Abstract
Colloidal glasses formed from hard spheres, nearly hard spheres, ellipsoids and platelets or their attractive variants, have been studied in great detail. Complementing and constraining theoretical approaches and simulations, the many different types of model systems have significantly advanced our understanding of the glass transition in general. Despite their early prediction, however, no experimental charged sphere glasses have been found at low density, where the competing process of crystallization prevails. We here report the formation of a transient amorphous solid formed from charged polymer spheres suspended in thoroughly deionized water at volume fractions of 0.0002–0.01. From optical experiments, we observe the presence of short-range order and an enhanced shear rigidity as compared to the stable polycrystalline solid of body centred cubic structure. On a density dependent time scale of hours to days, the amorphous solid transforms into this stable structure. We further present preliminary dynamic light scattering data showing the evolution of a second slow relaxation process possibly pointing to a dynamic heterogeneity known from other colloidal glasses and gels. We compare our findings to the predicted phase behaviour of charged sphere suspensions and discuss possible mechanisms for the formation of this peculiar type of colloidal glass.
Highlights
Glasses are amorphous solids exhibiting only short-range order and a finite shear rigidity
Note that we intentionally denote the short-range ordered solid state of the present system as “amorphous solid” rather than as “glass”, because we feel that i) many different colloidal glasses seem to co-exist in different experimental and theoretical approaches; ii) the issue of how exactly a glass can be unequivocally defined beyond being an amorphous solid remains unsolved; and iii) we are not sure about the exact nature and the ways in and out of this state
It is formed at such low particle concentrations, this system bears some strong resemblance to hard sphere or charged sphere glasses formed at elevated packing fractions
Summary
Glasses are amorphous solids exhibiting only short-range order and a finite shear rigidity. Most experiments on soft matter systems so far focused on colloidal hard spheres, which solely interact via excluded volume interactions These show a face centred cubic (fcc) crystalline phase above their freezing transition located at a packing fraction of ΦF = 0.495 and a transition to an amorphous solid state at elevated packing fractions around ΦG ≈ 0.57518,19. It is formed at such low particle concentrations, this system bears some strong resemblance to hard sphere or charged sphere glasses formed at elevated packing fractions It shows a liquid-like static structure factor and at the same time displays a finite shear rigidity. Speaking, a similar and presumably related phenomenology is known from other colloidal glasses
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