Writing on Superhydrophobic Nanopost Arrays: Topographic Design for Bottom-up Assembly

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Fig. 1, reproduced from [1]

Original Entry by Ryan Truby

AP 225 - Introduction to Soft Matter

November 10, 2012

Reference Information

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

Fig. 2, reproduced from [1]

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.

Summary

Fig. 3, reproduced from Figure 4B of [1]

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. When in contact with the silicon nanopost array, no water wets any portion of the nanoposts' sides; water only makes contact with the surface tips of the nanoposts. Considering gravitational, capillary, and dispersion forces that would seem to encourage films of water to assemble between neighboring nanoposts, the reader may find it surprising that it is energetically favorable for the water to interact with the nanoposts in this manner. This wetting behavior was first described for porous services by Cassie and Baxter in 1944.

The authors demonstrated their topography-induced precipitation method with the nanopost arrays by precipitating amorphous nanocrystals of calcium carbonate specifically on the nanoposts' tips (see Figure 2). To accomplish this, 40 µL of calcium chloride solution was dispensed on an array of nanoposts and desiccated in the presence of 10 g of ammonium carbonate. In the desiccator, the ammonium carbonate decomposed into ammonia, carbon dioxide, and water that then diffused into the drop of calcium chloride solution. Precipitation of calcium carbonate began at the silicon-water interfaces, resulting in the growth and assembly of calcium carbonate nanocrystals at the surfaces of the silicon nanoposts.

Aqueous solutions of polyvinyl alcohol (PVA) were added dispensed on the nanopost arrays as well, coating only the nanoposts' tips with layers of PVA. Solutions of poly(methyl methacrylate) nanospheres, polystyrene microspheres, and gold nanoparticles were also deposited on the nanopost arrays. Similar to the assembly of calcium carbonate nanocrystals on the nanopost tips, these colloidal micro-/nanoparticles were also deposited only on the tips of the nanoposts with which the solutions came in contact. The authors demonstrated that they could effectively "write" on the nanopost arrays with solutions of these colloidal materials, depositing colloids that then assembled on only the tips of the nanoposts on which the authors wrote.


Fig. 4, reproduced from Figure 5 of [1]
Fig. 5, reproduced from Figure 6 of [1]

Discussion and Relevance to Soft Matter

The superhydrophobic wetting properties observed for the nanopost arrays were explained by Cassie's law, which gives the contact angle on a heterogeneous, rough surface, <math>cos(\theta^*)</math>:

<math> cos(\theta^*) = r_f fcos(\theta_Y) + f - 1 </math>

where <math>r_f</math> is a roughness ratio of the wetted surface area (i.e., a ratio of the actual solid surface area to the apparent solid surface area resulting from surface heterogeneity), <math>f</math> is the fraction of surface area wetted by the liquid, and <math>\theta_Y</math> is the contact angle observed for the same liquid on a smooth surface of the same material as the rough surface (in this paper, <math>(\theta_Y)</math> is the contact angle of the liquid on a silicon wafer). This equation can also be expressed as

<math> \sigma_{lv} cos(\theta^*) = f(\sigma_{sv} - \sigma_{sl}) + (1-f)\sigma_{lv} </math>

Liquid thin films, such as water on nanopost arrays, that obey Cassie's equation are noted as being in a Cassie state.

Moreover, the energy of a thin liquid film with thickness <math>e</math>, <math>P(e)</math>, is given by

<math>P(e) = \sigma_{film} - \sigma_{lv} - \sigma_{sl}</math>

and, given the equations above, the energy of a thin film above the nanopost array could be approximated as

<math>P(e) = \sigma_{sv} - \sigma_{film} -[\sigma_{lv}(cos\theta^* -1)]/f</math>,

which shows that the energy of a thin liquid film of thickness <math>e</math> in a Cassie state increases in energy when <math>f</math> decreases, where such a decrease indicates a qualitative increase in surface roughness or a change in the nanopost array, such as a decrease in nanopost density. This is expected: a sufficiently low nanopost density will result in the thin film of water to find it energetically favorable to leave its high-energy Cassie state and wet the sides of the nanoposts and/or the bottom of the silicon wafer supporting the nanoposts.

The Cassie equation does not describe the energies associated with the formation of thin films (or a lack thereof) between the nanoposts. This suggests that, from an thin film stability and energetics perspective, that the Cassie equation does not provide a complete description of the superhydrophobic behavior of rough surfaces such as the nanopost arrays. As the paper discussed demonstrated with their novel approach for the bottom-up assembly of colloidal materials on nanopost arrays, exploiting superhydrophobic interactions of liquid thin films on surfaces offers a route to developing novel self-assembly and colloidal patterning techniques that could prove useful to the nanoelectronics, biological sensing, and semiconductor communities. Therefore, a greater understanding of superhydrophobicity would prove useful to the fields of materials science and soft matter physics.

References

[1] B. D. Hatton and J. Aizenberg. Writing on Superhydrophobic Nanopost Arrays: Topographic Design for Bottom-up Assembly. Nano Lett. 2012, 12, 4551-4557.