Viscoelastic phase separation

Abstract

Phase separation is one of the most fundamental phenomena that create spatially inhomogeneous patterns in materials and nature. It has so far been classified into three types: (i) solid, (ii) fluid, and (iii) viscoelastic phase separation [1]. The relevant transport processes are only diffusion for (i), diffusion and hydrodynamic convection for (ii), and diffusion, hydrodynamic convection, and mechanical stress for (iii). Here we discuss the physical mechanism of viscoelastic phase separation. This phase separation takes place universally if there is strong dynamic asymmetry between the components of a mixture. The origin of the dynamic asymmetry is either the size disparity between the components or the difference in the glass transition temperature between the components. For example, viscoelastic phase separation has been observed in polymer solutions, protein solutions [2], colloidal suspensions [3], and membrane systems [4]. Under a deep quench, a transient gel is formed by strong attractive interactions between the slower components. The connectivity of the network acts against phase separation and produces the mechanical stress in it. The coupling between the composition, velocity, and stress fields is the key to this phase separation. We also find phase-separation behaviour accompanying mechanical fracture [5]. Surprisingly, mechanical fracture becomes a dominant coarsening process of the phase separation. The behaviour of viscoelastic and fracture phase separation originates from a strong coupling between the composition and deformation field. We demonstrate that the same type of coupling between the density and deformation field leads to cavitation of fluid under shear and mechanical fracture of glassy liquids and solids under deformation [6]. We discuss a common physics underlying these apparently unrelated phenomena and a selection principle of the kinetic pathway of pattern evolution. For example, the only difference between phase separation and fracture may stem from whether deformation is produced internally by phase separation itself or externally by loading. The author thanks T. Araki, T. Koyama, Y. Nishikawa, and Y. Iwashita, and M. Tateno, for collaboration on viscoelastic phase separation and A. Furukawa for collaboration on cavitation and fracture.

Structure and optical function of amorphous photonic nanostructures from avian feather barbs: a comparative small angle X-ray scattering (SAXS) analysis of 230 bird species

“…The morphological similarities in the previously published EM images [3,25] and experimental X-ray scattering data reported here (figure 5a -d) for channel-and sphere-type amorphous barb nanostructures and synthetic self-assembled soft matter systems is congruent with the hypothesis that avian barb nanostructures probably self-assemble via arrested phase separation of polymerizing b-keratin from the cellular cytoplasm, as suggested earlier for a few avian species [25,38]. Although the channel-and sphere-type barb nanostructures, respectively, appear to be similar to morphologies observed during classical phase separation via SD and nucleation-and-growth, to conclude that they indeed develop via phase separation, let alone assign a particular mode of phase separation using just morphology is not straightforward [45][46][47]. The lack of a perfect agreement between channel-type barb nanostructures and classical spinodal morphologies perhaps suggests that there may be important differences between a biological soft matter system and a simple binary fluid de-mixing, perhaps involving some viscoelastic phase separation processes [45 -47].…”

“…However, phase separation phenomenology should be modified to include nonlinear viscoelastic mechanisms when the two nascent phases have distinct rheological (i.e. mechanical) properties, such as in mixtures of a network-forming or polymerizing component and a fluid [45][46][47]. In this scenario, polymerizing proteins may form networks that resist 21 ).…”

Modelling and numerical simulation of phase separation in polymer modified bitumen by phase-field method

“…As the assumption of dynamic symmetry (the same dynamics for the two components of a binary mixture) is hardly valid in various real viscoelastic matters, Tanaka [18][19][20] introduced the concept of dynamic asymmetry (one slow component and one fast component) and complemented the traditional models with a viscoelastic model for phase separation in binary mixtures of viscoelastic matter. In Tanaka's model, the dynamic asymmetry may physically originate from the large difference in molecular size or glass transition temperature between the two components.…”

“…In order to obtain the expression of the free energy, a double-well potential for PMB system can be determined on the basis of the Flory-Huggins free energy of mixing [18][19][20][23][24][25][26]. The Flory-Huggins theory was originally proposed for polymer solutions by taking into account their differences with an ideal solution, i.e.…”

Spinodal decomposition of asymmetric binary fluids in a micro-Couette geometry simulated with molecular dynamics

“…Dynamical asymmetry of the two mixed fluids, meaning that their viscosities differ significantly or that one component shows viscoelastic behavior, explains the complex phase separation processes and the "phase inversion" phenomenon observed in polymer-solvent mixtures and colloidal suspensions. [22][23][24] In these systems the "slow" component cannot keep up with the growth rate imposed by the "fast" component; hence, a network enriched in the slow component forms and subsequently succumbs to internal stresses to arrive again at a regular phase separated final configuration. An externally imposed shear flow both accelerates and hinders the phase separation process by the continuous transport, elongation, and resulting ruptures of the domains.…”