Fog Droplets Research Articles

Theoretical investigation on the oxidation mechanism of methylglyoxal in the aqueous phase

The oxidation mechanism of methylglyoxal (CH3COCHO) in the aqueous phase plays a crucial role in the formation of secondary organic aerosols (SOA). To date, the investigations of reaction mechanisms of MG in the aqueous phase still needs to be refined, and the oxidation mechanisms of MG in the existence of various oxidants (e.g., H2O2, O3, ∙NO3, etc.) are in controversy. In this paper, we investigated the hypothesis that small-molecule organic acids are the primary products in cloud water and fog droplets, while large-molecule organic acids and oligomers play crucial roles in wet aerosols. Specifically, the hydration reaction, oxidation mechanism and oligomerization reaction of MG in aqueous phase were investigated on a theoretical basis. It has been indicated that the hydration reaction is a significant initiating reaction of MG in the atmospheric aqueous phase, whose generated hydrated compounds played a critical part in the process of forming oligomers. The aqueous oxidation reaction of MG could form a variety of organic acids, including pyruvic acid, formic acid, acetic acid, and oxalic acid. In the presence of OH radicals, pyruvic acid was the main first-generation production, which undergoes further reactions to form acetic acid, oxalic acid, and mesoxalic acid. Acetic acid was mainly derived from the reaction of OH radicals with pyruvic acid, whereas oxalic and mesoxalic acids were mainly generated by the OH radical reaction for MG and pyruvic acid. Of these, the formation of acetic acid was thermodynamically most favorable. Additionally, the reactions of MG with other oxidants also provided the possible pathways for pyruvic acid production. At 298 K, we calculated the rate constants for the reaction of MGHY with NO3, OH, HO2 radicals, and O3 to be 4.48 × 108, 2.54 × 107, 1.26 × 10−2, and 4.38 × 10−4 M−1 s−1, with atmospheric aqueous phase lifetimes (τ) of 4.43, 3.12 × 103, 2.21 × 1011, and 3.17 × 108 h, respectively. The theoretical results from this work will facilitate the explanation for the MG reaction process in the aqueous phase so as to further correctly estimate the relationship between the aqueous phase chemistry of MG and the formation of SOA.

Leaf Vein-Inspired Superhydrophilic Microchannels for Sustainable Fog Collection.

Fog collection is a promising solution for mitigating the urgent water shortage around the world. Despite the delicate design of various bionic fog harvesting surfaces with prowess to enable fast fog capture and programmed water transport, achieving sustainable and efficient fog collection by regulating the macroscale surface refreshment efficacy remains rarely concerned yet is effective. Here, we proposed a bioinspired structural design to achieve significant improvement on the surface refreshment efficacy to 46.47%, nearly 5 times larger than that of conventional design. Specifically, we constructed superhydrophilic vein-like microchannels on a superhydrophobic brass surface by using laser texture technology and hydrothermal treatment. Our microchannel design acts as a "highway" for synergically transporting and converging the collected fog droplets, as well as rapidly refreshing large surface area for the subsequent fog collection, reminiscent of the leaf veins responsible for the persistent mass transport between plant tissues. The practical implementation also convinced our design of a maximum water collection efficiency of up to 506.67 mg cm-2 h-1 and a long-term performance stability within a 10 h test. Our design is generic to most of the fog harvesting materials, showing great application potential for efficient atmospheric fog collection.

The role of collision and coalescence on the microphysics of marine fog

Cloud microphysics fulfills a fundamental role in the formation and evolution of marine fog, but it is not fully understood. Numerous studies have addressed this by means of direct observations and modeling efforts. However, collision–coalescence of aerosols and fog droplets is a process often neglected. In this study we perform an analysis of the role of particle collections on the formation, development, and microphysical structure of marine fog. It was found that collisions open a path for aerosol activation by means of collisional activation. In addition, collisions contribute to the diffusional activation of fog particles by adding water mass to the growing aerosols, making them reach the required critical radius faster. Furthermore, collisions have a homogenizing effect on hygroscopicity, facilitating the activation of accumulation‐mode aerosols by increasing their diffusional growth.

Multi-scale modeling of fog harvesting using thin-fiber grids – Towards new design rubrics

Fog collectors with fibers thinner than 1 mm harvest fog droplets more effectively at a low airflow velocity around 1 m s-1 than thicker fibers. However, existing models and design rubrics are restricted to thick fibers at higher velocity. This paper reports a multi-scale numerical model for fog harvesting at airflow velocity of 1 m s-1 using thin-fiber grids with different fiber spacing and diameter. The numerical model can simulate fog harvesting at two extreme length scales that are comparable to collector scale at the large end and fiber scale at the small end. The results confirm two new and important effects of fiber grid geometries on water collection efficiency, which are not included in existing theories and design rubrics. First, dense thin-fiber grids negatively influence the collection efficiency because of wall effect caused by viscous boundary layers. Second, the sparse thin-fiber grids can benefit from isolated clogging waterdrops and maintain relatively high efficiency when clogging blocks multiple grid openings. The two identified effects are then included to develop a new performance map for fog collectors, thereby shaping new design rubrics for fog harvesting. This work extends the existing theories of fog harvesting and sheds light on the design of novel fog collectors.

Smart Lanceolate Surface with Fast Fog-Digesting Performance for Triboelectric Energy Harvesting.

Utilizing the ubiquitous fog in nature to create decentralized energy-harvesting devices, free from geographical and hydrological constraints, presents an opportunity to foster sustainable power generation. Extracting electrical energy from fog relies heavily on fog-digesting performance. Improving the efficiency of fogwater utilization remains a formidable challenge for existing fogwater energy-harvesting technologies. Inspired by the water-harvesting behavior of Tillandsia leaves, a smart lanceolate surface is developed to harvest triboelectric energy by rapidly digesting fog. Such a surface exhibits capabilities in fog management, encompassing precise fog capture, transportation, and critical droplet separation. Specifically, fog droplets condense at hydrophilic sites of acylated cellulose ester, subsequently migrating toward the rear under Laplace pressure, thereby producing energy as they traverse through the tail end. Such architecture yields a brief voltage restoration period (with an average of 9.36 s), can rush the capacitor to 11.59 V within 20 s, and achieves a water-digestion rate of up to 71.05 kg/m2 h. This biomimetic approach enhances the water-digestion efficacy of the atmospheric water energy apparatus and offers perspectives on mitigating deficiencies in power resources.

Synergistically Utilizing a Liquid Bridge and Interconnected Porous Superhydrophilic Structures to Achieve a One-Step Fog Collection Mode.

Conventional fog collection efficiency is subject to the inherent inefficiencies of its three constituent steps: fog capture, coalescence, and transportation. This study presents a liquid bridge synergistic fog collection system (LSFCS) by synergistically utilizing a liquid bridge and interconnected porous superhydrophilic structures (IPHS). The results indicate that the introduction of liquid bridge not only greatly accelerates water droplet transportation, but also facilitates the IPHS in maintaining rough structures that realize stable and efficient fog capture. During fog collection, the lower section of the IPHS is covered by a water layer, however due to the effect of the liquid bridge, the upper section protrudes out, while covered by a connective thin water film that does not obscure the microstructures of the upper section. Under these conditions, a one-step fog collection mode is realized. Once captured by the IPHS, fog droplets immediately coalesce with the water film, and are simultaneously transported into a container under the effect of the liquid bridge. The LSFCS achieves a collection efficiency of 6.5kg m-2 h-1, 2.3 times that of a system without a liquid bridge. This study offers insight on improving fog collection efficiency, and holds promise for condensation water collection or droplet manipulation.

Prediction of the dynamic pressure drop of filter media loaded with water mists based on the deposited droplet mass distributions and pore network model

Predicting the pressure drop of a filter is necessary, especially regarding the jump in the dynamic pressure drop of a filter without pre-filters when clogged by rain or fog droplets. However, the efficient prediction method for the pressure drop in cylinder filters loading with water mists in the pore network faces limitations due to the high requirements for microscopic images of materials and the need to consider for the increasing holding droplet capacity of filters. In this study, a systematic method that takes into account the distributions of the deposited droplet mass within the water-absorbed filter medium layers in the pore network model (PNM) was developed. The pore geometry and distributions were determined from 2D microstructure images, and the dynamic deposited droplet mass and water-absorbing capacity of fibers were combined with the changing saturation to optimize the PNM for calculating the dynamic pressure drop of the filter media. Specifically, the empirical equation of the self-defined equivalent single fiber filtration efficiency (ESFFE) describes the relationship between the changing single fiber filtration efficiency and the deposited droplet mass. The optimized PNM is suitable for predicting the behavior of bibulous wood cellulose fiber filter media. The dynamic pressure drop shows a slight increase before surging, following the typical “channel and jump” model. Although the calculations deviate slightly from the experimental results with 25 % deviations, this systematic method effectively predict the jump in pressure drop of the tested filter medium. Therefore, this systematic method for predicting pressure drop of filters holds promise in anticipating unsafe pressure drop jumps in filters installed in coastal areas.

Supersonic coaxial aerodynamic atomization dust removal technology

The high concentration of respirable dust pollution created during coal mining has long been known to be harmful to employee health. A water absorbing atomization device has certain advantages when spray trapping particle sizes of PM2.5–10. To further improve the PM0–2.5 trapping efficiency, supersonic coaxial pneumatic atomization dust removal technology has been developed. The effects of fog droplet characteristics on the instantaneous diffusion of dust were studied by numerical simulation and macro- and microexperiments for the first time, and the coupling characteristics of fog droplets and dust fields were obtained. The results show that with increasing aerodynamic pressure, the supersonic range extends along the axis toward the nozzle outlet, and the average velocity in the tube increases. Under different pressures, the droplet particle size in the supersonic coaxial atomizing nozzle is <6 μm and decreases with increasing aerodynamic pressure. When a droplet effuses from the nozzle, the transonic distribution of the flow field causes the droplet velocity to increase first and then decrease under the drag force. The droplet field characteristic distribution of supersonic coaxial atomization is determined by droplet evaporation, condensation, and migration. Under the same pressure, with increasing spray distance, the distribution of the droplet particle size increases in a stepwise manner, and the droplet velocity decreases according to an exponential function so that the droplet field produces a high-speed fine fog area. The range of droplets with higher velocities and smaller particle sizes is mainly influenced by migration and increases with increasing pressure. The coupling effect droplet field and dust field can be characterized by the instantaneous dispersion of dust and are determined by the distribution characteristics of the fog droplet field. The large range of high-speed fine fog produced by supersonic coaxial atomization technology easily traps respirable dust, and the classification efficiency of PM0–2.5 reaches 90%. In this study, a new dynamic microfog dust trapping technique was proposed, and a new method of instantaneous sampling combined with a microscopic dispersion test was adopted to characterize the temporal and spatial distributions of dust fields affected by fog droplet fields, reveal the generation of dynamic microfog and the mechanism of dust trapping, enrich the theory of fine dust trapping, and provide technical support for the safe production and utilization of coal.

Establishment of fog droplet distribution model and study on canopy deposition uniformity

In plant protection operations, the distribution of droplets affects the atomization effect. To make the distribution of fog droplets more uniform in the air field, a fog droplet distribution model was established based on a three-dimensional motion model of droplets and the particle size distribution function, combined with a two-dimensional normal distribution function. The effects of the initial incidence angle and the additional wind speed on the distribution of fog droplets were analyzed. The fog droplet distribution was simulated to analyze the droplet distribution in the spatial layer, which was compared with the experimental results. To investigate the impacts of different factors on the atomization distribution, the Lagrangian interpolation method was employed, and the optimal initial incidence angle and external wind speed were found. When the initial angle of incidence was 17°, the slope of the fitted curve was the smallest, with a coefficient of determination of 0.9622 and a relative error of 3.12%. With an additional wind speed of 0.1 m/s, the coefficient of determination was 0.9782, with an average error of 4.61%. The simulation results were consistent with the experimental findings, and the accuracy of the fog droplet distribution model was verified. In summary, this research provides a novel method to improve the uniformity of the droplet distribution, which can provide a theoretical basis for determining the operating parameters.

The research on infrared radiation affected by smoke or fog in different environmental temperatures

Infrared thermal imaging camera as a non-contact monitoring of the object to be measured is widely used in fire detection, driving assistance and so on. Although there are many related studies, there is a lack of research on the influence of fog or smoke on infrared imaging under different environmental temperatures. To address this shortcoming, The temperature of both the environment and the target in this experiment is controlled by PID technology. The smoke or fog environment is generated using a smoke cake or an ultrasonic fog machine. The temperature of the target was measured by infrared thermal imaging camera. It was observed that as the temperature of the environment increases, the measured temperature of the target also increases. However, the change in temperature is more pronounced in the fog environment compared to either the smoke environment or the normal environment. It has been found through research that environmental radiation causes temperature changes in fog droplets. Therefore, Infrared radiation is less affected in the smoke environment and more affected in the fog environment. Additionally, when the environmental temperature is close to the target's temperature, the infrared image becomes blurred.

Photochemistry of Imidazole-2-carbaldehyde in Droplets as a Potential Source of H2O2 and Its Oxidation of SO2.

Hydrogen peroxide (H2O2) plays a crucial role as an oxidizing agent within the tropospheric environment, making a substantial contribution to sulfate formation in hydrated aerosols and cloud and fog droplets. Field observations show that high levels of H2O2 are often observed in heavy haze events and polluted air. However, the source of H2O2 remains unclear. Here, using the droplets formed in situ by the deliquescence of hygroscopic compounds under a high relative humidity (RH), the formation of H2O2 by the photochemistry of imidazole-2-carbaldehyde (2-IC) under ultraviolet irradiation was explored. The results indicate that 2-IC produces IM-C•-OH and IM-C•═O radicals via H transfer itself to its excited triplet state and generates H2O2 and organic peroxides in the presence of O2, which has an evident oxidizing effect on SO2, suggesting the potential involvement of this pathway in the formation of atmospheric sulfate. H2O2 formation is limited in acidic droplets or droplets containing ammonium ions, and no H2O2 is detected in droplets containing nitrate, whereas droplets containing citric acid have an obvious promotion effect on H2O2 formation. These findings provide valuable insights into the behaviors of atmospheric photosensitizers, the source of H2O2, and the formation of sulfate in atmospheric droplets.

Dense Fog in the Netherlands: Composition of the Nuclei that Contribute Most to the Droplet Number Concentration

Dense fogs, with a visibility of less than 200 m, form a traffic hazard. Usually, models describing their formation use observations at the Cabauw super-site in the Netherlands for evaluation. A key parameter is the number of fog droplets and thus the number of aerosol particles on which the fog droplets form, the so-called fog nuclei (FN). No observational data are available for this key microphysical feature. An assumption is that this number scales with the concentration of the hygroscopic aerosol component sulfate. However, in the Netherlands nitrate and organics are the more important components of the total aerosol and thus possibly also of the FN. This short communication provides the first actual data via measurements with an aerosol mass spectrometer—AMS—for a period with dense fog events observed in November 2011. The aerosol in the relevant size range was composed of about half of the hygroscopic ammonium nitrate/sulfate. The other half consisted of organics; the low O/C ratio indicated that these compounds are rather hydrophobic; the hygroscopicity factor kappa of this mix was estimated at 0.3. This value implies that the activation diameter (the lowest diameter of the FN) was at least 150 nm. The mass distribution was converted into a number distribution which showed a sharp decrease as a function of size for diameters above this threshold. This result implies that the vast majority of the FN have diameters to the activation diameter. These smallest FN contained ammonium nitrate as the major hygroscopic compound. Currently, data for other dense fogs are evaluated to search for a possible generality of this finding.

Marine Stratus—A Boundary-Layer Model

A relatively simple 1D RANS model of the time evolution of the planetary boundary layer is extended to include water vapor and cloud droplets plus transfers between them. Radiative fluxes and flux divergence are also included. An underlying ocean surface is treated as a source of water vapor and as a sink for cloud or fog droplets. With a constant sea surface temperature and a steady wind, initially dry or relatively dry air will moisten, starting at the surface. Turbulent boundary layer mixing will then lead towards a layer with a well-mixed potential temperature (and so temperature decreasing with height) and well-mixed water vapor mixing ratio. As a result, the air will, sooner or later, become saturated at some level, and a stratus cloud will form.

Electrical measurements during fog in the United Arab Emirates

Distinct differences in electrical characteristics of the atmosphere are observed during clear and foggy laden air. The presence of droplets in the air causes the removal of natural cluster ions and hence, a change in the electrical properties, which is useful for fog detection, and potentially fog forecasting. In this study, we report on some of the first electrical measurements conducted during fog in the United Arab Emirates (UAE).The analysis indicates that the Potential Gradient (PG) values observed during fog in the UAE were substantially higher than those previously reported in the literature (ranging from −1247 V/m to 1400 V/m). Furthermore, the PG during fog was often negative, with 93% of cases recording negative PG values (with median PG value of −397 V/m), particularly during wintertime fog events. A comparison with fog measurements conducted in the UK showed a stark contrast in PG behaviour between the two sites, with only positive PG values reported during fog in the UK (as is the case for the majority of PG fog studies reported in the literature), and higher PG variability in the UAE fogs. It is hypothesized that the unusual polarity of PG observed in UAE fog events may be attributed to the deposition of fog droplets, during which positive charges are transported from the top of the fog layer downwards towards the surface, thereby modifying PG. This deposition process is expected to be particularly active during the latter stages of the fog, when the droplet size distribution has fully evolved.

Field evidence of brown carbon absorption enhancement linked to organic nitrogen formation in Indo-Gangetic Plain

Atmospheric brown carbon (BrC), a short-lived climate forcer, absorbs solar radiation and is a substantial contributor to the warming of the Earth's atmosphere. BrC composition, its absorption properties, and their evolution are poorly represented in climate models, especially during atmospheric aqueous events such as fog and clouds. These aqueous events, especially fog, are quite prevalent during wintertime in Indo-Gangetic Plain (IGP) and involve several stages (e.g., activation, formation, and dissipation, etc.), resulting in a large variation of relative humidity (RH) in the atmosphere. The huge RH variability allowed us to examine the evolution of water-soluble brown carbon (WS-BrC) diurnally and as a function of aerosol liquid water content (ALWC) and RH in this study. We explored links between the evolution of WS-BrC mass absorption efficiency at 365 nm (MAEWS-BrC-365) and chemical characteristics, viz., low-volatility organics and water-soluble organic nitrogen (WSON) to water-soluble organic carbon (WSOC) ratio (org-N/C), in the field (at Kanpur in central IGP) for the first time worldwide. We observed that WSON formation governed enhancement in MAEWS-BrC-365 diurnally (except during the afternoon) in the IGP. During the afternoon, the WS-BrC photochemical bleaching dwarfed the absorption enhancement caused by WSON formation. Further, both MAEWS-BrC-365 and org-N/C ratio increased with a decrease in ALWC and RH in this study, signifying that evaporation of fog droplets or bulk aerosol particles accelerated the formation of nitrogen-containing organic chromophores, resulting in the enhancement of WS-BrC absorptivity. The direct radiative forcing of WS-BrC relative to that of elemental carbon (EC) was ∼19 % during wintertime in Kanpur, and ∼ 40 % of this contribution was in the UV-region. These findings highlight the importance of further examining the links between the evolution of BrC absorption behavior and chemical composition in the field and incorporating it in the BrC framework of climate models to constrain the predictions.

Unraveling the Ultrafast Photochemical Dynamics of Nitrobenzene in Aqueous Solution.

Nitroaromatic compounds are major constituents of the brown carbon aerosol particles in the troposphere that absorb near-ultraviolet (UV) and visible solar radiation and have a profound effect on the Earth's climate. The primary sources of brown carbon include biomass burning, forest fires, and residential burning of biofuels, and an important secondary source is photochemistry in aqueous cloud and fog droplets. Nitrobenzene is the smallest nitroaromatic molecule and a model for the photochemical behavior of larger nitroaromatic compounds. Despite the obvious importance of its droplet photochemistry to the atmospheric environment, there have not been any detailed studies of the ultrafast photochemical dynamics of nitrobenzene in aqueous solution. Here, we combine femtosecond transient absorption spectroscopy, time-resolved infrared spectroscopy, and quantum chemistry calculations to investigate the primary steps following the near-UV (λ ≥ 340 nm) photoexcitation of aqueous nitrobenzene. To understand the role of the surrounding water molecules in the photochemical dynamics of nitrobenzene, we compare the results of these investigations with analogous measurements in solutions of methanol, acetonitrile, and cyclohexane. We find that vibrational energy transfer to the aqueous environment quenches internal excitation, and therefore, unlike the gas phase, we do not observe any evidence for formation of photoproducts on timescales up to 500 ns. We also find that hydrogen bonding between nitrobenzene and surrounding water molecules slows the S1/S0 internal conversion process.

Experiments and CFD simulation of downwash airflow and fog distribution discharged from a single-rotor UAV fitted with a pulse-jet thermal fogging machine

A single-rotor UAV fitted with a pulse-jet thermal fogging machine was developed. Computational fluid dynamics (CFD) was employed to simulate the downwash airflow and fog distribution of a single-rotor UAV fitted with a pulse-jet thermal fogger. The developed CFD models were validated in three steps by comparing the calculated results with the measurement experiments. Predicted air velocities of the single-rotor UAV downwash airflow agreed well with measured velocities. The model was also able to predict the fog droplet deposition density of the pulse-jet thermal fogger alone, and fitted to the single-rotor UAV, with the relative errors within 20% and 30%, respectively. The validated CFD model was then employed to investigate the effects of the headwind, crosswind and UAV operating height on the downwash airflow and the fog distribution. Results indicated that when the pulse-jet thermal fogging machine was mounted on the UAV, crosswinds needed to be avoided. At the UAV flight speed of 2 m s−1 and natural wind speed of 1 m s−1, a modest headwind appearance could help improve thermal fogging efficiency at different operating heights. The results have important research value and practical significance for improving the pesticide efficiency of UAV sprayers.

Optimization of the Air Cleaning Properties of Fog

Fog droplets are very often used as a cleaning agent when air pollution can be dangerous for health conditions and ecosystem. This work presents a new system to optimize the cleaning properties of fog by tuning its microphysical parameters. For this purpose, a newly developed system, which is based on the electromagnetic echo effect (EMEE) sensor, is used to detect the most efficient interaction between fog and impurities, i.e., which fog droplets can be used to most effectively clean a certain type of pollutant from the air. Fog droplet spectra controlled by the nozzle pressure system can be used to effectively remove pollutants from the air. For this purpose, an automated system for aerosol generation can allow an accurate control over the fog microphysical parameters and the use of fluids with specific concentrations of pulverized chemical compounds. Fog droplet size distribution is controlled by the feeding gas pressure at the nozzle and chemical simulants. The experimental results showed that the microphysical parameters (MP) are directly related to the impurity of species used in the cleanup simulation process. The MP parameters of fog are liquid water content (LWC), droplet mean radius (Rm), droplet number concentration (Nd), and both aerosol type and mass concentration. In the lab testing, harmless simulants of CBRN (chemical, biological, radiological and nuclear) species were used. During the tests, fog droplet size distribution is controlled by the air pressure at the nozzle and simulants. It is concluded that an integrated fog generator system (IFGS) with EMEE sensor developed in the current work can be utilized broadly to control fog microphysical parameters, leading to an optimum aerosol/chemical species’ cleaning process.

Anti-fogging/dry-dust transparent superhydrophobic surfaces based on liquid-like molecule brush modified nanofiber cluster structures

Transparent protective coatings capable of preventing fog and dust accumulation have broad application prospect in photovoltaic systems, optical devices and consumer electronics. Although a number of superhydrophobic coatings have been developed for self-cleaning purpose over the past three decades, there is still a lack of surfaces that can simultaneously possess high transparency, remarkable superhydrophobicity, and excellent fog and dust resistance. In this study, we have prepared surfaces featuring sub-wavelength nanofiber cluster structures through a facile plasma etching method, and further modified the surface with liquid-like perfluoropolyether (PFPE) brushes. The prepared PFPE modified nanofibrous surface (PFPE-NS) exhibits superior optical transparency (transmittance 90.4 % ± 0.7 %) and water repellency, with a water contact angle as high as 171.0° ± 0.6° and sliding angle down to 0.5° ± 0.1° (5 µL). More importantly, benefitted from the nanofiber cluster structures and the slippery liquid-like surface chemistry, the adhesion and accumulation of fog droplets and dust particles on PFPE-NS is greatly inhibited. As a consequence, PFPE-NS can keep excellent optical clearness after 2 h fogging test and maintain an average transmittance above 87 % after 24 h dusting test. Our study provides a promising strategy through constructing liquid-like nanofibrous coating for optical protection that could be applicable in practical rainy, foggy, and dusty environments.

One-Step Microfluidic Fabrication of Bioinspired Microfibers with a Spindle-Knot Structure for Fog Harvest.

Many biomimetic microfibers have been designed from spider silk to collect water efficiently from humid air as a result of its periodic spindle-knot structure, which enhances the direct movement and convergence of captured fog droplets. Here, a hydrodynamic flow-focusing microfluidic device with a theta-shaped tube is designed for the one-step fabrication of bioinspired microfibers with a spindle-knot structure for fog harvest. The morphology of the structured microfibers, including height, width, and spacing of spindle knots, can be adjusted readily by regulating the flow rate of specific phases. The production rate of these structured microfibers can reach 1100 cm/min. Moreover, the capture, transportation, and collection performance of fog droplets on various microfibers are investigated in a fog collection platform. It is demonstrated that our one-step microfluidic device presents a ready method for the fabrication of structured microfibers with spindle knots, which provide a significant facilitation on efficient fog capture and water collection.