This study concerns the use of rheology to obtain quantitative information on the microstructure of polyolefin-based materials. Such properties have crucial implications along both directions of the chain-of knowledge of polymeric materials. Moving backwards, in the direction of polymer synthesis, rheology can link the material response to the details of the molecular architecture. Moving forwards, in the direction of technological applications, rheology can provide a relevant link to the processing as well as to the final material properties. Determining the molecular structure of polyolefin-based materials is a relevant scientific and technological challenge. Molecular weight, its distribution, and structural details are often difficult to ascertain for industrial polymers. We attempt to address this challenge starting from the linear rheological response of polyolefins evaluated over the largest possible frequency range. To this end, linear rheology is measured in concentrated, entangled solutions, instead of melts, thus overcoming some intrinsic experimental difficulties encountered in measuring the latter. A time-concentration superposition principle is then applied to obtain rheological master curves. Molecular models and constitutive equations for entangled solutions are used to extract the quantitatively relevant microstructural information.

Blood is a complex suspension of red blood cells (RBCs), white blood cells (WBCs), and platelets in an aqueous solution known as plasma, containing dissolved proteins. While many works consider blood as a Newtonian fluid, it exhibits a pronounced non-Newtonian character. This is primarily explained by the ability of RBCs to aggregate/disaggregate, deform, and align to flow.

At low shear rates, blood proteins enhance the formation of a complex network consisting of column-like red cell aggregates, known as rouleaux. In almost stasis, these rouleaux form three-dimensional networks. As shear rates increase, these structures tend to disintegrate, leading to a state where red blood cells flow separately. The dynamic property of blood promoting this non-Newtonian nature necessitates the use of sophisticated rheological models to adequately capture its rheological response. The TEVP (Thixo-elastoviscoplastic) model, initially proposed by Varchanis et al. (2019) and later improved by Giannokostas et al. (2020) and Spyridakis et al. (2024), proves to be an effective model for this purpose.

This presentation will focus on the rheological modeling of blood and its application in hemodynamics, particularly in microcirculation (Giannokostas et al. (2022)).

References:

S Varchanis, G Makrigiorgos, P Moschopoulos, Y Dimakopoulos, J Tsamopoulos. "Modeling the rheology of thixotropic elasto-visco-plastic materials." Journal of Rheology 63 (4), 609-639 (2019).

K Giannokostas, P. Moschopoulos, S. Varchanis, Y. Dimakopoulos, J. Tsamopoulos. "Advanced Constitutive Modeling of the Thixotropic Elasto-Visco-Plastic Behavior of Blood: Description of the Model and Rheological Predictions." Materials, 13, 4184 (2020). https://doi.org/10.3390/ma13184184

A Spyridakis, P Moschopoulos, S Varchanis, Y Dimakopoulos, J Tsamopoulos. "Thixo-elastoviscoplastic modeling of human blood." Journal of Rheology 68 (1), 1-23 (2024).

K Giannokostas, Y Dimakopoulos, J Tsamopoulos. "Shear stress and intravascular pressure effects on vascular dynamics: Two-phase blood flow in elastic microvessels accounting for passive stresses." Biomechanics and Modeling in Mechanobiology 21 (6), 1659-1684 (2022).

Our work concentrates on the development of semiconductor lasers and integrated optics for applications in Quantum science and technology. We will present our research on coherent laser arrays operating in epitaxially grown semiconductor membrane quantum wells. The membranes are deposited by transfer on substrates of oxidised silicon and we record the real and reciprocal space of the laser emission. The Laser arrays operate in a lateral emission geometry and are waveguides lasers where the end mirrors are the end-facets of the cleaved membranes.  Cavities in the order of 100 microns are usually formed and we measure Laser thresholds down to 50 mW. We are able to form waveguide laser arrays and we use real and reciprocal space imaging to examine the emission characteristics of the lasing cavities. We discover that the Laser arrays are mutually coherent and the lasers can operate on a single frequency or multiple longitudinal modes. We will present how the emission of the Lasers and their coherence can be controlled using a digital micromirror device to position and shape the pump illumination, we will show control of threshold, coherence, frequency and possible control of phase. We will also discuss potential applications in integrated photonic circuits.

I will finally briefly present other parts of my research on semiconductor lasers, Terahertz spectroscopy and integrated optics and how I think my research activities fit in the environment of IESL and FORTH.  

Transmission Electron Microscopy (TEM) is a powerful tool for investigating the structural and electronic properties of materials at the nanoscale. In recent years, the integration of 2D materials as sample substrates in TEM has attracted significant attention due to their unique properties.   Graphene and hexagonal boron nitrate (h-BN) stand out as exceptional substrates for a diverse set of TEM applications.

The first part of the lecture addresses the exceptional structural, electronic and chemical properties of both h-BN and graphene, emphasizing their specific advantages like their atomic thickness, high thermal stability and chemical inertness. These features make both graphene and h-BN ideal substrates for supporting sensitive specimens during TEM analysis, reducing background noise and preventing unwanted interactions that may alter the intrinsic characteristics of the sample.

The second section explores the impact of two dimensional substrates on imaging resolution and contrast enhancement in TEM. Moreover, the structural and conductive properties of h-BN and graphene can alleviate radiation damage, allowing for prolonged observation of electron beam sensitive materials.

The third section explores the promise of graphene and h-BN in facilitating in-situ experiments within the TEM environment. Researchers can exploit the stable and inert nature of both 2D materials to study dynamic processes, such as phase transitions, chemical reactions, and mechanical deformations with unprecedented clarity and precision in their native environment.