In contrast, in the view of hydrocyclones, the above-mentioned examples also confirmed that the hydrocyclone separation could be effectively enhanced by the flotation. Therefore, recent years, to en- hance hydrocyclone separation by flotation, researchers introduced various approaches to add air bubbles into hydrocyclones. For instance, to enhance the oil-water separation in hydrocyclones, Bai et al. utilized an air-liquid mixing pump to produce 15–60 μm air bubbles and provide high probability for oil-bubble interaction. After air bubbles with larger diameter, which could decrease the separation efficiency, were separated using an air-liquid separation pot, the air bubbles, oil, and water were fed into the hydrocyclone together. Then the air bubbles were entrapped by a single oil droplet or oil droplets with flocculated structure (Fig. 17), which resulted in that the oil was separated more easily and rapidly than that without air bubbles. Results demonstrated that: (i) the air bubbles occurred in the water could ameliorate the separation efficiency of hydrocyclones. (ii) The separation efficiency reached the maximum when the air-liquid ratio was close to 1% . Zhao et al. injected the air into hydro- cyclones through the micro-pore section to combine them with dis- persed oil. Experimental results demonstrated that: (i) the air-injection hydrocyclone had two functions: hydrocyclone separation and floatation separation, and could certainly enhance the hydrocyclone separation. (ii) The vortex finder diameter of the air-injection hydro- cyclone should be larger than the hydrocyclone without air injection. (iii) The highest separation efficiency was obtained when the micro- pore diameter was smallest. The reason might be that, with the increase of the micro-pore diameter, the air velocity toward the center in- creased, whereas the residence time decreased. (iv) Nevertheless, the micro-pore diameter could not be too small. The reasons might be as follows: the larger the micro-pore diameter, the harder for the bubbles to be scattered into the fluid, and its capability carrying oil droplets increased; if the micro-pore diameter was too small, there would cer- tainly lead to blockage. (v) There did exist an optimum air-liquid ratio for hydrocyclone separation, which was supported by Bai et al.. (vi) The optimum separation efficiency could be got by the first one third fine-cone injection.3.7. Hydrocyclones enhanced by control particlesSimilar to the above-mentioned hydrocyclone enhanced by flota- tion, that is, the so-called “hydrocyclone enhanced by air bubbles”, the hydrocyclone enhanced by control particles are the hydrocyclone using control particles (i.e., the micro-particles in Fig. 18), including the solid particles, droplets, and gas-liquid composite drops, to enhance the hy- drocyclone separation, which was proposed by Wang et al. . Its principle is as follows. Generally, the ions, molecules, and their ag- gregates in fluids are easily to be adhered on the surfaces of large particles. Thus, we can first use the control particles with relatively large diameter (i.e., the micro-particles in Fig. 18) to adhere the ions, molecules, and their aggregates (i.e., the nano-particles in Fig. 18), and then enhance the separation of the ions, molecules, and their aggregates by separating the control particles with larger diameter (i.e., the micro- particles in Fig. 18). By this way, we can also enhance the separation by increasing the adsorption capacity of control particles. To date, how- ever, although the hydrocyclone enhanced by control particles has been successfully in several applications, further research is still re- quired to study its mechanism, and the range of its application should be significantly expanded.