Enhanced ice melting by strong Marangoni convection

• Enhanced ice melting mode by pushing ice cap into droplet’s interior is reported. • Strong Marangoni convection occurs when convective intensity exceeds 4056. • Droplet volumes over 10 µL advance the negative effect of natural convection. • Composite closed/open cells structured surface reduced melting duration by 21.3%. • Enhanced strong Marangoni convection can flip the ice cap into the droplet bottom. Conventional understanding holds that during the ice droplet melting process, the density difference between ice and water causes ice to float atop water until melting ends. Herein, we report a second melting mode driven by strong Marangoni convection (SMC) in which the ice cap is pushed down into the interior of the droplet, significantly accelerating the melting process. To reveal the mechanism of this second melting mode, we systematically explored the effects of three critical factors: heating rate, droplet volume, and contact angle. It was found that higher heating rates and larger contact angles favor the activation of SMC, which requires the convective intensity to reach a critical value of 4056. However, droplet volumes over 10 μl hinder its onset owing to dominant natural convection effects. A Rayleigh number equal to unity is a pivotal threshold that signals the transition in convective intensity and the non-negligible influence of natural convection. Structured surfaces demonstrate potential in enhancing SMC with closed- and open-cell structures that shortened the melting duration of 5-μl droplets by 21.3 % and increased the proportion of SMC by 143.1 %. Additionally, binary ethanol-water droplets triggered solute Marangoni convection during freezing, increasing the frozen droplet height and counteracting the SMC effect. In contrast, frozen saltwater droplets with increased surface tension can cause ice caps to flip and sink until the melting completes. This study enriches the understanding of hydrodynamic and thermal phenomena during phase changes and is expected to provide valuable insights for industrial applications.