Welcoming Remarks

Electrochemical anodization of titanium (Ti) in a static, bulk condition has been used widely to fabricate self-organized TiO₂ nanotube arrays. Such bulk approaches, however, require extended anodization time to obtain long TiO₂ nanotubes and produce only vertically-aligned nanotubes. To date, it remains challenging to develop effective strategies to grow long TiO₂ nanotubes in a short period of time and control the nanotube orientation. Here, we show that the anodic growth of TiO₂ nanotubes is significantly enhanced (~16-20 times faster) under flow conditions in microfluidics. Flow not only controls the diameter, length, and crystal orientations of TiO₂ nanotubes but also regulates the spatial distribution of nanotubes inside microfluidic devices. Strikingly, when a Ti thin-film is deposited on silicon substrates and anodized in microfluidics, both vertically- and horizontally-aligned (relative to the bottom substrate) TiO₂ nanotubes can be produced. Our results demonstrate previously unidentified roles of flow in the regulation of growth of TiO₂ nanotubes and provide powerful approaches to effectively grow long oriented TiO₂ nanotubes and construct hierarchical TiO₂ nanotube arrays on silicon-based materials.