Friday 18 July: (Leiden) Extra Fysisch Colloquium, Hans Wyss (14:00 hrs)



Hans Wyss, Harvard University, U.S.A.

Glass Formation and Structuring in Soft Materials

LION:  Time: 14:00 hrs, Location: De Sitterzaal - Oortgebouw 

Soft condensed matter consists of building blocks at the meso scale, larger than molecular but smaller than macroscopic. The typical energy scale that governs their behavior is the thermal energy kT. Examples are colloids, polymers, foams, emulsions, and a wide range of biological materials. They exhibit an extremely rich phase behavior that can be controlled by changing the interactions between the colloidal building blocks. This level of control also makes colloids ideal model systems to study and understand phenomena generic to all condensed matter. In addition, due to their softness and mesoscopic characteristic length scales their behavior can be studied with relatively simple techniques such as light scattering, light microscopy, and oscillatory rheology. As a first example I will present work on out-of-equilibrium behavior and the glass transition. Molecular glass formers show a wide range of behaviors in terms of their so-called fragility, a measure for how sensitively the viscosity changes as the glass transition is approached. The origin of these differences in fragility are poorly understood. In colloidal systems, only highly fragile behavior has been observed. However, we show that by varying the softness of our colloidal particles we can access the entire range of fragilities found in molecular glasses. We use polymeric microgel particles as a model system as their mechanical behavior can be easily controlled through changes in chemical composition. Our experiments suggests that elasticity is a key towards understanding the origin of fragility in glasses, both colloidal and molecular. In the second part of my talk I will show how glass formation and out-of-equilibrium processes can be exploited to create new structures in materials. Our experiments on weakly attractive colloids establish a new scenario of colloidal arrest, based on an interplay between phase separation and glass formation. This behavior is very generic and should be applicable to understand and induce structure formation in a wide range of materials. Such "out-of-equilibrium self-assembly" should be further explored in future work, combining structure-forming mechanisms induced by dynamical instabilities such as phase separation or convection with approaches to dynamically arrest these structures such as gelation or glass formation. The ability to control the structuring of these soft materials will also be essential for establishing an understanding of the relation between the local structure and the macroscopic mechanical behavior in soft matter systems. Investigating the rheological response of such systems at different length scales will require a combination of established methods and new experimental probes. Particular attention should be given to the nonlinear viscoelastic response, which is currently poorly understood but is intimately related to the relaxation dynamics of soft glassy materials.