Current research priorities on fog harvesting as a clean water resource: a bibliometric approach

. By 2050, more than 50% of the world's population will face water stress. Global demand for drinking water will also increase due to economic development, population growth and other needs. To alleviate this growing problem, the scientific community is called upon to explore all potential water resources and develop appropriate technologies. One such promising resource is fog, especially when climatic conditions are favorable and nearby conventional resources are insufficient. The present bibliometric study is an attempt to explore the topic of “fog harvesting” and understand the dynamics in this area. Indeed, three significant directions have been identified: 1-materials, their properties and characteristics; 2-biological mimicry to collect fog; 3-interaction with other nearby issues (energy, solar, desalination, etc.). In summary, “fog harvesting” is a relatively premature but promising area of research. Due to the strong dependence on climate and topography, this area is still hesitant in terms of collaboration. This may be a good avenue to explore.


Introduction
Nowadays, Access to clean water is considered one of the major worldwide challenge.According to the World Resources Institute [1], more than 50% of the world's population will face water stress, by 2050.Global demand for drinking water is also expected to increase by 20-25%, due to economic development and population growth.At the same time, 19% of watersheds will experience supply instabilities, notably due to the effects of climate change [2].Water scarcity would thus be the cause of social, economic and political problems, particularly in arid or semi-arid regions, where low or middle-income countries will face difficult situations in meeting the vital needs of their populations.Indeed, water stress is a complex and widespread phenomenon [3].This is not just about water shortages or inadequate infrastructure.It also concerns aspects linked to the quality of the water available, operating costs and the sustainability of the used techniques.For these reasons, water is a structuring issue in the United Nations' agenda for sustainable development [4].It is the subject of SDG 6 and is partially part of SDGs 1, 2, 3 and 10.
Thus, the scientific community is expressly called to explore all potential water resources and develop sustainable and efficient technologies to exploit them.Fog harvesting is one such option being studied by researchers.Some countries, like Chile, Yemen, South Africa, etc. [5], have installed and tested operational devices to collect fog with promising results.
This bibliometric study attempts to analyze recent scientific production in the field of "fog Harvesting".This is an exploratory approach, based on the theoretical foundations of bibliometrics [6]- [8] and on previous cases, carried out in several scientific fields, in particular the heuristic approach implemented to study desalination [9].It is, in fact, a matter of studying the behaviour of researchers (production, citations, subjects of interest, evolution, etc.) to understand the state of current knowledge and the possibilities for development that are emerging.This could help decision-makers to better assess the opportunity of this area of research and anticipate its main directions.

Atmospheric water, a path to sustainable development
Outside rainwater, surface or underground water, at least two large classes of alternative resources are identified to deal with water shortage [10], [11]: 1-salty, brackish or wastewater which requires energy-intensive and relatively complex treatment technologies, and 2atmospheric water, or air humidity, a sustainable resource which can be collected, using simpler techniques, but still seeking maturity.
Knowing that the global potential of atmospheric water is estimated at approximately 1.29 × 10 13 m 3 [12], [13], this humidity is encountered even in desert regions.In some cases, warm air can provide more humidity than cold air.However, this precious resource is not regularly distributed and the humidity amounts extracted depend on geographic and climatic conditions [14].To collect this resource, several methods are used: steam condensation, cooling of ambient air, interception of fog or dew, etc.These techniques may be combined to improve yields, using physical phenomena such as sorption, thermoelectric cooling (Peltier effect) or vapour compression refrigeration [10].
Paradoxically, climate change, causing an increase in air temperature and relative humidity, favours the conditions for using these Sustainability an opportunity to develop them further.In addition, these techniques do not affect the hydrogeology and do not generate significant residues.They also respect the hydrological cycle because most of the humidity extracted from the air will be returned to it, using natural processes (evaporation, transpiration, etc.).Furthermore, unlike traditional techniques, such as desalination or membrane distillation, they are relatively easy to implement, do not require large infrastructure and do not depend on conventional water sources [11].Thus, given the ambitions in terms of SDGs [3] and the potential of air humidity, researchers and stakeholders must take an interest in improving the techniques, current conditions as well as their sustainability and viability.

The opportunity of "Fog Harvesting"
Fog is a natural and localized phenomenon which depends on climatic and geographical conditions.It generally occurs when humid air is cooled, for example, due to long-wave radiation or the forced rise of air facing a relief (e.g.mountain).This cooling causes supersaturation of the air and fog is thus formed (droplets with a diameter of 10 to 50 μm, at a concentration of 10 to 100 drops per cm 3 [15]).
Fog harvesting is based on a surface impaction process.It consists of intercepting the flow of supersaturated air, using a mesh (artificial nets equipped with gutters) stretched against the wind bringing the droplets [16].Fog is collected directly, using physical collection techniques.However, extracting water vapour requires the air to be supersaturated (~100% relative humidity) and cooled below the dew point temperature.This involves additional energy expenditure compared with direct capture of fog or dew.
Then, when topographical and humidity conditions are favourable, fog collection is inexpensive and can be easily deployed, even in landlocked regions.It can thus complement traditional resources [17], [18], particularly in shortage areas, improve the autonomy of isolated populations in terms of agriculture and reforestation and possibly offer an alternative emergency, in the event of contamination.

Bibliometric analysis of fog harvesting
Given its importance for sustainable development, research on fog collection has undergone remarkable developments.Several attempts to establish the state of the art in this area are published.However, bibliometric studies on this subject are rare.An attempt to understand current research directions in this sector would make it possible to identify promising avenues and better construct a vision of the priority questions in this research field.This bibliometric study aims to explore the scientific literature on fog as an alternative or complementary water source.As a result, the corpus to be studied is selected in such a way as to avoid cross-disciplinary work that concerns the collection of atmospheric water, in general: An explicit focus is placed on the fog.

Methodological approach
The corpus studied is extracted from the Scopus bibliographic database due to its broad scope and its interest in regional issues [19], which is the case of fog.Fig. 1. summarizes the adopted approach, inspired by the PRISMA system [20].This involves extracting all bibliographic references that explicitly deal with fog collection.The bibliographic fields queried are Title, Abstract and Keywords.A cleaning of "Noise" is then applied, such as that linked to out-of-scope concepts, such as "fog computing" or "Freezing of gait (FOG)".
The final query was launched on the 18th of November 2023 and gave an overall result of 962 publications.This corpus contains 795 original articles, 65 literature reviews, 53 conference articles, 30 book chapters and other minority types.The oldest article found is "Collection of Fog Particles with Fine Fiber and Infra-red Absorption of Fog Particles".It is a publication, from 1952, of the journal "Papers in Meteorology and Geophysics", still active now.
To carry out a bibliometric analysis of this corpus, we adopt the exploratory approach suggested by [9].We also draw inspiration from the methodologies of some previous exercises [21], [22].This is based on the theoretical foundations of bibliometrics [8], [23]- [25] which allow us to carry out reductions/deepenings of the corpus, based on successive interpretations, suggesting a fruitful heuristic approach.This allows, intuitively and rapidly, to fuel reflection on trends and research priorities in the subject of fog collection.
The first stage consists of limiting the selection of publications, from 2018, of the following types: original articles, literature reviews, conference articles and book chapters.In total, 569 publications were retained, including 473 original articles and 53 literature reviews.

Main Results of the Bibliometric Analysis of Fog
After collecting and cleaning the corpus, we used the Scival analytical platform to explore it.Indeed, Scival (Elsevier) has several modules for analysing bibliometric data, with good details of entities and better contextualization of publications [26], [27].This serves the suggested heuristic approach and promotes the efficiency and outcome of the analytical effort.

Potential and "Maturity"
Generally speaking, the corpus of 569, with +31% in Open Access, involved nearly 2030 authors and received 9144 citations (Nov.18,2023).This generated an average impact index of 1.34 (Field-Weighted Citation Impact -FWCI1 ).This is 34% more efficient than the international average.This shows the general interest in this area of research.Indeed, 44% of this corpus is published in the top 10% of Scopus journals (in the CiteScore2 sense).Furthermore, 75% of contributions in this field are published in journals in the first quartile (Q1) and +86% cumulatively in Q1 and Q2.However, only 17% of publications are among the top 10% of publications worldwide (in terms of FWCI).This may suggest a sort of gap between the topicality of the subject, reflected by the interest of publishers in this subject (44% in the top 10% journals), and a sort of late reaction of researchers to capture this interest, reflected by the gradient in terms of FWCI (only 17% in the top 10%).This could also be seen as a precursor signal of an evolution in perspective, to be expected or encouraged.

Academic and industrial collaboration
At the international level, only 15 countries contributed more than 10 publications to the corpus studied (Table 1.).Except for India, Chile and Iran, few countries, potentially concerned by the fog (semi-arid and middle or low income countries), are present in this group of 15 countries .Besides, almost a third of the publications (178) were produced by researchers belonging to the same institution, while 24 publications were produced by a single author.The 22.9% of the corpus produced internationally has an FWCI of 1.66, while the 41.7% published in national collaboration achieve an exceptional performance (FWCI=1.41).
Only 17 documents, including 7 from China, were published in collaboration with researchers from industrial companies.This collaboration also generated a modest FWCI of 0.95, while academic collaborations (552 Pub.) performed better, with an FWCI of 1.35.
On the other hand, China has a central position, with its participation of 50% in the global corpus.This is reinforced by its performance in FWCI (1.6) and its first position in terms of number of publications in collaboration with companies (7/17 publications), while its international collaboration does not exceed 17.6% of its corpus.Some countries, such as Iran, India, Italy, the United States, etc., which are part of the world's top 15, in terms of publications, are also not very open to international collaboration (< 40%).This leads us to note a certain weakness, in terms of international collaboration and corporate interest.This also corroborates the observation in part (4.2.1) on the phase shift between the importance of this subject and its maturity.This suggests that local specificity is strongly present, with an open question on the technological and industrial potential of this theme.It is worth noting that only 10 publications concerned the inventive activity, with 14 citations emanating from 10 patents (6 USA, 2 France, 1 Australia, 1 Chile), without any Chinese!Table 1.Top 15 countries (+10 Pub.) in "fog harvesting".

Disciplinary Profile: Three Emerging Branches
The analysis of the subject areas of publications allows us to make a "macroscopic" approach to the disciplinary system, which expresses the structural orientation in this field.Fig. 2. shows that two major disciplinary spectra describe this research: 1-The field of prolific disciplines, such as materials science, chemistry, engineering and the environment, which each contribute to more than 134 publications to the corpus.These disciplines also show good performance in the sense of the FWCI (+1.3).This disciplinary core (1; a) is structuring this area of research; 2-The field of disciplines that are not very prolific but have remarkable dynamism, displaying a performing FWCI (+1.3).This field consists of two classes.The first (2; b),  made up of energy, agriculture and biology, with a relatively modest volume.The second (2; c), not very prolific, is made up of the disciplines "Biochemistry, Genetics and Molecular Biology", "mathematics" and "multidisciplinary", but have a high FWCI.This second disciplinary spectrum reveals some breakthroughs that researchers are increasingly interested in [28]- [31].

Main research questions: Topics 3
Scival also allows the studied corpus to be projected into the topics space, bringing together publications dealing with a unique and particular subject.These topics provide information on the dynamics of the subject, through the notion of prominence [32].
Thus, the corpus studied participates in 86 Topics (clusters), with only 11 with more than 5 publications.In barely 4 of these 11, the corpus studied contributes more than 1% to the publications of the Topic.Nevertheless, we note a very strong contribution (33.73% and 26.51%) to the two Topics T.27547 and T.17632, named "Liquid; Peristome; Nepenthes Alata" and "Cloud Water; Weather report; California Institute of Technology" (Table 2.).
Thus, Table 2. shows an orientation of the corpus for themes belonging to the established "macroscopic" structure (Fig. 2.), with a strong specialization in Topics T.27547 and T.17632 (33.73 and 26.51%).Furthermore, the modest contribution of the corpus in the Top 1% Topics in terms of prominence (T.21, T.48012, T.1779, T.4718) displays, in return, a very remarkable performance in terms of FWCI.

Keywords Cloud
Fig. 3. represents the cloud of the most relevant keywords from the overall corpus (word size), constituting recent terminological trends (green).This provides additional clarification of the directions of research in this area: 1-The study of the characteristics of materials (hydrophobicity, hygroscopy, hydrophilic, wetting, porous, etc.) to be for fog (Harvesting, Collecting, etc.) and devices, their forms and manufacturing techniques (Mesh, Harp, fibre, nanofiber, Contact angle, Threedimensional Printing, hierarchical Structure, Electrospinning, Lubricating Grease, etc.); 2-The study of biological systems to draw inspiration from them in the design of new devices (Weather bio-inspired, Biomimetic, Beetles Peru, Cactaceae, Janus insect, etc.); 3-The interaction between the subject of fog collection and other issues, such as dew collection (dew), electrical issues (Electric Corona, Femtosecond Laser, Wire, etc.), the environment (Atmospheric, Desert, etc.) and the relationship/comparison with other water recovery techniques (Desalination).
This analysis of keywords reflects the main investigated topics.According to font size, the main keywords reinforces the previous proposals and suggests that there are good perspectives for the evolution of this research in interaction with nearby and current topics (T.21, T.48012, T. 1779, T.4718).This further defines a probable trend in this resolutely dynamic research.

Focus on Topic T.27547
The Topic T.27547 (Liquid; Peristome; Nepenthes Alata) is among the top 2% most prominent Topics worldwide (2022).It is a structuring area in the corpus studied (fog harvesting).65% of the overall corpus belongs to this Topic.Conversely, the 369 publications concerned constitute 33.73% of the entire cluster (T.27547).
Relative to the general corpus, the 369 publications, belonging to T.27547, are only 27.6% Open Access, nevertheless with a higher FWCI (1.5 vs 1.34).Likewise, almost 50% of the 369 are published in the top 10% of Scopus journals (+88% in Q1 and Q2) and more than 20% of the 369 are in the top 10% of global publications (in the sense of FWCI).In addition, 13 of the 17 industrial collaborations belong to T.27547, as well as 7 of the 10 publications that have been cited by patents, with 12 citations out of a total of 14.This indicates the high importance of this Topic in the corpus studied.
The leading institutions, having signed more than 5 publications in this corpus, are mainly Chinese (14/17), the remaining three belong to the USA, Poland and Chile.In this group, the Chinese institution "Dalian University of Technology", which authored 12 publications, achieved a remarkable FWCI of 3.67, while the "Chinese Academy of Sciences" was present in 53 of the publications.The same observation is made at the level of prolific authors (> 4).Chinese are omnipresent in this corpus, in volume and quality.
All these elements demonstrate the effective strength and interest of this structuring topic of research on fog collection.They also show a certain Chinese "hegemony" in this area, accompanied by a closure towards international collaboration.

Discussion and Conclusion
The theoretical foundations of bibliometrics, notably described in a subsequent study on desalination [9] make it possible to carry out efficient explorations by reducing the amount of the corpus studied.This made it possible to explore the theme of "fog harvesting" and to approach the logic of scientific orientations in this field.Indeed, three significant directions are emerging: 1-materials/devices and their properties and characteristics; 2-the inspiration of biological systems for collecting fog and 3-the interaction with other technical systems (energy, solar, desalination, etc.).The bibliometric analysis also suggests a fourth option, in interaction with the topics (T.21, T.48012, T.1779, T.4718).In sum, "fog harvesting research" is a dynamic niche, heavily dominated by China.Particular prematurity was revealed, suggesting an opportunity for growth that must be closely observed.This niche is relatively sealed, concerning international and industrial collaboration.It is probably due to the domestic context, with a strong dependency on climate and topography.Nevertheless, the perspective of creating unconventional collaborations, linking Chinese researchers with their counterparts from Mediterranean and sub-Saharan countries, is an avenue to explore.The present work was limited to the collection of "fog", focusing on the bibliometric analysis of a specific corpus.Knowing the interaction of this topic with the general theme of air humidity could reduce the scope of the observed findings.A holistic study to place this theme in its general context would make building a broader vision of this subject possible.