Controlled Wrinkle Patterning on Thin Films to Improve Hydrophobicity.

Controlling surface morphology is one of the main strategies used to tune surface hydrophobic and icephobic properties. Taking advantage of coating growth by initiated chemical vapor deposition, random and ordered wrinkles were induced on a thin film of polyperfluorodecyl acrylate (pPFDA) deposited on polydimethylsiloxane (PDMS) to simultaneously modify surface chemistry and morphology. A range of wrinkles of different wavelengths were studied, and how the wrinkle characteristics change with varying coating thickness. Ordered wrinkles enhanced hydrophobicity more when compared to random wrinkles, with a noticeable effect for coating thickness on the order of hundreds of nanometers. An insight into the mechanism of surface wrinkling and its effect on freezing delay is also provided, and promising results were found on ordered wrinkles, where a freezing delay was observed.

Interfacial Nanoblisters Formed in Water Serving as Freestanding Platforms for Measuring Elastic Moduli of Polymeric Nanofilms.

Polymeric nanofilms have been widely utilized in diverse cutting-edge technologies, yet accurately determining their elastic moduli remains challenging. Here we demonstrate that interfacial nanoblisters, which are produced by simply immersing substrate-supported nanofilms in water, represent natural platforms for assessing the mechanical properties of polymeric nanofilms using the sophisticated nanoindentation method. Nevertheless, high-resolution, quantitative force spectroscopy studies reveal that the indentation test must be performed on an effective freestanding region around the nanoblister apex and meanwhile under an appropriate loading force, to obtain load-independent, linear elastic deformations. The nanoblister stiffness increases with either decreasing its size or increasing its covering film thickness, and such size effects can be adequately rationalized by an energy-based theoretical model. The proposed model also enables an exceptional determination of the film elastic modulus. Given that interfacial blistering is a frequently occurring phenomenon for polymeric nanofilms, we envision that the presented methodology would stimulate broad applications in relevant fields.

Chitosan-Based Biomaterials: Insights into Chemistry, Properties, Devices, and Their Biomedical Applications.

Chitosan is a marine-origin polysaccharide obtained from the deacetylation of chitin, the main component of crustaceans' exoskeleton, and the second most abundant in nature. Although this biopolymer has received limited attention for several decades right after its discovery, since the new millennium chitosan has emerged owing to its physicochemical, structural and biological properties, multifunctionalities and applications in several sectors. This review aims at providing an overview of chitosan properties, chemical functionalization, and the innovative biomaterials obtained thereof. Firstly, the chemical functionalization of chitosan backbone in the amino and hydroxyl groups will be addressed. Then, the review will focus on the bottom-up strategies to process a wide array of chitosan-based biomaterials. In particular, the preparation of chitosan-based hydrogels, organic-inorganic hybrids, layer-by-layer assemblies, (bio)inks and their use in the biomedical field will be covered aiming to elucidate and inspire the community to keep on exploring the unique features and properties imparted by chitosan to develop advanced biomedical devices. Given the wide body of literature that has appeared in past years, this review is far from being exhaustive. Selected works in the last 10 years will be considered.

Hierarchical Structure of Cellulose Nanofibril-Based Foams Explored by Multimodal X-ray Scattering.

Structural characterization techniques are fundamental to correlate the material macro-, nano-, and molecular-scale structures to their macroscopic properties and to engineer hierarchical materials. Here, we combine X-ray transmission with scanning small- and wide-angle X-ray scattering (sSWAXS) to investigate ultraporous and lightweight biopolymer-based foams using cellulose nanofibrils (CNFs) as building blocks. The power of multimodal sSWAXS for multiscale structural characterization of self-assembled CNFs is demonstrated by spatially resolved maps at the macroscale (foam density and porosity), at the nanoscale (foam structural compactness, CNF orientation in the foam walls, and CNF packing state), and at the molecular scale (cellulose crystallite dimensions). Specifically, we compare the impact of freeze-thawing-drying (FTD) fabrication steps, such as static/stirred freezing and thawing in ethanol/water, on foam structural hierarchy spanning from the molecular to the millimeter scale. As such, we demonstrate the potential of X-ray scattering imaging for hierarchical characterization of biopolymers.

Contactless Transport and Mixing of Liquids on Self-Sustained Sublimating Coatings.

Controlled handling of liquids and colloidal suspensions as they interact with surfaces, targeting a broad palette of related functionalities, is of great importance in science, technology, and nature. When small liquid volumes (drops on the order of microliters or nanoliters) need to be processed in microfluidic devices, contamination on the solid/liquid interface and loss of liquid due to adhesion on the transport channels are two very common problems that can significantly alter the process outcome, for example, the chemical reaction efficiency or the purity and the final concentration of a suspension. It is, therefore, no surprise that both levitation and minimal contact transport methods-including nonwetting surfaces-have been developed to minimize the interactions between liquids and surfaces. Here, we demonstrate contactless surface levitation and transport of liquid drops, realized by harnessing and sustaining the natural sublimation of a solid-carbon-dioxide-coated substrate to generate a continuous supporting vapor layer. The capability and limitations of this technique in handling liquids of extreme surface tension and kinematic viscosity values are investigated both experimentally and theoretically. The sublimating coating is capable of repelling many viscous and low-surface-tension liquids, colloidal suspensions, and non-Newtonian fluids as well, displaying outstanding omniphobic properties. Finally, we demonstrate how sublimation can be used for liquid transport, mixing, and drop coalescence, with a sublimating layer coated on an underlying substrate with prefabricated channels, conferring omniphobicity using a simple physical approach (i.e., phase change) rather than a chemical one. The independence of the surface levitation principle from material properties, such as electromagnetic, thermal or optical, surface energy, adhesion, or mechanical properties, renders this method attractive for a wide range of potential applications.

Detergency and Its Implications for Oil Emulsion Sieving and Separation.

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil-water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water-soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro-/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh-and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of w ≈ d is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter-without any additional surface engineering or functionalization-to separate oil droplets (d < 100 µm) from a surfactant-stabilized emulsion with a flux of ∼11,000 L m-2 h-1 bar-1. We believe these findings can inform the design of future oil separation materials.

Water Touch-and-Bounce from a Soft Viscoelastic Substrate: Wetting, Dewetting, and Rebound on Bitumen.

Understanding the interaction between liquids and deformable solid surfaces is a fascinating fundamental problem, in which interaction and coupling of capillary and viscoelastic effects, due to solid substrate deformation, give rise to complex wetting mechanisms. Here we investigated as a model case the behavior of water drops on two smooth bitumen substrates with different rheological properties, defined as hard and soft (with complex shear moduli in the order of 10(7) and 10(5) Pa, respectively, at 1 Hz), focusing both on wetting and on dewetting behavior. By means of classical quasi-static contact angle measurements and drop impact tests, we show that the water drop behavior can significantly change from the quasi-static to the dynamic regime on soft viscoelastic surfaces, with the transition being defined by the substrate rheological properties. As a result, we also show that on the hard substrate, where the elastic response is dominant under all investigated conditions, classical quasi-static contact angle measurements provide consistent results that can be used to predict the drop dynamic wetting behavior, such as drop deposition or rebound after impact, as typically observed for nondeformable substrates. Differently, on soft surfaces, the formation of wetting ridges did not allow to define uniquely the substrate intrinsic advancing and receding contact angles. In addition, despite showing a high adhesion to the soft surface in quasi-static measurements, the drop was surprisingly able to rebound and escape from the surface after impact, as it is typically observed for hydrophobic surfaces. These results highlight that measurements of wetting properties for viscoelastic substrates need to be critically used and that wetting behavior of a liquid on viscoelastic surfaces is a function of the characteristic time scales.

Physics of icing and rational design of surfaces with extraordinary icephobicity.

Icing of surfaces is commonplace in nature and technology, affecting everyday life and sometimes causing catastrophic events. Understanding (and counteracting) surface icing brings with it significant scientific challenges that requires interdisciplinary knowledge from diverse scientific fields such as nucleation thermodynamics and heat transfer, fluid dynamics, surface chemistry, and surface nanoengineering. Here we discuss key aspects and findings related to the physics of ice formation on surfaces and show how such knowledge could be employed to rationally develop surfaces with extreme resistance to icing (extraordinary icephobicity). Although superhydrophobic surfaces with micro-, nano-, or (often biomimetic) hierarchical roughnesses have shown in laboratory settings (under certain conditions) excellent repellency and low adhesion to water down to temperatures near or below the freezing point, extreme icephobicity necessitates additional important functionalities. Other approaches, such as lubricant-impregnated surfaces, exhibit both advantages and serious limitations with respect to icing. In all, a clear path toward passive surfaces with extreme resistance to ice formation remains a challenge, but it is one well worth undertaking. Equally important to potential applications is scalable surface manufacturing and the ability of icephobic surfaces to perform reliably and sustainably outside the laboratory under adverse conditions. Surfaces should possess mechanical and chemical stability, and they should be thermally resilient. Such issues and related research directions are also addressed in this article.

On the nanoengineering of superhydrophobic and impalement resistant surface textures below the freezing temperature.

The superhydrophobic behavior of nano- and microtextured surfaces leading to rebound of impacting droplets is of great relevance to nature and technology. It is not clear however, if and under what conditions this behavior is maintained when such surfaces are severely undercooled possibly leading to the formation of frost and icing. Here we elucidate key aspects of this phenomenon and show that the outcome of rebound or impalement on a textured surface is affected by air compression underneath the impacting drop and the time scale allowing this air to escape. Remarkably, drop impalement occurred at identical impact velocities, both at room and at very low temperatures (-30 °C) and featured a ringlike liquid meniscus penetration into the surface texture with an entrapped air bubble in the middle. At low temperatures, the drop contact time and receding dynamics of hierarchical surfaces were profoundly influenced by both an increase in the liquid viscosity due to cooling and a partial meniscus penetration into the texture. For hierarchical surfaces with the same solid fraction in their roughness, minimizing the gap between the asperities (both at micro- and nanoscales) yielded the largest resistance to millimetric drop impalement. The best performing surface impressively showed rebound at -30 °C for drop impact velocity of 2.6 m/s.

Supercooled water drops impacting superhydrophobic textures.

Understanding the interaction of supercooled metastable water with superhydrophobic surface textures is of fundamental significance for unraveling the mechanisms of icing as well as of practical importance for the rational development of surface treatment strategies to prevent icing. We investigate the problem of supercooled water drops impacting superhydrophobic textures for drop supercooling down to -17 °C and find that increased viscous effects significantly influence all stages of impact dynamics, in particular, the impact and meniscus impalement behavior, with severe implications to water retention by the textures (sticky versus rebounding drop) and possible icing. Viscous effects in water supercooling conditions cause a reduction of drop maximum spreading (∼25% at an impact speed of 3 m/s for a millimetric drop) and can significantly decrease the drop recoil speed when the meniscus partially penetrates into the texture, leading to an increase of the contact time up to a factor of 2 in supercooling conditions compared to room temperature. We also show that meniscus penetration upon drop impact occurs with full penetration at the center, instead of ring shape, common to room temperature drop impact. To this end, we describe an unobserved mechanism for superhydrophobicity breakdown: unlike for room temperature drops, where transition from bouncing to sticky (impaled) behavior occurs sharply at the condition of full texture penetration, with a bubble captured at the point of impact, under supercooled conditions, the full penetration velocity threshold is increased markedly (increasing by ∼25%, from 2.8 to 3.5 m/s) and no bubble is entrapped. However, even though only partial texture penetration takes place, failure to completely dewet because of viscous effects can still prohibit complete supercooled drop rebound.

Wetting characterisation on complex surfaces by an automatic open-source tool: DropenVideo.

HYPOTHESIS: Investigating solid-liquid interactions to determine advancing and receding contact angles, and consequently contact angle hysteresis, is crucial for understanding material wetting properties. A reliable, automated, and possibly open-source tool is desirable, to standardize and automatize the measurement and make it user-independent. EXPERIMENTS: This study introduces an open-source software, DropenVideo, as an extension of Dropen. DropenVideo automates frame-by-frame video analysis for the advancing and receding contact angle determination, by considering needle presence, contrast tuning, and compensating for missing drop edge data. Contact angles are calculated using convolution mask, circle, and polynomial fittings. An innovative feature in DropenVideo is the automatic protocol for identifying advancing and receding contact angles: (i) the advancing contact angle is determined as the average value during drop inflation; and (ii) the receding contact angle is determined from the frame of incipient motion during drop deflation. FINDINGS: Exploring the application of DropenVideo across a range of complex surfaces as representative test cases, we highlight existing challenges in interpreting wetting measurements by addressing different wetting scenarios. Our study demonstrates that employing frame-by-frame automatic analysis of contact angle measurement videos using DropenVideo significantly mitigates the potential risks of subjective bias associated with manual interpretation and enhances the precision of identified wetting characteristics.

Advanced Self-Cleaning Surfaces.

Hydrophobicity, olephobicity, hemophobicity, amphiphobicity, omniphobicity, icephobicity [...].

Withdrawn: Low ice adhesion on soft surfaces: Elasticity or lubrication effects?

This article has been withdrawn at the request of the author(s) and/or editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/policies/article-withdrawal.

Discontinuity-enhanced icephobic surfaces for low ice adhesion.

HYPOTHESIS: Passive low ice-adhesion surfaces are frequently composed of soft materials; however, soft materials potentially present durability issues, which could be overcome by fabricating composite surfaces with patterned rigid and soft areas. Here we propose the innovative concept of discontinuity-enhanced icephobic surfaces, where the stress concentration at the edge between rigid and soft areas, i.e. where discontinuities in elasticity are located, facilitates ice detachment. EXPERIMENTS: Composite model surfaces were fabricated with controlled rigid-soft ratios and discontinuity line lengths. The ice adhesion values were measured while recording the ice/substrate interface, to unravel the underpinning ice detachment mechanism. The experiments were complemented by numerical simulations that provided a better understanding of the ice detachment mechanism. FINDINGS: It was found that when a surface contains rigid and soft areas, stress is concentrated at the edge between soft and hard areas, i.e. at the discontinuity line, rather than all over the soft or rigid areas. An unexpected non-unidirectional crack propagation was observed for the first time and elucidated. When rigid and deformable materials are present, the crack occurs on the discontinuity line and propagates first on rigid and then on soft areas. Moreover, it was demonstrated that an increase in discontinuities promotes crack initiation and leads to a reduction of ice adhesion.

Icephobic Gradient Polymer Coatings Deposited via iCVD: A Novel Approach for Icing Control and Mitigation.

Materials against ice formation and accretion are highly desirable for different industrial applications and daily activities affected by icing. Although several concepts have been proposed, no material has so far shown wide-ranging icephobic features, enabling durability and manufacturing on large scales. Herein, we present gradient polymers made of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4D4) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) deposited in one step via initiated chemical vapor deposition (iCVD) as an effective coating to mitigate ice accretion and reduce ice adhesion. The gradient structures easily overcome adhesion, stability, and durability issues of traditional fluorinated coatings. The coatings show promising icephobic performance by reducing ice adhesion, depressing the freezing point, delaying drop freezing, and inhibiting ice nucleation and frost propagation. Icephobicity correlates with surface energy discontinuities at the surface plane resulting from the random orientation of the fluorinated groups of PFDA, as confirmed by grazing-incidence X-ray diffraction measurements. The icephobicity could be further improved by tuning the surface crystallinity rather than surface wetting, as samples with random crystal orientation show the lowest ice adhesion despite high contact angle hysteresis. The iCVD-manufactured coatings show promising results, indicating the potential for ice control on larger scales and various applications.

PFAS-free superhydrophobic chitosan coating for fabrics.

In view of health and environmental concerns, together with the upcoming restrictive regulations on per- and polyfluoroalkyl substances (PFAS), less impactful materials must be explored for the hydrophobization of surfaces. Polysaccharides, and especially chitosan, are being explored for their desirable properties of film formation and ease of modification. We present a PFAS-free chitosan superhydrophobic coating for textiles deposited through a solvent-free method. By contact angle analysis and drop impact, we observe that the coating imparts hydrophobicity to the fabrics, reaching superhydrophobicty (θA = 151°, θR = 136°) with increased amount of coating (from 1.6 g/cm2). This effect is obtained by the combination of chemical water repellency of the modified chitosan and the nano- and micro-roughness, assessed by SEM analysis. We perform a comprehensive study on the durability of the coatings, showing good results especially for acidic soaking where the hydrophobicity is maintained until the 8th cycle of washing. We assess the degradation of the coating by a TGA-IR investigation to define the compounds released with thermal degradation, and we confirm the coating's biodegradability by biochemical oxygen consumption. Finally, we demonstrate its biocompatibility on keratinocytes (HaCaT cell line) and fibroblasts (HFF-1 cell line), confirming that the coating is safe for human skin cells.

Chitosan derivatives as dynamic coatings for transferrin glycoform separation in capillary electrophoresis.

Chitosan and its derivatives are interesting biopolymers for different field of analytical chemistry, especially in separation techniques. The present study was aimed at testing chitosan water soluble derivatives as dynamic coating agents for application to capillary electrophoresis. In particular, chitosan was modified following three different chemical reactions (nucleophilic substitution, reductive amination, and condensation) to introduce differences in charge and steric hindrance, and to assess the effect of these physico-chemical properties in capillary electrophoresis. The effects were tested on the capillary electrophoretic separation of the glycoforms of human transferrin, an important iron-transporting serum protein, one of which, namely disialo-transferrin (CDT), is a biomarker of alcohol abuse. Chitosan derivatives were characterized by using NMR and 1H NMR, HP-SEC-TDA, DLS, and rheology. The use of these compounds as dynamic coatings in the electrolyte running buffer in capillary electrophoresis was tested assessing the peak resolution of the main glycoforms of human transferrin and particularly of disialo-transferrin. The results showed distinct changes of the peak resolution produced by the different derivatives. The best results in terms of peak resolution were achieved using polyethylene glycol (PEG)-modified chitosan, which, in comparison to a reference analytical approach, provided an almost baseline resolution of disialo-transferrin from the adjacent peaks.

Results of an interlaboratory study on the working curve in vat photopolymerization.

The working curve informs resin properties and print parameters for stereolithography, digital light processing, and other photopolymer additive manufacturing (PAM) technologies. First demonstrated in 1992, the working curve measurement of cure depth vs radiant exposure of light is now a foundational measurement in the field of PAM. Despite its widespread use in industry and academia, there is no formal method or procedure for performing the working curve measurement, raising questions about the utility of reported working curve parameters. Here, an interlaboratory study (ILS) is described in which 24 individual laboratories performed a working curve measurement on an aliquot from a single batch of PAM resin. The ILS reveals that there is enormous scatter in the working curve data and the key fit parameters derived from it. The measured depth of light penetration Dp varied by as much as 7x between participants, while the critical radiant exposure for gelation Ec varied by as much as 70x. This significant scatter is attributed to a lack of common procedure, variation in light engines, epistemic uncertainties from the Jacobs equation, and the use of measurement tools with insufficient precision. The ILS findings highlight an urgent need for procedural standardization and better hardware characterization in this rapidly growing field.

Rational Design and Characterization of Materials for Optimized Additive Manufacturing by Digital Light Processing.

Additive manufacturing technologies are developed and utilized to manufacture complex, lightweight, functional, and non-functional components with optimized material consumption. Among them, vat polymerization-based digital light processing (DLP) exploits the polymerization of photocurable resins in the layer-by-layer production of three-dimensional objects. With the rapid growth of the technology in the last few years, DLP requires a rational design framework for printing process optimization based on the specific material and printer characteristics. In this work, we investigate the curing of pure photopolymers, as well as ceramic and metal suspensions, to characterize the material properties relevant to the printing process, such as penetration depth and critical energy. Based on the theoretical framework offered by the Beer-Lambert law for absorption and on experimental results, we define a printing space that can be used to rationally design new materials and optimize the printing process using digital light processing. The proposed methodology enables printing optimization for any material and printer combination, based on simple preliminary material characterization tests to define the printing space. Also, this methodology can be generalized and applied to other vat polymerization technologies.