Dimitris Vlassopoulos

Dimitris Vlassopoulos

2019 Bingham Medalist

University of Crete and IESL-FORTH

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Video of Bingham Lecture (members only)
Citation

For outstanding achievements in the field of experimental soft matter rheology; including revolutionary discoveries in the key areas of polymer blends, branched and ring polymers, and soft colloids; and for the development of powerful rheometric and rheo-optical tools.

Professor Dimitris Vlassopoulos received his Diploma in Chemical Engineering from the NTU Athens in 1983, and then did his graduate work at Princeton where he worked for Bill Schowalter, primarily on the topic of drag reduction by dilute polymer solutions, completing his thesis in 1990. After a year at Mobil Research and Development Corporation in Paulsboro, he elected to return to Greece, initially as a contract researcher at the FORTH Institute in Crete, but in 1998 he simultaneously joined University of Crete, initially in the Department of Physics and subsequently in the Department of Material Science and Engineering. He has also held visiting professorships at the University of Delaware (USA), the University of California, Santa Barbara (USA), the ETH in Zürich (Switzerland), and held the Michelin Chair at the ESPCI (France).

Scientifically, he has published extensively on many subjects in rheology (more than 200 papers to date), and he has become an intellectual leader on a number of the most important topics in rheology. Beyond that, however, he has been active and a leader in rheology. He has served on the Executive Committee of the SOR. He has twice been on the editorial board of Rheologica Acta and served as an editor from 2006-2011. He is a member of the editorial board of JOR and also Physics of Fluids, and is associate editor of Soft Matter. He has been recognized twice by SOR for his research, once via the Publication Award in 2011 and this past year by election as a Fellow of the SOR. He is a former president of the Hellenic Society of Rheology. In 2015, he was awarded the Weissenberg Award of the European Society of Rheology.

Dimitris’ research focuses on molecular engineering of soft matter with emphasis on fundamental aspects of polymer and suspension rheology as well as on bridging the gap between these two disciplines. His approach consists of devising strategies based on molecular design of model systems with adaptable molar mass and architecture or tunable interactions (from hard to ultrasoft), and developing appropriate protocols and rheometric tools for obtaining molecular insights into the rheology of polymers, supramolecular assemblies, and soft colloids. Some highlights include:

Interplay of thermodynamics and rheology in polymer blends

By identifying and quantifying the contribution of enhanced pre-transitional composition fluctuations to the viscoelasticity of partially miscible polymer blends, Dimitris showed that rheology is a very sensitive tool for determining both the bimodal and spinodal phase boundaries of blends. This approach is now being used routinely. The role of dynamic asymmetry of the components has been elucidated, along with the role of shear in inducing mixing or demixing. Recently, a universal rheological diagnostic scheme for phase transitions was proposed.

Molecular rheology of branched polymers

Dimitris’ work represents the most comprehensive experimental study of the role of branches on entangled polymer rheology. This was achieved by designing and obtaining a library of well-characterized branched polymers with precise molar masses and architecture (size, position and distribution of branches). To test and advance tube-model theories, different model polymers like pom-pom and Cayley trees were investigated and the mechanism of hierarchical relaxation assessed, but the cornerstone of this effort is the comb polymers paradigm. Their methodological study under linear and nonlinear shear and extensional deformation, along with targeted modeling to correct tube theories, revealed important molecular insights into their rheology, such as the interplay of branch and backbone relaxation, the double shear stress overshoot for large branches, or the importance of dynamic dilution in both nonlinear damping and extension hardening. This work has motivated further academic and industrial developments. For example, the state-of-the-art BoB tube model has been successfully applied to branched polymers with marginally entangled branches, whereas molecular design parameters for tailoring the rheology of comb polymers have been proposed recently.

Ring polymers

Dimitris’ discovery that in the absence of free ends, entangled polymers do not form a network with a plateau modulus but instead exhibit a power-law stress relaxation, resolved a 30-years-old mystery, and revealed the crucial importance of appropriate material characterization for molecular rheology, opening the route for exploration of important biological problems such as the dynamics in chromosome territories. This has been one of the outstanding challenges in polymer physics and rheology. The extraordinary sensitivity of rings dynamics to traces of unlinked polymeric chains sets them apart from any other polymer and reflects the extreme sensitivity of rheology as a molecular probe. His most recent work focuses on their unusual nonlinear shear response, which is characterized by weaker thinning compared to their linear precursors, their uniaxial extensional rheology, as well as their remarkably efficient use as rheology modifiers of linear polymer matrices.

From polymers to colloids

Due to their inherent density heterogeneity, certain types of hyperbranched polymers, like multi-arm stars in the melt, exhibit a complex viscoelastic response with distinct polymeric and colloidal contributions. As a result, appropriately designed star polymers with controlled number and size of arms have been established as model soft colloids with tunable interactions. This work forms the foundation of what has emerged as the field of solvent-free colloids and has impacted significantly the large field of nanocomposites. Colloidal star polymers (in melt or solution) constitute one of the two pillars in the field of soft colloids (the other being microgels), encompassing the key features of softness (shape adjustment and interpenetration) that affect their rheology.

Soft colloids: metastability, tunable rheology and flow instabilities

Extensive work by Dimitris and his colleagues over several years with colloidal stars as the prime paradigm has addressed outstanding challenges associated with the flow of yield-stress fluids, colloidal glasses, pastes, and gels. Highlights include the observation of liquid-to-solid transition upon heating, the link among aging, yielding, and shear banding in soft colloidal glasses, the unusual two-step mechanisms of aging, yielding, and slow dynamics in glassy hairy nanoparticles, which are often studied by means of specially developed protocols, the shear-induced crystal-to-crystal transition without intermediate melting, and the analysis and exploitation of osmotic pressure effects in soft colloidal mixtures (which drive both particle compression and depletion). The latter has opened Pandora’s box and enabled the use of appropriately designed mixtures for tailoring the flow properties of soft composites, driving the field into new territories. His most recent work addresses the questions of jamming and shape effects in hairy particle glasses.

Rheometry

The redesign of cone-partitioned plate geometry for strain-controlled rheometers has revitalized the field of nonlinear shear rheology and, combined with enhanced molecular understanding, holds the premise for understanding the transient polymeric response in strong shear and especially the contributions of convective constraint release, stretching, and tumbling. More recently, both the first and second normal stresses were measured in melts of linear polymers at high temperatures. His most recent efforts are focusing on further improving the measurement of second normal stress differences at high temperatures.

Other works in very important areas that Dimitris has accomplished include his work on associating polymers and recent work on polymer blends with architectural dispersity, including the use of interaction chromatography as an indispensable tool in molecular rheology along with advanced rheometry for true molecular understanding of polymer viscoelasticity and physics.