Ana Oprisan

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. We performed two separate experiments, one with 20 nm gold and the other with 200 nm silica colloids, and extracted both the structure factors and the correlation time during the early stages of concentration fluctuations. The temporal evolution of fluctuations was also qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. We found that the correlation time versus wavenumber for gold nanocolloids is concave shaped, whereas, for silica colloids, it is convex shaped. The difference in correlation time behavior is not only due to the size of the particle, but also to possible plasmonic interactions in gold colloids.

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. From τ, we determined the mutual mass diffusion coefficient, D, very near and above the critical point of Me-CC* mixtures. We also numerically estimated both the background and critical contributions to D and compared the results against our experimental values determined from τ. We found that the experimental value of D is close to the prediction based on Stokes-Einstein diffusion law with Kawasaki’s correction.Graphical abstract

Non-equilibrium concentration fluctuations in superparamagnetic nanocolloids

Abstract.We investigate non-equilibrium concentration fluctuations during the free diffusion of a colloidal suspension against pure water. We investigate Fe2O3 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. We found no correlation between the kinematic viscosity coefficient and the external magnetic field. Conversely, we found that the mass diffusion coefficient decreases in the presence of the external magnetic field and slowly rebounds after the magnetic field was turned off.Graphical abstract

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 3.6 mK, until it crossed its critical temperature and phase-separated into gas and liquid domains. Both full view (macroscopic) and microscopic view images of the sample cell unit were analyzed to determine the changes in the distribution of liquid droplet diameters during phase separation. Previously, dimple coalescences were only observed in density-matched binary liquid mixture near its critical point of miscibility. Here we present experimental evidences in support of dimple coalescence between phase-separated liquid droplets in pure, supercritical, fluids under microgravity conditions. Although both liquid mixtures and pure fluids belong to the same universality class, both the mass transport mechanisms and their thermophysical properties are significantly different. In supercritical pure fluids the transport of heat and mass are strongly coupled by the enthalpy of condensation, whereas in liquid mixtures mass transport processes are purely diffusive. The viscosity is also much smaller in pure fluids than in liquid mixtures. For these reasons, there are large differences in the fluctuation relaxation time and hydrodynamics flows that prompted this experimental investigation. We found that the number of droplets increases rapidly during the intermediate stage of phase separation. We also found that above a cutoff diameter of about 100 microns the size distribution of droplets follows a power law with an exponent close to −2 , as predicted from phenomenological considerations.Graphical abstract

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). The system was already at liquid-vapor equilibrium slightly below the critical temperature and further cooled down by a 0.2-mK temperature quench that produced a new phase separation. Both full view and microscopic views of the direct observation cell were analyzed to determine the evolution of the radii distributions. We found that radii distributions could be well approximated by a lognormal function. The fraction of small radii droplets declined while the fraction of large radii droplets increased over time. Phase separation at the center of the sample cell was visualized using a 12× microscope objective, which corresponds to a depth of focus of about 5 μm. We found that the mean radii of liquid droplets exhibit a t1/3 evolution, in agreement with growth driven by Brownian coalescence. It was also found that the mean radii of the vapor bubbles inside the liquid majority phase exhibit a t1/2 evolution, which suggest a possible directional motion of vapor bubbles due to the influence of weak remaining gravitational field and/or a composition Marangoni force.

Near-critical fluid boiling: Overheating and wetting films

The heating of coexisting gas and liquid phases of pure fluid through its critical point makes the fluid extremely compressible, expandable, slows the diffusive transport, and decreases the contact angle to zero (perfect wetting by the liquid phase). We have performed experiments on near-critical fluids in a variable volume cell in the weightlessness of an orbiting space vehicle, to suppress buoyancy-driven flows and gravitational constraints on the liquid-gas interface. The high compressibility, high thermal expansion, and low thermal diffusivity lead to a pronounced adiabatic heating called the piston effect. We have directly visualized the near-critical fluid’s boundary layer response to a volume quench when the external temperature is held constant. We have found that when the system’s temperature T is increased at a constant rate past the critical temperature Tc, the interior of the fluid gains a higher temperature than the hot wall (overheating). This extends previous results in temperature quenching experiments in a similarly prepared system when the gas is clearly isolated from the wall. Large elliptical wetting film distortions are also seen during these ramps. By ray tracing through the elliptically shaped wetting film, we find very thick wetting film on the walls. This wetting film is at least one order of magnitude thicker than films that form in the Earth’s gravity. The thick wetting film isolates the gas bubble from the wall allowing gas overheating to occur due to the difference in the piston effect response between gas and liquid. Remarkably, this overheating continues and actually increases when the fluid is ramped into the single-phase supercritical phase.

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. Under such conditions, the pure fluid sulphur hexafluoride (SF6) near its critical point also benefits from the universality of phase separation behavior and critical slowing down of dynamics. Initially, the fluid was slightly below its critical temperature with the liquid matrix separated from the vapor phase. A 0.2 mK temperature quench further cooled down the fluid and produced a double phase separation with liquid droplets inside the vapor phase and vapor bubbles inside the liquid matrix, respectively. The liquid droplets and the vapor bubbles respective distributions were well fitted by a lognormal function. The evolution of discrete bins of different radii allowed the derivation of the transition rates for coalescence processes. Based on the largest transition rates, two main coalescence mechanisms were identified: (1) asymmetric coalescences between one small droplet of about 20 μm and a wide range of larger droplets; and (2) symmetric coalescences between droplets of large and similar radii. Both mechanisms lead to a continuous decline of the fraction of small radii droplets and an increase in the fraction of the large radii droplets. Similar coalescence mechanisms were observed for vapor bubbles. However, the mean radii of liquid droplets exhibits a t1/3 evolution, whereas the mean radii of the vapor bubbles exhibit a t1/2 evolution.

Quantitative estimates of the electric charge for a classical electrostatics experiment

Despite the widespread use of the charged tape method to demonstrate electrostatic interaction and the advances in affordable sensors that can directly measure electric charges, there is no detailed analysis of a quantitative model for such experiments. As a result, we present two different theoretical approaches that allowed us to quantitatively estimate the amount of electric charge present on transparent tape. Our first model assumed that the entire electric charge was concentrated at the center of mass of the tape and gave a simple algebraic solution, which came close to the order of magnitude of the actual electric charge measured with a PASCO charge sensor. Our second model approximated the sticky tape as a linear uniformly charged object. The estimated electric charge based on the uniform charge density model is closer to the measured value of electric charge than the value found by approximating each piece of tape as a point charge. Based on the statistical test, the experiments did not support the predictions of the point charge model. At the same time, the statistical test validated the uniform charge density model with a p-value of 0.01.

Finding the temperature using image analysis techniques

Many experiments used light scattering to visualize the fluctuations of fluids density. Fluids near the critical point are affected by gravity because the compressibility of the fluid is very large near the critical point. Therefore, microgravity experiments allowed new phenomena to be discovered by reducing convection, sedimentation and buoyancy phenomena. In order to study, fluctuation and phase separation processes near the critical point of pure fluids without the influence of the Earths gravity, a number of experiments were performed in microgravity. Our results refer to a set of experiments that studied local density fluctuations by illuminating a cylindrical cell filled with sulfur hexafluoride, near its liquid-gas critical point. Using image analysis, we estimated the temperature of the fluid in microgravity from the recorded images showing fluctuations of the transmitted and scattered light. Our method has the advantage of avoiding any reference to the spatial correlation of the pixels in the recorded images. We assumed that the variation of the scattered light intensity is proportional to the average value of the gray levels. Furthermore, we also assumed that a small fluctuation of the fluid density induces a change in the scattered light intensity that can be measured from average gray scale intensity of the image. We found that the histogram of an image can be fitted to a Gaussian relationship and by determining its width we were able to estimate the position of the critical point.

Computational Model of Population Dynamics Based on the Cell Cycle and Local Interactions

Our study bridges cellular (mesoscopic) level interactions and global population (macroscopic) dynamics of carcinoma. The morphological differences and transitions between well and smooth defined benign tumors and tentacular malignat tumors suggest a theoretical analysis of tumor invasion based on the development of mathematical models exhibiting bifurcations of spatial patterns in the density of tumor cells. Our computational model views the most representative and clinically relevant features of oncogenesis as a fight between two distinct sub‐systems: the immune system of the host and the neoplastic system. We implemented the neoplastic sub‐system using a three‐stage cell cycle: active, dormant, and necrosis. The second considered sub‐system consists of cytotoxic active (effector) cells — EC, with a very broad phenotype ranging from NK cells to CTL cells, macrophages, etc. Based on extensive numerical simulations, we correlated the fractal dimensions for carcinoma, which could be obtained from tumor ima...