Viscoelastic phase separation in soft matter and foods

Abstract

Phase separation is a fundamental phenomenon that produces spatially heterogeneous patterns in soft matter and foods. We argue that phase separation in these materials generally belongs to "viscoelastic phase separation", where the morphology is determined by the mechanical balance of not only the thermodynamic force (interface tension) but also the viscoelastic force. The origin of the viscoelastic force is dynamic asymmetry between the components of a mixture, which can be caused by either a size disparity or a difference in the glass transition temperature between the components. Such dynamic asymmetry quite often exists in foods, which are typically mixtures of big molecules (polymers, proteins, etc.) and liquids (water, oil, etc.). We show examples of mechanically driven pattern formation in foods, in which dynamic asymmetry plays crucial roles, including the formation of network and cellular patterns in foods (e.g., breads, sponge cakes, butter, chocolates, etc.) and crack pattern formation (dried foods, cooked meat, etc.). Collapsing of these structures upon heating or moisture uptake is also discussed. We also argue that heterogeneous gels are in general formed as a consequence of dynamical arrest of the viscoelastic phase separation. Finally we mention an intimate link of viscoelastic phase separation, where deformation fields are spontaneously generated by phase separation itself, to mechanical instability and fracture induced by externally imposed strain fields. Such mechanical instability and nonlinear rheology may be relevant to food processing and also to separation and fracture of foods. We propose that all these phenomena can be understood as mechanically driven inhomogeneization with the concept of dynamic asymmetry in a unified manner.

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

“…When the two components have very different glass transition temperatures, the strong dependency of mobility on phase composition may have similar effects on the phase separation process as the stress-diffusion coupling [20].…”

“…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.…”

Understanding Food Structures

“…The morphology of the phase-separated emulsion system is determined by a mechanical balance of the thermodynamic forces (interfacial tension) and the viscoelastic forces associated with the flocculated droplet network. This is an example of a general structure-forming phenomenon in soft matter systems known as "viscoelastic phase separation" (Tanaka, 2012). Depletion flocculation of a protein-stabilized emulsion, and any associated viscoelastic phase separation, may also be induced by various kinds of non-adsorbing nanoparticles and micellar species (Dickinson, 2010a).…”

A Multi-scale Parallel Numerical Solver for Modeling of Two-phase Viscoelastic Fluids Based on the OpenFOAM

“…Many natural and synthetic fluids exhibit viscoelastic characteristics; notable examples include blood, DNA solutions, polymer melts and solutions, and fiber suspensions [1][2][3][4]. The most challenging part of modeling the viscoelastic fluids is to determine the non-linear coupling between the macroscopic rheological responses and the microscopic evolution of molecular configurations under flow, thus modeling the viscoelastic fluids intrinsically is a multiscale problem.…”