The DLVO theory has long been accepted as the standard theory of colloidal dispersions. It can describe quantitatively well the stability and instability of concentrated dispersions. Recently, however, it turns out impossible to explain a variety of phenomena such as ordering formation and phase transitions in dilute dispersions of highly charged particles. Those phenomena require the colloid particles to have the interaction potential with the strong medium-range repulsion and weak long-range attraction. We review critically the existing theories of ionic solutions, argue over a new standard theory of colloid interaction which can describe the characteristics of dilute as well as concentrated dispersions, and propose experiments of laser diffraction and CCD photography in the Kibo module of ISS to verify the theory.
We report on the onboard apparatus for the Japanese colloid “Kikuchi-Kossel” experiment on the ISS/KIBO. The system was devoted to the KIBO utilization research theme “Study on the mutual interaction process between colloids”, as proposed by Prof. Sogami and started in 2011, where the main objective is observation of structure formation process in colloid dispersions under micro-G condition to verify existence of a long-range attraction force among the colloid particles. Based on the mission requirement, we have implemented a Kikuchi-Kossel laser-diffractometer and void-observation systems, where the former allows structural analysis of the colloid crystal structure and the latters allow the structure formation study at a mesoscopic (~10-100 μm) and macroscopic scale(~1mm) in colloid dispersions. The system also includes space-unique subsystems such as a stirring system and cuvettes etc. The paper summarizes system descriptions based on the Preliminary Design Review (PDR) phase. In December 2014, several colloid themes by other international partners are also planned for the ISS. It is expected that CCOF will contribute significantly to these colloid research community
Kikuchi-Kossel diffraction method of colloidal crystal is developed for automatic mesurement experiment under microgravity. This article describes two important aspects of the project: Kossel diffraction camera that acquires indepth information of crystal growth, and sample cuvette that maintains the sample condition over a long period of time.
We examined influences of the particle volume fraction ϕ and effective charge number Zeff on the crystal structures of charged colloids, by means of the reflection spectroscopy and Kikuchi-Kossel diffraction measurements. We employed dilute salt-free aqueous dispersions of charged polystyrene (PS) particles (the diameter d = 110 to 120 nm; Zeff = about 1300) and colloidal silica (d = 120 nm, Zeff = about 550). The PS colloids had the BCC structure at ϕ ≤ 0.01 and exhibited the structural phase transition into the FCC lattice on increasing ϕ. The silica colloids, which had lower Zeff, took the BCC structures at all ϕs examined (ϕ≤ 0.04).
Microgravity experiments on the order formation of silica particles under the critical concentration for colloidal crystallization were planned and prepared. Optical system was constructed for the following three experiments; (i) voids detection by measuring intensity of transmitted-light run through dispersions, (ii) observation of individual particles with diameters of 1.0 and 3.0 μm at lower concentration than 1.0 vol% by an optical microscope and (iii) position identification and size estimation of larger grains of colloidal crystals by an ultramicroscope with ring light source consisted of LEDs of different wavelengths.
Charged polystyrene particles, which were initially sedimented uniformly on a glass wall, spontaneously transformed into twodimensional gas, liquid, and solid phases in aqueous solvents. Their relative stabilities could be determined by a common phase diagram consisting of gas-liquid and liquid-solid coexistence lines under the conditions of a large particle radius (a ≫κ -1 ) and intermediate screening length (a=1.5 μm and 10 nm10σ) decay lengths were found. The latter was attributed to the charge density wave (CDW) due to alternating layers of oppositely charged colloids and counterions along the radial coordinate. Because the charged glass wall had a similar surface charge density and sign as the particles, the repulsive levitation at the wall against gravity assisted the thermal rearrangement during phase separation. Further evaluation is promising for investigating the complicated physics of particle-wall interactions.
A molecular model is proposed for the charged colloidal dispersion. This molecular system consists of 256 particles with the charge 25e and 6400 particles with charge -e, where e is the unit charge. Molecular dynamics simulations are performed by NVT ensemble on a wide aria of (V,T) phase space, where T is the temperature and V is the volume of the system. The system has gas, liquid and solid phases and the charge 25e-particles have FCC structure in the liquid and solid phases.
Deionized dilute charged colloidal systems exhibit gas-liquid and gas-solid coexistence and re-entrant transitions depending on suspensions parameters such as volume fraction, charge density and ionic strength. These observations constitute evidence for the existence of long-range attraction between like-charged colloids. Monte Carlo (MC) simulations using Sogami-Ise pair-potential, which has a long-range attractive term, could explain the above mentioned experimental observations. MC simulations, which correspond to zero gravity conditions, revealed coexistence of liquid-like droplet with rare phase (gas-like) and voids coexisting with dense (solid-like) phase clusters. These predictions will be verified by the project of microgravity experiments in space and the outcome is expected to provide evidence for the existence of long-range attraction between like charged colloids.
This paper reports the space experiment definition and operation planning of the Japanese space colloid project “Kikuchi-Kossel”. The project was selected as one of candidates of space experiment in KIBO in 2010. Since then, the researchers and project members have tackled with the various issues to realize the space experiment in ISS/KIBO module, where these issues must be treated under the operations constraints as well as restrictions of the onboard apparatus. To ensure the mission success, we are adopting a milestone review approach for the on-board projects, issues and missions design referring to the experiment operations are coordinated with it. The paper will provide basic information to the researchers who aspire for scientific space experiments.
A microgravity experiment system using a high altitude balloon has been developed. In order to accommodate payloads larger than previous system which employed three- dimensional drag-free control, one-dimensional drag-free control has been applied. The first test flight was conducted in Aug. 2014. A gravity level below 10-3 G was obtained for more than 30 seconds during the free-fall of the capsule. A combustion experiment was conducted during the low gravity condition.
We report on a compact laser-heated aerodynamic levitation facility which can be used for studies of thermophysical properties like density, surface tension and viscosity of liquid oxides at high temperatures. The facility is characterized by employing an acoustic excitation system for controlled excitation of droplet oscillations which is key for measurements of surface tension and viscosity. Results of measurements of density and viscosity of pure liquid Al2O3 as well as CaAl2O4 are presented. The data are discussed in the context of available literature data. It is shown that with the present setup the experimentally accessible temperature ranges could be enlarged. For viscosity high precision data for high melting point and highly reactive oxide melts can now be obtained using aerodynamic levitation.