Turbidity data obtained from image analysis in near critical hydrogen.

Video images are being used with increased frequency in science, supplanting current methods such as light scattering by statistical evaluation of the images. In this study we use light turbidity data due to density-induced refractive index fluctuations to obtain critical amplitudes from image analysis. In order to bring hydrogen (H_{2}) very close to its critical point, we place the sample cell under weightlessness using a magnetic levitation device.

Manipulating Charge Transfer from Core to Shell in CdSe/CdS/Au Heterojunction Quantum Dots.

The photophysics of charge-transfer and recombination mechanisms in a heterojunction structure of CdSe/CdS/Au quantum dots (QDs) are studied by temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL). We manipulate the charge transfer from core to shell surface by varying the tunneling barrier height resulting from temperature variation and the barrier width resulting from shell thickness variation. The charge-transfer process, which can be described by a tunneling transmission model, is manifested by two competitive recombination processes, an intrinsic exciton emission and a trap emission in the near-infrared (NIR) range.

Measuring the Transition Rates of Coalescence Events during Double Phase Separation in Microgravity.

Phase transition is a ubiquitous phenomenon in nature, science and technology. In general, the phase separation from a homogeneous phase depends on the depth of the temperature quench into the two-phase region. Earth's gravity masks the details of phase separation phenomena, which is why experiments were performed under weightlessness.

Pattern Evolution during Double Liquid-Vapor Phase Transitions under Weightlessness.

Phase transition in fluids is ubiquitous in nature and has important applications in areas such as the food industry for volatile oils' extraction or in nuclear plants for heat transfer. Fundamentals are hampered by gravity effects on Earth. We used direct imaging to record snapshots of phase separation that takes place in sulfur hexafluoride, SF₆, under weightlessness conditions on the International Space Station (ISS).

Non-equilibrium concentration fluctuations in superparamagnetic nanocolloids.

We investigate non-equilibrium concentration fluctuations during the free diffusion of a colloidal suspension against pure water. We investigate FeO superparamagnetic nanocolloids with sizes between 1 and 10 nm by means of a shadowgraph apparatus to determine the mixture mass diffusion coefficient and kinematic viscosity. The experiments were performed in three distinct conditions: Experiment 1 is without any magnetic field; Experiment 2 with a vertical magnetic field; Experiment 3 after turning off the magnetic field.

Direct imaging of long-range concentration fluctuations in a ternary mixture.

We used a direct imaging technique to investigate concentration fluctuations enhanced by thermal fluctuations in a ternary mixture of methanol (Me), cyclohexane (C), and partially deuterated cyclohexane (C*) within 1mK above its consolute critical point. The experimental setup used a low-coherence white-light source and a red filter to visualize fluctuation images. The red-filtered images were analyzed off-line using a differential dynamic microscopy algorithm that allowed us to determine the correlation time, τ, of concentration fluctuations.

Dimple coalescence and liquid droplets distributions during phase separation in a pure fluid under microgravity.

Phase separation has important implications for the mechanical, thermal, and electrical properties of materials. Weightless conditions prevent buoyancy and sedimentation from affecting the dynamics of phase separation and the morphology of the domains. In our experiments, sulfur hexafluoride (SF6) was initially heated about 1K above its critical temperature under microgravity conditions and then repeatedly quenched using temperature steps, the last one being of 3.

Imaging critical fluctuations of pure fluids and binary mixtures.

We use optical microscopy techniques to directly visualize the structures that emerge in binary mixtures and pure fluids near their respective critical points. We attempt to understand these structures by studying the image formation using both a phase contrast and a dark field filter to our microscope. We found that images of critical fluctuations for both liquid-liquid and liquid-gas critical systems have gray level intensity histograms with Gaussian shape.

Dynamic structure factor of density fluctuations from direct imaging very near (both above and below) the critical point of SF(6).

Large density fluctuations were observed by illuminating a cylindrical cell filled with sulfur hexafluoride (SF(6)), very near its liquid-gas critical point (|T-T(c)|< 300 μK) and recorded using a microscope with 3 μm spatial resolution. Using a dynamic structure factor algorithm, we determined from the recorded images the structure factor (SF), which measures the spatial distribution of fluctuations at different moments, and the correlation time of fluctuations. This method authorizes local measurements in contrast to the classical scattering techniques that average fluctuations over the illuminating beam.

Dynamics of a wetting layer and Marangoni convection in microgravity.

Near the liquid-vapor critical point in pure fluids, material and thermal properties vary considerably with temperature. In a series of microgravity experiments, sulfur hexafluoride (SF6) was heated ∼1 K above its critical temperature, then quenched below the critical temperature in order to form gas and liquid domains. We found a power law exponent of 0.

Experimental study of nonequilibrium fluctuations during free diffusion in nanocolloids using microscopic techniques.

We report quantitative experimental results regarding concentration fluctuations based on a small-angle light-scattering setup. A shadowgraph technique was used to record concentration fluctuations in a free-diffusion cell filled with colloids. Our experimental setup includes an objective attached to the CCD camera to increase the field of view.

Universality in early-stage growth of phase-separating domains near the critical point.

We present both the experimental and computational methods and results of phase-separating experiments performed with sulfur hexafluoride (SF6) close to its critical density. These experiments were performed in microgravity to suppress buoyancy and convection-driven effects. Phase separation under reduced gravity is analyzed for both 0.

Wetting film dynamics during evaporation under weightlessness in a near-critical fluid.

By performing near-critical fluid experiments in the weightlessness of an orbiting space vehicle, we have suppressed buoyancy-driven flows and gravitational constraints on the liquid-gas interface of a large gas bubble. At equilibrium, the liquid completely wets the walls of a cylindrical cell, and the bubble is pushed to the sidewall. In these experiments the system's temperature T is increased at a constant rate past the critical temperature T(C), pushing it slightly out of equilibrium.