Soft colloids make strong glasses
Edited by Qichao Hu
September 27th, 2010
There are some similarities between hard sphere colloids and molecular systems, when it comes to glass formation. For hard sphere colloids, the glass phase transition is controlled by an increase in volume fraction, and for molecular systems, it is controlled by a decrease in temperature. The difference is that molecular glasses exhibit a wider variety of behavior when the supercooled molecular liquids approach glassy state. These behaviors include viscosity and structural relaxation time, or in general called "fragility".
For fragile molecular liquids, the relaxation time is highly sensitive to changes in temperature, while for non-fragile, or strong molecular liquids, the relaxation time has a lower temperature dependence. On the other hand, hard sphere colloids are characterized as fragile, and limited by their volume fraction dependence. This fragile nature limits the use of hard sphere colloids in glass formation.
This paper demonstrates that instead of using hard sphere colloids, soft sphere colloids (deformable) can exhibit the same amount of variation in fragility by varying their concentration at fixed temperature as molecular liquids by varying the temperature at fixed volume. The fragility of these soft sphere colloids is determined by the elastic properties of the individual particles. This relation between fragility and elasticity also has an analogy in molecular liquids.
In molecular liquids, the concept of fragility is understood as the log slope in an Arrhenius plot when temperature is the glass transition temperature. For colloidal systems, fragility needs to have a concentration dependence rather than a temperature dependence.
Several notations used in this study are:
<math>\eta</math>: particle concentration
<math>\phi</math>: volume fraction
<math>V_P</math>: volume of each particle
<math>n</math>: number density of each particle
For hard sphere colloids, the total volume fraction is simply
However, for soft sphere colloids, the volume is not fixed and we need to use concentration