Capillary Filling of Anodized Alumina Nanopore Arrays
Original entry: Caspar Floryan, APPHY 225, Fall 2010
K. J. Alvine, O. G. Shpyrko, P. S. Pershan, K. Shin, and T. P. Russell, Capillary Filling of Anodized Alumina Nanopore Arrays,Phys. Rev. Lett. 97, 175503 (2006).
Nanopores have many applications in areas including DNA translocation, nanofluidic transistors, nanoparticle self-assembly, catalysis and chemical sensors. Most of these applications involve the migration of liquid into the pore, suggesting that an enhanced understanding of this migration process may have important benefits.
This paper describes nanopore wetting experiments using the solvent perfluoro-methyl-cyclohexane and observed with small angle x-ray scattering. The pores are patterned in anodized alumina (aluminum trioxide, Al2O3) and are approximately 20nm in diameter. The pores were fabricated in a 1cm thick alumina membrane with a hexagonal pattern and nearest-neighbour spacing of approximately 58nm. The experiments were carried out in an environmental chamber which allows precise control over the amount of solvent condensed inside the pores. Absorption into pores was controlled by varying the chemical potential between the sample and the liquid using a thermal offset. The pores are depicted in figure 1 below.
Adsorption and desorption into the nanopores were measured and expressed as a fraction of the pore filled. These results are shown below in figure 2 (labelled fig 3). As the temperature, and hence chemical potential, increase, the volume fraction decreases (adorption, filled circles in the figure) . When desorption happens, there is an observable hysteresis effect (open circles in the figure). The results agree closely with Cohan's theoretical model, though it does not explain the hysteresis. The Cole & Saam and Derjaguin & Kelvin methods agree with the experimental results within the experimental error.