Which Effective Viscosity?
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CORE Metadata, citation and similar papers at core.ac.uk Provided by Bournemouth University Research Online Mineralogical Magazine, April 2009, Vol. 73(2), pp. 167–191 Which effective viscosity? N. PETFORD* School of Conservation Sciences, Bournemouth University, Poole, Dorset BH12 5BB, UK [Received 7 January 2009; Accepted 21 May 2009)] ABSTRACT Magmas undergoing shear are prime examples of flows that involve the transport of solids and gases by a separate (silicate melt) carrier phase. Such flows are called multiphase, and have attracted much attention due to their important range of engineering applications. Where the volume fraction of the dispersed phase (crystals) is large, the influence of particles on the fluid motion becomes significant and must be taken into account in any explanation of the bulk behaviour of the mixture. For congested magma deforming well in excess of the dilute limit (particle concentrations >40% by volume), sudden changes in the effective or relative viscosity can be expected. The picture is complicated further by the fact that the melt phase is temperature- and shear-rate-dependent. In the absence of a constitutive law for the flow of congested magma under an applied force, it is far from clear which of the many hundreds of empirical formulae devised to predict the rheology of suspensions as the particle fraction increases with time are best suited. Some of the more commonly used expressions in geology and engineering are reviewed with an aim to home in on those variables key to an improved understanding of magma rheology. These include a temperature, compositional and shear-rate dependency of viscosity of the melt phase with the shear-rate dependency of the crystal (particle) packing arrangement. Building on previous formulations, a new expression for the effective (relative) viscosity of magma is proposed that gives users the option to define a packing fraction range as a function of shear stress. Comparison is drawn between processes (segregation, clustering, jamming), common in industrial slurries, and structures seen preserved in igneous rocks. An equivalence is made such that congested magma, viewed in purely mechanical terms as a high-temperature slurry, is an inherently non- equilibrium material where flow at large Pe´clet numbers may result in shear thinning and spontaneous development of layering. KEYWORDS: viscosity, rheology, fluid motion, magma, engineering, shear rate. Introduction with time- and rate-dependent properties (Fig. 1). WHILE significant progress has been made in Especially problematic is that factors that govern quantifying the flow properties of silicate melt in the flow of magma, including heat transfer, phase terms of viscosity and density over a wide transitions, coupled deformation of both solid and compositional range (see Shaw, 1965; Giordano fluid phases, seepage phenomena and chemical et al., 2008 amongst many others), a fundamental processes, occur simultaneously and interdepen- treatment that captures simultaneously the dently. Of critical importance is that for multi- rheology and flow behaviour of magma as a phase materials, the viscosity is not a single- fluid-particle suspension remains elusive. This valued function nor necessarily isotropic, but is lack of understanding of what may appear at first instead a macroscopic property derived from the glance a relatively trivial problem arises from the suspension microstructure. It is this fact that magmas are complex multiphase flows microstructure-viscosity connection that defines the complex rheological properties of magma, which in turn controls its rate and style of * E-mail: [email protected] emplacement, both within and on the surface of DOI: 10.1180/minmag.2009.073.2.167 the earth and other planets (e.g. Dingwell and # 2009 The Mineralogical Society N. PETFORD Webb, 1990; Kargel et al., 1991; Melnik and dealing explicitly with magma rheology include Sparks, 2005; Reese and Solomatov, 2006; Hale McBirney and Murase, 1984; Ryerson et al. 1988; et al., 2007; Cordonnier et al., 2008; Whittington Pinkerton and Stevenson, 1992; Lejeune and et al., 2009). Richet, 1995; Lavallee´ et al., 2007. Finally, this More generally, a better understanding of the review deals exclusively with two-phase flow and motion of particle-fluid mixtures is important for a ignores the complicating effects of gas bubbles on large number of applications in chemical engi- magma rheology known to provide an additional neering, polymer science and biophysics, where degree of freedom on microstructre evolution the complexity of phenomena has attracted much (e.g. Rust and Manga, 2002; Pal, 2003). attention. A crude classification of the subject is carried out by the specification of: (1) the solids volume content of the suspension, magmatic or Background otherwise, defined as the volume occupied by the Stickel and Powell (2005) provide a highly particles as a fraction or percentage of the total accessible introduction to dense-suspension volume of the suspension; (2) the hydrodynamics rheology based on dimensional analysis and the of particle-particle interactions; (3) the mean shear conservation of linear momentum. For complete- rate of the flow; and (4) the mean particle size. As ness, some of their analysis is reproduced here. to (1), this review is concerned primarily with The viscosity of a suspension, magmatic or estimates of flow rheology where the magma otherwise, is a function of a range of variables crystal content (solidosity) is equal to or exceeds including particle diameter (d), dynamic melt 40% by volume. Such materials are referred to in viscosity (Z0), deformation rate (expressed either the micromechanics literature as ‘congested’ or as as shear rate or shear stress), thermal energy and ‘dense slurries’, and magmas during flow where time (see Table 1 for a list of symbols and cooling and crystallization dominate, provide meaning). In general, these terms can be brought excellent natural examples (e.g. Marsh, 1996). together such that the suspension viscosity, also With respect to (2), neutral (non-colloidal) referred to as the effective viscosity, Ze,isa particles in Newtonian fluids are envisaged, and function of: (3) the shear rate is sufficiently small so that no Z = f[(f,d,r)(Z ,r ,kT,tg_,t)] (1) turbulent effects, associated with fluid inertia, take e 0 0 place. Finally, (4) the particles (crystals in where all the terms inside the first bracket relate suspension) are sufficiently large (>0.1 mm) that to properties of the suspension, and those in the Brownian motion can be ignored. Despite these second the fluid phase. Conveniently the variables restrictions, which correspond to the high-Pe´clet, in equation 1 can be arranged into the following 3 low-Reynolds number limit, there is potential to dimensionless groups: Zr = Z/Z0, f =(4p/3)fd , 3 gain insight into a wide range of classical rr = r/r0 and tr = tkT/Z0d . Note that fluid magmatic-emplacement phenomena by treating deformation can be defined either in terms of the problem as one would a dense suspension. shear stress (t) or shear rate, the latter defined as This review is split broadly into two parts À a the time rate of shear strain (g_) with units 1/s. background introduction to the rheology and flow Strain (e) is defined non-dimensionally as e = tg_. of both dilute and dense suspensions that includes In this study, both shear rate and shear stress are a survey of some of the better known empirical used as the deformation term. A further expressions put forward to capture the important simplification can be made by assuming that the effect of increasing viscosity due to particle dense suspension is neutrally buoyant (settling loading, and a second, more discursive section due to mass density is inhibited to some extent by that looks afresh at the origin of layering and particle interactions), and is steady state, such that other structures preserved in igneous rocks using Z = f(f, Pe,Re) (2) concepts informed by fluid-mechanical studies of e dense slurries. The first part in particular draws Two important dimensionless numbers with heavily on previous but highly relevant treatments bearing on the suspension rheology during shear of suspension flow by Zapryanov and Tabakova are the particle Reynolds (Reg_) and Pe´clet (Peg_) (1999) and Stickel and Powell (2005). Interested numbers readers should look to these works and references 2 r0d g_ therein for further in-depth analysis (see also Reg_ ¼ ð3Þ Jeffrey and Acrivos, 1996). Other relevant studies Z0 16 8 WHICH EFFECTIVE VISCOSITY? and T|me dependency 3 6pZ0d g_ Peg_ ¼ ð4Þ Suspensions with volume concentrations in excess kT of ~40% are candidate examples of non- Newtonian fluids (e.g. Zapryanov and Tabakova, The ratio of the Pe´clet and Reynolds numbers is 1999). The degree of non-linearity can be related the Schmidt number Peg_/Reg_. For a given rate of to the degree of hydrodynamic diffusion (diffu- shear, Sc >>1 tends to indicate Newtonian sion due to interaction with other particles as behaviour. Together, these dimensionless opposed to thermal motion), to advection of numbers provide insight into the rheology and particles in the flow through the Pe number, kinds of structures that may form in dense magma which must be of O(1) for flow to disrupt slurries undergoing shear. significantly the suspension microstructure and TABLE 1. Selected list of symbols and units used in text. Symbol Meaning Unit A0 pre exponential factor in equation 8 A parameter in equation 9 Pa A kinetic parameter in equations 14 and 15 Pam B parameter