A dwarf – nanos in Greek – is a mythical creature of a short sturdy stature residing in arcane depths of mountains and mines beyond the fringes of the known world. Unlike its imaginary namesakes, the nano-technology goes even deeper: into the very structure of the matter. It focuses on manipulation of matter on molecular scale and development of technologies, the functional components of which range from about 1 to 100 nanometres (1 nm = 10-9 m). Subsequently, the nanotechnology branched into numerous fields of study including hydrodynamics. Whereas the classical hydrodynamics helps us to understand the behavior of fluids (i.e. gases and liquids) in structures with characteristic dimensions allowing to model fluids as a continuous mass discounting their molecular composition, the nano-hydrodynamics takes an opposite approach. Since the molecules of fluids and the flow domains are similarly sized in the range of nanometres, it is not possible to employ continuum models of flow. Thus, nanohydrodynamics is indeed other and in a more distant future will not only focus on the analysis and the prediction of flowing of molecules through nanostructures, but as speculated for now, it will create an environment for molecular assembly and thus facilitate the molecular manufacturing.
The classical hydrodynamics, which has been developed since the 17th century, studies the flow of fluids by means of internal and external forces in spaces of imaginable size. The classical hydrodynamics has been essential for solving of a broad spectrum of scientific and engineering tasks, e.g. forecasting weather, studying oceanic currents, streamlining of airplanes and cars, describing the flow of blood in human body or the flow of steam through blades of steam turbines. Nanohydrodynamics, the other hydrodynamics, focuses on the movement of fluids in nanoscale domains. The flow in such flow space is governed primarily by interactions between the molecules of the fluid with the surface of the domain. These mutual interactions are, e.g. of the paramount importance in development of nanopore membranes made of carbon nanotubes, which may be employed to a desalination of seawater or to a removal of CO2 from natural gas. It is of an interest that the permeation of fluids through such nanotube membranes is higher by several orders of magnitude than what the classical hydrodynamics would predict for equivalent arrangements.
The flow of gas molecules, the mean free path of which is about several tens of nanometres, or liquids, with the distance between molecules in the range of nanometres, through a flow structure composed of nanoscale channels or pores, can be modeled by a numerical integration of a system of kinetic equations for individual moving molecules. The experimentally determined flow rates through nanostructures are assessed by semi-empirical relations determined by means of a molecular dynamic simulation. In particular, various diffusion equations are used to describe the permeation of gases through nanofluidics and modified integrated flow dependencies, which take into account small distances of liquid molecules and their strong mutual interactions, are applied to assess the permeation of liquids.
Thanks to a continual development of nanotechnologies, novel procedures of manufacturing of permeable nanostructures and possibilities of their application emerge. One of those are nanofluidics. These are artificial flow structures composed of nanoscale channels and pores the surface of which can selectively react with a permeating fluid. E.g. electrically charged surfaces of nanofluidics are suitable for a selective separation of molecules of the fluid or for a transmission of the mechanical energy of the flowing molecules to electrical energy. Similarly, the laboratories-on-a-chip (called broadly the labs-on-a-chip) integrate several laboratory functions on a single integrated circuit commonly called a chip. Their chief advantages lie in the possibility to perform several different analyses with a sample of a very low volume, or to detect an extremely rare event in a large volume. The latter is indispensable e.g. for detection of circulating tumor cells, which are of a considerable importance for tumor diagnosis, staging and evaluation of therapy. Alternatively, specifically designed labs-on-chip are self-containing devices, which can be readily used on spot. Therefore, by obviating the need of transferring samples to a central laboratory, the analysis of samples can be sped up, which can be beneficial in emergency situation or when such a transport would be impractical or impossible due to e.g. missing infrastructure.
The manufacturing process of nanofluidics and labs-on-chip in not elementary and it requires exact techniques and special equipment. Such structures can be manufacture by a top-down approach, when the channels into glass, silica or various polymers are being created by photolithography, etching or casting. Recently, an ion beam is being used for creating of more complex nanofluidics. Vice versa, a bottom-up approach can assemble nanofluidics from primary components such as living cells or polymer nanoparticles printed by special printers. Nanofluidics can be made of carbon nanotubes as well. This is a particularly advantageous approach, since the carbon nanotubes can be differentially assembled and variously modified, which vastly increases the range of their possible practical applications. We at the Institute for Hydrodynamic together with the Centre for Polymer Systems of the Thomas Bata University in Zlín focused on this approach. The nanofluidics made of entangled network of carbon nanotubes have been prepared by vacuum filtration of a carbon nanotube suspension and further chemically or physically modified. Thus we have been able to alter e.g. the polarity, porosity, electrical conductivity or channel sizes of such carbon nanotube nanofluidics by means of, e.g. nanotube oxidation, addition of silver nanoparticles, carbon fibers or carbon black.
Research at the Institute of Hydrodynamics
In collaboration of four institutes from three countries we have determined that various chemical vapors specifically alter the electrical conductivity of the forest of nanowalls (Fig. 1). Thus, the arranged nanowalls may serve as sensors for a detection of volatile organic compounds .
In collaboration of two research institutes we have established that an application of an electrical voltage specifically modulates the permeability of the entangled network of carbon nanotubes towards various chemical vapors of different polarities (Fig. 2). This allows separating gasses by regulating their respective diffusion rates through a carbon nanotube membrane .
In collaboration of three research institutes from two countries we have assessed conditions of the longitudinal flow of epoxide monomer through a layer of entangled carbon nanotubes. The layer of nanotubes was a part of a layered composite and after polymerization served as a fully integrated sensor to measure composite deformation continuously .
Figure 1: Forest of nanowalls from top (A) and from side (B). Nanowalls and scanning electron microscope micrographs were prepared by collaborators at the Nagoya University.
Figure 2. View of membrane (A), micrograph of the surface of a membrane from entangled carbon nanotubes (B) and a micrograph of cross-section of a membrane of thickness 50 µm (C). Membranes and scanning electron micrographs were prepared by the collaborators at the Centre of Polymeric Systems at the Tomas Bata University, Zlín.
Slobodian, P., Říha, P., Olejník, R., (2018). Electrically-Controlled Permeation of Vapors Through Carbon Nanotube Network-Based Membranes. IEEE Transactions on Nanotechnology, 17, 332-337.
Slobodian, P., Lloret Pertegás, S., Říha, P., Matyáš, J., Olejník, R., Schledjewski, R., Kovář, M., (2018). Glass fiber/epoxy composites with integrated layer of carbon nanotubes for deformation detection. Composites Science and Technology. 156, 61-69.
Slobodian, P., Cvelbar, U., Říha, P., Olejník, R., Matyáš J., Filipič, G., Watanabe, H., Tajima, S., Kondo, H., Sekine, M., Hori, M., (2015). High sensitivity of carbon nanowalls based sensor for detection of organic vapours. Royal Society of Chemistry: Advances. 5, 90515–90520.