Writing on Superhydrophobic Nanopost Arrays: Topographic Design for Bottom-up Assembly
Original Entry by Ryan Truby
AP 225 - Introduction to Soft Matter
November 10, 2012
Authors: B. D. Hatton, J. Aizenberg
Citation: B. D. Hatton and J. Aizenberg. Writing on Superhydrophobic Nanopost Arrays: Topographic Design for Bottom-up Assembly. Nano Lett. 2012, 12, 4551-4557.
Related Course Keywords: wetting, superhydrophobicity
Background and Introduction
In the Aizenberg Biomineralization and Biomimetics Lab at Harvard, researchers have fabricated arrays of vertical, nanoscale silicon posts to create i) novel actuating surfaces that serve as sensor and stimuli-response surfaces, ii) freeze-resistant surfaces, as well as iii) patterned substrates for studying bacterial assembly and cell mechanics. The nanopost arrays are created via a Bosch process, a method of deep reactive-ion etching, in silicon wafers that yields vertical nanoposts of tunable densities, aspect ratios, and cross-sectional areas. The Aizenberg Lab has also found that the same nanopost arrays demonstrate intriguing wetting properties. When in contact with water, the nanopost arrays cause water to bead-up on the nanoposts' highest surfaces, such that no water enters the space between the nanoposts or comes in contact with any portion of the nanoposts' vertical surfaces.
The field of soft matter physics has a broad interest in the formation, stability, dynamics, and wettability of thin films of liquids. In fact, numerous studies reported over several decades have attempted to describe the enhanced hydrophobic characteristics observed on surfaces that specifically exhibit non-uniform topologies, such as rough and porous surfaces. Thus, understanding the superhydrophobicity of the nanopost arrays fabricated and studied in Professor Aizenberg's lab presents an interesting lesson in soft matter physics.
In the paper presented here, the authors demonstrate the superhydrophobic properties of silicon nanopost arrays and exploit them to assemble and pattern colloidal materials and polymers via a bottom-up, dynamic writing method. The nanopost arrays also allowed the authors to deposit precipitates on the surfaces of the nanoposts, a technique they coin as TIP, or topography-induced precipitation. Given below are a summary of this work, a physical explanation of the superhydrophobicity phenomenon observed with the nanopost arrays, as well as a discussion on the relevance of this work to the field of soft matter physics.
As mentioned previously, the authors found that arrays of silicon nanoposts fabricated on silicon wafers via a Bosch deep reactive ion etching process demonstrate superhydrophoic properties. In Figure 3, water is shown on a silicon wafer (upper left), a silicon wafer coated with polyvinyl alcohol (PVA, upper right), an array of silicon nanoposts (bottom left), and an array of silicon nanoposts whose tips have been coated with PVA (bottom right). The distinct increase in contact angle resulting from the controlled etching of the silicon wafer to create the nanoposts is apparent, as is a clear decrease in the contact angle of the water was observed for silicon nanopost/wafer surfaces functionalized with hydrophilic PVA.
Discussion and Relevance to Soft Matter
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 B. D. Hatton and J. Aizenberg. Writing on Superhydrophobic Nanopost Arrays: Topographic Design for Bottom-up Assembly. Nano Lett. 2012, 12, 4551-4557.