A number of analytical techniques are available for evaluating particle

A number of analytical techniques are available for evaluating particle size of a suspension, in addition to N-(p-amylcinnamoyl) microscopy; such as DLS, centrifugal liquid sedimentation, and small-angle X-ray scattering, to name a few [36]. In this study, particle size distribution of nanofluid samples was attained by DLS measurements with a zetasizer. Although, the size distribution of nanoparticles is mostly referred to as particle size distribution, in reality, aggregation of particles is mostly significant in a dispersion. In that case, one may talk about cluster of particles, rather than individual particles. Fig. 5 presents an idea about the clustering mechanism. In that case, the effective diameter is not the diameter of a single particle. Rather, it is the diameter of a group of aggregated particles.
Consideration of Fig. 5 with FESEM and TEM images given in this study (and in the literature) together is required in order to understand the effective diameter for a particular mixture, and geometrical structure of aggregates. Results for the average cluster sizes with respect to different ultrasonication durations are presented in Fig. 6, together with literature data collected for different ultrasonication durations. It could be noted that the primary nanoparticle diameter was around 21nm, according to the information provided by the manufacturer. Here in Fig. 6, the cluster sizes collected in this study were within the range of 167–315nm after different ultrasonication durations, which was greater than the average size of a single nanoparticle.
Results presented in Fig. 6 are indicative of decrease of mean cluster sizes with ascending ultrasonication period. When the data of the present study is examined, it can be seen that the use of intense ultrasonic energy resulted in a more than two times decrease in cluster size, by breaking down the nanoparticle aggregates. While appreciable decrease of cluster sizes for ultrasonication durations up to 150min can be seen in Fig. 6, no significant improvements were observed in particle size decrease with longer ultrasonication durations (i.e., 180min). It is also observed in Fig. 6 that cluster sizes sharply decreased right after the application of ultrasound energy. The tendency of the decrease in cluster size was greater in the beginning, in comparison to the latter parts of ultrasonication period. The results of the present study is also compared with the data of Sadeghi et al. [11] and Mahbubul et al. [28], in Fig. 6. The data of Sadeghi et al. [11] and Mahbubul et al. [28] had similar decreasing trends in cluster size rate with ultrasonication as reported in MTOC study. During the first 30min of ultrasonic treatment, the rate of decrease in average cluster size was the highest for the data in [11], compared to the data of present study, and those of [28]; in comparison to the latter parts of the ultrasonication period. The average cluster size data obtained in [11] were higher than those of the present study. The reason behind this outcome is most probably due to the fact that the primary nanoparticle size in [11] was 25nm, and nanoparticles might initially have high level of agglomeration. Their achievable minimum cluster size was about 158nm after 180min of ultrasonication for Al2O3–H2O nanofluid. The data reported in [28] were the lowest compared to the data of the present study and those in [11]. Mahbubul et al. [28] reported 110nm of cluster size after 180min of ultrasonication, while the primary nanoparticle size of their samples was 13nm. Chen et al. [18] found a lowest aggregate size of ∼140nm for TiO2 nanoparticles, after 20h of ultrasonication, where the primary particle size was 25nm. Therefore, aggregate size depends more on initial primary size than the sonication power [37]. Also, the differences in the decreasing rates can be attributed to the effect of the nanoparticle (or cluster) material types, which may affect the tendency of the primary nanoparticles to form agglomerates, and break down to a mono-dispersed condition if previously agglomerated.