Soft colloidal particles play an important role in large-scale and microfluidic industrial processing applications, as well as in a variety of cosmetic and pharmaceutical products. Such systems undergo a transition to a glassy state through a process known as “caging”: owing to crowding, a given particle is trapped by its nearest neighbors in a virtual cage. Understanding the details of the caging process is key to controlling the stability and efficiency of commercially available oils, creams and gels.

In a development that could have significant implications to the way future hybrid soft composite materials are designed, a team of scientists from Europe and Canada has discovered a new asymmetric glassy state, which arises specifically as a result of the soft interactions between the particles [Mayer et al., Nature Materials (2008) 7, 780].
“We have found it is possible to tune the viscosity of a liquid or the elasticity of the corresponding amorphous solid, simply by changing the amount and size of the added polymer in solution”, explains Emanuela Zaccarelli, from the Department of Physics at the University “La Sapienza” of Rome, Italy. “The elasticity and viscosity of the resulting solution can be greatly affected, by means of an anisotropic re-organization of the particles at a microscopic level”.
Zaccarelli stresses the asymmetric glass state can only arise in soft mixtures, as opposed to the more commonly studied hard-sphere mixtures. As a result, her team focused its attention on star polymers - molecules that are multiply branched from a single point - since they allow the softness of the interaction to be easily tuned.
“This work demonstrates that what might at first appear as a modest increase in complexity over well-studied hard-sphere mixtures - softness of interaction - can turn out a number of surprises”, comments professor Johan Bergenholtz, from the University of Gothenburg in Sweden. “The star polymer systems are superb model systems that allow a very detailed understanding of the complex structures and dynamics in concentrated solutions of soft colloids”.
The authors continue their work in the laboratory