Abstract
The leaf of the lotus plant provides an elegant example of how a natural surface can remain clean even in the dirtiest of environments. The leaf’s fine-scale surface structure combined with its hydrophobic chemistry ensures that water droplets bead off its surface, carrying away contaminant particles [1]. An alternative dirt-shedding surface, also found in nature, is based on hydrophilic surface chemistry. Water droplets that come into contact with hydrophilic surfaces spread, forming a thin film of water and lifting contaminant from the surface. There is significant motivation to pursue both of these types of behaviour for artificial systems because of the wide range of possible applications ranging from self-cleaning paints to dirt-resistant clothing. However, for some applications, pertinent to the system in question, the synthesis of a self-cleaning surface is indeed a difficult task to achieve, and one needs to first elucidate the precise properties required to generate a certain level of resistance to the adhering contaminant. Atomistic simulations provide a useful tool to gain such insight and add value to surfaces where stay-clean properties are highly desirable. Here, using force-field-based molecular mechanics and dynamics, we explore the properties of polymer surfaces in order to develop design aspects for self-cleaning industrial paint-coatings. A polyester surface model is constructed, on the basis of a realistic cured paint coating [2] while various carbon models are selected [3–6], to emulate commonly encountered atmospheric dirt particulates [7–10]. The purpose of this work is to gain a fundamental understanding of the nature of interactions between contaminant and polymer coating in various environments, including aqueous conditions [11], and explore nanoscale modifications of the coating that help reduce the strength of the adhering contaminant. The modifications are based on hydrophobic or alternatively hydrophilic surface treatments. Here we demonstrate that the chemistry, morphology, and stability of the surface play a vital role in resisting adhesion of contaminant particles. Nanoscale surface modification combined with fine-scale roughness reduces adhesion between coating and contaminant by up to 21% [12]. The newly formed surface functional groups comprise “heavy” atoms, which repel the carbon contaminant at a close proximity to the surface via van der Waals interactions. Meanwhile the atomic-scale surface roughness reduces the effective contact area between surface and contaminant [6], in accordance with the Cassie–Baxter construction [13]. However, our modelling suggests that flexible polymer surfaces undergo significant rearrangement [4, 14], even at ambient conditions, in agreement with ageing [15] and hydrophobic recovery studies [16]. A s a result of this, the physical and chemical properties of the coating that initially help shield the surface from the adhering contaminant are diminished with time. E in an aqueous environment, the mobility of the polymer chains plays a vital role in the long-term functionality of the surface [11]. We propose a surface cross-linking procedure, aimed at improving the hardness of the outer region of the coating as a preventative measure against ageing [17]. F or surface cross-linking of the polymer with isophorone di-isocyanate (IPDI) molecules, we observe a much improved stability of the coating’s outer surface, and consequently a ~47% weaker adhesion with our contaminant particle. The rigid outer surface prevents the polymer chains from wrapping around the contaminant, improving the coating’s dirt-resistance capabilities. We anticipate that our modelling studies will be a starting point for the fabrication of a polymer coating which exhibits permanent dirt-shielding qualities. The synthesis of such a coating will require careful control of the chemistry, atomic-scale roughness, and stability of the surface.