2D colloids in rotating electric fields: A laboratory of strong tunable three-body interactions.

Many-body forces play a prominent role in structure and dynamics of matter, but their role is not well understood in many cases due to experimental challenges. Here, we demonstrate that a novel experimental system based on rotating electric fields can be utilised to deliver unprecedented degree of control over many-body interactions between colloidal silica particles in water. We further show that we can decompose interparticle interactions explicitly into the leading terms and study their specific effects on phase behaviour. We found that three-body interactions exert critical influence over the phase diagram domain boundaries, including liquid-gas binodal, critical and triple points. Phase transitions are shown to be reversible and fully controlled by the magnitude of external rotating electric field governing the tunable interactions. Our results demonstrate that colloidal systems in rotating electric fields are a unique laboratory to study the role of many-body interactions in physics of phase transitions and in applications, such as self-assembly, offering exciting opportunities for studying generic phenomena inherent to liquids and solids, from atomic to protein and colloidal systems.

Mean-field model of melting in superheated crystals based on a single experimentally measurable order parameter.

Melting is one of the most studied phase transitions important for atomic, molecular, colloidal, and protein systems. However, there is currently no microscopic experimentally accessible criteria that can be used to reliably track a system evolution across the transition, while providing insights into melting nucleation and melting front evolution. To address this, we developed a theoretical mean-field framework with the normalised mean-square displacement between particles in neighbouring Voronoi cells serving as the local order parameter, measurable experimentally. We tested the framework in a number of colloidal and in silico particle-resolved experiments against systems with significantly different (Brownian and Newtonian) dynamic regimes and found that it provides excellent description of system evolution across melting point. This new approach suggests a broad scope for application in diverse areas of science from materials through to biology and beyond. Consequently, the results of this work provide a new guidance for nucleation theory of melting and are of broad interest in condensed matter, chemical physics, physical chemistry, materials science, and soft matter.

Direct Experimental Evidence of Longitudinal and Transverse Mode Hybridization and Anticrossing in Simple Model Fluids.

A significant number of key properties of condensed matter are determined by the spectra of elementary excitations and, in particular, collective vibrations. However, the behavior 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 a significant advance in understanding excitations of fluids, opening new perspectives 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 and molecular and complex fluids.

Defect-governed double-step activation and directed flame fronts.

Defects play a crucial role in physics of solids, affecting their mechanical, electromagnetic, and chemical properties. However, influence of thermal defects on wave propagation in exothermic reactions (flame fronts) still remains poorly understood at the molecular level. Here, we show that thermal behavior of the defects exhibits essential features of double-step exothermic reactions with preequilibrium. We use experiments with monolayer complex (dusty) plasma and find that it can show a double-step activation thermal behavior, similar to chemically reactive media. Furthermore, we demonstrate capabilities to control flame fronts using defects and the different dynamic regimes of the thermal defects in complex (dusty) plasmas, from a nonactivated one to being sound and self-activated (like in active soft matter). The results suggest that a range of challenging phenomena at the forefront of modern science (e.g., defect activation, flame front dynamics, reaction waves, etc.) can now be experimentally interrogated on a microscopic scale.

Experimental validation of interpolation method for pair correlations in model crystals.

Accurate analysis of pair correlations in condensed matter allows us to establish relations between structures and thermodynamic properties and, thus, is of high importance for a wide range of systems, from solids to colloidal suspensions. Recently, the interpolation method (IM) that describes satisfactorily the shape of pair correlation peaks at short and at long distances has been elaborated theoretically and using molecular dynamics simulations, but it has not been verified experimentally as yet. Here, we test the IM by particle-resolved studies with colloidal suspensions and with complex (dusty) plasmas and demonstrate that, owing to its high accuracy, the IM can be used to experimentally measure parameters that describe interaction between particles in these systems. We used three- and two-dimensional colloidal crystals and monolayer complex (dusty) plasma crystals to explore suitability of the IM in systems with soft to hard-sphere-like repulsion between particles. In addition to the systems with pairwise interactions, if many-body interactions can be mapped to the pairwise ones with some effective (e.g., density-dependent) parameters, the IM could be used to obtain these parameters. The results reliably show that the IM can be effectively used for analysis of pair correlations and interactions in a wide variety of systems and therefore is of broad interest in condensed matter, complex plasma, chemical physics, physical chemistry, materials science, and soft matter.