HA, HA→PW→HA, and HA→PW→MA. The results show that both the sequence of the voltage-modulated anodizing modes and the application of the intermediate PW step led to unique three-dimensional morphology and hierarchy of the nanopore structures of the anodic alumina beyond the conventional two-dimensional cylindrical pore geometry. It suggests that the stepwise anodizing process regulated by the sequence of the anodizing modes and the intermediate PW step can allow the design and fabrication of various types of nanopore structures, which can broaden the applications of the nanoporous anodic alumina with greater efficacy and versatility.

Contamination and carryover free handling of complex fluids using lubricant-infused pipette tips

Cross-contamination of biological samples during handling and preparation, is a major issue in laboratory setups, leading to false-positives or false-negatives. Sample carryover residue in pipette tips contributes greatly to this issue. Most pipette tips on the market are manufactured with hydrophobic polymers that are able to repel high surface tension liquids, yet they lack in performance when low surface tension liquids and viscous fluids are involved. Moreover, hydrophobicity of pipette tips can result in hydrophobic adsorption of biomolecules, causing inaccuracies and loss in precision during pipetting. Here we propose the use of lubricant-infused surface (LIS) technology to achieve omniphobic properties in pipette tips. Using a versatile and simple design, the inner lumen of commercially available pipette tips was coated with a fluorosilane (FS) layer using chemical vapor deposition (CVD). The presence of FS groups on the tips is confirmed by x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) tests. After lubrication of the tips through a fluorinated lubricant, the omniphobicity and repellent behaviour of the tips drastically enhanced which are revealed via static and hysteresis contact angle measurements. The repellency of the lubricant-infused pipette tips against physical adsorption is investigated through pipetting a food coloring dye as well as human blood samples and are compared to the untreated tips. The results show significantly less amount carryover residue when the lubricant-infused tips are utilized compared to commercially available ones. We also demonstrate the lubricant-infused tips reduce bacteria contamination of the inner lumen by 3 to 6-log (over 99%, depending on the tip size) after pipetting up and down the bacteria solution.

Recent Progress in the Fabrication and Optical Properties of Nanoporous Anodic Alumina

The fabrication of a thick oxide layer onto an aluminum surface via anodization has been a subject of intense research activity for more than a century, largely due to protective and decorative applications. The capability to create well-defined pores via a cost-effective electrochemical oxidation technique onto the surface has made a major renaissance in the field, as the porous surfaces exhibit remarkably different properties compared to a bulk oxide layer. Amongst the various nanoporous structures being investigated, nanoporous anodic alumina (NAA) with well-organized and highly ordered hexagonal honeycomb-like pores has emerged as the most popular nanomaterial due to its wide range of applications, ranging from corrosion resistance to bacterial repelling surfaces. As compared to conventional nanostructure fabrication, the electrochemical anodization route of NAA with well-controlled pore parameters offers an economical route for fabricating nanoscale materials. The review comprehensively reflects the progress made in the fabrication route of NAA to obtain the material with desired pore properties, with a special emphasis on self-organization and pore growth kinetics. Detailed accounts of the various conditions that can play an important role in pore growth kinetics and pore parameters are presented. Further, recent developments in the field of controlling optical properties of NAA are discussed. A critical outlook on the future trends of the fabrication of NAA and its optical properties on the emerging nanomaterials, sensors, and devices are also outlined.