The research group of Professors Mikhail A. Anisimov and Jan V. Sengers at IPST is concerned with theoretical and experimental studies of mesoscopic fluctuations in soft matter, both in molecular fluids and in complex fluids.
What is mesoscopic thermodynamics ?
Mesoscopic and namnoscale thermodynamics is a field of science that can be defined as a semi-phenomenological approach to systems and phenomena in which a length scale, intermediate between the atomistic scale and the macroscopic scale, emerges and where this intermediate length scale does affect the physical properties profoundly. This is still thermodynamics, as the phenomena are still governed by statistical physics, but, in contrast to "macro-thermodynamics" it explicitly contains a mesoscopic length scale associated with the structure of soft matter.
Study of critical phenomena
In molecular fluids thermal fluctuations are known to become mesoscopic near critical points. Light-scattering experiments in their laboratory have contributed to the experimental verification of the concept of universality of critical behavior in fluids and fluid mixtures, including electrolyte solutions. With the concept of isomorphism of critical behavior, a large variety of apparent different experimental observations can be transformed into a single universal representation in terms of appropriate scaling fields.
What are hydrodynamic fluctuations and complex fluids?
Complex fluids differ from molecular fluids by possessing a mesoscopic structure even far away from critical points. Such a mesoscopic structure will induce coupling between different hydrodynamic fluctuations. Light-scattering experiments in polymer solutions in their laboratory at IPST have shown that the intensity of the fluctuations is affected by a competition between the spatial extent of the critical fluctuations and the radius of gyration. Moreover, the dynamics of critical fluctuation is strongly affected by a coupling with viscoelastic relaxation of the polymer chains. Hence, dynamic light-scattering experiments can provide microrheological information about the hydrodynamic environment of macromolecules in solution.
Study of "smooth" interfaces
Another interesting topic is an effect of fluctuations on the behavior of smooth (or "fuzzy") interfaces. Smooth interfaces are ubiquitous in soft matter. The examples include near-critical vapor-liquid and liquid-liquid interfaces in simple and complex fluids, interfaces in polymer solutions and polymer blends, liquid membranes and vesicles. A smooth interface is characterized by the interfacial density/concentration profile with a characteristic length scale, or "thickness" of the interface. Such interfaces are mesoscopic, extending from nanometers to microns. One cannot define a droplet size smaller than the thickness of its interface. The surface tension of a smooth interface is usually very low; hence the interface undergoes strong fluctuations. Fluctuations fundamentally change thermodynamics of smooth interfaces. In particular, the curvature correction to the surface tension, known as Tolman's length, may become as large as the thickness of the smooth interface itself. This effect crucially depends on the degree of asymmetry in the fluid phase coexistence, which is properly described by "complete scaling". A broader impact of this research includes filtration through microporous media in oil recovery, microfluidics, nanoscale liquid bridges, and nucleation phenomena: wherever science and technology deal with fluid droplets at submicron and nano scales.