Rheology of Gels and Yielding Liquids

Abstract

In this review, today’s state of the art in the rheology of gels and transition through the yield stress of yielding liquids is discussed. Gels are understood as soft viscoelastic multicomponent solids that are in the incomplete phase separation state, which, under the action of external mechanical forces, do not transit into a fluid state but rupture like any solid material. Gels can “melt” (again, like any solids) due to a change in temperature or variation in the environment. In contrast to this type of rheology, yielding liquids (sometimes not rigorously referred to as “gels”, especially in relation to colloids) can exist in a solid-like (gel-like) state and become fluid above some defined stress and time conditions (yield stress). At low stresses, their behavior is quite similar to that of permanent solid gels, including the frequency-independent storage modulus. The gel-to-sol transition considered in colloid chemistry is treated as a case of yielding. However, in many cases, the yield stress cannot be assumed to be a physical parameter since the solid-to-liquid transition happens in time and is associated with thixotropic effects. In this review, special attention is paid to various time effects. It is also stressed that plasticity is not equivalent to flow since (irreversible) plastic deformations are determined by stress but do not continue over time. We also discuss some typical errors, difficulties, and wrong interpretations of experimental data in studies of yielding liquids.

Underused Marine Resources: Sudden Properties of Cod Skin Gelatin Gel

“…The loss modulus G″ for all studied samples steadily increases in the region ω > 1 rad/s, but up to the last studied value ω (240 rad/s) does not exceed G′.Thus, all the studied hydrogels demonstrate the solid-like behavior, typical for physical Frequency dependencies of the real (storage modulus) G (ω) and the imaginary (loss modulus) G (ω) components of complex elastic modulus were obtained in the linear region (γ = 1%) (Figure 8a). The loss modulus G for all studied samples steadily increases in the region ω > 1 rad/s, but up to the last studied value ω (240 rad/s) does not exceed G .Thus, all the studied hydrogels demonstrate the solid-like behavior, typical for physical gels in the range of stresses up to the yield stress σγ [70,71]. For the hydrogels of pork gelatin (sample 1) and extracted cod fish (sample 3) the dependences G (ω) demonstrate almost constant values of the storage modulus G (quasi-equilibrium module on the plateau G pl ) throughout the studied range of ω (from 0.001 to 240 rad/s).…”

Liquid Metal Nanoparticles Physically Hybridized with Cellulose Nanocrystals Initiate and Toughen Hydrogels with Piezoionic Properties

“…17 In comparison, for the gel without LM that contains molecular cross-linking, G′ is constant and dominates across all frequencies, consistent with a near-ideal elastic gel. 51,52 Gels containing LM-CNC are similar but have a G′ that increases slightly at low frequency (resulting in a slight increase in tan σ). Such long time scales of relaxation are presumably due to the use of secondary ionic bonds to cross-link the network (in contrast to the covalent cross-linkers used in the more ideal gels without LM).…”

Luminescent Supramolecular Metallogels: Drug Loading and Eu(III) as Structural Probe

“…This predominance of G' over G'' is characteristic of gels. [57] An exception to this behavior is the EuIDA 1 : 3 gel in which G'' is higher than G' for frequencies below 12 rad/s. Increasing the amount of ligand IDA resulted in a decrease in the values of both G' and G''.…”