S3E2

Mrityunjay Kothari

Shaoting Lin

Yue Zheng

Abstract of Talk 1

Phase separation is observed across scales and in a variety of physical processes that range from the fields of biology to metallurgy. Liquid-liquid phase separation and subsequent species migration is understood to be a crucial mechanism for regulating the normal bodily functions of a cell. For instance, P-granule assembly is believed to employ phase separation as a mechanism to polarize the cell along its anterior-posterior axis as a precursor to cell differentiation. The dynamics of this process is dependent on the cytoplasm and its properties. Similarly, phase separation phenomena are commonly observed in metals. In the case of metals, elastic fields in the matrix are well known to affect the phase transformations; they can either inhibit or promote the growth of precipitates. While important work has been carried out, both experimentally and theoretically, in understanding the elastic effects on phase separation and species migration in soft-material systems, a quantitative understanding of the dynamics of these processes still presents many challenges. In practical applications, the matrix properties are often heterogeneous, and the phase separation process is sensitive to these non-uniformities. In this talk, we will present the theory of phase separation with nonlinear elastic effects and apply it to provide quantitative predictions for the phase separation of a binary liquid mixture in a crosslinked polymer [1]. We will discuss how elasticity arrests phase separation in such a system, leading to a distribution of uniform-sized droplets in the polymer. Over a longer timescale, this distribution becomes unstable in a heterogeneous sample, resulting in a propagating droplet mixing front (as experimentally reported in [2]). We will discuss how elasticity controls the droplet size and the speed of the front —a potentially important mechanism for biological systems, like cells, to actively control the rate of material transport.

Biosketch of Speaker 1

Abstract of Talk 2

Polymer networks are pervasive in biological organisms and engineering materials. Topological defects such as cyclic loops and dangling chains are ubiquitous in polymer networks. While fracture is a dominant mechanism for mechanical failures of polymer networks, existing models for fracture of polymer networks neglect the presence of topological defects. Here, we report a defect-network fracture model that accounts for the impact of various types of topological defects on fracture of polymer networks. We show that the fracture energy of polymer networks should account for the energy from multiple layers of polymer chains adjacent to the crack. We further show that the presence of topological defects tends to toughen a polymer network by increasing the effective chain length, yet to weaken the polymer network by introducing inactive polymer chains. Such competing effects can either increase or decrease the overall intrinsic fracture energy of the polymer network, depending on the types and densities of topological defects. Our model provides theoretical explanations for the experimental data on the intrinsic fracture energy of polymer networks with various types and densities of topological defects.

Biosketch of Speaker 2

Abstract of Talk 3

Nematic elastomers exhibit many unusual mechanical behaviors as the rotation of mesogens and the deformation of polymer networks in molecular scale are often tightly coupled. In the presentation, I will present some unusual mechanical deformation phenomena of nematic elastomers recently observed in the experiment. The first one is a simple uniaxial stretch of a monodomain nematic elastomer film. We found that when the angle between the orientation of the mesogens and the direction of the tensile force is large than a critical value, the neck can form and propagate in the film with the increase of the deformation of the specimen. The long neck can completely disappear after unloading and reappear again during the second-time reloading. The second experiment is the inflation of a nematic elastomer balloon. We found that the inflation behavior of a nematic elastomer balloon is drastically different from that of a routine elastomer balloon. Based on the quasi-convex free energy function previously proposed for nematic elastomer, we can provide a theoretical explanation of those phenomena, which quantitatively agree with our experimental measurements.

Biosketch of Speaker 3