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across particle suspensions is outweighed by direct imaging. Figure 6a displays the year of highest occurrence of each of the main keywords. Interestingly, a shift from solutions (2006), frequently meaning laborato- rial aqueous systems, to wastewater (2014) can be observed. Also, remarkably, even considering the consistent increase in the number of publications on the subject, cellulose derivatives were mostly men- tioned back in 2007. In contrast, current trends and future research seem to aim more at nanocelluloses, not necessarily involving chemical modifications, than at etherification or esterification of anhydroglu- cose units. These seem to be displaced by crosslinking and/or grafting of side chains. Bibliometric studies are effective in identifying not only the current and past research trends, but also which keywords do not appear in the literature, and thus hinting which fields are not being studied and
keywords that are found tightly related. In this case, those flocculants are cellulose derivatives or even cellulose nanocrystals (Eyley et al. 2015). It is worth mentioning that papers on this application are rela- tively recent, thus showing a current research trend (Fig. 5b).
Trends and missing keywords
Other than cellulose , coagulation and fl occulation , i. e. , the terms of the search string, the nine main keywords are presented in Fig. 6, showing their key statistics. Scanning electron microscopy is the most frequently mentioned technique along the 460 pub- lications (Fig. 6b) and the most popular method to analyze morphology and the structure of membranes . Besides turbidity , which is closely related to solutions and polymers (Fig. 6c), the use of light scattering
Fig. 6 Selection of nine major keywords, highlighting the year of highest popularity ( a ), their individual occurrence ( b ) and their most usual combinations ( c )
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