Large-Scale Nanofiber Manufacturing| A Janus fibrous membrane with asymmetricwettability and surface potential for dualenhanced fog collection from theatmosphere

Researcher Zheng Yuming from Chinese Academy of Sciences: Asymmetric Wettability Janus Fibrous Membrane with Surface Potential for Efficient Atmospheric Fog Water Harvesting

The scarcity of freshwater resources has become increasingly severe in recent years, posing a serious threat to human health and sustainable socioeconomic development. Atmospheric water accounts for approximately 10% of Earth's total freshwater, making it a highly potential new freshwater resource and providing a practical solution to alleviate water crises. As one of the important forms of atmospheric water harvesting, fog collection demonstrates significant application potential in addressing water resource issues.

Efficient fog harvesting imposes stringent requirements on material performance. Materials must not only possess strong fog droplet capture capability to precisely and abundantly trap tiny water droplets from mist but also exhibit efficient moisture transport and collection performance to ensure rapid and smooth transfer and collection of captured water. However, current fog collection materials primarily focus on constructing single functionalities to promote water collection, failing to effectively achieve synergy among multiple fog collection processes, thereby limiting overall harvesting efficiency. How to construct high-efficiency fog water collection materials that simultaneously enhance key performances through simple preparation processes remains a major challenge in this field.

Recently, Researcher Zheng Yuming's team at the Institute of Urban Environment, Chinese Academy of Sciences, published their latest research titled "A Janus fibrous membrane with asymmetric wettability and surface potential for dual enhanced fog collection from the atmosphere" in Separation and Purification Technology. The first author of the paper is PhD student Li Chenxi, and the corresponding authors are Researcher Zheng Yuming and Associate Researcher Zhong Lubin. The researchers employed a simple sequential electrospinning technique to prepare a poly(vinylidene fluoride-trifluoroethylene)/polyacrylonitrile (PVDF-TrFE/PAN) Janus fibrous membrane with asymmetric wettability and surface potential for efficient fog collection.

Through the asymmetric wettability and microstructure regulation between the bilayer structure of the fibrous membrane, an efficient interlayer directional water transport process and water desorption process in the hydrophilic layer were achieved for Janus system fog collection. Additionally, the surface charges formed during high-voltage electrospinning generated additional electrostatic adsorption effects on fog droplets, enhancing the capture efficiency on the hydrophobic side. The prepared Janus fibrous membrane significantly contributes to the entire continuous process required for effective fog collection, from precise droplet capture to guided directional water transport and final desorption collection. Based on this design, the Janus membrane exhibited excellent water collection efficiency (up to 1572 mg cm⁻² h⁻¹). Moreover, its fabrication process is simple and feasible, opening new avenues for material design in atmospheric water harvesting and offering potential to alleviate water scarcity in practical applications.

Figure 1. Preparation process, cross-sectional SEM image, and schematic diagram of fog water collection for electrospun Janus fibrous membrane.

Using a simple sequential electrospinning process, an ultra-hydrophilic PAN fiber membrane was first deposited on the collector, followed by a hydrophobic PVDF-TrFE fiber layer, resulting in a Janus bilayer fibrous membrane with asymmetric wettability and surface potential. SEM images further confirmed the bilayer structure of the electrospun membrane, where the two layers had different fiber diameters but were tightly bonded. This unique structural characteristic is highly advantageous for interlayer water transport based on asymmetric wettability, efficiently promoting directional water transfer between the layers.

Figure 2. Effect of hydrophobic layer structure on water transport performance and water collection efficiency.

By regulating the microstructure of the hydrophobic layer, the water transport behavior of the Janus membrane was optimized. Analysis using the Young-Laplace equation revealed that when the hydrophobic layer had larger pore sizes and moderate thickness, the optimal interlayer directional water transport behavior could be achieved by constructing significant wettability differences between layers.

Figure 3. Effect of hydrophilic layer on water collection efficiency of Janus bilayer fibrous membrane.

The study also found that increasing the water storage capacity of the hydrophilic layer enhanced the collection efficiency. As storage capacity increased, the efficiency initially rose and then stabilized. Considering both efficiency and cost, JM-7 was selected for subsequent experiments.Furthermore, comparative studies on the collection efficiency of hydrophilic PAN fibers and hydrophobic PVDF-TrFE membranes demonstrated that the Janus PVDF-TrFE/PAN bilayer membrane exhibited superior performance, highlighting the unique advantages of the bilayer design. 

Figure 4. Effect of electrostatic adsorption on water collection efficiency of fibrous membranes.

When surface charges were present on all three membranes, collection efficiency significantly improved. Notably, the Janus membrane showed a 27.3% increase in efficiency after charge removal, primarily attributed to the potential difference formed between the charged membrane surface and surrounding fog droplets, generating electrostatic adsorption that enhanced droplet capture and overall performance.

In summary, this study utilized a two-step sequential electrospinning technique to directly prepare a Janus fibrous membrane with asymmetric wettability and surface potential. The membrane achieved high collection efficiency through excellent droplet capture, interlayer water transport, and hydrophilic layer desorption. This work provides an innovative membrane design solution to address freshwater shortages and is expected to play a significant role in future water resource applications.