Head: Prof. Dr. habil. Krzysztof Witold Wojciechowski

### Research goals

Investigations of counterintuitive phenomena, their description and explanation. In particular:

- studies of models and materials exhibiting anomalous properties (e.g. negative Poisson's ratio, negative thermal expansion, and negative mechanical compliance - local or directional) or extreme properties (e.g. containing instable inclusions),
- studies of the influence of various microscopic factors on macroscopic properties of the systems under consideration,
- the identification of the phenomena and mechanisms behind the specific properties of matter that offer the possibility of practical applications.

### Research profile

The research concerns micro- and macroscopic models of complex systems in the context of their extraordinary physical properties: structural, dynamic and thermodynamic. Our studies concentrate on the so-called *auxetics*, i.e. systems exhibiting anomalous (negative) Poisson's ratio, as well as on materials with instable inclusions, systems exhibiting negative thermal expansion and magneto-elastic systems. Beside some theoretical methods, various computer simulation methods and numerical techniques are used in the department, e.g., molecular dynamics, Monte-Carlo, mesh methods (finite element method) and meshless methods (fundamental solutions method).

### Research projects

- Project of the Polish National Center for Science:
*Computer modeling of auxetic foams*(2013-2017, project leader: MSc. Eng. A.A.Pozniak, project supervisor: prof. Dr. Habil. K.W. Wojciechowski) - Projects of the Ministry of Science and Higher Education:
*Influence of the particles' size polydispersity on the elastic properties of classical model systems*(2010-2013, project leader Dr. Habil. K.W. Tretiakov)*Influence of the particles' size polydispersity on the elastic properties of selected molecular model systems*(2007-2009, project leader: Prof. Dr. Habil. K.W. Wojciechowski, PhD student: J.W. Narojczyk)

### Scientific Achievements

- Microscopic studies, light transmission, dielectric and Hyper-Rayleigh scattering for two derivatives of bipiromidine showed high variety of physicochemical properties for both compounds. It has been discovered that both compounds exhibit liquid crystal properties in a certain range of temperatures. Besides strong fluorescence and two-photon absorption, these materials exhibit second-harmonic generation while subjected to laser illumination [S. van Cleuvenbergen, P. Kedziora, et al., Angew. Chem. Int. Ed. 56, 9546-9550 (2017)].
- Time evolution of magnetic domains in magnetic auxetics with Ising interactions (where magnetic interaction depends on the lattice constant) were studied by computer simulations. It has been shown that mechanical deformation of the structure impacts on the domains evolution and the rate of the domains increase does not fulfill the power dependence related to Ising model with fixed lattice constant. It has been also shown that by altering the rate of mechanical deformation for the studied model, one can control the rate of the magnetic domains increase and, in the consequence, magnetic properties of the system. This may be of significance for certain practical applications [K.K. Dudek, et al. Phys. Stat. Solidi RRL 11 (8) 1700122 (2017)].
- Selected structures of planar composites with periodic elliptical inclusions have been studied by computer simulations using finite element method. It has been shown that introducing non-auxetic macroinclusions in a non-auxetic matrix one can obtain an auxetic composite with high Young's modulus [A.A. Pozniak, K.W. Wojciechowski, J.N. Grima, L. Mizzi, Composites Part B 94, 379-388 (2016)].
- With the use of high scale computer simulations (1 million CPU hours) it has been shown how to obtain the thinest auxetic yet proposed, from a graphene sheet. The mechanism behind the auxetic properties has been explained [J.N. Grima et al., Adv. Mater. 27, 1455 (2015)].
- A novel, effective method for calculations of elastic constants for systems with long range inter-particle interactions has been proposed and tested for Lennard-Jones interactions [K.V. Tretiakov, K.W. Wojciechowski, Computer Phys. Commun. 189, 77 (2015)].
- The structure and the properties of thin smectic layers placed in water has been studied using McMilan mean field method. It has been concluded that the transition of molecules from smectic A phase to an isotropic phase, commonly referred to as "melting", starts from the center of the system and moves towards the surfaces [I. Śliwa, A.V. Zakharov, J. Chem. Phys. 141, 194706 (2014)].
- It has been shown that in the systems which are far from thermodynamic equilibrium, the nature favors the realization of states of lower energy dissipation. States with higher energy dissipation are also possible but less likely to occur [K.V. Tretiakov et al., Angewandte Chemie Intern. Ed. 52, 10304 (2013)].
- By finite element methods it has been shown that after uniform compression and the removal of internal tensions the planar model foams spontaneously transfer into the auxetic state (their Poisson's ratio becomes negative). This means that methods for the foam auxetization used to date, are the special case of a general strategy [A.A. Poźniak et al., Smart Mater. Struct. 22, 084009 (2013)].
- It has been shown that Poisson's ratio of two-dimensional system of polydisperse hard discs tends to +1 in the high pressure limit, for any non-zero polydispersity. This implies that Poisson's ratio is not continuous in this limit, when the polydispersity tends to zero [K.V. Tretiakov, K.W. Wojciechowski, J. Chem. Phys. 136, 204506 (2012)].
- It has been shown that carbon nanotube heterojunctions are able to filter specific vibration modes, what can be useful in construction of novel stress nano-sensors or vibration filters [F. Scarpa et al., Nanotechnology 22, 465501 (2011)].
- It has been shown that below a certain critical value of the Poisson's ratio, auxetic systems can exhibit locally negative mechanical compliance. This encourages one for applying auxetics as nano inclusions to produce materials with extreme mechanical properties (hardness, dumping, etc.) [A.A. Poźniak et al., Rev. Adv. Mater. Sci. 23,169 (2010)].
- The propagation of soliton waves in auxetic plates has been studied. The results can be helpful in a non-destructive analysis of materials (especially composites) with the use of soliton impulses that are significantly less suppressed compared to harmonic vibrations P. Kolat et al., J. Non-Cryst. Solids 356, 2001 (2010)].
- It has been shown that: (i) particle size polydispersity leads typically to an increase of the Poisson's ratio, which (ii) can be significantly lower for anisotropic systems than for isotropic systems. certain mechanisms leading to a decrease of Poisson's ratio has been identified [J.W. Narojczyk, PhD Thesis (2009)].
- The measurements of nonlinear dielectric effect relaxation in the isotropic, cholesteric and blue phases have been carried out for the first time [P. Kędziora, K.W. Wojciechowski, J. Phys. Chem. B 113, 9123 (2009)].

During the period of 2009-2017 three research projects were conducted. Two of them concerned the influence of particle size polydispersity on elastic properties of the studied model systems. Auxetic properties were observed in a number of systems. It has been also shown that (at high pressure limit) the disorder causes an increase of the average Poisson's ratio to the extreme positive value which is possible for isotropic materials. The third project concerned computer modeling of auxetic foams. Among others, it has been shown that by compressing model foams of positive Poisson's ratios and removing the internal stresses one obtains auxetic foams.