Balancing bubble dynamics and liquid replenishment in a two-phase closed thermosyphon via geometrically tuned Bi-philic evaporator surfaces

Abstract

Bi-philic surfaces are known to enhance pool boiling, yet their application in liquid-supply-limited systems like two-phase closed thermosyphons (TPCTs) remains underexplored. This study experimentally investigates the thermal performance and flow dynamics of a rectangular TPCT featuring vertically aligned bi-philic evaporator stripes. Using a masking technique with alkali etching and nanoparticle dip-coating, four configurations with varying hydrophilic/hydrophobic stripe widths (2/2, 2/4, 4/2, and 4/4 mm) were fabricated. High-speed visualization revealed that bubble dynamics are governed by the interplay between contact-line pinning on hydrophobic stripes and capillary-driven liquid replenishment along hydrophilic channels. The 4/2 pattern (4 mm hydrophilic / 2 mm hydrophobic) achieved optimal performance, reducing overall thermal resistance by 30% compared to a bare copper surface. Visualization results demonstrated that limiting the hydrophobic width to 2 mm induced contact-line pinning, constraining bubble growth (similar to 2.3 mm), while wider 4 mm hydrophilic stripes minimized hydraulic resistance for liquid return, preventing local dry-out. This geometric optimization shifted the system's thermal bottleneck to the condenser. These findings establish critical design criteria for bi-philic TPCTs: hydrophilic widths must be sufficient to sustain capillary supply, while hydrophobic widths must be constrained to regulate bubble growth under liquid-limited conditions.