Mechanism of interfacial nanolayer thickness on couple-stress penta-hybrid nanofluids over a stretching sheet for solar thermal applications

Abstract

The impact of interfacial nanolayer thickness is critical in modeling heat transfer in nanofluids, especially when nanoparticle sizes are below 10 nm. These nanolayers, which naturally form around nanoparticles, significantly enhance thermal conductivity and heat flux, making them essential in applications like biomedical cooling, microfluidics, and drug delivery. This study examines the unsteady boundary-layer flow of a Boussinesq couple-stress fluid through a Darcy–Forchheimer porous medium, utilizing a penta-hybrid nanofluid while explicitly accounting for nanolayer effects. Modified Maxwell-type models are used to account for nanolayer-to-radius ratios, thereby improving the estimation of effective thermophysical properties for all five nanoparticles. The model includes the effects of thermal radiation, Joule heating, viscous dissipation, velocity slip, and magnetic fields. Similarity transformations reduce the governing equations to ordinary differential form, solved numerically via the shooting method with a fourth-order Runge–Kutta scheme. The framework is applicable to solar thermal collectors with flexible absorbers exposed to radiative and electromagnetic loading. Results show that velocity decreases with increasing Casson number, couple stress, porosity, and suction, whereas stretching enhances flow. Temperature increases with radiation and Biot number but decreases with stronger heat generation. Penta-hybrid nanofluid consistently outperforms simpler nanofluids in both skin friction and heat transfer efficiency, validating its thermal superiority.