# A Review on the Some Issues of Multiphase Flow with Self-Driven Particles
## 2 Interactions between Self-Propelled/Rotary Particles and Passive Particles  
Self-driven particles frequently collide with passive particles in flow. For example, spermatozoa, algae, bacteria and movement proteins collide with intracellular vesicles or dead bacteria; artificial active particles collide with pollutants. There are some pieces of research on interactions between self-driven particles and passive particles. [PERSON] et al. [7] studied the interaction between one self-rotary rotator and two passive particles, and found that when the rotator acted on passive particles, the Saffman force was larger than the Magnus force, which caused the passive particles to simultaneously rotate about rotator and revolve on its axis. In a small Reynolds number range, velocities of passive particles were the superposition of large amplitude and small amplitude pulsations, but the former would disappear if the Reynolds number was large enough. However, studies of interactions between a few self-driven and passive particles are insufficient to understand the properties of particle diffusion and aggregation. In addition, self-driven particles usually translate and rotate simultaneously, rather than only one of them doing so. Based on the experimental results, [PERSON] et al. [8] pointed out that increasing the number of self-driven particles would change the structural and dynamic characteristics of passive particles. As the number density of passive particles increased, the particles would aggregate with a self-diffusing function (as shown in Figure 3). In their study, self-driven particles carried out translational self-diffusion under the function of electrophoresis.  
Natural and synthetic self-driven particles usually move near the wall. Some papers have concluded that cells would move along the circumferential direction, Chlamydomonas cells would disperse or oscillate along the wall [9] (as shown in Figures 4 and 5), tumble of _Escherichia coli_ would be restrained, Volvox would become double beat, spermatozoa  
Figure 3: (**a–d**): Passive particles (red) aggregated under the function of self-driven particles (blue) [8]. Adapted with permission from [PERSON] et al. (2015).  
would change to backward swimming [10], and there existed three kinds of motion states for Chlamydomonas cells after touching the wall surface, i.e., escaping from the wall and swimming parallel with the wall [11].  
Research on the influence of wall surface on the swimming properties of self-driven particles can provide benefits for controlling the natural biological and synthetic particles such as biofilm formation [12], guiding spermatozoa pass through the fallopian tube [13], biochemical sensing [14], targeting drug delivery [15], environmental modification [16], and improving the velocities of self-diffusion electrophoresis particles [17]. [PERSON] et al. [18] discovered that hydrodynamic attraction led to Escherichia coli aggregation near the wall. However, [PERSON] et al. [9] believed that despite hydrodynamic attraction enhancing particle aggregation, it also appeared without hydrodynamic attraction. Near the wall, particle movements were related to the flow regime caused by the interaction between fluid and the wall, which depended on the wettability of the wall surface [19] and could be described by slip length \(ls\) (as shown in Figure 6). In addition, the slip length \(ls\) was close to 0 on the hydrophilic surface but a few tens of nanometers away on the smooth hydrophobic surface. Moreover, \(ls\) could be increased to the micron scale on a surface coated with hydrophobic molecular film or a surface on which bubble formation took place on the inner concave place. [PERSON] et al. [4] found that under the condition of \(ls\
eq 0\), the trajectories of particles would change greatly with the increase in \(ls\), and the results obtained from such cases were totally different from the case of \(ls\approx 0\)[20]. The values of \(ls\) changed along the wall for the rough surface [21].