Mechanism for sonocoating a polymer surface with nano-hydroxyapatite

Abstract

A mechanism is described for coating a polymer surface with nanoparticles (NPs) using high-power ultrasound acting on NPs suspended in liquid. The process is called “sonocoating”. Nano-hydroxyapatite (HAP) NPs are used for sonocoating in this study. In the sonocoating process, according to the known “throwing stones” mechanism, liquid microjets created when cavitation bubbles implode randomly throw NPs towards the surface. In our studies, the nano-HAP layer grows laterally, forming dendrite-like patterns. We propose that the liquid streams generated by cavitation bend close to the surface and move the HAP NPs parallel to the surface. The layer nucleates when the NPs gather at surface imperfections. Subsequently, the layer grows laterally by preferential attraction of HAP NPs to the layer edges. The layer structure depends on the size of the HAP NPs and the coating duration. NPs with a mean diameter of 15 nm form a homogeneous layer faster than those with a mean diameter of 45 nm. Sonocoating is an extraordinary method for producing nano-HAP layers, since the coating takes place in water at 30 °C and the process duration is only 8 min.