Trixotropia Barnes (1996)

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<ul><li><p>ELSEVIER J. Non-Newtonian Fluid Mech., 70 (1997) 1-33 </p><p>Jear~lof Nea-l~.wtmi~ </p><p>l~id Medmks </p><p>Rev iew </p><p>Thixotropy a review </p><p>Howard A. Barnes </p><p>Unilever Research Laboratory, Bebington, Merseyside L63 3JW, UK </p><p>Received 16 November 1996; received in revised form 16 December 1996; accepted 6 January 1997 </p><p>Abstract </p><p>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, microstruc- tural 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. 1997 Elsevier Science B.V. </p><p>Keywords: Microstructure; Stress; Strain; Viscoelastic </p><p>1. Introduction </p><p>The growing use of clay-based structurants together with the increasing presence of floccu- lated structures in home, personal and chemical products and precursors has led to the appearance o f th ixotropy in a widening range of situations, quite apart f rom its presence in systems long known to display the phenomenon. Difficulties then arise in mixing and handl ing these materials because thixotropic structures progressively break down on shearing and slowly rebuild at rest. The time-scales involved can range from many minutes in the case of breakdown to many hours in rebuilding. </p><p>0377-0257/97/$17.00 1997 Elsevier Science B.V. All rights reserved. PII S0377-0257(97)00004-9 </p></li><li><p>2 H.A. Barnes/J. Non-Newtonian Fluid Mech. 70 (1997) 1-33 </p><p>Thixotropy has been deliberately built into products to make them usable by non-experts-- with the best-known example being thixotropic paints--however, as will be shown here, what is usually wanted in these cases is extreme shear-thinning. However the way in which this is brought about usually introduces thixotropy as well, which is then almost always an unwanted nuisance. However the phenomena still has to be understood, and hence the need for an up-to-date review. </p><p>Major post-war reviews of thixotropy have been produced by Bauer and Collins, 1967 [1], Mewis, 1979 [2], Cheng, 1982 [3], and Godfrey, 1983 [4]. While the general areas they cover are also dealt with here, they are well worth consulting for interesting examples of thixotropic systems not cited here. </p><p>2. A history of thixotropy </p><p>2.1. Origins </p><p>In 1923, Schalek and Szegvari found that aqueous iron oxide "gels have the remarkable property of becoming completely liquid through gentle shaking alone, to such an extent that the liquified gel is hardly distinguishable from the original sol. These sols were liquified by shaking, solidified again after a period of time ... the change of state process could be repeated a number of times without any visible change in the system" [5]. The term thixotropy was then coined by Peterfi in 1927 [6], in the first paper that properly described the phenomenon. The work combines the Greek words thixis (stirring or shaking) and trepo (turning or changing). </p><p>Although no mention of the phenomenon appeared in the seminal rheology text of the day 'The Viscosity of Liquids', by Emil Hatschek [7], (especially the chapter on colloidal solutions), by 1935 Freundlich had published a book called 'Thixotropie' [8] devoted to the subject, having been the first to introduce it into the title of a paper when he described the flow properties of aluminium hydroxide gels. Freundlich and co-workers soon found thixotropic effects manifested by a whole variety of systems including vanadium pentoxide sols, starch pastes, gelatin gels, pectin gels and many more. </p><p>Thixotropy originally therefore referred to the reversible changes from a flowable fluid to a solid-like elastic gel. Previously these kinds of physical changes had only been known to occur by changing the temperature, when such gels would melt on heating and then re-solidify on cooling. It was believed that a new kind of phase change had been found. </p><p>2.2. Progress </p><p>Early work in this area in the USA is exemplified by a series of three papers by McMillen in 1932 [9], reporting the results of his doctoral investigations into the thixotropy of a large number of flocculated paints. He showed that the fluidity (the inverse of viscosity) as a function of rest time decreased in some cases by four orders of magnitude, showing almost a quadratic dependence on rest time. </p><p>Writing in the UK in 1942, Scott-Blair [10] stated that 'the whole subject [of thixotropy] is so very new'. But then went on to list over 80 papers on the subject (see pp. 61-64). (In the second </p></li><li><p>H.A. Barnes/J. Non-Newtonian Fluid Mech. 70 (1997) 1-33 3 </p><p>edition of this book published in 1949, nearly 120 papers on thixotropy are cited.) Among the examples of thixotropic materials he gives are clays and soil suspensions, creams, drilling muds, flour doughs, flour suspensions, fibre greases, jellies, paints, carbon black suspensions and starch pastes. He also lists a number of papers on so-called thixotrometers, instruments specially devised to characterise the phenomenon. In this respect he raised some interesting points, among them whether thixotropy ought to be studied at constant rate of shear or at constant stress? This is still a most controversial question. </p><p>Scott-Blair quotes Hamaker's explanation of thixotropy as being due to the secondary minimum so that 'particles can form a loose association which is easily destroyed by shaking but re-establishes itself on standing'. This explanation still stands. With our present knowledge of microstructural changes, it is probably safe to say that all materials that are shear thinning are thixotropic, in that they will always take a finite time to bring about the rearrangements needed in the microstructural elements that result in shear thinning. As Scott-Blair concluded all those years ago "If this recovery is very rapid, the phenomenon is observed as structural viscosity [shear thinning]; if slow, it is observed as thixotropy". However even Scott-Blair sometimes confused thixotropy with shear thinning, as in his example of the importance of thixotropy for drilling muds that must be runny [sic] when lubricating the drill, but "of a high enough consistency at rest to avoid settling of suspended matter". </p><p>An important point he made concerned a suggestion that certain results of flow in capillary tubes of suspensions--that we now believe showed migration of particles away from the wall and thus, have an easier flow in small rather than large tubes--was due to thixotropy. He refuted this by showing that doubling the tube length halved the flow rate for a given driving pressure. </p><p>Pryce-Jones [11] (the first well-known Welsh rheologist) studied about 250 paints all in a state of light flocculation, using his own thixotrometer [12]. He noted that "It is a well-established fact that thixotropy is more pronounced in systems containing non-spherical particles", this is obviously so because they have to find themselves in the best 3D structure by rotation as well as movement, and progress from a solid gel to a freely flowing liquid due to complete microstructural breakdown, see Fig. 1. </p><p>Thixotropy is one of the few original technical terms used in pre-war, European rheology circles that has survived, unlike 'structural viscosity' (Strukturviskositaet, which we now understand as shear thinning) and 'false body' (now understood as extreme shear thinning with thixotropy) which have fallen by the wayside ~. </p><p>However, as late as 1953, Roscoe [13] still referred to 'false body' as different from thixotropy. The 'false body' had an apparent yield stress [stress at low shear rate following shearing at a high shear rate] that recovered quickly, while the thixotropic material takes some times before relatively fast recovery takes place. Today we understand that they are both manifestations of thixotropy. False bodies were taking a long time to die. </p><p>Jobling and Roberts in 1957 [14] commented that "thixotropy now has an even less distinct connotation. Electronic methods of measurement have shown that the time-lag required before </p><p>Readers with an interest in the historical derivation of scientific expressions are directed to Scott-Blair [10], p. 52. All Scott-Blair's books were written as personal memoirs and are very evocative of the man himself for those who knew him. </p></li><li><p>4 H.A. Barnes/J. Non-Newtonian Fluid Mech. 70 (1997) 1-33 </p><p>the original structure is regained may be very short indeed and it then becomes difficult to distinguish between a thixotropic material with a very short recovery time and a material whose viscosity falls with increasing rate of shear and depends for all practical purposes only on the instantaneous rate of shear. The latter effect is frequently called 'structural viscosity'". They went on to say "We endorse Pryce-Jones's plea that in the absence of authoritative definitions, terms such as ... thixotropy should not be used unless the intended meaning is made clear". In the Discussion section of this paper, Marcus Reiner notes that 'structural viscosity' and 'thixotropy' are seen as the same thing by some, with structural viscosity seen as a material with "nearly zero time of recovery". </p><p>The full extent of thixotropy was maintained by Bauer and Collins in their 1967 review [1]: "When a reduction in magnitude of rheological properties of a system, such as elastic modulus, yield stress, and viscosity, for example, occurs reversibly and isothermally with a distinct time dependence on application of shear strain, the system is described as thixotropic". They went on to say that thixotropy was "usually conceived as an unusual property of very special materials, sol-gel systems such as aqueous iron oxide dispersions, thixotropy in the sense described above has been found to be exhibited by a great many and a large variety of systems. Along with the breakdown in structure, other non-rheological features change, such as conductivity and dielectric constant". Lastly they noted that "The terms used by Freundlich are now seen to be archaic, viz liquefaction, re-solidification, sol. These had some obvious meaning for the qualitative changes brought about in low concentration dispersions of highly insoluble oxides of needle-like crystals such as iron oxide and vanadium pentoxide in low-viscosity aqueous media". </p><p>Nowadays thixotropy is sometimes used to include all time effects in a movement to non-linear behaviour, see for instance Cheng [15], but especially Lapasin and Pricl [16], who illustrate thixotropic behaviour by the transient response of viscosity and normal force of </p><p>S~KING / Ski,,. . . . . / e </p><p>Completely structured- giving ~)~4?&gt;'ft'~,/~--T:~--~.~_2LI ~'~-'~'A ~"~//~\~,~,,V~f ' elastic, so,d-,ke response ." I I ."/. </p><p> Partly structured - giving '-....._~" ~-~'~- yt~f ~/ t /~-" ; I "-k viscoelastic response i/,~'~/-"~/',.-,~-.,~#~ ~" ~ ~,~ ~i </p><p>Completely unstructured ~ '~/4~ ~ - giving viscous, shear </p><p>-thinning response </p><p>Fig. 1. Breakdown of a 3D thixotropic structure. </p></li><li><p>H.A. Barnes / J. NonNewtonian Fluid Mech. 70 (1997) 1-33 5 </p><p>polymer solutions. They then noted that the stress overshoot on start-up increases with increasing rest time. This is an interesting point--build up in polymer solutions is usually considered to be rapid, and rest times are rarely considered necessary. Weakly cross-linked gels would give the same thixotropic effect as a flocculated system. 2.3. How is" thixotropy generally understood today7 </p><p>One of the first definitions of thixotropy was given by Freundlich and Rawitzer [17] who stated that "By thixotropy is meant the phenomena of concentrated gels ... which solidify to gels which may again be liquified to sols. The resolidification occurs repeatedly, at constant temperature with a constant speed". (This would be very far from the kind of definition offered today.) However, Pryce-Jones [11] soon afterwards stated that the true meaning of thixotropy was "an increase of viscosity in a state of rest and a decrease of viscosity when submitted to a constant shearing stress". </p><p>It is clear that people using the word thixotropy today fall into two camps: first those who understand it in the latter (Pryce-Jones) sense as the time response of the microstructure brought about by shearing or resting, respectively, and the rheological effects arising therefrom. In these circles it is often used in a very narrow sense of viscosity changes only, with no reference to the reversible transition from gel-like to fluid-like behaviour. </p><p>Secondly there are those--often in industrial circles--who understand thixotropy in its original Freundlich and Rawitzer sense, as stated above, of conferring gel-like properties to a liquid which disappear on shaking but reappear on standing. This particular property adds considerable advantage to the practical use of materials such as paints, adhesives and coatings. Unless this different use is borne in mind, severe misunderstand will arise on reading the general literature. The former group would understand the use of thixotropy by the latter group as conferring extreme shear thinning to a liquid by, for instance, the addition of so-called thixopropes or by inducing flocculation. On the other hand the latter group only see the temporal properties of thixotropy as an irritation, because the desired reversible gelled state they want takes some time to disappear on shearing or reappear on standing. It is not surprising therefore that thixotropy has, on occasions, been confused with shear thinning. </p><p>There are various definitions of thixotropy offered in the current general scientific literature, e.g., scientific dictionaries and encyclopedias, that reflect these two points of view. Some are misleading, with even the best being incomplete. The following are a selection that illustrate the situation: </p><p>Oxford Encyclopedic Dictionary of Physics, [18]--"Thixotropy: Certain materials behave as solids under very small applied stresses but under greater stresses become liquids. When the stresses are removed the material se...</p></li></ul>