Experiments
In my soft condensed matter (liquids, colloids, polymers, critical fluids and a number of biological materials) research lab I conduct low angle scattering experiments on nanocolloidal suspensions.

My research lab also shares and analyzes unique experimental data on supercritical fluids recorded in microgravity through a fruitful collaboration with Dr. John J. Hegseth (University of New Orleans) and the Drs. Yves Garrabos, Carole Lecoutre-Chabot, and Daniel Beysens (Centre National de la Researche Scientifique, Universite Bordeaux, and Commissariat a l'Energie Atomique, Grenoble, France).

Signal and Image Processing
On the computational side, I use a wide range of signal and image processing techniques to enhance and denoise experimental data and then characterize the density or concentration fluctuations.

Concentration Fluctuations in Nanocolloids - Low-angle Light Scattering Experiments


Background
Recent improvement of dynamic light scattering methods allowed visualization of macroscopic fluctuations. Giant fluctuations in low molecular weight liquid mixtures (water-urea and water-glycerol), protein solution (lysozyme), suggested that the fluctuations induced by a gradient of concentration have universal character. Low angle light scattering methods take advantage of very high pixel density (1000 x 1000 or higher) of modern CCD cameras to gain statistical accuracy by providing 10^6 independent measurements at one shot [Wong and Wiltzius, 1993; Ferri, 1997; Cipelletti and Weitz, 1999]. Such experimental setups allow direct measurement of ensemble averages of the spatial and temporal fluctuations of fluid properties. Therefore, a direct comparison of theoretically estimated correlation function and structure factor against experimental results is possible [Brogioli et al., 2008].

Experiments
We use two different experimental setups designed and build at the College of Charleston in order to study free diffusion in nanocolloids [Vailatti and Giglio, 1997, 1998; Croccolo, 2005; Croccolo et al., 2006, 2007]. Early on (2008-2009), we investigated concentration fluctuations using a HeNe laser and a schlieren optical method. Lately we moved to a different experimental setup a superluminescent diode and a shadowgraph optical method (after 2011).

Experiment: Light scattering using a low-coherence superluminescent diode (after 2011):
Experimental Setup Fluctuating Image
Experimental Setup Fluctuating Image

Experiment: Low-angle light scattering experiment using a HeNe laser (Oprisan et al, Applied Optics, 2010):
Experimental Setup Fluctuating Image Structure Factor and Correlation Time
Experimental Setup Fluctuating Image Structure Factor and Correlation Time


Data Acquisition and Image Processing
I used LabView to recorded experimental data continuously at different rates (from 30 frames/s down to 1 frame/s). I analyzed the recorded images with in-house Matlab code. The computational image processing side of the project involves, among others, the computation of :
  1. static and dynamic structure factor,
  2. temporal sections through fluctuation images,
  3. convolutions and correlations, denoising,
  4. correlation time of fluctuations,
  5. diffusion coefficient of nanoparticles.


Results
With the noise-sensitive experimental setup we investigate:
  1. early stages of diffusion process. We were interested in capturing not only the concentration fluctuations, but also propagating modes developed during the early stages of diffusion.
  2. the timeframe for fluctuation evolution. Do the fluctuations develop after 10, 1000, 10000 seconds?
  3. the evolution of concentration fluctuations in different colloids (water-soluble silica and gold - 2008 & 2009 experiments; water-soluble silica, gold and silver - experiments I conducted after 2011).


Density Fluctuations and Phase Separation - Microgravity Experiments


Background
When pure fluids in microgravity, or two-component density matched mixture on earth, are quenched from the homogeneous one-phase state through their critical point, they evolve to their final two-phase equilibrium state by the formation and growth of domains of different phases. The volume fraction determines whether the system follows a spinodal decomposition or a nucleation path. The presence of solid walls and the wetting effects dramatically modify phase separation dynamics.

Experiments
One of our experimental setups and excerpts from our published results (Oprisan et al, Physical Review E, 2011):
Microgravity Sample Cell Unit Early Fluctuations Phase Separation
Microgravity Sample Cell Unit Early Fluctuations Phase Separation


Results
We investigate phase separation phenomenon at a coexistence temperature of gas and liquid phases in sulfur hexafluoride (SF6) in microgravity.
We found that interconnected domains occurred during the early stage (ES) of phase separation in a supercritical pure fluid quenched through its critical point. By the end of the ES, a wetting film that separates the copper wall of the sample cell unit (SCU) and the gas bubble is clearly visible.

During the intermediate stage (IS) of the phase separation process the wetting layer thickness rapidly increases. Based on our experimental data, we found a power law exponent for the growth of the average thickness of the wetting layer. Our experimentally determined power law exponent is close to the value obtained from other experiments investigating the growth of wetting layer in binary mixtures.
The closest theoretical prediction of a power law for the growth of wetting films belongs to Steiner and Klein and is based on a diffusion-limited mechanism.

We also found that at some point during the phase separation, the wetting layer increases very slowly - a regime we call late stage (LS) phase separation. During the LS of the phase separation we observed thermocapillary migration both in microscopic and full view images of the SCU.

In addition to droplets formation during phase separation, convective flows may occur. Thermocapillary migration is an important topic in material processing that is usually studied in experiments by inducing a temperature gradient inside a cell containing fluids or melted alloy. We experimentally measured the velocity of the liquid droplet embedded inside the gas phase and found a very good agreement with the predicted value of 0.386 microns/s based on Young's theory of the thermocapillary effect. Our measurements show a periodic motion of the large gas bubble seen in the macroscopic (full) view compatible with thermocapillary-induced (Marangoni) convection.