Arrested demixing opens route to bigels

Abstract

Understanding and, ultimately, controlling the properties of amorphous materials is one of the key goals of material science. Among the different amorphous structures, a very important role is played by colloidal gels. It has been only recently understood that colloidal gels are the result of the interplay between phase separation and arrest. When short-ranged attractive colloids are quenched into the phase-separating region, density fluctuations are arrested and this results in ramified amorphous space-spanning structures that are capable of sustaining mechanical stress. We present a mechanism of aggregation through arrested demixing in binary colloidal mixtures, which leads to the formation of a yet unexplored class of materials--bigels. This material is obtained by tuning interspecies interactions. Using a computer model, we investigate the phase behavior and the structural properties of these bigels. We show the topological similarities and the geometrical differences between these binary, interpenetrating, arrested structures and their well-known monodisperse counterparts, colloidal gels. Our findings are supported by confocal microscopy experiments performed on mixtures of DNA-coated colloids. The mechanism of bigel formation is a generalization of arrested phase separation and is therefore universal.spinodal decomposition | DNA-coated colloids | programmable interactions | amorphous self-assembly T he properties of a self-assembled material are ultimately controlled by the interactions among its building blocks and by the conditions in which they are prepared. It is by tuning these two properties that different structures can be obtained. Shortranged attractive colloidal systems, for example, can form crystals, two glasses of different origin, or gels. The latter have great technological importance. Colloidal gels find applications in synthetic colloid porous materials (1, 2), functionalization of surfaces and films production (3, 4), ceramics processing (5, 6), protein assemblies (7, 8), food science (9, 10), and soft matter (11, 12). Although they have been known for some time (13,14), it has only recently been understood that the colloidal gels arise as a result of arrested phase separation (15-18).The gels are characterized by a ramified amorphous spacespanning structure that is capable of sustaining mechanical stress. The colloidal density plays a crucial role in the aggregation and therefore in the resulting structure. At low densities, irreversible aggregation leads to fractal gels. At intermediate densities more compact porous structures are observed, whereas a homogeneous glass emerges when the solute occupies more than 50% of the volume (11,10,14,19).It has been proven that when colloidal particles are quenched into the gas-liquid phase separation region, gelation occurs as a consequence of dynamic arrest that interferes with phase separation (15, 18). After the quench, the system is thermodynamically unstable and strong density fluctuations set in, favoring the separation of the fluid into two ...

Fractal-like structures in colloid science

“…In this context, of specific interest is again the fractal geometry and its ability to create percolating structures. An interesting result in this direction has been reported by Varrato et al in demonstrating the possibility of creating bigels structures using DNA [103].…”

Local structure of percolating gels at very low volume fractions

“…In order to predict the mechanical properties of gels, it is important to know both their local [13][14][15] and global [16][17][18] structure, but a deep understanding of both remains today a challenge. For example, in the very dilute limit, the study of gel formation via molecular dynamics is challenged by the very long times required to form aggregates, with equilibration times that easily exceed 10 8 integration steps 19 .…”

Measuring local volume fraction, long-wavelength correlations, and fractionation in a phase-separating polydisperse fluid

“…The results show emphatically that complex fractionation is involved right from the beginning of polydisperse phase separation, local composition relaxing alongside local density rather than long after it. It can therefore play a role in the formation of nonequilibrium structures which arrest before coming to equilibrium 27,28 , such as when gels form from a polydisperse colloid 2 . As well as fractionation, the correlation functions introduced could be of general use in characterising structural effects of polydispersity.…”

“…There is scant data on how these systems behave whilst evolving towards their fractionated equilibria [24][25][26] . This may be especially important where phase separation serves as a route to some nonequilibrium arrested state 27,28 -in such cases, the true compositional equilibrium (in terms of fractionation) may never be reached. In other cases, fractionation is required in order to even access the equilibrium phase 11 , so that the dynamics of fractionation directly influences whether the system can equilibrate in any meaningful sense 2 .…”