Film Surface Research Articles

Surface modification of geopolymer coatings with polymethylhydrosiloxane-tetraethyl orthosilicate (PMHS-TEOS) for inhibiting efflorescence

Geopolymer coatings, as novel environmentally friendly inorganic coating, hold promising prospects in the field of concrete protection due to their characteristics such as corrosion resistance and fire resistance. However, its vulnerability to efflorescence can alter surface colouration and damage the structural integrity of the surface film, thereby diminishing its concrete protective efficacy. To inhibit efflorescence in geopolymer coatings, this study reports a surface modification method using polymethylhydrosiloxane-tetraethyl orthosilicate (PMHS-TEOS) for the first time. To explore the underlying mechanism, the effects of PMHS-TEOS on the geopolymer gel structure, mechanical properties, and ion leaching concentration were investigated. The experimental results indicate that PMHS-TEOS can penetrate the surface of geopolymer coatings and participate in the activation reaction, generating a new gel phase that effectively fills pore structure. Moreover, its hydrophobic groups facilitate the establishment of a waterproof structure, thereby inhibiting water infiltration and alkaline ion leaching. After PMHS-TEOS modification, the contact angle of the geopolymer coating reached 136.5°, demonstrating excellent wear resistance. The dissolution rate of alkaline cations in the modified geopolymer coatings was reduced by approximately 90 % and no efflorescence was observed in the long-term accelerated tests. This method is convenient for practical engineering of geopolymer coatings.

Multi‒scale numerical investigations on the atomization performance of liquid‒liquid pintle injector

Three‒dimensional atomization field of the radial annular slot type liquid‒liquid pintle injector under different total momentum ratio (TMR) is simulated based on the volume of fluid to discrete particle model (VOF to DPM) method and the octree adaptive mesh refinement (AMR). The typical spray morphology of the injector is obtained, the variation of spray angle, breakup length of liquid film, amplitude of disturbance waves before liquid film breaking and its causes are analyzed. The results show that the injector can form a hollow conical spray during operation, and there are two recirculation zones near the inner surface of liquid film. According to the characteristics of the spray morphology, the spray can be divided into four areas: undisturbed liquid film, disturbed liquid film, fluctuating breakup zone and small droplets accumulation zone. The simulated spray angle is close to the model proposed by Heister when TMR is smaller than 0.6 and is close to the model proposed by Boettcher when TMR exceeds 0.6, with the maximum relative error of 9.5%. TMR has little influence on the amplitude of disturbance waves before liquid film breaking, while the breakup length of liquid film increases at first and then decreases with the increase of TMR. The breakup length is relatively large when TMR is close to 1, within the range of 20∼22 mm, while it is relatively small when TMR is far away from 1, within the range of 12∼15mm.

Biodegradable active composite film based on pea protein isolate, sage seed gum, and cumin essential oil: Fabrication and characterization

The present study perused the physicochemical, mechanical, thermal, barrier, and optical properties of pea protein isolate (PPI)-Sage seed gum (SG) (3:1, 1:1, and 1:3) composite films incorporated with cuminum essential oil (CEO, 0, 1, and 2 %). Results indicated that the thickness, contact angle (CA), yellowness index (a*), total color differences (∆E), and opacity (OP) significantly enhanced with increasing PPI portion and CEO concentration (p < 0.05), as well as heterogeneous and inconsistent structure were increased in film surface due to protein agglomeration and oil droplets. However, the moisture content (MC), water solubility (WS), and swelling ratio (SR) decreased. Results also indicated that the antioxidant property significantly increased (P < 0.05) following the increase in CEO concentration. The highest tensile strength (TS) and elastic modulus (EM) and the least elongation at break (EAB) in PPI 3-SG 1–0 % were estimated. The potential antimicrobial property of CEO containing film was confirmed, and the maximum inhibition zone for E.coli, S.aureus, C.albicans, and A.niger were measured in film containing 2 % CEO. The barrier properties of the film against water and oxygen (WVP, OXP) diminished with increases the SG ratios, due to the hydrophilic nature of SG. The FTIR analysis identified the functional groups in film components and the interaction between film components. The DSC thermogram indicated the thermal stability of films and the highest decomposition temperature in PPI 3-SG 1–2 % film. XRD revealed a semi-crystalline structure of biopolymer active films. With the increase of SG portion and CEO concentration, the crystallinity of the films was enhanced. Results also indicated that prepared films were more than 90 % biodegradable.

Enhanced Photocatalytic Performance under Ultraviolet and Visible Light Illumination of ZnO Thin Films Prepared by Modified Sol-Gel Method.

Semiconductor oxides are frequently used as active photocatalysts for the degradation of organic agents in water polluted by domestic industry. In this study, sol-gel ZnO thin films with a grain size in the range of 7.5-15.7 nm were prepared by applying a novel two-step drying procedure involving hot air treatment at 90-95 °C followed by conventional furnace drying at 140 °C. For comparison, layers were made by standard furnace drying. The effect of hot air treatment on the film surface morphology, transparency, and photocatalytic behavior during the degradation of Malachite Green azo dye in water under ultraviolet or visible light illumination is explored. The films treated with hot air demonstrate significantly better photocatalytic activity under ultraviolet irradiation than the furnace-dried films, which is comparable with the activity of unmodified ZnO nanocrystal powders. The achieved percentage of degradation is 78-82% under ultraviolet illumination and 85-90% under visible light illumination. Multiple usages of the hot air-treated films (up to six photocatalytic cycles) are demonstrated, indicating improved photo-corrosion resistance. The observed high photocatalytic activity and good photo-corrosion stability are related to the hot air treatment, which causes a reduction of oxygen vacancies and other defects and the formation of interstitial oxygen and/or zinc vacancies in the films.

Plant-Based Films for Food Packaging as a Plastic Waste Management Alternative: Potato and Cassava Starch Case.

The escalating environmental impact of plastic packaging waste necessitates sustainable alternatives in food packaging. This study explores starch-based films derived from cassava and potato as viable substitutes, aiming to mitigate plastic pollution and enhance environmental sustainability. Utilizing a casting method, formulations optimized by CCRD were characterized for their physical, physicochemical, and morphological properties. Comprehensive analysis revealed both cassava and potato starch films to exhibit robust structural integrity, high tensile strength (up to 32.6 MPa for cassava starch films), and semi-crystalline morphology. These films demonstrated low water vapor permeability and moderate solubility, akin to conventional low-density polyethylene used in packaging. Differential scanning calorimetry indicated glass transition temperatures between 116.36 °C and 119.35 °C, affirming thermal stability suitable for packaging applications. Scanning electron microscopy confirmed homogeneous film surfaces, with cassava starch films (C4-15) exhibiting superior transparency and uniformity. X-ray diffraction corroborated the films' semi-crystalline nature, unaffected by sorbitol content variations. Despite their mechanical and thermal suitability, further enhancements in thermal degradation resistance are essential for broader thermoprocessing applicability. These findings underscore the potential of starch-based films to be used as lids or other part of a food package, decreasing the plastic dependency in food packaging, contributing decisively to waste reduction and environmental preservation.

Effect of oxygen doping on cutting performance of CVD diamond coated milling cutters in zirconia ceramics machining

Diamond thin films were deposited on WC-Co tools using the hot-filament chemical vapor deposition (HFCVD) technique at varying pressures and oxygen doping levels. As the oxygen doping content increased, the purity of the diamond phase improved, and the density of the <111> crystal plane on the film's surface increased. The growth rate of diamond films initially increased and then decreased under low pressure, with a corresponding decrease in residual compressive stress. Conversely, at high pressure, increased oxygen doping enhanced the diamond phase purity and growth rate while reducing residual compressive stress. Cutting experiments on pre-sintered zirconia ceramics for milling linear slots and engraving dentures demonstrated that diamond-coated milling cutters exhibited superior cutting performance under consistent oxygen and carbon source concentrations and stable air pressure conditions. The oxygen-doped diamond-coated milling cutter grown at a pressure of 1 KPa exhibited the longest service life, attributed to its reduced surface roughness, lower cutting force, and increased density of the <111> crystal plane orientation. This cutter could machine up to 1300 crowns, making it approximately 80% more durable than non‑oxygen-doped diamond-coated milling cutters and about 20 times more durable than uncoated ones.

Enhanced performance of ambient-air processed CsPbBr3 perovskite light-emitting electrochemical cells via synergistic incorporation of dual additives

Metal halide perovskite light-emitting electrochemical cells (MHP-LECs) are promising due to their facile solution processability and exceptional optoelectrical properties. However, commercialization is hindered by poor thin-film coverage and the need for glovebox processing. This study addresses these issues by incorporating poly(ethylene oxide) (PEO) and potassium hexafluorophosphate (KPF6) into cesium lead bromide perovskite (CsPbBr3) emitting layers (EML), processed under ambient-air conditions. These perovskite composite thin films were spin-coated on a preheated substrate, and the device structure was ITO/PEDOT:PSS/EML/Al. Notably, the MHP-LEC based on CsPbBr3:PEO:KPF6 (100:65:0.2, weight ratio) exhibited pure-green emission with a peak at 524 nm and a narrow full width at half-maximum of 27 nm. Compared to a reference device (CsPbBr3:PEO (100:65, weight ratio)), the new device showed 1.5-, 4-, and 5-fold improvements in current density, electroluminescence (EL) intensity, and lifetime, respectively. These enhancements are attributed to reduced non-radiative current leakage, improved film surface coverage and passivation, and balanced charge carrier injection and transportation. This study offers a straightforward approach for processing CsPbBr3 thin films under ambient-air conditions, enhancing the EL performance of MHP-LECs.

Comparative study of the sensitivity of H2S and NO2 in CuInSe2 gas sensors that are fabricated using screen printing and MEMS Integration

This study compares the gas response efficiency of a gas sensor substrate that is produced using screen-printing (SP) and micro-electro-mechanical systems (MEMS) technology, followed by deposition of CuInSe2 (CIS) film layers by radio-frequency (RF) sputtering and hyperthermal annealing for gas sensing applications. The SP substrates are used for CIS film deposition for RF sputtering, followed by various hyperthermal annealing treatments. SEM observations of the CIS film surfaces after annealing show surface particles with a size distribution of 155 to 30 nm, which demonstrate a phenomenon that is similar to Ostwald ripening and which creates structural variation and unevenness after annealing. The CIS film/MEMS substrates (MEMSs) gas sensor substrates are evaluated using a NIR thermal camera to determine the optimal operating temperature for direct annealing, which is 203.2 °C. Gas sensing response tests show that only H2S or NO2 gas rapidly and significantly elicit a response but a CIS film/SPs gas sensor exhibits significant resistance signal noise. The CIS film/MEMSs gas sensor is more stable than CIS film/SPs (after annealing) so it is more suitable for gas sensing. SEM images confirm that the film remains intact, so it is suitable for gas sensing applications.

Fish-Mimicking Hydrophilic and Hygroscopic Transparent Films with Long-Lasting Anti-Oil Adhesion and Its Application to PET Bottles

With the recent ban on the production and use of long-chain perfluorinated compounds, the development of alternative approaches to prepare liquid-repellent surfaces that avoids the use of such compounds has become an urgent issue. We have succeeded in the development of fish-mimicking hydrophilic transparent hydrogel-based films with long-lasting anti-oil adhesion properties. Such films could be prepared by simply mixing poly(vinylpyrrolidone) (PVP), nanoclay particles (NCPs), and a waterborne aminosilane (AOS) using an integral blend (IB) method. When submerged in water, these films displayed underwater superoleophobicity (advancing and receding contact angles (CAs) of diiodomethane were ~171°/~163°) with low CA hysteresis (less than 8°), because the hydrophilic nature of the films promoted the formation of a thin layer of adsorbed water on the topmost film surfaces, similar to fish scales. Furthermore, when our films were coated onto the inside of poly(ethylene terephthalate) (PET) bottles and pre-wetted using 80 °C hot water vapors, these film surfaces could effectively repel various oils and were able to maintain their oil-repellent properties for more than 5 weeks. These water-driven, non-perfluorinated transparent hydrogel-based films are expected to increase recycling of PET bottles for oils that are generally incinerated or landfilled.

Reconstructing Helmholtz Plane Enables Robust F-Rich Interface for Long-Life and High-Safe Sodium-Ion Batteries.

Hard carbon (HC) is the most commonly used anode material in sodium-ion batteries. However, the solid-electrolyte-interface (SEI) layer formed in carbonate ester-based electrolytes has an imperceptible dissolution tendency and a sluggish Na+ diffusion kinetics, resulting in an unsatisfactory performance of HC anode. Given that electrode/electrolyte interface property is highly dependent on the configuration of Helmholtz plane, we filtrated proper solvents by PFBE (PF6 - anion binding energy) and CAE (carbon absorption energy) and disclosed the function of chosen TFEP to reconstruct the Helmholtz plane and regulate the SEI film on HC anode. Benefiting from the preferential adsorption tendency on HC surface and strong anion-dragging interaction of TFEP, a robust and thin anion-derived F-rich SEI film is established, which greatly enhances the mechanical stability and the Na+ ion diffusion kinetics of the electrode/electrolyte interface. The rationally designed TFEP-based electrolyte endows Na||HC half-cell and 2.8 Ah HC||Na4Fe3(PO4)2P2O7 pouch cell with excellent rate capability, long cycle life, high safety and low-temperature adaptability. It is believed that this insightful recognition of tuning interface properties will pave a new avenue in the design of compatible electrolyte for low-cost, long-life, and high-safe sodium-ion batteries.

Bayesian Optimization of Low-Temperature Nonthermal Plasma Jet Sintering of Nanoinks.

In response to the escalating demand for flexible devices in applications such as wearables, sensors, and touch panels, there is a need for innovative fabrication approaches for devices made from nanomaterial-based inks. Subsequent to ink deposition, a pivotal stage in device manufacturing typically involves high-temperature sintering, posing challenges for heat-sensitive substrates. Nonthermal plasma jet sintering utilizing an atmospheric pressure dielectric barrier discharge (DBD) plasma jet enables sintering at room temperature and standard pressure, facilitating the sintering of printed nanoparticle films without compromising substrate or film surface integrity. However, determining optimal plasma jet sintering conditions can be challenging due to multiple processing variables with intricate interrelationships. This work employed Bayesian optimization (BO) and machine learning (ML) to identify optimal values for seven primary plasma jet sintering variables. Optimization yielded a 99.2% increase in the measured electrical conductivity for plasma jet-sintered indium tin oxide (ITO) films after five rounds of experiments. Moreover, the optimal sintering conditions achieved an electrical conductivity that was 81.4% of conventional furnace sintering at 300 °C, but was three times faster and with a peak substrate temperature below 47 °C. This result demonstrates the prospect of applying BO to optimize processing techniques for emerging low-temperature requirements.

High-entropy amorphous FeCoCrNi thin films with excellent electrocatalytic oxygen evolution reaction performance

High-entropy amorphous FeCoCrNi thin-film catalysts with different Cr contents were prepared on the surface of copper substrate via electrodeposition. Their electrocatalytic performance for oxygen evolution reaction (OER) in alkaline solution was investigated. The catalyst with the optimal performance was obtained when the concentration of Cr3+ in the electrodeposition solution was 200 g/L. It achieved a current density of 10 mA⸱cm−2 with a low overpotential of merely 295 mV and demonstrated a Tafel slope of 69.52 mV⸱dec−1, indicating favorable reaction kinetics. Furthermore, this catalyst showed remarkable stability, maintaining its catalytic performance after 1000 cycles of cyclic voltammetry tests in the alkaline solution. The reasons for its excellent catalytic performance are as follows: (1) The uniform cracks and cellular structure on the film surface result in a larger active area; (2) The amorphous structure is more prone to surface reconstruction during the catalytic process, exposing more active sites; (3) Cr element regulates the valence states of Fe and Co ions as well as the existence forms of Ni and Co elements during the catalytic process, which accelerates the OER process and improves the catalytic performance.

Preparation and Properties of Fluorinated Poly(aryl ether)s with Ultralow Water Absorption and Dielectric Constant by Cross-Linked Network Strategy.

Poly(aryl ether) materials are used in a wide range of applications in the communications and microelectronics fields for their outstanding mechanical and dielectric properties. In order to further improve the comprehensive performance, this work reports a series of cross-linkable poly(aryl ether)s (UCL-PAEn) containing trifluoroisopropyl and perfluorobiphenyl structures using 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 2,2'-diallyl bisphenol A, and perfluorobiphenyl as starting materials. Their chemical structures and the effect of changes in the allyl content on the properties are thoroughly investigated. Owing to the introduction of fluorine atoms and cross-linked networks, the cross-linked poly(aryl ether) films present low dielectric constants (Dk = 1.93-2.24 at 1 MHz), low water absorption (0.14% -0.25%), and hydrophobic film surfaces (94.3-99.4°). Additionally, because of the presence of cross-linked networks, the CL-PAEn films exhibit superior thermal stability, with the 5% weight loss temperatures all above 445 °C and the maximum thermal decomposition rate temperatures all above 550 °C. The cross-linked films also demonstrate excellent mechanical properties, with tensile strength in the range of 57.1 -146.7 MPa and tensile modulus in the range of 1.8 GPa-4.5 GPa.

Durable photobioreactor antibiofouling coatings for microalgae cultivation by photoreactive poly(2,2,2-trifluoroethyl methacrylate)

To improve the durability of the photobioreactor antibiofouling surface for microalgal cultivation, a series of photoreactive poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) were successfully synthesized and used to modify ethylene-vinyl acetate (EVA) films by a surface coating and UV light grafting method. Fourier transform infrared (FT-IR) spectra, X-ray photoelectron spectroscopy analysis (XPS) and fluorescence microscopy results indicated that PTFEMA were fixed successfully onto the EVA film surface through a covalent bond. During the microalgal adhesion assay, the number of EVA-PTFEMA film-adhered microalgae was 41.4% lower than that of the EVA film. Moreover, the number of microalgae attached to the EVA-PTFEMA film decreased by 61.7% after cleaning, while that of EVA film decreased by only 49.1%. It was found that the contact angle of EVA-PTFEMA film surface increased, and remained stable when immersed in acid and alkali solution for up to 90 days. HIGHLIGHTS Durable photobioreactor antibiofouling surfaces for microalgal cultivation were prepared successfully. The contact angle of antibiofouling coating surface remained stable in acid and base environment for 90 days. The attached microalgae on antibiofouling surface decreased 41.4% than those of unmodified surface. The attached microalgae on antibiofouling surface could be cleaned by 61.7% through changing the flow velocity of microalgal suspension.

Molecule Anchoring Strategy Promotes Vertically Homogeneous Crystallization and Aligned Interfaces for Efficient Pb-Sn Perovskite Solar Cells and Tandem Device.

Narrow-bandgap (NBG) Pb-Sn perovskites are ideal candidates as rear subcell in all-perovskite tandem solar cells. Because Pb-Sn perovskites contain multiple components, the rational regulation of vertical structure and both interfaces of the film is primarily crucial to achieve high-performing NBG perovskite solar cells (PSCs). Herein, a molecule anchoring strategy is developed to in situ construct Cs0.1MA0.3FA0.6Pb0.5Sn0.5I3 perovskite film with vertically aligned crystals and optimized interfaces. Specifically, l-alanine methyl ester is developed as an anchoring additive to induce the vertical crystal growth, while PEA2PbI3SCN film is introduced to promote the homogeneous crystallization at the buried interface via SCN- anchoring with cations. Further ethylenediamine dihalides (EDA(I/Cl)2) post-treatment leads to the gradient energy level alignment on the film surface. Pb-Sn PSCs based on such film show efficient charge transport and extraction, producing a champion power conversion efficiency (PCE) of 22.3% with an impressive fill factor of 82.14%. Notably, combining with semitransparent 1.78 eV wide-bandgap PSCs, the four-terminal all-perovskite tandem device achieves a PCE of 27.1%. This work opens up a new pathway to boost the performance of Pb-Sn PSCs and their tandem devices.

Bioinspired multilayer mulch film integrating mud repulsion, adhesion reduction, and antifouling for high-efficiency resource recycling

Resource recycling is a promising solution to the current problem of residual film pollution. However, the challenge that remains to be addressed in the resourcefulness of mulch films lies in mitigating heavy water and power consumption during the cleaning process. Herein, inspired by the unique structure of typical soil-dwelling insects, the mass production of high-strength multilayer mulch (Bio-HAM) films is achieved through a sequential combination of multilayer coextrusion, blow molding, roll-to-roll imprinting, and spray coating. The regular micro-nano hierarchical structure on the Bio-HAM films surface enables it to effectively inherit the antifouling, slurry repulsion, and adhesion reduction functions of the prototypes, even when exposed to ultraviolet radiation, deformation, and liquid or solid impact. Benefiting from the aforementioned properties, the Bio-HAM films exhibit low impurities adhering and soil wrapping during the entire planting cycle. The impurity content of the recovered Bio-HAM films wrapping is approximately 12.7 g/m2, which is a 67.4% reduction in impurity content compared to the basic films. Moreover, the recovered film also possesses favorable mechanical and thermal properties, thereby ensuring water and energy conservation as well as high-value regeneration. The proposed approach is expected to address the excessive consumption and persistent ecological pollution of residual film recycling.

On the reconstruction of LEIS spectra after distortion by an electrostatic energy analyzer

The study attempts to reveal the influence of the transmission function of electrostatic deflector analyzers with linear voltage scan on the shape of the LEIS energy spectra and the arising error in surface elemental composition and film thickness analysis. The practical applicability of several analytical and numerical approaches for reconstructing true energy distributions of particles is studied using examples of simulated LEIS spectra. The presented methodology can be extended to a wider range of applications involving electrostatic or magnetic analyzers of charged particles. The study proposesa freely available and ready-to-use tool for simulating distortions of spectra caused by the transmission function of spectrometers and for reconstructing true distributions of arbitrary spectra.

Modifying back contact by silver to Enhance the performance of carbon-based Sb2S3 solar cells

Antimony sulfide (Sb2S3) has drawn significant attention due to its excellent photoelectric properties. However, the photon conversion efficiency of Sb2S3 solar cell is limited by its magnificent value band offset between absorber layer and carbon electrode. Herein, a back surface optimization strategy is developed to reduce the barrier at the back contact. By introducing Ag at back contact before post-selenization process, the Valence-Band Maximum (VBM) of Sb2S3 is elevated under the action of spin–orbit interaction between Sb-5 s orbit and Ag-d orbit, leading to improve the extraction of holes. Consequently, the electrical conductivity of the film undergoes a significant improvement, rising from 1.20 × 10-5 to 1.59 × 10-5 S/cm. Additionally, the film surface morphology is refined, exhibiting a reduction in roughness from 17.1 nm to 13.6 nm, which leads to less leak current generated. Finally, the device based on FTO/SnO2/CdS/Sb2S3/Carbon structure achieved the power conversion efficiency (PCE) of 5.51 %, representing a 16 % improvement compared to the device without Ag treatment.

Biomass carbon nanosphere-based piezoresistive flexible pressure sensors for motion capture and health monitoring

Biomass carbon-based flexible sensors have shown significant potential in various applications. Indeed, biomass carbon-based flexible sensors, such as carbonized cotton fabrics, exhibit limitations in terms of sensitivity and response speed. Herein, a kind of biomass carbon nanosphere-based flexible pressure sensor was proposed and developed by embedding biomass carbon nanospheres on the skin-hair structures of the polydimethylsiloxane surface. The carbon nanospheres were obtained through the carbonization of cuttlefish ink. Owing to the skin-hair structures fabricated on the surface of the composite films, the sensors achieve a sensitivity of 135.2 kPa−1 in the pressure range of 0–1 kPa. In addition, the sensors exhibit a short response (43 ms) and recovery time (23 ms). With these properties, the sensors can capture and monitor various motions as well as the characteristic waveforms of the wrist pulse, such as the percussion wave (P wave), tidal wave (T wave), and diastolic wave (D wave).

A novel coaxial air-R134a spray cooling for heat transfer enhancement of laser dermatology

Efficient cooling is essential in laser dermatology to prevent thermal damage, but current methods are often hindered by thermal barrier film deposition. This study presents coaxial air-R134a spray cooling for heat transfer enhancement and deposited-film mitigation. Surface temperature was measured using a TFTC thermocouple, while heat flux was estimated through the improved Duhamel theorem. High-speed camera visualization with the Mie-scattering technique captured the spray and film behavior. The results showed uniform and effective cooling via transient surface heat transfer and film behaviors. The boiling curve identified three distinct cooling phases: transition boiling, nucleate boiling, and single-phase film deposition. Airflow adjustments not only enhance the heat transfer coefficient but also control spray dynamics, droplet rebounding, and film spreading. Self-organizing maps revealed that medium-to-high nozzle spacing (y/D) reduces surface temperature and increases the heat transfer coefficient. Film resistance time is minimized by either high airflow and low y/D or low airflow and high y/D. For critical heat flux, low y/D and airflow are optimal. A y/D = 32 and 60 L/min airflow provided appropriate operating conditions. However, airflow increased frost deposition during off-duty, suggesting moisture-free air in the future.