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Tunable Strong Coupling Phenomena at Atomistic Level

Final Report Summary - TUSCOPAL (Tunable Strong Coupling Phenomena at Atomistic Level)

The fellow has successfully completed the objectives of the TUSCOPAL project (2013-2016) at the Harvard University, USA and Heinrich-Heine University, Germany by following the project outline. During this project, the fellow has been trained extensively on cutting-edge technologies on soft materials as well as performed his own experiments and analyzed data with software of optimized algorithms of particle location and tracking methods.

Initial training: In this time period the fellow was trained with different techniques of polymerization, emulsion, microfluidics to synthesize colloidal particles. At the same time, he gained experimental expertise on other techniques such as confocal microscope, static and dynamic light scattering, scanning electron microscopy (SEM), atomic force microscopy (AFM), soft lithography, rheology and large scale image analysis, etc. The fellow took unique research facilities of Centre for Nanosciences (CNS) at Harvard University. The fellow also received certificates of “Responsible conduct of research” by Harvard University.

Quasi-two-dimensional complex plasma containing spherical particles and their binary agglomerates
Summary: A new type of quasi-two-dimensional complex plasma system was observed which consisted of monodisperse microspheres and their binary agglomerations (dimers). The particles and their dimers levitated in a plasma sheath at slightly different heights and formed two distinct sublayers. The system did not crystallize and may be characterized as disordered solid. The dimers were identified based on their characteristic appearance in defocused images, i.e. rotating interference fringe patterns. The in-plane and inter- plane particle separations exhibit nonmonotonic dependence on the discharge pressure, which agrees well with theoretical predictions.
Reference: Phys. Rev. E, 93, 053202 (2016)

Dust interferometers in plasmas
Summary: A novel interferometric diagnostic has been proposed to measure diameter of individual spherical dust particle instantly inside the plasma chamber. The technique is based on the defocus image analysis of both spherical particles and their binary agglomerates (dimers). Stationary and rotational fringes on defocus images are the characteristics of spheres and dimers respectively. With increasing particle diameter, the fringe pattern appears distinct and the number of fringes increases. Two critical diameters of the particles have been identified, one of which corresponds to the fringe appearance on spherical particles and the other on binary agglomerates respectively.
Reference: Phys. Rev. E, 93, 031201(R) (2016)

Triple point at the triple junction in colloidal films microscopically resolved
Summary: The triple point associated with face-centered-cubic (fcc) crystal, body-centered-cubic (bcc) crystal and liquid phases for repulsive Yukawa (screened Coulomb) systems has been explored experi- mentally at the single particle level using charge stabilized PMMA colloidal suspension in a nonpolar solvent. The triple point represents the triple junction, which develops under mechanical equilibrium due to the intersection of solid-liquid interface and bcc-fcc grain boundary. The grain boundary groove develops at the triple junction as a morphological change along with dihedral angle. Following Young’s equation, it is found that the bcc-fcc grain boundary energy is almost 1.3 times higher than fcc-liquid interfacial energy close to the triple point.
Reference: to be submitted for publications

Tunable strong coupling phenomena with transparent aqueous core-shell microgels: from liquid to crystal to glass
Summary: The compressibility of core-shell microgels have been tuned to explore the strong coupling phenomena as- sociated with liquid, solid and glass phases with in- creasing volume fraction. The crystal structures al- ways appear to be close packed with clearly identified solid-liquid interface. Contrary to conventional phase space behavior of colloidal suspension, the microgels exhibit supercooled liquid and subsequent glass phases at higher volume fraction due to their higher compressibility characteristics. However it is found that the glass behavior of microgels resembles to those of hard spheres.
Reference: to be submitted for publications

Direct visualization of diffusionless BCC→FCC phase transformations in 3D colloidal crystals
Summary: Gravitation induced slowly developed uniaxial stress produces phase transformations within three dimensional sedimented colloidal crystals for highly charged PMMA particles in nonpolar solvents under steady state condition. The complete phase space of the suspension has been explored in a single sample with supernatant at the top followed by liquid, low density BCC structures to close packed FCC structures at the bottom. The solid-solid transition zone intermediate between BCC and FCC crystals has been identified with a characteristic structure.
Reference: to be submitted for publications

Experiments on classical many body strongly correlated systems: from dusty plasmas to colloids
Summary: Dusty (complex) plasma is the plasma state of soft matter, which shares unique complementary features with popular soft matter system “colloids”. Both these systems can be used to explore statistical physics of classical many body strongly correlated systems due to their experimental simplicities. Hard sphere colloids have been used extensively for a long time where volume fraction is the only controlling parameter to explore entire phase space. However, more interesting features are associated with soft colloids where the suspension can be stabilized by adding either charge or polymers on the surface of individual particle. Highly charged colloidal particles form “Wigner crystals” which are similar to that observed in dusty plasma experiments in the form of plasma crystal. In this report a comparative study has been made to explore three dimensional melting transition for two different types of soft colloids: sterically stabilized aqueous core-shell (CS) microgels and charge stabilized PMMA particles in nonpolar solvents.
Reference: to be submitted for publications

Socio-economic impacts of the project:
During this project, the fellow was trained on cutting edge technologies such as synthesis and characterization of colloidal nanoparticles, soft lithography, microscopy, microfluidics, and particle tracking algorithms used in colloids and plasma physics. The training and experiences will help him to work on next generation semiconductor devices, where both plasma and photolithography techniques are used to produce smaller and faster chips. In this way, the fellow will successfully contribute to the European semiconductor industry leading to tremendous impact on next generation technologies.

Acknowledgement: The fellow sincerely thanks REA, EU (Marie-Curie IOF) for financial support, Harvard University and Heinrich-Heine University (HHU) for providing excellent academic environments to conduct research activities along with administrative supports.