The interaction of bioconvection caused by gyrotactic micro-organisms and settling of small solid particles

Purpose - To investigate numerically the settling of small solid particles in a suspension of motile gyrotactic micro-organisms in order to evaluate the possibility of using bioconvection to slow down settling and enhance mixing between particles. Design/methodology/approach - Numerical computations are performed at the North Carolina Supercomputing Center utilizing an Origin 2400 workstation. A conservative finite-difference scheme is used to discretize the governing equations. A staggered uniform grid with the stream function and vorticity stored in one set of nodes and the number densities of micro-organisms and solid particles stored in another set of nodes is utilized. CPU time required to investigate plume development until it attains steady-state for 36 x 36 uniform mesh is about 50 h. Findings - It is established that small solid particles that are heavier than water slow down bioconvection. Extremely small particles (nanoparticles) that have negligible settling velocity do not have any noticeable impact on bioconvection, very large particles (that have negligible diffusivity), or very heavy particles (that have very large settling velocity) also do not have any impact on bioconvection because they simply settle at the bottom. However, if the particles are of the optimal size and density (gravitational settling must compete with Brownian diffusion to create an exponential number density distribution of solid particles with the maximum at the bottom of the chamber), these particles can effectively slow down bioconvection. Research limitations/implications - The question how solid particles may affect the wavelengths of bioconvection patterns requires further investigation. Practical implications - The finding that solid particles slow down bioconvection may be important in using bioconvection to enhance mixing in fluid microvolumes. Originality/value - The paper provides a model and numerical data about the effect of bioconvection on mixing of small solid particles. These data are valuable for researches working in fundamental fluid mechanics, multiphase flow, and applications of bioconvection.