Scientific instrument for studying of colloid systems in microgravity
Disperse systems are currently the subject of intensive studies due to unique effects that appear in these systems and wide field of their possible applications [1,2]. Colloid suspensions are actively used in biomedical applications for development of new methods in targeted drug delivery and growth of tissues as well as for formation of new structures for optoelectronic devices, namely photonic crystals . Moreover, suspensions of monodisperse particles can be used as model systems for studies of crystalline structures since dispersed particles can be viewed in certain conditions as distinct atoms in the modelled structure .
The colloid suspensions are affected by gravitational force in terrestrial conditions that leads to sedimentation of dispersed particles and foliation of suspensions. These processes significantly complicate studies of physical-chemical processes that proceed in such systems. Furthermore, particle sedimentation also limits the possibility of formation of colloid systems based on particles of different sizes and makes impossible growth of colloid structures from particles with sufiiciently large size by phase transitions, namely sol-gel transition. At the same time, in microgravity the influence of gravitational force becomes negligible and interaction of dispersed particles comes to the fore. This provides an opportunity for studying the mechanisms that lie beneath these processes, as well as to obtain disperse systems with large particles and control their structural properties by use of weak external electric and magnetic fields. Furthermore, formation and subsequent study of disperse systems not only in liquids, but also in gas and vacuum, such as Coulomb clusters  and plasma crystals , becomes possible in microgravity.
Hereby main construction features of scientific instrument that is developed by Laser Systems LLC in collaboration with S. P. Korolev Rocket and Space Corporation Energia for the space experiment "Dispersia" proposed by Moscow Aviation Institute for carrying out on Russian Orbital Segment (ROS) of International Space Station (ISS) are reviewed.
METHODS FOR STUDYING OF COLLOIDAL SYSTEMS
Colloid systems are commonly studied by optical microscopy, static and dynamic light scattering as well as Bragg scattering. Choice of the methods that are realized in a particular scientific instrument is related to both features of the colloid suspension that have to be analyzed, namely the concentration and mean diameter of the dispersed particles, and the possibility of simultaneous use of the different methods. The main restrictions for the scientific equipment on board of ISS are overall size and mass of the instrument.
Two methods for colloid systems analysis are realized in the SI "Dispersia": recording of high resolution video along with capturing photo images of the studied dispersions and detection of signal from static light scattering in a wide range of angles.
The analysis of data extracted from video and photo images is one of the most common and relatively simple methods for analysis of the colloids. It provides an opportunity for study of changes in the colloid system structure, in particular colloidal crystal growth from the initially homogeneous suspension, arrays of aggregates or fractal gel . The information obtained from the images allows to evaluate the rate of these processes and to determine the type and the general features of the structures formed. All this requires the use of high-speed camera with high optical resolution since lateral size of structures formed in colloid suspensions lies beneath tens of micrometers. The characteristic times of formation of structures, especially at the initial time of the process can be of up to hundreds of microseconds. Moreover, simultaneous video and microphotography capturing in the certain moments with maximum resolution allows to obtain maximum of experimental results. These constrains determine the choice of the camera and the format of data.
Static light scattering method is based on detection and subsequent analysis of angle distribution of intensity of the scattered light after it passed through the sample. The analysis of the scattering indicatrix and the ratio between unscattered and scattered light provides information about the concentration of the suspension and the phase transitions that are proceeding in the sample.
When the concentration of the suspension under study is relatively low (less than 0,01%) only one scattering event occurs (in comparison to multiple scattering in highly-concentrated suspensions). In this case Mie or Fraunhofer model can be applied to determine particle size from the indicatrix of the scattered light. For monodisperse suspension its indicatrix has specific structure (Fig.1(1,2)) determined by particle size. However, in polydisperse suspensions features of the indicatrix become less prominent and analysis of the particles size requires an additional algorithm that allow to obtain the information about particle size distribution.
THE SCHEME OF MEASUREMENTS
Study of colloid suspensions by static light scattering method requires the complicated optical scheme that allows detection of scattered light with high dynamic range in different directions with minimum aberrations. The optical scheme for analysis of colloid suspensions on the base of static light scattering that is realized in scientific instrument "Dispersia" is shown in Fig.2.
Three diode lasers are used as light sources. Two lasers, with wavelengths λ=445 nm и λ=660 nm, are combined together by the dichroic beamsplitter and directed onto the cuvette tilted by ~1° in order to avoid the backscattering. We use as a light sources fiber-coupled diode lasers. Single-mode fiber plays the role of spatial filter, making laser beam profile nearly equal to the Gaussian beam. The radiation from the third laser (with wavelength λ=445 nm) is directed at the angle of 37° relatively to the first two lasers what leads to widening of the range of angles at which the scattered light can be detected. This allows one to obtain more correct information about size distribution in the studied sample since accuracy of the size distribution calculations determined by range of angles of detection. Angle sectors of scattered light detection by multi-element sensor of IS "Dispersia" are shown on Fig.1.
After passing the cuvette, the transmitted light is partially scattered and detected by matrices of the photoreceiving system (Fig.2, Pos.9), whereas the intensity of the unscattered part of laser radiation is detected by the photodiode (Fig.2, Pos.11). The ratio of scattered and unscattered parts allows to estimate the concentration of the analyzed suspension assuming that the lateral size of the particles is initially known.
The scheme of the device is based on reverse Fourier optics. The sample is illuminated not by collimated but by converging laser beam formed by system of convex lenses (Pos.6,7). As a result, light, scattered at certain angle by different particles from different parts of the volume of cuvette, converges at a the same point of the matrix.
The photo sensor array is one of the most important parts of measurement scheme that provides the possibility to detect patterns of the scattered light with high resolution in a wide area of angles. The photo sensor array of scientific instrument "Dispersia" consists of 8 complementary metal-oxide-semiconductor (CMOS) matrices (Fig.2) with 1920Ч1200 elements and pixel size of 5.86 µm. This allows to detect scattered light with high angle resolution. Moreover, use of these matrices provides additional advantage related to possibility of applying of the binning procedure that increases the dynamic range and the signal-to-noise (SNR) ratio combining the signal from nearby pixels. Since intensity of light, scattered by particles with relatively small diameters (of up to 1 µm), is significantly low and the indicatrix structure does not demand high spatial resolution (Fig.1,1), a high binning factor can be applied. On the contrary, in the case of large particles (with lateral size more than 10 µm) the intensity of the scattered light is significantly higher but the indicatrix structure is very fine within small angles of scattering. As a result, detection of the scattered light pattern with high spatial resolution is required in this case to observe all its characteristic features. Furthermore, the control of the matrices binning factor also allows to optimize data flow rate.
The photoreceiver matrices are connected to the main computer by RS‑485 interface with use of expansion card that provides possibility to collect simultaneously the data flow from 8 matrices with low frame rate and large exposition time. With the frame rate 2 fps and the minimal binning, the data flow from each matrix is within 8 Mbit/s. Slow but noise immune and simple RS‑485 interface can be applied for this purpose. All matrices are controlled by single control channel that is also based on data exchange interface RS‑485.
In addition the color video camera with GigE Vision interface allows to capture video- and photoimages of the sample. JPEG-LS encoder is used to transfer video data compression without loss. The camera is controlled by user data protocol (UPD) that is also applied for transfer of the compressed video data. Depending on the compression ratio, the bit rate from the camera with 1920Ч1200 resolution and frame rate of 25 fps is within the range from 265 to 890 Mbits/s.
ARRANGEMENT OF EQUIPMENT OF THE "DISPERSIA" SCIENTIFIC INSTRUMENT
Fig.3 demonstrates the arrangement of equipment of the "Dispersia" scientific instrument. All elements are mounted on the single platform that provides the necessary stiffness of the whole structure. The upper section mounted on vibration isolation comprises the elements of the optical scheme and (Fig.4) external influences equipment. The bottom section contains electronics and data-storage system. In order to protect the equipment from the shock and vibration loads during transportation to the orbit the main platform is suspended by vibration isolation mounts.
Some restrictions for the materials used for construction of the equipment arise due to presence of external electric and magnetic fields, heating of the sample etc. The cuvette windows are made from transparent polymethyl methacrylate (PMMA) whereas other elements – from polyamide. For the experiments with external heating of the cuvette the duralimin frame instead of polyamide is used as a central part of the cuvette, providing heat transfer from the heating elements to the sample volume. The construction of the experimental cuvette and the equipment for application of an external fields are shown on Fig.4.
Application of the external electric field on the cuvette is provided by applying electric potential to two flat plates located at the top and at the bottom of the cuvette (Fig.4, Pos.3). External magnetic field is formed by the inductance coils, located on the both sides of the cuvette (Fig.4, Pos.4). Heat treatment of the studied suspension is carried out by the use of ohmic heating elements (Fig.4, Pos.5). Controllable shaking of the sample is realized by use of vibro engine (Fig.4, Pos.7) based on solenoid that provide oscillating movement of the cuvette around the hinges that are fixed in the cylindrical protective screen (Fig.4, Pos.6).
The control and data system (CDS) of SI "Dispersia" is based on complex of specialized program and instrumental sources that realize power supply and control for all functional elements as determined by the experiment timeline as well as provides data gathering and storage. The CDS in SI "Dispersia" has two-level structure. The bottom level of CDS is presented by power control module that regulates parameters of the processes that proceed during the experiments and controls executive systems. The upper level of CDS is based on high-performance single board computer that is designed for controlling lasers and matrices of photoreceivers, gathering data, performing formed timelines and data storage during the experiment. Graphical interface of the CDS software and data input is provided by the use of sensor monitor with size of 6.5".
Scientific instrument "Dispersia" developed for studying of various colloid systems with the presence of external influencing factors, namely weak electric and magnetic fields, heating and vibrations, in microgravity is reviewed. The scientific instrument under review provides opportunities for studying of colloidal crystallization, polymerization, spinodal decomposition and processes of aggregation of dispersed phase and in liquid-polymer systems, in the presence of the external influence. SI combines the following methods: static light scattering, and video and photo data capturing. The device is robust in order to endure the harsh conditions of transportation by the launch vehicle to the board of ISS.