SiO2 is a crucial material for various applications ranging from daily life to industrial production and scientific research [1,2]. Due to its exceptional high-temperature resistance, low expansion, non-toxicity, and chemical stability, SiO2 finds extensive use in fields such as flame retarding [3], filler of coating [4], and adsorbents for water pollutants removal [5]. Despite its wide-ranging applications, raw SiO2 typically possesses an untreated surface with an abundance of highly active dangling bonds that can easily generate a significant number of silicon hydroxyl groups when exposed to water and oxygen in the air. This leads to the hydrophilicity of SiO2, which severely limits its application range [6]. One viable solution to this problem is to modify SiO2 with low surface energy compounds to change its wetting features from hydrophilicity to hydrophobicity or even superhydrophobicity [7,8].

Numerous enchanting phenomena exist in nature, and superhydrophobic surfaces are one of them, which are characterized by a water contact angle (CA) above 150° and a sliding angle (SA) beneath 10° [9,10]. These surfaces have captured the attention of scholars due to their exceptional qualities, including anti-adhesion [11], anti-corrosion [12], fluid drag reduction [13], and oil-water separation [14]. Recent studies have revealed that superhydrophobic surfaces require both low surface energy materials and micro/nano rough features [15,16]. Currently, two main approaches can be used to create artificial superhydrophobic surfaces. The first method involves creating micro/nano rough structures on hydrophobic surfaces [17,18], while the second entails directly modifying hydrophilic micro/nano structures with hydrophobic materials [19,20]. So far, numerous studies have been conducted to fabricate superhydrophobic SiO2 surfaces by utilizing fluorine-containing compounds as low surface energy modifiers. For example, Wu et al. adopted a one-step foaming process to create a superhydrophobic polyurethane foam with exceptional oil-water separation properties by adding fluorinated SiO2 into a foam matrix derived from lignin [21]. Cao et al. developed a superhydrophobic SiO2 coating by consecutive deposition of SiO2 and fluorosilane on the candle ash pre-deposited stainless steel. The resulting coating exhibited a remarkable 90 % reduction in ice adhesion strength compared to bare surfaces [22]. Li et al. successfully fabricated durable superhydrophobic cotton fabrics with self-healing performances by three alternating sprays of perfluorooctylethyltrimethoxysilane (FAS) modified SiO2 and FAS/fluorinated copolymers [23]. Although superhydrophobic SiO2 surfaces have been successfully fabricated, the fluorine-containing compounds employed in these studies are easily accumulated in living organisms and pose a serious health hazard to humans [24]. Therefore, it is worthwhile and appealing to fabricate fluorine-free superhydrophobic SiO2 surfaces. At the same time, most of the previously reported superhydrophobic SiO2 surfaces only exhibit high repellency to room temperature water droplets, a few reported superhydrophobic SiO2 surfaces with high stability to hot water are also prepared by fluorine-containing processes [25], which greatly restricts the application of superhydrophobic SiO2 surfaces. In light of the above, there is an urgent need to propose a new method for preparing a fluorine-free and hot water-resistant SiO2 superhydrophobic surface.

With the rapid advancement in computer science and technology in recent years, molecular dynamics simulation (MD) has been widely used in the research of solid/liquid interface interactions [[26], [27], [28]]. Compared to traditional macroscopic experiments, molecular dynamics research methods can provide more information at the molecular level, allowing for an in-depth understanding of SiO2 surface modification and the associated water-wetting mechanism. In previous reports, Aleksandr et al. [29] gradually saturated the hydroxylated quartz surface with pentyl in the simulation. The results suggested that the CA on the quartz surface varied from 10° to 180° as the grafting density of the pentyl group rose from 0.29 to 3.18 C5H11/nm2. From these results, the authors inferred that the concentration of surface alkyl chains may significantly affect the wettability performance. Cui et al. [30] investigated the interfacial characteristics of a modified surface from the perspective of hydrogen bonding and surface interaction energy. The findings revealed that the interaction energy at the interface after grafting was significantly lower compared to that of the unmodified interface, leading to a reduction in interfacial hydrogen bonds. Wu et al. examined the impact of temperature on the wetting behavior of water droplets on solid surfaces and discovered that higher temperatures notably increased the wettability and diffusion rate of water droplets [31]. However, there remains a paucity of studies that combine experimental investigations and molecular dynamics simulations on hot water repulsive superhydrophobic SiO2 surfaces.

Based on the above considerations, in this work, a fluorine-free and hot water-resistant superhydrophobic SiO2 surface was prepared by using a modifier of dodecyltrimethoxysilane (DTMS). By varying the mass ratio of DTMS to SiO2, the water repellency of SiO2 can be adjusted, which was further investigated by MD simulations. Additionally, the variation of the wettability of pristine and modified SiO2 with water droplet temperature was also explored, and the results of the simulation are consistent with the experimental data. The discoveries of this study are anticipated to provide microscopic insight into how the hydrophobicity of silica surfaces is controlled.