Stability and bioconvection dynamics of water-based AA7075 nanofluid with gyrotactic microorganisms: flow separation

Purpose Despite numerous studies on nanofluid flows where multiple solutions are identified, limited research addresses multiple solutions for highly nonlinear coupled equations. The purpose of this study is to conduct a numerical analysis for the first time to examine the effects of gyrotactic microorganisms on natural convective flows of AA7075 water nanofluid toward an exponentially shrinking sheet. Design/methodology/approach The flow governing equations are converted into highly coupled nonlinear ordinary differential equations using suitable transformations and are solved using the bvp4c routine that implements the three-stage Lobatto-IIIa formula. Stability analysis has been performed to examine the consistent solution. Findings The study observes that multiple solutions arise only in opposing buoyancy cases. Notably, flow separation is delayed, which benefits applications requiring smooth flow. Solution bifurcation has been recognized as saddle-node bifurcation. A decrease in the Eckert number leads to a 16.83% and 23.69% enhancement in heat transfer under shrinking and suction, respectively. The Sherwood number rises by 39.69% with an increasing Schmidt number for the first solution. Bioconvection parameters boost microbial density, with a 38.46% increase observed when the bioconvection Schmidt and Peclet numbers rise for a constant bioconvection Rayleigh number. The study reveals that stable flow patterns can be achieved while significantly enhancing heat and mass transfer performance, demonstrating the potential of bioconvective AA7075-water nanofluid systems for advanced microfluidic, biomedical and thermal management applications. Originality/value This study explores the combined effects of gyrotactic microorganisms, AA7075 nanoparticle volume fraction, viscous dissipation and buoyancy forces in a highly coupled system. The findings enhance understanding of bioconvective nanofluid flow over shrinking surface, with key implications for thermal management, biotechnology and biological fields, including cell separation, enzyme biosensors and culture purification.