Over the past decade, graphene has emerged as a promising candidate for exploring the viscous nature of electronic flow facilitated by the availability of extremely high-quality devices employing a graphene channel encapsulated within dielectric layers of hexagonal boron nitride (hBN). However, the level of disorder in such systems is mainly determined by the device fabrication protocols, making it impossible to obtain a tunability between the impurity-dominated and viscous transport within the same device. In this work, using a combination of ultraviolet (UV) radiation and gate electric field, we have demonstrated a dynamic modulation of charge hydrodynamics quantified in the thermal and electrical transport by the extent of departure from the Wiedemann-Franz (WF) law in monolayer graphene devices at room temperature. We achieved this by tuning the disorder level continuously and reversibly using UV light to create transient trap states in the encapsulating hBN dielectric. With progressive UV radiation, we observed a dramatic increase in the momentum-relaxing scattering relative to that between the electrons and to the Lorentz number by nearly a factor of ten, with increasing disorder, thereby approaching the restoration of the WF law in highly disordered graphene. Our experiments outline a potent strategy to tune the fundamental mechanism of charge flow in state-of-the-art graphene devices.

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• Electron thermal conductivity
• Light-matter interaction
• Optoelectronics
• Graphene
• Hexagonal boron nitride
• Quantum fluids & solids
• Drude model
• Noise measurements
• Relativistic hydrodynamics