The most important procedures for the production of nanopowders and/or deposition of thin layers from the liquid phase, include the sol gel procedure and the electro-chemical deposition. Both procedures are further suitable for the building of nanoporous materials. Nanophased systems in liquids (nano-suspensions and emulsions) were already investigated in different microgravity research projects. Topics of interest are, for example, the adsorption dynamics and the mass transfer on individual liquid / liquid boundary surfaces, droplet/droplet interactions as well as stability and phase inversion of model emulsions. Apart from realizations within the basic research range, also approaches for the optimization of wet-chemical procedures for terrestrial applications are expected to be realized by means of microgravity experiments.
Figure 1. Measurements of the particle concentration in a flame under terrestrial and microgravity conditions.
Studying How the Effects of Gravity on Wet-Chemical and Electrophoretic Deposition (EPD) Processes Could Improve Process Techniques and Materials
Although the gravimetric sedimentation of very small particles is to be neglected in relation to the random Brownian movement, gravity effects can play a role in wet-chemical processes in view of the long duration of particle aggregation, as well as the impact of gas bubbles on Electrophoretic Deposition (EPD) processes, which arise under the influence of the gravity force. The investigation of the influence of gravitational effects on the formation and characteristics of nanomaterials by sol gel processes (e.g. of aerogels), could serve a better understanding of the gel aggregation with the gelling process, which could be used in principle for the optimization of the appropriate process technologies and materials.
Benefits of Studying the Gelling Process of Aerogel Formation under Microgravity Conditions
In a NASA research project, an appropriate measuring device is currently developed, helping to examine the gelling process of aerogel formation by means of optical measuring procedures under microgravity conditions. Furthermore, there are indications that the production of aerogels under microgravity can lead to improved material properties. Thus, microgravity experiments are conducted by the University of Wisconsin with the aim of reducing the pore sizes of aerogels to obtain transparent, colourless aerogels, which would be better suited for technical applications. Conventional aerogels with pore sizes of up to 200 nm appear frequently bluish and/or translucent due to light scattering effects, which limit the technical applicability.
Microgravity Research into Using Electrophoretic Deposition (EPD) to Produce Nanomaterials
Also, the electrophoretic deposition (EPD) for the production of nanomaterials offers starting points for microgravity research. Although structures with an unusually high degree of order can be produced under terrestrial conditions by means of EPD, a more or less strong deviation from the ideal course of the nanoparticles along the electrical field occurs by gravitational effects. This could lead to disturbances in the microstructure of the deposited nanomaterials. For fundamental investigation with the purpose of understanding the mechanisms of the EPD process, this means that an accurate empirical analysis is not possible due to gravitational influence, so that a verification of the postulated models and simulations is likewise not accurate.
Electrophoretic Deposition (EPD), Electrophoretic Impregnation and Electrophoretic Infiltration (EPI) Processes in Aqueous and Organic Solvents
In principle, both electrophoretic deposition and impregnation can be accomplished in aqueous as well as organic solvents. Although these two basically different procedures exhibit specific advantages, the use of organic solvents is usually not economical due to the clearly higher process times and the environmental incompatibility, so that aqueous dispersions have to be used for technically relevant processes. However, the electrolytic decomposition of water, applying voltages above approximately 2 V within EPD and Electrophoretic Infiltration (EPI) processes, leads to the formation of gas bubbles on the electrodes, which disturbs the particle movement and causes defects in the microstructure of the deposited materials. While defects in the microstructure can mostly be avoided by applying ion permeable membranes, an investigation of the movement of the dispersed nano-particles in the electrical field is, however, influenced through gas bubbles. Thus, the investigation of the electrophoretic caused movement and deposition of nanoparticles under microgravity could lead to a crucial contribution to understand the relevant mechanisms, and thus accelerate the conversion into industrial development significantly.
Gravitational Effects on the Self-Organization Processes of Biological Molecules
A further relevant topic field is the self-organization of molecules in liquids, which will significantly gain importance in the future for bottom-up strategies for the production of nanomaterials. Experiments under microgravity showed that gravitational effects had a crucial influence on self-organization processes of biological molecules. Although the influence of gravity on a single particle is only small, effects could arise in systems of a multiplicity of particles, which lead to a macroscopic self-organization into so-called dissipative structures. Thus, in microgravity experiments it was observed that molecules of the protein, tubulin, arrange themselves in a completely irregular form, while, under the same conditions, ordered structures arise in a terrestrial laboratory. The investigation of such self-organization phenomena under microgravity could be relevant for the development of nanostructured materials by self-organization processes. Within the nanotechnology scene, however, no concrete approaches are at present recognizable to take up this topic in the context of their own research activities.
Investigation of Thermal Transportation Phenomena in Ferrofluids
For the investigation of the physical properties of ferrofluids micrograviy experiments are utilized to examine thermal transportation phenomena and magnetic effects without distortion through gravity influences. Ferrofluids, which consist of a suspension of magnetic nanoparticles (approx. 10 nm diameters) in a carrier liquid (e.g. oil or water), offer potential e.g. for the employment in thermal control elements, since their physical properties (e.g. the viscosity, thermal conductivity) can be controlled by exterior magnetic fields. Experiments under microgravity are expected to lead to a better understanding and controllability of mass and heat transport processes, which is necessary for potential technical applications of ferrofluids. Appropriate investigations are accomplished e.g. by the Center for Applied Space Technology and Microgravity (ZARM) in Bremen with parabolic flights and drop tower experiments in the frame of an ESA Map project. At a later time, experiments in the space shuttle and on the International Space Station (ISS) are also planned.