Nanofluid bioconvection in water-based suspensions containing nanoparticles and oxytactic microorganisms: oscillatory instability

TLDR

Adding motile microorganisms to a nanofluid enhances mass transfer, microscale mixing, and stability, and the resulting bioconvection can become oscillatory through the interaction of oxytactic cells, thermal gradients, and nanoparticle density stratification. This study proposes a new nanofluid comprising nanoparticles and oxytactic microorganisms and investigates its stability in a shallow horizontal layer. The authors model the system with conservation equations for mass, momentum, heat, nanoparticles, microorganisms, and oxygen, incorporate slip mechanisms such as Brownian motion and thermophoresis, and solve the eigenvalue problem analytically using the Galerkin method. The analytical solution reveals key physical insights and delineates the conditions under which the system exhibits oscillatory instability.

Abstract

The aim of this article is to propose a novel type of a nanofluid that contains both nanoparticles and motile (oxytactic) microorganisms. The benefits of adding motile microorganisms to the suspension include enhanced mass transfer, microscale mixing, and anticipated improved stability of the nanofluid. In order to understand the behavior of such a suspension at the fundamental level, this article investigates its stability when it occupies a shallow horizontal layer. The oscillatory mode of nanofluid bioconvection may be induced by the interaction of three competing agencies: oxytactic microorganisms, heating or cooling from the bottom, and top or bottom-heavy nanoparticle distribution. The model includes equations expressing conservation of total mass, momentum, thermal energy, nanoparticles, microorganisms, and oxygen. Physical mechanisms responsible for the slip velocity between the nanoparticles and the base fluid, such as Brownian motion and thermophoresis, are accounted for in the model. An approximate analytical solution of the eigenvalue problem is obtained using the Galerkin method. The obtained solution provides important physical insights into the behavior of this system; it also explains when the oscillatory mode of instability is possible in such system.