The ensuing mechanical response to stressing or straining a structured liquid results in various viscoelastic phenomena, either in the linear region where the microstructure responds linearly with respect to the stress and strain but does not itself change, or in the nonlinear region where the microstructure does change in response to the imposed stresses and strains, but does so reversibly. The complication of thixotropy arises because this reversible, microstructural change itself takes time to come about due to local spatial rearrangement of the components. This frequently found time-response of a microstructure that is itself changing with time makes thixotropic, viscoelastic behaviour one of the greatest challenges facing rheologists today, in terms of its accurate experimental characterisation and its adequate theoretical description. Here a history of thixotropy is given, together with a description of how it is understood today in various parts of the scientific community. Then a mechanistic description of thixotropy is presented, together with a series of applications where thixotropy is important. A list of different examples of thixotropic systems is then given. Finally the various kinds of theories that have been put forward to describe the phenomenon mathematically are listed.

Thixotropy—a review

Polymeric materials near the liquid-solid transition (LST) exhibit a very distinct relaxation pattern. The reference point for analyzing these patterns is the instant of LST at which relaxation becomes self-similar over wide ranges of the relaxation time. The universality of this transition and its consequences have been explored extensively during the past decade. This study will present an overview of rheological implications inherent in liquid-solid transitions of polymers. The LST can be most reliably detected in a dynamic mechanical experiment in which the frequency independence of the loss tangent marks the LST. A wide variety of rheological observations of materials in the vicinity of an LST are discussed with respect to their universality. It is shown that polymer chemistry, molecular weight, stoichiometry, temperature, inhomogeneities, etc. greatly influence the material behavior near the LST. However, the characteristic self-similar relaxation is shown by all investigated materials, independent of the nature of the LST (e.g., both, physically and chemically crosslinking polymers). Several theories predict chemical and rheological properties in the vicinity of an LST. They are briefly discussed and compared with experimental results. A variety of applications for polymers near LST are presented that either already exist or can be envisioned. The self-similar relaxation behavior which results in a power law relaxation spectrum and modulus is not restricted to materials near LST. Different classes of polymers are described that also show power law relaxation behavior. What makes the self-similar relaxation specific for materials at LST is its occurrence at long times with the longest relaxation time diverging to infinity.

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Rheology of Polymers Near Liquid-Solid Transitions

Publisher Summary This chapter describes the process of the gelation of proteins Gelation is a phenomenon; therefore, its definition and that of its product, a gel, is dependent on the perspective of the observer and the technique(s) used to observe it For example, gels may be defined by their ability to immobilize a liquid, their macromolecular structure, or their textural or rheological properties The chapter reviews a variety of protein gel systems with a broad perspective stressing similarities between them, yet not without mentioning the important differences that make each unique It describes the multicomponent protein gels The rheological and optical properties of thermally irreversible gels are the outcome of two events First, a change in protein structure is needed that permits protein–protein interactions The subsequent aggregation process, which is highly influenced by the solvent environment, may produce the gel microstructure, which is responsible for the optical and rheological properties The physical integrity of a gel is maintained by counterbalancing forces of attraction and repulsion among polymer molecules and between the polymer network and the surrounding solvent

The Gelation Of Proteins

Nanoemulsions are kinetically stable biphasic dispersion of two immiscible liquids typically stabilized by an emulsifier with droplet sizes in the range of 10–200 nm. Present work deals with the formulation and characterization of stable oil-in-water nanoemulsions using nonionic surfactant (Tween 40) and light mineral oil for their application in enhanced oil recovery. The stability study of the nanoemulsions formed by high energy and low energy method was accomplished by bottle testing method. The emulsions were characterized in terms of droplet size, morphology and inner structure, surface charge, interfacial tension, and rheology. Droplet sizes of 18–31 nm obtained by dynamic light scattering analysis and surface charge values above −35 mV obtained by ζ potential measurement prove the higher kinetic stability of the formed emulsions. Cryo-TEM micrographs reveal the surface morphology and inner structure of nanoemulsions. A miscibility test was performed to determine the dissolving ability of the nanoem...

Surfactant Stabilized Oil-in-Water Nanoemulsion: Stability, Interfacial Tension, and Rheology Study for Enhanced Oil Recovery Application

We measure the complex rheological behavior of nearly critical gels and analyze the data by searching for characteristic patterns and abstracting those patterns into a self-consistent model. The sample is a linear, flexible, nearly monodisperse polybutadiene which gets cross-linked on its vinyl side groups. The dynamic mechanical storage and loss moduli of cross-linking polymers change smoothly during the liquid-solid transition, while equilibrium rheological properties (e.g., zero-shear viscosity and equilibrium compliance) diverge. During gelation, the relaxation occurs in a distinct pattern which can be described in a quantitative way with a minimum number of parameters. The pattern can be understood as a combination of the BSW spectrum (representing the precursor relaxation behavior) and the self- similar Chambon-Winter gel spectrum (modeling the terminal relaxation due to growing clusters). The spectrum is cut off at the material's longest relaxation time, Imax. Our model parameters, Imax and Ge (equilibrium modulus), exhibit characteristic scaling behavior with respect to the distance from the gel point,jp - pcj. The relaxation exponent, n, in the terminal zone is a function of the extent of reaction. Hence, dynamic scaling (requires constant n values) is not valid for our system. The proposed model passes the self-consistency test by predicting the mechanical behavior (at different frequencies) as a function of the extent of reaction and other rheological observations during the sol-gel transition.

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Relaxation Patterns of Nearly Critical Gels

Rheology is a sensitive measure of the evolving molecular structure in a crosslinking polymer. Dynamic mechanical experiments in small amplitude oscillatory shear give the storage modulus G′(ω, p) and the loss modulus G″(ω, p) as a function of frequency ω. The extent of crosslinking, p(t), changes with reaction time. Dynamic mechanical experiments allow detection of the gel point (GP) and give a macroscopic description of the critical gel state (network polymer at GP). This critical gel state is used as a reference for describing the entire evolution of rheology. The most surprising discovery of these experiments was that critical gels exhibit stress relaxation in a power law, i. e. the relaxation modulus is given as G()=St −n. The relaxation exponent, n, depends on network structure. The power law behavior is an expression of mechanical self similarity (fractal behavior). The range of self similarity is defined between an upper and a lower frequency limit. The lower frequency limit (reciprocal of characteristic relaxation time) corresponds to an upper scaling length, the correlation length, which is of the order of the linear size of the largest molecular cluster (of pre-gel) or of the largest remaining percolation cluster (of post-gel). High frequencies probe relaxation within single chains. The upper frequency limit corresponds to a lower scaling length, the glass length, which is given by the dimension of the molecular network units responsible for glassy behavior. The correlation length and, hence, the characteristic relaxation time increase in the approach of the gel point, diverge to infinity at the gel point, and then decrease again with increasing extent of crosslinking. The critical gel has no characteristic length or time scale. All observations are restricted to polymers at a temperature above the glass transition temperature and at frequencies much below the glass frequency.

H. H. Winter

Papers

Evolution of rheology during chemical gelation