A hybrid immersion cooling assisted by liquid-liquid encapsulation: A game changer for thermal management of transient hotspots.

HYPOTHESIS: Transient hotspots increasingly limit next-generation electronic and energy systems, where localized heat bursts can outpace conventional immersion cooling. Single-phase immersion cooling lacks sufficient localized cooling capacity, whereas two-phase immersion cooling is difficult to activate precisely at a hotspot. We hypothesize that adding a low-boiling-point activator drop to a single-phase water immersion system creates a hybrid strategy that couples localized phase-change activation with bulk-liquid heat dissipation, enabling rapid, spatially targeted hotspot suppression. EXPERIMENTS: The cooling performance was evaluated by synchronized infrared thermography and high-speed imaging and benchmarked against conventional immersion strategies across varying heat flux, immersion depth, and activator-drop size. A constrained gravity-driven liquid-liquid encapsulation route was developed to fabricate encapsulated activator drops in situ and regulate their boiling behavior. FINDINGS: Selectively activating boiling at the hotspot suppressed transient temperature excursions by pairing rapid phase-change absorption with efficient convection in the surrounding water; at 14.4 W/cm2, the hotspot center cooled by ∼17 °C within 15 s. Gravity-driven encapsulation further enabled programmable boiling through interfacial engineering, broadening the strategy's flexibility across diverse hotspot scenarios. More broadly, this work establishes programmable liquid-liquid interfaces as a versatile route to controlling localized phase change in advanced thermal management.