# A Review on the Some Issues of Multiphase Flow with Self-Driven Particles
## 1 Introduction
For example, how does the hydrodynamic interaction between organisms affect swimming efficiency? How do the different characteristics between the flows near the wall and far away from the wall influence the distribution and swimming properties of organisms? Secondly, exploring the energy conversion mechanism of self-driven particles in the flow is beneficial for the development of artificial active materials, such as cell tissue and phosphorus-containing colloid. Moreover, research on the influence of particle shapes on the flow properties contributes to the design of the miniature swimming devices and improvement of self-organization technology of rotating body in a water environment. Understanding the interaction between self-driven particles and passive particles also contributes to the development of catalytic active particles and achieving of the biodegradation of pollutants and environmental remediation in water treatment and water protection. Thirdly, exploring the influence of fluid physical property on particle motion contributes to the realization of the propagation and diffusion mechanism of active organisms in the human system, e.g., manipulating drugs to kill germs and control infections. Finding out the influence of wall surface on the properties of particle motion can ensure that particles give full play to the function of being biomarkers and contrast media, and contribute to particle localization and identification, targeted drug delivery, accuracy improvement of non-invasive surgery, screening of diseased cells and development of detection devices. Figure 2: Synthesized self-driven particles. (**a**) Artificial cell; (**b**) biomarkers; (**c**) miniature swimmer. The energy injected into the fluid by self-driven particles is dependent on the particle driving modes. The self-driven particles described in this paper are the self-propelled and self-rotary particles and squirmers, which, as the most typical particles in natural and synthetic particles, directly interact with fluid by oscillating the cilia around the body. Based on the different stress applied to the fluid, squirmers can be divided into three kinds of modes, i.e., pushers (e.g., _Escherichia coli_, spermatozoa), pullers (e.g., Chlamydomonas) and neutral squirmers (e.g., paramecium), respectively. In past research, although the research on the motion of self-propelled/rotary particles and squirmers has made some achievements, there are still some problems that have not been clarified. For example, the effect of number density ratio of self-driven particles to passive particles on the motion of passive particles was not taken into account; the settlement characteristics of squirmers under the combined influence of driving force, gravity, hydrodynamic force, wall effect and Reynolds number are not clear; the combined effect of Reynolds number, power law index, and driving modes on the swimming velocities and hydrodynamic efficiency of squirmers in shear-thickening fluids are still unknown; there is still a lack of studies taking particle volume fraction, shape factor, self-driving mode, and fluid viscosity into account to explore the formation mechanisms of self-organization of squirmers. Therefore, the following part of the paper summarizes the research progress of these related contents.