Cavitation dynamics in micro-scale hydrodynamic reactors: effect of green solvents on intensification of graphene exfoliation

Official URL: https://dx.doi.org/10.1016/j.cep.2026.110998

This study investigates the influence of solvent physical properties on cavitating flow dynamics in micro-scale hydrodynamic cavitation (HC) reactors using green solvent systems (de-ionized water (DIW)–isopropyl alcohol (IPA) and DIW–ethanol mixtures). The primary focus of this work is to resolve cavity dynamics in the presence of graphite and relate them to process intensification of graphene exfoliation. Experiments were conducted in two reactor configurations—a long diaphragm and a micro-step design—using four micro-scale HC reactors under varying solvent conditions. High-speed visualization revealed that alcohol-based mixtures generate vapor structures with reduced thickness and coherence compared to DIW, with IPA forming a confined and stable vapor core, while ethanol produces broader and more oscillatory cavities. At 3.45 MPa in Reactor 4, the IPA-based solution led to an approximately 30% lower mean void fraction than the EtOH-based solution, with dominant frequencies of 1.0–1.5 kHz for IPA and approximately 2.0 and 4.5 kHz for EtOH. Compared to long diaphragm design, the micro-step configuration enhances cavitation localization and shear-layer development. Raman and UV–Vis analyses show that IPA-based systems yield more uniform graphene flakes with lower relative layer thickness, whereas ethanol leads to higher edge-related disorder. The IPA-based solution achieves ∼10% higher exfoliation yield, particularly in the micro-step reactors. These results indicate an association of the solvent-dependent cavitation dynamics with the exfoliation performance, while the possible contributions of the localized collapse and shear-layer effects are inferred from the combined hydrodynamic and material-characterization results. The findings establish HC-on-a-chip as an energy-efficient, scalable, and surfactant-free route for intensified graphene production.