Regulating Droplet Dynamic Wetting Behaviors Using Surfactant Additives on High-Temperature Surfaces

Surfactants are commonly used to lower the surface tension between two mediums by acting as additives, emulsifiers, wetting, foaming agents. In recent years, surfactants have been used to extensively change the wettability of solid surfaces in numerous agricultural and industrial processes. Different important parameters affecting the contact conditions and the impaction behavior between liquid droplets and solid surfaces are always key considerations in enhancing the mass and heat transfer rates. On surfaces with high temperatures, the direct contact between the droplets and solid surfaces is prevented by the formation of vapor pockets when the impacted droplets vaporize, a phenomenon known as the Leidenfrost point. Consequently, the Leidenfrost point can be regulated by modifying the surface and droplet properties. This includes the addition of flexible polymers such as poly(ethyleneoxide) that significantly reduced the Leidenfrost point of water droplets due to the large energy dispersion caused by the polymer molecules.

Previous research findings have reported that the dynamic behaviors of droplets on the heated surfaces are greatly influenced by surfactant additives. For instance, in applications such as spray cooling processes, the addition of surfactant sodium dodecyl sulfate (SDS) enhances the heat transfer efficiency in liquid droplets by promoting the vapor bubble nucleation and foaming. Nevertheless, the relationship between the droplet dynamic behavior and heat transfers as well as the underlying mechanisms have not been fully explored in the literature. To this note, Dr. Peipei Zhang, Dr. Baoxu Peng, Dr. Xiaoxiao Yang, Professor Jingming Wang, and Professor Lei Jiang from Tsinghua University investigated the dynamic behaviors of water droplets with surfactant additives on heated surfaces using different chemical compounds. Their research work is currently published in the journal, Advanced Materials Interfaces.

In their approach, three chemical compounds: cationic surfactant molecule hexadecylcetyltrimethylammonium bromide (CTAB), anionic surfactant molecule sodium dodecyl sulfate (SDS), and nonionic surfactant molecule poly(ethylene glycol) 1000 (PEG-1000) were used to regulate droplet properties such as density, viscosity, and surface tension. The effects of these surfactant additive compounds on the Leidenfrost point were analyzed and compared at low and high temperatures.

The authors reported that both ionic and nonionic surfactants could regulate the dynamic behaviors of water droplets on heated smooth surfaces with different compounds. At low temperatures, the surfactant additives enhanced the spreading of droplets on hydrophilic and hydrophobic surfaces. This was attributed to improved surfactant adsorption. Consequently, the bubble jet and bubble explosion processes on the hydrophilic surfaces at high temperatures resulted in improved heat transfer efficiency. Furthermore, unlike PEG-1000, both SDS and CTAB significantly reduced the Leidenfrost point on hydrophilic and hydrophobic surfaces due to the high effects of the concentration variation. Generally, the Leidenfrost point increases with a corresponding decrease in the ionic surfactant molecule concertation.

In summary, the study reported the regulation of droplet dynamic wetting behaviors using surfactant additives on heated surfaces. Results showed that both ionic and nonionic surfactants could regulate the dynamic behaviors of droplets of heated surfaces. Whereas ionic surfactants such as SDS and CTAB reduced the Leidenfrost point of both hydrophilic and hydrophobic droplets, it was increased by nonionic surfactants like PEG-1000. According to the authors and in a statement to Advances in Engineering, they said their study provided useful insights that would guide the selection of suitable operating temperature range for surfactants droplets on heated surfaces, thus promoting their application in different fields.