ABSTRACT:The capillary rise of liquid on a surface, or "wicking", has potential applications in biological and industrial processes such as drug delivery, oil recovery, and integrated circuit chip cooling. This paper presents a theoretical study on the dynamics of wicking on silicon nanopillars based on a balance between the driving capillary forces and viscous dissipation forces. Our model predicts that the invasion of the liquid front follows a diffusion process and strongly depends on the structural geometry. The model is validated against experimental observations of wicking in silicon nanopillars with different heights synthesized by interference lithography and metalassisted chemical etching techniques. Excellent agreement between theoretical and experimental results, from both our samples and data published in the literature, was achieved.

No abstract

The dynamics of droplet spreading on two-dimensional wicking surfaces were studied using square arrays of Si nanopillars. It was observed that the wicking film always precedes the droplet edge during the spreading process causing the droplet to effectively spread on a Cassie-Baxter surface composed of solid and liquid phases. Unlike the continual spreading of the wicking film, however, the droplet will eventually reach a shape where further spreading becomes energetically unfavorable. In addition, we found that the displacement-time relationship for droplet spreading follows a power law that is different from that of the wicking film. A quantitative model was put forth to derive this displacement-time relationship and predict the contact angle at which the droplet will stop spreading. The predictions of our model were validated with experimental data and results published in the literature.

Articles you may be interested inPotassium hydroxide polishing of nanoscale deep reactive-ion etched ultrahigh aspect ratio gratings J. Vac. Sci. Technol. B 31, 06FF02 (2013); 10.1116/1.4820901 Long nanoscale gaps on III-V substrates by electron beam lithography J. Vac. Sci. Technol. B 30, 06F305 (2012); 10.1116/1.4766881 Formation of large-area GaN nanostructures with controlled geometry and morphology using top-down fabrication scheme J. Vac. Sci. Technol. B 30, 052202 (2012); 10.1116/1.4739424Ultradense gold nanostructures fabricated using hydrogen silsesquioxane resist and applications for surfaceenhanced Raman spectroscopy

When a liquid droplet is deposited onto an array of nanostructures, a situation may arise in which the liquid wicks into the space between the nanostructures surrounding the droplet, forming a thin film that advances ahead of the droplet edge. This causes the droplet to effectively spread on a flat, composite surface that is made up of the top of the nanostructures and the wicking film. In this study, we examined the effects of structural and chemical anisotropy of the nanostructures on the dynamics of droplet spreading on such two-dimensional (2D) wicking surfaces. Our results show that there are two distinct regimes to the process, with the first regime characterized by strong anisotropy in the droplet spreading, following the asymmetric structural or chemical cues provided by the nanostructures. The trend reverses in the second regime, however, as the droplet adopts an increasingly isotropic shape with which it eventually comes to rest. Based on these findings, we formulated a quantitative model that accurately describes the behaviour of droplet spreading on 2D wicking surfaces over a wide range of conditions.