Low Surface Tension Fluids Research Articles

Rheological behavior and solution pH response properties of nanoparticle-regulated low surface tension systems.

Regarding the rheological properties of fluids, certain nanoparticles can markedly modify the rheological behavior of low surface tension solutions by interacting with surfactant molecules. In this work, a low surface tension fluid with cetyltrimethylammonium chloride was prepared, and the silica nanoparticles were uniformly dispersed into it by ultrasonic dispersion. By adjusting the size, shape, and concentration of nanoparticles, the fluid behavior can be changed from Newtonian to non-Newtonian with finely tuned viscosity and characterized by a shear-thinning rheological behavior. In addition, this work explored how variations in environmental temperature and solution pH affect the rheological responses of the low surface tension suspension system. The experimental findings revealed that increasing the temperature substantially decreases the system's viscosity and induces a shear-thickening behavior. It is particularly significant that, under extreme pH conditions (either strongly acidic or alkaline), the viscosity of the nanoparticle suspensions was markedly enhanced at a particle concentration of 10 000 ppm. This interesting result coincided with a notable reduction in the zeta potential and an increase in the average particle size, suggesting an intensified aggregation of particles within the suspension system. A mechanism detailing the interaction between silica nanoparticles and surfactant micelles was proposed. This work indicates that the incorporation of nanoparticles into surfactant solutions offers a powerful approach to modulating fluid rheology across various conditions.

Patterned Quasi‐Liquid Surfaces for Condensation of Low Surface Tension Fluids

AbstractExtensive research concerns dropwise condensation of low surface tension fluids to promote energy efficiency and decarbonization in thermal energy systems. However, it is challenging as these fluids typically result in filmwise condensation. Drawing inspiration from the Namib desert beetle that enhances condensation through patterned wettability, conventional beetle‐inspired surfaces excel in water condensation but flood when condensing low surface tension fluids. In this work, a patterned quasi‐liquid surface is reported that achieves exceptional dropwise condensation of low surface tension fluids. The surface consists of alternating stripes with low surface energy, that is, a perfluoropolyether (PFPE) and fluorinated quasi‐liquid surface (FQLS), that shows ultralow contact angle hysteresis for ethanol and hexane. The PFPE stripes are slightly more slippery, acting as slippery bridges that accelerate droplet coalescence and removal. It is experimentally demonstrated that the striped PFPE‐FQLS pattern exhibits a heat transfer coefficient 85%, 330%, and 550% higher than that of PFPE, fluorinated silane, and filmwise condensation, respectively. This study reveals that a high contact angle is desired to sustain dropwise condensation, irrespective of contact angle hysteresis. These findings provide a new paradigm for promoting the dropwise condensation of low surface tension fluids and offer valuable insights into surface design for energy sustainability.

Charge Protection in Electret Air Filtration Nonwoven Materials

AbstractNonwoven media used as electret air filters are often embedded with charges to improve particle capture efficiency. These charged filters are invariably exposed to low surface tension fluids such as oils and alcohols leading to charge loss. In this study, filtration media are endowed with charge protection through increased surface repellency using melt additives that can migrate to the surface during processing. Nonwovens containing fluorochemical melt additives are produced, and examined to determine the relationship between surface chemistry, isopropyl alcohol (IPA) repellency, resultant charge retention, and filtration characteristics. Surface fluorine/carbon (F/C) ratios of ≈0.2 are sufficient to protect filtration performance from vapor discharging methods. Samples with bulk additive loadings of 1.2% or higher are found to achieve the necessary repellency to resist discharging independent of the migration state of the sample, while samples loaded at the 0.6% level required sufficient migration to achieve the requisite F/C ratio of 0.2 in order to be protected. Samples that achieved the necessary surface chemistry to provide significant IPA repellency retained > 80% of electret charge and corresponding filtration performance. These results have special significance in the design of filtration media relevant in global healthcare and other industrial settings.

Subcooled Pool Boiling on Hierarchical Micro- and Nanostructure-Modified Copper Surfaces in HFE-7100 Dielectric Liquid

ABSTRACT Hierarchical surfaces comprised both microscale and nanoscale structures have been previously studied as a means of targeting multiple length scales to achieve superior pool boiling performance. However, preceding studies have focused almost exclusively on high surface tension working fluids, while technologically important low surface tension fluids have remained largely unexplored. Due to their significantly lower surface tension these liquids tend to push out the air trapped in surface cavities of the heating surface, resulting in fewer nucleation sites compared to the same surface in water at low to moderate superheats. Thus, developing effective surface modification techniques for pool boiling in dielectric liquids and understanding the multiphase physics behind them is a pressing need in order to overcome these performance limitations and accelerate their adoption. In this work, we utilize scalable manufacturing techniques to realize four separate surface types (planar, nanoscale-modified, microscale-modified, and hierarchical) and experimentally determine their respective pool boiling performance within the low surface tension commercial working fluid HFE-7100. A maximum heat transfer enhancement of 125% at 38 K of superheat was observed for the best performing samples, which interestingly were nanoscale-modified and not those of the hierarchical type. Visual observations via high-speed video analysis of vapor bubble behavior are utilized to explain the underlying multiphase physics as to why these samples performed so well and future directions for achieving surface optimization across multiple length scales.

OPTIMALIZATION OF INTEGRAL FIN TUBE IN OUTSIDE CONDENSATION BASED ON FLOODING ANGLE

Increasing heat transfer in condensation for low surface tension fluid is dominated by using extended area (fin). One of the extended area on pipe is integral fin tube. Designing of integral fin tube should be conducted due to generate optimal condition such as designing fin spacing. Fin spacing has significantly impacted to heat transfer enhancement. Whereas the material of pipe is copper since has higher heat conductivity than others and fluid working is methanol. To predict optimal geometry of integral fin tube is conducted by using mathematical simulation by dividing area into flooded and unflooded. The active area or unflooded area represents the heat transfer area adn enhancement. In this experiment is found that the optimum fin spasing is 1 mm

Perspectives on superhydrophobic surface durability

Superhydrophobicity is a remarkable adaptation observed in various natural surfaces, which can potentially play a crucial role in the efficient functioning of numerous applications. The attainment of long-lasting or “durable” superhydrophobicity has significant implications for global energy production, water conservation, and the overall carbon footprint of industrialized and developing nations. Many applications requiring superhydrophobicity have been limited to laboratory scale experiments due to poor durability. In this article, we discuss important recent advancements and challenges to the development of durable superhydrophobic surfaces, including scalable fabrication, efficacy with low surface tension fluids, and the absence of standardized durability testing methods. To help address these challenges that hinder the implementation of superhydrophobicity, we conclude the article by providing a roadmap that outlines the crucial areas where the community should focus its research and development efforts, both from a fundamental and applied perspective.

Facile fabrication of transparent slippery coatings with dual self-healing ability

Slippery surfaces are highly effective in repelling low-surface tension fluids due to their oleophobicity, anti-adhesive, and drag-reduction properties. However, it should be noted that while these surfaces show promising performance under normal conditions, they may lose their efficacy under harsh conditions, particularly in outdoor applications. In this study, we developed a transparent slippery coating with dual self-healing abilities by a 3D-grafting method. The coating can be formed on various substrates, and the coated glass is highly transparent. Additionally, the coating shows exceptional slipperiness on water and most oils, making it highly effective at repelling liquids. Moreover, the coating exhibits remarkable intrinsic and surface self-healing capabilities, enabling it to restore its slippery effect even in the presence of scratches or damage. This outstanding self-healing property results in a long-lasting, durable coating that can resist wear and tear. The exceptional slipperiness and self-healing properties of coating make them suitable for various industrial, medical, and consumer products.

One-step spraying preparation of omniphobic membrane with anti-wetting behavior for membrane distillation

Membrane distillation (MD) with the application of omniphobic membranes shows much promise for desalinating saline wastewater containing low surface tension contaminants. However, the complicated membrane production steps and the wetting of the membranes have hindered its industrial application. This study reported a simple and highly efficient air spraying strategy to fabricate an omniphobic membrane by spraying a coating solution comprising fluorinated multi-walled carbon nanotubes (F-MWCNTs) onto a commercial polytetrafluoroethylene (PTFE) membrane. The contact angles for water and hexadecane on the surface reached 173.0° and 161.2°, and the water flux remained stable for 12 h when tested in DCMD with low surface tension fluids (SDS and STAC). The omniphobic membrane showed higher wetting resistance in direct membrane distillation (DCMD) experiments than the PTFE membrane when feeding low surface tension feed solutions. During spraying, F-MWCNTs randomly accumulated, and solvent evaporation resulted in micron-level mounds and surface roughness. The presence of air gaps on the membrane surface prevented the surfactant molecules from directly contacting the membrane surface and enabled the membrane to operate generally in the liquid with surfactant for a long time. The membrane was not only resistant to wetting of liquids containing low surface tension but was also easy to prepare and was expected to be used for large-scale desalination of low surface tension wastewater.

Molecularly Capped Omniphobic Polydimethylsiloxane Brushes with Ultra-Fast Contact Line Dynamics.

Droplet friction is common and significant in any field where liquids interact with solid surfaces. This study explores the molecular capping of surface-tethered, liquid-like polydimethylsiloxane (PDMS) brushes and its substantial effect on droplet friction and liquid repellency. By exchanging polymer chain terminal silanol groups for methyls using a single-step vapor phase reaction, the contact line relaxation time is decreased by three orders of magnitude-from seconds to milliseconds. This leads to a substantial reduction in the static and kinetic friction of both high- and low-surface tension fluids. Vertical droplet oscillatory imaging confirms the ultra-fast contact line dynamics of capped PDMS brushes, which is corroborated by live contact angle monitoring during fluid flow. This study proposes that truly omniphobic surfaces should not only have very small contact angle hysteresis, but their contact line relaxation time should be significantly shorter than the timescale of their useful application, i.e., a Deborah number less than unity. Capped PDMS brushes that meet these criteria demonstrate complete suppression of the coffee ring effect, excellent anti-fouling behavior, directional droplet transport, increased water harvesting performance, and transparency retention following the evaporation of non-Newtonian fluids.

ANALISA TRANSFER PANAS PADA KONDENSASI LUAR DENGAN MENGGUNAKAN REFRIGERANT R-134a

An experimental study in condensation has been conducted to analyse the condensation phenomena. The enhancement heat transfer performance is as always highlighted phenomenon for being investigated. The modified surface is a common method for enhancing the performance of condensation since inducing the droplet on the surface. The droplet leads to increasing heat-transfer area, decreasing thermal resistance, and shorten the condensing cycle. The condensation by using water successfully induces droplets on the surface and enhances the heat transfer. But water and refrigerant have different fluid properties which induce different phenomena whereas refrigerant is widely used in industry. In this case, the condensation is conducted on the modified surface by using a commercial promotor to investigate the phenomena. The investigation results show that the surface tension immensely influences the condensate on the surface. Refrigerant is low surface tension fluid which leads to the difficulty of fluid for inducing droplets and almost no enhancement heat transfer performance since the results almost fitted well to Nusselt bare tube prediction around 2443-3063 W/m2 oC with subcooled temperature 2 up to 5 oC.

Chlorotrimethylsilane treatment affords long-lasting contaminant-proof and transparent slippery covalently attached liquids surfaces

Slippery lubricant-infused porous surfaces (SLIPs) are effective in repelling low-surface tension fluids given their omniphobicity, anti-adhesive, and drag reduction properties. However, over time, SLIPs are subjected to lubricant losses and undergo dust fouling. In this research, we address these challenges by developing a new slippery covalently attached liquids (SCAL) surface. SCALs was composed of a molecularly-thick lubricant layer covalently bound to the solid support, generating long-lasting anti-fouling, hydrophobic and slippery effects. We show that our SCALs exhibit ultra-low contact angle hysteresis for a wide range of fluids and enable reduced colloidal forces in the vertical and lateral directions (adhesion and friction, respectively). In parallel, the SCALs exhibit a high resistance to contamination (from molecular adhesion to dust fouling), mechanical integrity, and light transmission. Overall, we introduce a facile, cost-effective, and scalable process to synthesize a SCALs system suitable to the demands of application in bioengineering, power generation and water harvesting.

Opportunities in Nano-Engineered Surface Designs for Enhanced Condensation Heat and Mass Transfer

AbstractRecent advancements in surface nano-engineering have spurred intense interests in their implementation for enhancing condensation heat transfer. When appropriately designed, nano-engineered surfaces not only lead to highly efficient transport mechanisms not achievable with conventional dropwise condensation (DWC), they also demonstrate the possibility of augmenting condensation of low surface tension fluids widely used in industry. These advantages are further enhanced by the development of highly scalable nanofabrication methods, which enable the potential transition from laboratory-scale prototypes to real-world industrial applications. In this review, we discuss the progress, opportunities, and challenges of enhancing condensation heat and mass transfer with nano-engineered surfaces. This article provides an overview of the recent developments in micro/nanoscale coating and structure fabrication techniques and performs a thorough comparison of their condensation performance, elucidating the complex interfacial transport mechanism involved. Surface structuring methods that are durable, scalable, and low-cost are essential attributes for large-scale industrial implementation. Here, the methods used to improve surface durability and demonstrations of nanostructure-enhanced meter-scale condensers are presented. Limitations are discussed and the potential techniques to overcome these challenges are summarized. Given the recent development of metal additive manufacturing (AM) technology and its growing relevance in manufacturing processes, we end this review by providing our perspectives on the opportunities in enabling surface nanostructuring of metal additive manufactured materials and the potential of nanometric–millimetric codesign optimization for the development of next-generation additively manufactured condensers.

Polydimethylsiloxane‐Silane Synergy enables Dropwise Condensation of Low Surface Tension Liquids

AbstractDespite decades of research on promoting the dropwise condensation of steam, achieving dropwise condensation of low surface tension liquids remains a challenge. The few coatings reported to promote dropwise condensation of low surface tension liquids either require complex fabrication methods, are substrate dependent or have poor scalability. Here, the rational development of a coating, which is applicable to all conventionally used condenser metals, is presented by combining a low contact angle hysteresis polydimethylsiloxane with a low surface energy silane using atmospheric vapor phase deposition. The siloxane‐silane coating enables the dropwise condensation of fluids with surface tensions as low as 15 mN m−1 in pure vapor conditions. This siloxane‐silane coating enables a 274%, 347%, and 636% heat transfer enhancement during ethanol, hexane, and pentane condensation, respectively, when compared to filmwise condensation on the same un‐coated surfaces. Furthermore, this coating exhibits 15 days of steady dropwise condensation with no apparent signs of coating degradation. This study not only demonstrates the possibility of achieving stable dropwise condensation of low surface tension fluids on scalable, structure‐less surfaces, it also develops design principles for creating facile, substrate‐independent, durable, and scalable omniphobic coatings for a plethora of applications.

Binary mixture droplet wetting on micro-structure decorated surfaces

Liquid surface tension as well as solid structure play a paramount role on the intimate wetting and non-wetting regimes and interactions between liquids droplets and solid substrates. We hypothesise that the coupling of these two variables, independently addressed in the past, eventually offer a wider range of understanding to the surface science and interfacial communities.In this work, intrinsically hydrophobic micro-pillared surfaces varying in the spacing between structures, and pure ethanol, pure water and their binary mixtures (as well as acetone–water and ethylene glycol–water mixtures) are utilised, accessing a wide range of substrate solid fractions and liquid surface tensions experimentally. Wettability measurements are carried out at different azimuthal directions to exemplify the wetting/non-wetting behaviour as well as the droplet asymmetry function of both liquid composition and structure spacing.Our findings reveal that high water concentration droplets, i.e., high surface tension fluids, sit in the Cassie-Baxter regime while partial non-wetting Wenzel or mixed-mode regimes with enhanced droplet asymmetry ensuing for medium and high ethanol concentrations, i.e., low surface tension fluids, below certain micropillar spacing. Beyond micropillar spacing s ≥ 40 µm, the impact of the surface structure on the droplet shape is negligible, and droplets adopt a similar contact angle and circular shape as on a flat smooth hydrophobic surface. Wetting and non-wetting regimes are then supported by classical wetting theories and equations. A wetting regime map for a wide range of surface tension fluids and/or their mixtures on a wide domain of solid fractions is then proposed.

Non-wetting Liquid-Infused Slippery Paper.

Liquid-infused slippery surfaces have replaced structural superhydrophobic surfaces in a plethora of emerging applications, hallmarked by their favorable self-healing and liquid-repelling characteristics. Their ease of fabrication on different types of materials and increasing demand in various industrial applications have triggered research interests targeted toward developing an environmental-friendly, flexible, and frugal substrate as the underlying structural and functional backbone. Although many expensive polymers such as polytetrafluoroethylene have so far been used for their fabrication, these are constrained by their compromised flexibility and non-ecofriendliness due to the use of fluorine. Here, we explore the development and deployment of a biodegradable, recyclable, flexible, and an economically viable material in the form of a paper matrix for fabricating liquid-infused slippery interfaces for prolonged usage. We show by controlled experiments that a simple silanization followed by an oil infusion protocol imparts an inherent slipperiness (low contact angle hysteresis and low tilting angle for sliding) to the droplet motion on the paper substrate and provides favorable anti-icing characteristics, albeit keeping the paper microstructures unaltered. This ensures concomitant hydrophobicity, water adhesion, and capillarity for low surface tension fluids, such as mustard oil, with an implicit role played by the paper pore size distribution toward retaining a stable layer of the infused oil. With demonstrated supreme anti-icing characteristics, these results open up new possibilities of realizing high-throughput paper-based substrates for a wide variety of applications ranging from biomedical unit operations to droplet-based digital microfluidics.

Delayed Lubricant Depletion of Slippery Liquid Infused Porous Surfaces Using Precision Nanostructures.

Slippery liquid infused porous surfaces (SLIPS) are an important class of repellent materials, comprising micro/nanotextures infused with a lubricating liquid. Unlike superhydrophobic surfaces, SLIPS do not rely on a stable air-liquid interface and thus can better manage low surface tension fluids, are less susceptible to damage under physical stress, and are able to self-heal. However, these collective properties are only efficient as long as the lubricant remains infused, which has proved challenging. We hypothesized that, in comparison to a nanohole and nanopillar morphology, the "hybrid" morphology of a hole within a nanopillar, namely a nanotube, would be able to retain and redistribute lubricant more effectively, owing to capillary forces trapping a reservoir of lubricant within the tube, while lubricant between tubes can facilitate redistribution to depleted areas. By virtue of recent fabrication advances in spacer defined intrinsic multiple patterning (SDIMP), we fabricated an array of silicon nanotubes and equivalent arrays of nanoholes and nanopillars (pitch, 560 nm; height, 2 μm). After infusing the nanostructures (prerendered hydrophobic) with lubricant Krytox 1525, we probed the lubricant stability under dynamic conditions and correlated the degree of the lubricant film discontinuity to changes in the contact angle hysteresis. As a proof of concept, the durability test, which involved consecutive deposition of droplets onto the surface amounting to 0.5 L, revealed 2-fold and 1.5-fold enhancements of lubricant retention in nanotubes in comparison to nanopillars and nanoholes, respectively, showing a clear trajectory for prolonging the lifetime of a slippery surface.

Effect of wettability on nucleate pool boiling heat transfer of a low surface tension fluid outside horizontal finned tubes

Nucleate pool boiling heat transfer of a low surface tension fluid (R134a) is tested outside the surfaces with polymer coating. Nanoscale Parylene films are coated on the surfaces with Plasma Enhanced Chemical Vapor Deposition(PECVD) method. The thickness of the coating is about 10 nm. Heat flow rate intensity varies in the scope of 15–150 kW/m2. Saturation temperatures are 6, 10 and 16 °C, respectively. According the investigation, for the smooth tube, it is found that the roughness of the surface reduced after the coating and the pool boiling heat transfer performance is also deteriorated. The boiling heat transfer can be enhanced to a certain degree for two finned structures. For the saw-teeth tube, the heat transfer coefficient(HTC) can be 1.1–1.3 times higher than that without coating. For the reentrant cavity tube, at lower heat flux, the effect of coating on the performance is insignificant. While, the heat transfer can be enhanced up to a factor of 1.2 at higher heat flux. In general, the effect of the Parylene polymer coating on the pool boiling of the low surface tension fluid R134a are insignificant at the moderate heat flux

Dropwise condensation of low surface tension fluids on lubricant-infused surfaces: Droplet size distribution and heat transfer

The use of lubricant-infused surfaces and slippery liquid-infused porous surfaces to promote dropwise condensation of low surface tension fluids has received increasing attention in recent years due to the high condensate droplet mobility and low droplet contact angle hysteresis enabled by these coatings. Though multiple studies have focused on developing models to predict heat transfer during water vapor condensation on hydrophobic lubricant-infused surfaces, a lack of understanding exists for condensation of low surface tension fluids, where the surface becomes wettable to the condensate. In this study, we develop a theoretical model to predict the heat transfer during dropwise condensation of low surface tension fluids (ethanol and hexane) on lubricant-infused surfaces. Using numerical analysis, we develop a relationship between droplet Nusselt number and Biot number to accurately predict individual droplet heat transfer. The droplet size distribution of ethanol and hexane were experimentally determined, with result showing that the size distribution density depends on both the working fluid properties and droplet contact angle behavior. To validate our model, experiments were conducted by condensing pure ethanol and hexane vapor on different diameter tubes coated with lubricant-infused surfaces. From the experimental measurements, two correlations which characterized the distribution density of ethanol and hexane were developed. Using the droplet distribution correlations as input parameters, our developed model predicts the heat transfer coefficients for low surface tension fluid condensation more accurately when compared to the existing models, where the maximum deviation between the prediction and experimental results is less than 15%.

Migration Effects of Fluorochemical Melt Additives for Alcohol Repellency in Polypropylene Nonwoven Materials.

The use of bulk polymer melt additives provides a facile, industrially relevant approach to tailor properties of polymer surfaces for many different applications. These melt additives, when blended with polymers prior to melt spinning, migrate to the fiber surface and influence surface functionality. While the use of bulk polymer melt additives to impart hydrophilicity or oleophobicity is well studied, the impact of the fiber formation process on additive migration and resultant repellency of nonwoven media products remains largely unexplored. In this study, we produce fluorochemical melt additive containing meltblown nonwovens, and establish methods for characterization of fiber mat surface composition and repellency. Repellency of low surface tension fluids is a significant challenge and is of particular importance in the creation of medical garments such as surgical gowns and masks which must perform as liquid barriers even when exposed to alcohol based solutions. Similarly, melt additives are also used in the production of electret air filtration devices. Electret filters are imbued with charges to enhance particle capture performance, but this charge can be negated through wetting by low surface tension fluids. To address this challenge, the changing composition of fiber surfaces due to the migration of additives is monitored via X-ray Photoelectron Spectroscopy, then related to repellency of alcohol solutions by contact angle analysis. We demonstrate that for the samples tested a fluorine to carbon (F/C) ratio of 0.35 is sufficient to prevent wicking of isopropanol droplets, and higher surface tension fluids could be repelled by fiber mats with lower fluorine content. Through the use of cross-sectional ToF-SIMS analysis, we find that migration of additives is key to the performance of samples with low additive loadings, and that these phenomena are heavily influenced by many nonwoven manufacturing parameters including fiber size, die-to-collector distance, and polymer resin melt flow rates.

Recent developments, challenges, and pathways to stable dropwise condensation: A perspective

Dropwise condensation (DWC) on non-wetting surfaces has remarkable potential to enhance heat transfer performance compared to filmwise condensation on wetting substrates. In this article, we discuss important recent developments and challenges in the field of DWC, including durability of DWC-promoting coatings, DWC of low surface tension fluids, physical mechanisms governing DWC, unconventional methods to achieve DWC, and promising metrology techniques for DWC. We end the article by providing a road map detailing where we believe the community should direct both fundamental and applied efforts in order to solve the identified century-old challenges that limit DWC implementation.