Using two-dimensional (2D) complex plasmas as an experimental model system, particle-resolved studies of flame propagation in classical 2D solids are carried out. Combining experiments, theory, and molecular dynamics simulations, we demonstrate that the mode-coupling instability operating in 2D complex plasmas reveals all essential features of combustion, such as an activated heat release, two-zone structure of the self-similar temperature profile ("flame front"), as well as thermal expansion of the medium and temperature saturation behind the front. The presented results are of relevance for various fields ranging from combustion and thermochemistry, to chemical physics and synthesis of materials.

A method for phase identification (MPI) in two-dimensional (2D) condensable systems is proposed on the basis of the analysis of the Voronoi cells’ characteristics. A simple algorithm is developed for determination of particles belonging to a condensate, a gaseous state, and an interface between them (“surface”). The efficiency and stability of the developed method is studied using molecular dynamics simulations and calculation of binodals in 2D systems with both short- and long-range attraction between particles. To illustrate prospectives of the method for experimental studies, 2D clusters of 2.12 μm diameter colloidal silica particles assembled in a rotating external electric field have been analyzed using the MPI. The results prove that the MPI possesses high accuracy and can be applied to analyze in detail both simulation data (obtained by molecular dynamics or Monte Carlo methods) and results of particle-resolved experimental studies (with colloidal suspensions and complex (dusty) plasmas) of solids and liquids when the considered multiparticle systems can undergo spinodal decomposition. The developed method can be generalized to analyze three-dimensional and multicomponent systems, and therefore, the presented results are of relevance for a broad range of problems in physical chemistry, chemical physics, and materials science.

A review of experimental studies on waves, phonon dispersion relations, and mode-coupling instability in two-dimensional complex plasma crystals is presented. An improved imaging method allowing simultaneous measurements of the three wave modes (compression in-plane, shear in-plane, and out-of-plane) is given. This method is used to evidence the formation of hybrid modes and the triggering of the mode-coupling instability due to wake-mediated interactions. The main stages of the mode-coupling instability are analyzed. In the early stages, synchronization of microparticle motion at the hybrid mode frequency is reported. The spatial orientation of the observed synchronization pattern correlates well with the directions of the maximal increment of the shear-free hybrid mode. When the instability is fully developed, a melting front is formed. The propagation of the melting front has similarities with flame propagation in ordinary reactive matter. Finally, it is experimentally demonstrated that an external mechanical excitation of a stable 2D complex plasma crystal can trigger the mode-coupling instability and lead to the full melting of a two-dimensional complex plasma crystal.

A significant number of key properties of condensed matter are determined by the spectra of elementary excitations and, in particular-collective vibrations. However, behaviour and description of collective modes in disordered media (e.g. liquids and glasses) remains a challenging area of modern condensed matter science. Recently, anticrossing between longitudinal and transverse modes was predicted theoretically and observed in molecular dynamics simulations, but this fundamental phenomenon has never been observed experimentally. Here, we demonstrate the mode anticrossing in a simple Yukawa fluid constructed from charged microparticles in weakly-ionized gas. Theory, simulations, and experiments show clear evidence of mode anticrossing that is accompanied by mode hybridization and strong redistribution of the excitation spectra. Our results provide significant advance in understanding excitations of fluids, opening new prospectives for studies of dynamics, thermodynamics, and transport phenomena in a wide variety of systems from noble gas fluids and metallic melts to strongly coupled plasmas, molecular, and complex fluids.

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