Translational diffusivity of non-bottom-heavy squirmers in a semi-dilute suspension with c =0.1. Where, lines with slope 1 and 2 are drawn for comparison

 Conclusion In this paper, we have reviewed our fundamental studies on properties of a suspension of micro-organisms. Some of the continuum properties, such as the viscosity and diffusivity, were clarified in a semi-dilute regime. We expect to expand the present analysis to a denser suspension, which is the subject of current. Taking into account biological characteristics in the squirmer model will be another important challenge. Though the biological reactions were a minor factor in the particular cell-cell interaction, discussed in section 2.1, they are likely to be important for many types of cell. Also most mi-cro-organisms show some kinds of taxis, such as geotaxis, phototaxis, chemotaxis, rheotaxis or magnetotaxis, which dominate the preferred di-rection of cell's orientation. In order to develop a more realistic model of a suspension of micro-organisms, one may need to model both hydrody-namical and biological phenomena. There are still plenty of unsolved phenomena in a suspension of mi-cro-organisms. We expect that many researchers will be interested in this

28 research field and will clarify the natural beauty exhibited by micro-organisms in suspension in the near future