Pushed soft-matter: Living polymer and Membrane under external fields
Dr. Prathyusha K. R. (TIFR Centre for Interdisciplinary Sciences, Hyderabad INDIA)

Soft matter systems encompass a variety of materials such as polymers, surfactants, colloids, biological materials and liquid crystals. They are easily deformable by weak external forces such as shear, electric field, mechanical stress etc. In this talk I will be discussing about Molecular Dynamics simulation studies of shear induced structural transition in living polymer and undulating membranes in the presence of electric-field.

The rheological properties of living polymer are non-trival and they exhibit fascinating shear induced structures. We developed a coarse-grained model for living polymer systems to understand the shear induced structure formation and also associated rheological transition. In this model, basic polymer segment contains three beads connected by harmonic springs and end beads undergo scission and recombination. We use both Dissipative Particle Dynamics simulation and Langevin Dynamic simulation to study the dynamics of the system. Barrier in the segmental interaction potential, which corresponds to the end-cap energy of micelles, was found to play a major role in determining the shear

induced structure. The effect that the range of interaction potential, viscosity of the

background solvent and living polymer density have on shear induced structures will be

discussed.

One of the striking phenomena exhibited by lipid membranes is the formation of vesicles when an external electric field is applied to a multi-lamellar stack of bilayers. This is called ‘electroformation’. There is very little understanding of the large-scale deformation of a membrane induced by an electric field. We simulated the bilayer system in the presence of explicit solvent using a coarse-grained model called, MARTINI. It was found that the membrane develops static transverse undulations when an electric field is applied along its normal. The amplitude of these undulations were found to increase with increasing electric field. At large electric fields the membrane ruptures through formation of pores with the membrane-water interface aligning parallel to the electric field. We found that the mechanical parameters of a freely-floating membrane are unaffected by the presence of an external electric field. There was a significant decrease in the area compressibility modulus. Though the reason for the dramatic change in the compressibility modulus is unclear, this may be one of the important mechanisms for the electroformation of vesicle.