Salt Crystallization and Lift-Off during Brine Droplet Evaporation on Low Contact Angle Hysteresis Surface

Evaporating brine droplets exhibit complex internal dynamics due to evolving thermal and solutal gradients. We present the first study of brine droplet evaporation on quasi-liquid surfaces (QLS), with ultra-low contact angle hysteresis (~1°), to passively manage mineral scaling. Sessile droplets (15–25 wt% NaCl) were evaporated on QLS under controlled substrate temperatures (30–70 °C) and relative humidity (10–70%). At T ≥ 40 °C and RH ≥ 35%, we observe spontaneous salt crystal lift-off at the end of evaporation. This behavior arises from the interplay of Marangoni stresses and buoyancy-driven (Rayleigh) convection. Thermal and solutal gradients drive surface-tension-driven flow toward the apex and density-driven flow in the opposite direction, establishing internal circulation. Our experimental results suggest Marangoni effects dominate, promoting central salt accumulation and leg-like crystal growth that lifts the crystal upward. However, even trace contamination or steep solute gradients can reverse flow direction, deviating from theoretical predictions. This sensitivity makes brine evaporation a complex, coupled transport problem requiring deeper analysis. Our study aims to identify the dominant convection mechanism and optimal conditions for passive scaling control.