Natural Nano Space Network showing Molecular Gas Flow

The rice husk has a three layer structure - two cellulose layers and an amorphous silica layer. It was reported that the gas flux (H₂, CO₂, CH₄) through the silica layer showed a molecular flow (or Knudsen flow) while the BET measurement indicated an existence of 4-5 nm pores. The silica layer seems to act as a protection layer against virus and it has been strongly suggested there are holes throughout in the layer. In this study, the structure of the holes or spaces in silica layer was proposed based on the TEM images and the characteristics of the gas flow through the system was explained.

The inner structure of the silica layer was investigated by a 200 KeV TEM. To observe the structure of the layer, the rice husks were embedded in Epofix resin and cured at 25°C for 12 hrs. The microtomed sections (approximately 100–150 nm) were cut from the embedded specimen by an ultramicrotome using a diamond knife at room temperature and then collected onto 200-mesh fomba copper grids. The specimen was milled with ions.

TEM images of the silica layer showed the connections of (void) spaces. Even though the images changed with the time of electron irradiation and focusing depth, they could be analyzed as a space network having space edges and space nodes. The mean length of the edges was 3-5 nm and the number of the edges at a node was about 3-4. From the observed TEM images, we can assume the space network was spread to the silica layer.

If a gas molecule travels a space edge of length l, the time passing through a space edge is l/v + l·τ_e/λ, where λ, v, and τ_e are the mean free path of the molecule, the speed of molecule, and the interaction time of the molcule on the surface of space during collision, respectively. Similarily, the time retained in a space node of a molecule becomes  (4π/Ω_open)(d/v+τ_n) where Ω_open, d, and τ_n are the solid angle of opening of the node, diameter of the node, and the delay of the colliding molecule on the surface of the node, respectively. Then the overall time to pass a space network with N space node and edge pairs will be roughly N{(1+4πd/Ω_open)/v+l·τ_e/λ+4π·τ_n/Ω_open}. Since the mean free path of gas molecule is larger than the length of the space edge under the experimental condition and the interaction of the molecules with silica surface is negligible (d/v>τ_n), the gas flux shows a molecular flow.

From the observation, it is suggested that a well designed space network in nano scale can be used for the separation of gas mixture and for the control of gas molecule movement in a tiny system.