After its predecessor IVIDIL, SODI COLLOID comes into play. This will be the third and last experiment of the Selectable Optical Diagnostics Instrument (SODI) project, performed inside NASA facility (MSG) as well.
- Mission ULF4, Increment 23/24: Septiembre 10th 2010 – planned upload and physical configuration during launch.
- Mission ULF5, Increment 25/26: ~November 2011 – estimated download of the Flash Disks.
This experiment stems from the proposal "Advanced Photonic Devices in Microgravity" submitted in response to the International Announcement of Opportunity of 2000. As suggested by its title, the focus is here on materials that have a special interest in photonics, with emphasis on nano-structured, periodic dielectric materials, known as photonic crystals, which possess appealing properties and make them promising candidates for new types of optical components. Since the key property of photonic crystals lies in the periodic variation of refractive index in two or three dimensions on a length scale that is equivalent to that of visible wavelength, the building block are (often mixtures of) colloidal particles of micrometric dimension. The overall goal of the experiment is therefore to study self assembly of colloidal systems, trying to tackle the puzzling and still not fully understood mechanisms that lie behind, with the benefit of a reduced gravity environment.
Within such framework, the experiment here proposed is focused on a peculiar type of aggregation that allows a high degree of control particularly suitable for space experiments. The system is composed by spherical colloidal particles suspended in a binary solvent mixture with two-phase coexisting region. The interest of such a system lies in the fact that, in the homogeneous phase of the solvent, the adsorption of one of the liquid species depends on temperature difference from the host phase separation line. The preferential adsorption of the liquid component on the colloidal particles weakens the repulsive electrostatic double layer thereby modifying the colloidal stability. The effective interparticle interaction changes from repulsive to attractive, as the two phases coexistence region is approached. The advantage of this system lies in the fact that the interparticle interaction is tunable by controlling a macroscopic parameter such as the temperature.
Specific goal of the experiment is to observe nucleation and the early stages of aggregation, where the first nuclei with supercritical size are formed, studying the growth rate and size distribution over time.
The basic scheme of the COLLOID experiment is to acquire NFS batches at different temperatures. In this way, it is possible to follow the formation of aggregates and growing structures by a quantitative check of different features of the power spectra of the scattered radiation. 5 cells will be probed by NFS, grouped in an array, where the NFS setup can be positioned in any of the 5 cells, being probed 4 of them as experimental cells and one as reference cell.
Different temperatures will be probed for every cell, the values depending on the sample composition. For every temperature, the idea behind the experiment protocol is to acquire NFS batches and real space images as long as the system is undergoing a detectable change. The experiment is divided in the following blocks:
"Aggregation Temperature Detection Run", where the aggregation temperature and temporal duration is probed cell by cell. The decision of keeping the same temperature or going to the next temperature step is taken on the basis of a comparison between quantities extracted from the images. This decision making process is performed by the SODI COLLOID software.
"Reference cell run", where an NFS batch is acquired at different temperatures in the reference cell.
"Experiment run", where the NFS batches are acquired during aggregation duration at a desired temperature. This forms the core part of the COLLOID experiment.