Mechanics and Physics of Soft Materials

Abstract

Materials where thermal energy is comparable to the interaction energy between molecules are called soft materials. Soft materials are everywhere in our life: food, rubber, polymer diaper, our own body, etc. The thermal fluctuation endows soft materials with fundamentally different behavior comparing to hard materials like metals and ceramics. This dissertation studies three aspects of the mechanics and physics of soft materials, as is reviewed below.

First, soft materials are generally swellable and viscous. The combination of diffusion and viscous flow gives rise to a length scale we called poroviscous length. The emergence of a length scale results in size dependent relaxation. We show that the coupling between diffusion and viscous flow explains the Brownian motion in supercooled liquids, where the classical result of Stokes-Einstein relation generally fails. The concurrent diffusion and viscous flow cannot be described by the classical hydrodynamics, where all the material transport is lumped into velocity field. We formulated a continuum theory to modify the classical hydrodynamics. In particular, the new theory predicts a new bulk viscosity that could exist in incompressible material. We generalize this idea of bulk viscosity to binary systems and study the mixing of materials that is limited by local structural rearrangement instead of diffusion. This model develops formulation of non-equilibrium thermodynamics by removing the common assumption of local equilibrium.

Second, capillarity has strong influence on the morphology of soft materials. The competition between capillarity and elasticity gives rise to the elastocapillary length, which is defined as surface tension over the shear modulus. We show that elastocapillary effect explains the complex nucleation of crease, a widely observed surface instability in soft elastic materials. We also explore the possible competition between capillarity and osmosis in gels, which defines the osmocapillary length, the surface tension divided by osmotic pressure. We show that at small enough length scale or for a gel that is nearly fully swollen, surface tension can pull liquid solvent out from the gel phase, a phenomenon we termed osmocapillary phase separation.

Third, soft materials are nearly incompressible. The incompressibility and softness makes elastomers ideal for the design of seals. Although the failure of seals has been studies for decades, existing studies mainly focus on the damage and degradation of materials. Here we study the leak of a seal due to elastic deformation without any damage. We call such a failure mode the elastic leak. We point out that elastic leak is involved in any leak event no matter whether material is damaged or not. We also show that the reversible nature of the elastic leak enable seal series to achieve higher sealing capability.