The evaporation of saltwater droplets is a fundamental physical and chemical phenomenon that has key applications in separation technology, seawater desalination, and crystal engineering. During the evaporation process, complex flows are formed inside the droplets, which determine the final distribution and crystalline morphology of the dissolved substances. The flow inside droplets is mainly controlled by two key mechanisms: Rayleigh convection driven by density differences and Marangoni effects driven by surface tension gradients. However, there is still a lack of systematic quantitative research on the interaction and dominant status transition between these two mechanisms under different interface thermal conditions. Therefore, a deep understanding of these intrinsic flow mechanisms is crucial for achieving precise control over crystalline deposition morphology.

Recently,Professor He Gaohong and Professor Jiang Xiaobin from the State Key Laboratory of Fine Chemical Engineering and the Center for Advanced Science in Intelligent Materials of Dalian University of TechnologyIn the field of chemical engineering, international journals《Chemical Engineering Science》Published an article titled“Flow transitions in evaporating saline droplets: interplay between Rayleigh convection and Marangoni effects”The research paper.

This study combines systematic experiments with computational fluid dynamics (CFD) simulations to investigate the internal circulation mechanism during the evaporation of saltwater droplets under different geometric configurations and thermal conditions. The study revealed different cyclic patterns controlled by the interaction between Rayleigh convection and Marangoni effect.Among them, the research team utilized Mofang Precision Surface Projection Micro Stereoscopic Lithography (P μ SL) technology (nanoArch) ® S140 Pro， Precision: 10 μ m) 3D printing system is used to prepare droplet micro platforms.

Research has found that at room temperature, the interior of droplets is mainly driven by Rayleigh convection, and the size of their contact angle determines the final deposition morphology: edge deposition is formed when it is less than 90 °, while central deposition is formed when it is greater than 90 °. By precisely regulating the temperature, a fundamental transformation of the internal flow pattern has been achieved through research. A key finding is that when the temperature difference exceeds a specific critical threshold, the thermal Marangoni effect overwhelms Rayleigh convection, leading to a reversal of the flow pattern. In addition, the heat transfer coefficient at the solid-liquid interface has been confirmed to be a key parameter for regulating the dominant flow mechanism. When its value is below 10 W/m2 · K, the flow is dominated by Rayleigh convection; In the range of 100-1000 W/m2 · K, the Thermalango effect dominates. Ultimately, this work constructed a comprehensive phase diagram based on dimensionless Rayleigh number (Ra) and Marangoni number (Ma), providing a powerful quantitative tool for predicting and controlling flow state transitions under different evaporation conditions, thus laying a theoretical foundation for precise control of crystal deposition morphology through interface thermal regulation.

To ensure the accuracy of the simulation, the research team built a synchronous characterization system that includes optical and infrared thermal imaging (Figure 1), and used it to validate the CFD model. As shown in Figure 2, the simulation results are highly consistent with experimental data in key parameters such as droplet geometry evolution, interface temperature distribution, and internal flow field. For example, the relative difference between the interface temperature distribution of simulation and experiment is only 0.05114%, which fully confirms the reliability of the numerical model.

Research has shown that the contact angle of droplets is a key factor determining the flow mode and final deposition morphology (Figure 3). When the contact angle is less than 90 °, the maximum evaporation rate occurs at the three-phase contact line, which drives clockwise flow dominated by Rayleigh convection and solute Marangoni effect, ultimately forming a "coffee ring" shaped deposition. When the contact angle is greater than 90 °, the evaporation rate shifts to the droplet vertex, causing a dual circulation flow of clockwise in the upper part and counterclockwise in the lower part. This unique flow field structure leads to crystal aggregation in the central region.

The contact radius of droplets also affects the crystallization process (Figure 4). Research has found that at a contact angle of 120 °, as the contact radius increases, the internal concentration gradient driven by buoyancy effect is strengthened. The longer circulation path provides sufficient solute absorption time for the crystal nucleus, which helps to form larger crystals. This phenomenon has been confirmed in both simulated and experimental scanning electron microscopy images.

By adjusting the thermal boundary conditions, internal flow can be actively controlled (Figure 5). Research has found that the interfacial heat transfer coefficient (h) is the key factor determining the flow mode, and a low heat transfer coefficient (h<10 W/m2 · K) is beneficial for maintaining clockwise flow dominated by Rayleigh convection. When the heat transfer coefficient increases to the range of 100-1000 W/m2 · K, the temperature difference caused by substrate heating is sufficient to overpower other effects, reversing the flow mode to counterclockwise circulation.

Under heating conditions, the internal flow undergoes a dynamic transition from Rayleigh dominated to Marangoni dominated (Figure 6). This study accurately quantified the critical temperature difference at which this transition occurs, which is 0.94 K for droplets with a contact angle of 120 °, while for droplets with a contact angle of 60 °, a higher temperature difference (1.34 K) is required to achieve reversal due to the stronger solute Marangoni effect. This conclusion provides quantitative guidance for precise control of internal flow through temperature.

In order to systematically predict the flow state under different conditions, the research team constructed a flow phase diagram based on dimensionless Rayleigh number (Ra) and Marangoni number (Ma) (Figure 7). These phase diagrams clearly divide the Rayleigh dominated (pink region), Marangoni dominated (blue region), and transitional flow regions under the combined action of the two. As shown in Figure 8, the transition boundaries between different flow states can be accurately described by power law relationships, providing a powerful tool for quantitatively controlling the internal flow of droplets.

This study systematically revealed the competitive and synergistic mechanisms between Rayleigh convection and Marangoni effect in evaporated saltwater droplets, and found that droplet geometry and thermal boundary conditions have a decisive impact on internal circulation and final crystallization mode.The study has clarified the correspondence between flow patterns and sedimentary morphology at different contact angles: when the angle is less than 90 °, a "coffee ring" shaped sediment is formed, while when it is greater than 90 °, a centralized sediment is formed. In addition, the study quantified the control effect of thermal conditions on flow patterns and determined the critical temperature difference and interface heat transfer coefficient range for achieving the dominant position of the thermal Marangoni effect. The final Ra Ma phase diagram provides a reliable tool for predicting cyclic modes under different thermal and geometric conditions. These achievements provide a theoretical basis for optimizing the evaporation crystallization process through precise control of interface conditions, and have potential applications in fields such as protein crystallization, chemical microreactors, and biological analysis platforms.