# A Review on the Some Issues of Multiphase Flow with Self-Driven Particles
## 3 Aggregation, Phase Separation and Sedimentation of Squirmers  
Natural and synthetic self-driven particles, such as Escherichia coli or Brevibacterium with honeycomb or rafted structure (as shown in Figure 7), artificial Janus granules and synthetic photoactivated colloidal particles, often show the phenomena of aggregation, phase separation and sedimentation [5; 22]. Clarifying and controlling such mechanisms can help improve the capability of screening diseased cells, developing detection devices  
Figure 4: Chlamydomonas cells disperse near the wall [9]. Adapted with permission from [PERSON] et al. (2016).  
Figure 5: Squirmer oscillating along the wall [9]. Adapted with permission from [PERSON] et al. (2016).  
Figure 6: Slip length.  
together with particle localization and identification. There are several explanations for the aggregation of squirmers in previous studies. Some researchers believed it was caused by Brownian motion [23; 24]; some accepted that high concentration and strong driving properties led to the increase in particle collision rate [25]; some supported that it was caused by the comprehensive influence of geometric dimensions of particles, self-driven properties and collision between particles [26]; and others attributed to the hydrodynamic interactions between particles and fluid [27; 28].  
For the particles with different phases, particle aggregation may result in phase separation. [PERSON] et al. [29] discovered that phase separation would be enhanced when the hydrodynamic interaction between fluid and particles was weak and would disappear when the interaction was strong. [PERSON] et al. [30] concluded that hydrodynamic force suppressed phase separation. Phase separation is also related to the driving modes of particles. [PERSON] et al. [31] found that pushers had the largest degree of phase separation, neutral squirmers followed and pullers were the last. However, [PERSON] et al. [32] and [PERSON] et al. [33] discovered that neutral squirmers had the largest degree of phase separation. The reason for the different conclusions could be attributed to the different treatment of change in fluid density. If the compressibility of the flow was not considered, the fluid transport induced by squirmers would lead to the uneven distribution of fluid density. The driving characteristics of squirmers would be weakened in the areas with large fluid density, resulting in squirmers being stable within the group without phase separation. The research of [32; 33] belonged to such cases, but the effect of fluid compressibility was taken into account in the research of [31]. The effect of fluid compressibility is related to the number density of particles. The above conclusions were based on the assumption that number density of particles is constant.  
Near the wall, particles would present different settlement patterns, such as a stable state together with a fixed position and orientation [34], upright state together with floating above the wall [35], etc. The appearance of such states is correlated with the influence of gravity. The inertia of particles could not be ignored if settling velocities of particles were large enough [36]. Accordingly, it is necessary to explore the settlement characteristics of squirmers under the combined influence of driving force, gravity, hydrodynamic force, wall effect and inertia of particles.