Effect of Viscosity on Vibration-Induced Motion of a Spherical Particle Suspended in a Fluid Cell

Production of protein and semi-conductor crystals with advanced quality and properties is possible under microgravity conditions due to the suppression of convection effects. However, aboard space platforms, g-jitter induced motions of solid particles can cause unsteady convection that may result in degradation of the properties of crystals produced. There are different effects of g-jitter on small particles suspended in a fluid cell which are not fully understood. To investigate these small vibration effects, ground experiments were conducted by suspending a spherical particle with a thin wire in a rectangular fluid cell and subjecting the cell and particle to horizontal vibrations with different frequencies and amplitudes. The fluid viscosity was varied to investigate the attraction or repulsion force induced in the direction normal to the direction of the vibration. The force was found to change from attraction to repulsion with an increase in the fluid viscosity and increase with the increasing vibration frequency and amplitude.     

Twenty Years of Surface Tension Measurements in Space

A brief excursus is presented on the development of techniques and methods for surface tension measurements in microgravity conditions, with a specific reference to the author’s Group contributions. The two main techniques already available, namely the “Capillary Tensiometry” and the “Levitated Drop Method” are presented, and their application to systems both at ambient temperature and in the high temperature ranges critically discussed. After a survey of the principal results already obtained in real microgravity conditions, an analysis of the potential such methods still have for future applications is given.     

A New Paradigm of Crystallization Arising from Non-standard Nucleation Pathways

There is increasing evidence that large classes of colloid materials crystallize via a non-standard nucleation mechanism involving intermediate metastable phases. In this paper recent work on the microscopic derivation of the phase diagram and free energy barriers in the nucleation of protein crystals, and on the kinetics of growth of solid particles in the post-nucleation regime is reviewed. The extent to which combined structural and density fluctuation give rise to favourable crystallization pathways involving an intermediate fluid phase is assessed and the connection with experiments in microgravity at ISS (PROMISS-2 and NANOSLAB-2) is discussed.     

Agglomeration control of hydroxyapatite nano-crystals grown in phase-separated microenvironments

Materials synthesis processes that require high temperatures consume large quantities of energy that generate an environmental burden. We attempted to synthesize hydroxyapatite (HAp) nano-crystals without firing or melting. “Water in oil” (W/O) emulsions were employed as microreactors for HAp formation. The surfactant-bounded water mediated HAp crystal nucleation, and HAp nano-crystallites were obtained. The obtained particles were aggregates composed of plate-like nano-crystals and monodisperse tiny crystals. Utilization of the W/O emulsions resulted in tunable nucleation frequency and the reactant provision, and yielded HAp nano-crystals with characteristic agglomeration properties.     

The Protein Crystallisation Diagnostics Facility (PCDF) on Board ESA Columbus Laboratory

Crystals of proteins or macromolecules are at the basis of X-ray crystallography to reveal structural information necessary for the understanding of their likely mode of action. However, the structural resolution is strongly dependent on the crystalline quality, which is known to be related to gravity dependent processes. The facilities and instruments used so far to grow crystals in space have mostly focused on the growing of crystals for detailed post-flight analysis on ground, and less on the understanding of phenomena associated to the crystallisation processes. The Protein Crystallisation Diagnostics Facility (PCDF), developed by Astrium under contract of the European Space Agency (ESA), allows to study with several diagnostics means in situ the crystallisation of macromolecules over long periods in microgravity. In addition, several ground models with PCDF similar capabilities were developed to allow scientists to prepare their experiments. The PCDF is installed in the European Drawer Rack (EDR), on board ESA’s Columbus Laboratory module launched in February 2008 to the International Space Station (ISS) for research in microgravity on protein nucleation and assembling sequences. The PCDF configuration for this first mission accommodates four reactors, using the batch crystallization technique. Individual process control for temperature and concentration will allow several crystallizations of solutions to be performed. Each reactor will be observed by several optical diagnostics, including video microscopy, dynamic light scattering, and Mach–Zehnder interferometry. This paper presents the overall PCDF design and details the different diagnostics allowing the scientific community to use the PCDF in orbit for microgravity research on molecule assemblies grown from solutions.     

Optimization of the process of continuous mass crystallization of salts from solutions

Crystallization of a polydisperse system of crystals in the case of continuous organization of the process in a cascade of crystallizers is investigated. The process is assumed to occur in a kinetic regime. The influence of fluctuations in the rate of crystal growth is taken into consideration. Basic computational formulas are obtained and certain limiting situations are analyzed. Russian Scientific Center “Applied Chemistry”, St. Petersburg, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 5, pp. 707–713 September-October, 1997.     

Acid corrosion analysis of fibre glass

The scanning electron microscope and the energy dispersive X-ray based analytical techniques have been used to study the sulphuric acid corrosion resistance of standard “E” fibre glass and PPG developed and patented 1201 [1], an “ECR” acid-resistant fibre glass. While it is well agreed by the experts in the field that direct exposure of fibre glass to 0.5 m sulphuric acid is much too severe a condition to simulate service conditions of most filament wound fibre glass products, nevertheless, the above corrosive environment was used to demonstrate a significant superiority of PPG 1201 glass in acid resistance over the standard “E” glass regardless of the source of its origin. It is also demonstrated that not only boron oxide leached out during acid exposure but a significant amout of calcium, aluminium, and magnesium oxides also were depleted from the “E” glass composition during acid treatment. It is also demonstrated that heat treatment of “E” glass products reduced the rate of acid attack. It did not, however, eliminate it completely. Heat treatment affects the strength properties of this fibre glass adversely.     

Vibration-induced particle drift in a fluid cell under microgravity

A theoretical investigation into the effect of small vibrations on the behaviour of a small particle contained in a fluid cell under microgravity is presented. Diffusion-controlled material processing such as protein crystal growth can be adversely affected by small vibrations called g-jitter, if a relative motion is induced between the particle and surrounding fluid. When a fluid cell containing a small particle such as a protein crystal is vibrated parallel to the wall nearest to the particle, the particle oscillates with a certain amplitude and a hydrodynamic force in the direction normal to the wall is induced. Theoretical models based on an inviscid fluid assumption are used to predict the particle amplitude variation and drifting motion. Due to an external vibration such as g-jitter, the oscillating particle is predicted to drift towards the wall and the particle oscillation amplitude to decrease slightly as the distance between the particle and wall is reduced. The reduction in particle ampitude also depends on the particle-to-fluid density ratio. The particle drift towards the nearest wall acclerates due to an increasing attraction force, and the drifting speed increases with both the vibration frequency and particle diameter. Even for small protein crystals with a density close to that of the fluid, the time required to drift from the center of the fluid cell to the wall is predicted to be much shorter than the growth time.     

Protein crystallization in space

The microgravity environment of space is an ideal place to study the complicated protein crystallization process and to grow good-quality protein crystals. A series of crystal growth experiments of 10 different proteins was carried out in space on a Chinese re-entry satellite FSW-2 in August, 1992. The experiments were performed for about two weeks at a temperature of 18.5 +/- 0.5 degrees C using a tube-like crystallization apparatus made in the Shanghai Institute of Technical Physics, Academia Sinica. More than half of 48 samples from 6 proteins produced crystals, and the effects of microgravity on protein crystal growth were observed, especially for hen-egg white lysozyme and an acidic phospholipase A2 from the venom of Agkistrodon halys Pallas. Analyses of the crystallization of these two enzymes in this mission showed that the microgravity environment in space may be beneficial to improve size, external perfection, morphology, internal order, and nucleation of protein crystals. Some of these positive microgravity effects were also demonstrated by the growth of protein crystals in gelled solution with the above two enzymes. A structural analysis of the tetragonal lysozyme crystal grown in space is in progress.     

The effective oxidation pressure of indium–oxygen system

A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid Indium as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under both low total pressure (Knudsen regime) and inert atmospheric pressure (Fick regime) conditions, confirming that an oxide removal regime occurs under an oxygen partial pressure much higher than the equilibrium one (the “Effective Oxidation Pressure”). Experimental results are reported which give a further insight on the relative importance of the various processes due to the oxygen mass transport between the liquid metal and the gas phase. The critical aspects involved in surface tension measurements of liquid metals, related to the problem of liquid metal–oxygen interactions, are also underlined.     

A micromechanical model of collapsing quicksand

The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e.  at the particle scale) of granular/soil materials. We present a contact dynamics method, accounting for the cohesive nature of fine powders and soils. A modification of the model adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of loose systems is able to simulate “quicksand” behavior of a collapsing soil material, in particular of a specific type, which we call “living quicksand”. We investigate the penetration behavior of an object for varying density of the material. We also investigate the dynamics of the penetration process, by measuring the relation between the driving force and the resulting velocity of the intruder, leading to a “power law” behavior with exponent 1/2, i.e.  a quadratic velocity dependence of the drag force on the intruder.     

Vaporization of CdS single crystals and formation of negative whiskers

The mechanism and kinetics of vaporization of A^II-B^VI compounds (mainly of CdS single crystals) in hydrogen flow are studied. It is found that a liquid forming agent (gold) evaporated on the crystal surfaces significantly increases vaporization rates and tends to transfer the vaporization regime from a kinetic to a diffusion one. Formation mechanism of negative whiskers (crystallographically oriented cavities underneath the crystal surfaces) is investigated. It is concluded that the negative whiskers are formed under the action of the liquid phase which lowers nucleation barriers on the liquid-solid interface.     

Temperature-dependence on the optical properties and the phase separation of polymer–fullerene thin films

A detailed study on the thermal transition of poly(3-hexylthiophene) (P3HT) and blends was investigated by differential scanning calorimetry, while the morphological, phase separation and the transformation in the optical properties were probed by thermal-atomic force microscopy (AFM), polarized optical microscopy (POM) and spectroscopic ellipsometry (SE). The inclusion of fullerenes on the polymer structure confirms the formation and evolution of a new endothermic transition at high temperatures. SE revealed that the refractive index and extinction coefficient of the films increased with annealing temperature up to 140 °C due to the suppressed diffusion of PCBM molecules into the blend. Annealing above 140 °C resulted in a decrease in the optical constants due to the formation of large “needle-like” crystals. This is due to the depletion of PCBM clusters near the “needle-like” structures; resulting from the diffusion of the PCBM molecules into the growing PCBM crystals or “needle-like” crystals as is evidenced by in situ thermal-AFM and POM. These findings indicate that annealing temperature of 140 °C is suitable for a P3HT:PCBM film to obtain the desired phase separation for solar cell application.     

On the glass–crystal transformation kinetics in materials which fulfil the conditions of “site saturation”. Application to the crystallization of some alloys of Sb–As–Se and Ge–Sb–Se glassy systems

A theoretical procedure has been developed for the kinetic study of the glass–crystal transformation under continuous heating regime in materials involving formation and growth of nuclei. The quoted procedure obtains an evolution equation with the temperature for the actual volume fraction transformed at non-isothermal reactions. In this procedure an extended volume of transformed material has been defined and spatially random transformed regions have been assumed in order to obtain a general expression of the actual volume fraction transformed as a function of the temperature using differential scanning calorimetry. The kinetic parameters of the quoted transformation have been deduced, assuming that the crystal growth rate has an Arrhenius-type temperature dependence, and the nucleation frequency is negligible, condition of “site saturation”, and using the following considerations: the condition of maximum crystallization rate and the quoted maximum rate. The procedure developed has been applied to the analysis of the crystallization kinetics of some semiconducting alloys, prepared in our laboratory, corresponding to the Sb–As–Se and Ge–Sb–Se glassy systems, and which fulfil the condition of “site saturation”. The obtained values for the kinetic parameters satisfactorily agree with the calculated results by the non-isothermal technique of maximum-value. This fact confirms the reliability and accuracy of the theoretical procedure developed.     

Disruption of an Aligned Dendritic Network by Bubbles During Re-melting in a Microgravity Environment

The Pore Formation and Mobility Investigation (PFMI) utilized quartz tubes containing succinonitrile and 0.24 wt% water “alloys” for directional solidification (DS) experiments which were conducted in the microgravity environment aboard the International Space Station (ISS; 2002–2006). The sample mixture was initially melted back under controlled conditions in order to establish an equilibrium solid-liquid interface. During this procedure thermocapillary convection initiated when the directional melting exposed a previously trapped bubble. The induced fluid flow was capable of detaching and redistributing large arrays of aligned dendrite branches. In other cases, rapidly translating bubbles originating in the mushy zone dislodged dendrite fragments from the interface. The detrimental consequence of randomly oriented dendrite arms at the equilibrium interface upon reinitiating controlled solidification is discussed.     

Convective Boiling Between 2D Plates: Microgravity Influence on Bubble Growth and Detachment

The experiment detailed in this paper presents results obtained on the nucleation, growth and detachment of HFE-7100 confined vapour bubbles. Bubbles are created on an artificial nucleation site between two-dimensional plates under terrestrial and microgravity conditions. The experiments are performed by varying the shear flow by changing the convective mass flow rate, and varying the bubble nucleation rate by changing the heat flux supplied. The experiments are performed under normal (1 g) and reduced gravity (μg). The distance between the plates is equal to 1 mm. The results of these experiments are related to the detachment diameters of bubbles on the single artificial nucleation site and to the associated effects on the heat transfer by the confinement influence. The experimental device allows the observation of the flow using both visible video camera and infrared video camera. Here, we present the results obtained concerning the influence of gravity on the bubble detachment diameter and the images of 2D bubbles obtained in microgravity by means of an infrared camera. The following parameters: nucleation site surface temperature, bubble detachment diameter and bubble nucleation frequency evidence modifications due to microgravity.     

“Nanotube”: Scanning microscope probe as a controlled source of microwave radiation

A nanoparticle (scanning microscope probe) can be effectively heated by equilibrium thermal radiation in a vacuum chamber. The particle also can re-radiate energy via near-field modes to the sample surface. A possible design of an oscillating source of microwave radiation (“nanotube”) based on these phenomena is considered.     

Theory and simulation of buoyancy-driven convection around growing protein crystals in microgravity

We present an order-of-magnitude analysis of the Navier-Stokes equations in a time-dependent, incompressible and Boussinesq formulation. The hypothesis employed of two different length scales allows one to determine the different flow regimes on the basis of the geometrical and thermodynamical parameters alone, without solving the Navier-Stokes equations. The order-of-magnitude analysis is then applied to the field of protein crystallization, and to the flow field around a crystal, where the driving forces are solutal buoyancy-driven convection, from density dependence on species concentration, and sedimentation caused by the different densities of the crystal and the protein solution. The main result of this paper is to provide predictions of the conditions in which a crystal is growing in a convective regime, rather than in the ideal diffusive state, even under the typical microgravity conditions of space platforms.     

Theory and simulation of buoyancy-driven convection around growing protein crystals in microgravity

We present an order-of-magnitude analysis of the Navier-Stokes equations in a time-dependent, incompressible and Boussinesq formulation. The hypothesis employed of two different length scales allows one to determine the different flow regimes on the basis of the geometrical and thermodynamical parameters alone, without solving the Navier-Stokes equations. The order-of-magnitude analysis is then applied to the field of protein crystallization, and to the flow field around a crystal, where the driving forces are solutal buoyancy-driven convection, from density dependence on species concentration, and sedimentation caused by the different densities of the crystal and the protein solution. The main result of this paper is to provide predictions of the conditions in which a crystal is growing in a convective regime, rather than in the ideal diffusive state, even under the typical microgravity conditions of space platforms.     

Ultrahigh density modulation of aligned single-walled carbon nanotube arrays

Controlling the densities of aligned single-walled carbon nanotube arrays (SWNTs) on ST-cut quartz is a critical step in various applications of these materials. However the growth mechanism for tuning SWNT density using the chemical vapor deposition (CVD) method is still not well understood, preventing the development of efficient ways to obtain the desired results. Here we report a general “periodic” approach that achieves ultrahigh density modulation of SWNT arrays on ST-cut quartz substrates—with densities increased by up to ∼60 times compared with conventional methods using the same catalyst densities—by varying the CH₄ gas “off” time. This approach is applicable to a wide range of initial catalyst densities, substrates, catalyst types and growth conditions. We propose a general mechanism for the catalyst size-dependent nucleation of SWNTs associated with different free carbon concentrations, which explains all the observations. Moreover, the validity of the model is supported by systematic experiments involving the variation of key parameters in the “periodic” CVD approach.