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New materials for analogue experiments: Preliminary tests of magnetorheological fluids
TECTO-126321; No of Pages 6 Tectonophysics xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tectonophysics journal homepage: www.elsevier.com/locate/tecto
New materials for analogue experiments: Preliminary tests of magnetorheological fluids C. Cavozzi a,⁎, F. Storti a, Y. Nestola a, F. Salvi b, G. Davoli b a b
NEXT — Natural and Experimental Tectonics Research Group, Dipartimento di Fisica e Scienze della Terra “Macedonio Melloni”, Università degli Studi di Parma, Italy Eni E&P, Via Emilia 1, I-20097 San Donato Milanese, MI, Italy
a r t i c l e
i n f o
Article history: Received 11 January 2014 Received in revised form 7 May 2014 Accepted 12 May 2014 Available online xxxx Keywords: Analogue modelling MR fluid Shale tectonics Decollement layer
a b s t r a c t New materials and related apparatuses are welcome to advance analogue modelling techniques. In this contribution, we report on a first attempt to use magnetorheological (MR) fluids as analogue materials for simulating the mechanical behavior of mobile décollement layers that change their mechanical properties during deformation. For this purpose, a specific sandbox was designed to include the possibility of quickly applying and removing a magnetic field below a MR fluid layer, in order to induce an instantaneous change from a frictional to a viscous behavior in the basal décollement material. The simulation of gravitational gliding and sediment progradation above a basal mobile shale layer provided results that compare well with analogue models produced with other experimental techniques, and with natural structures like those developed in the Niger delta region. This pilot study thus encourages further research for optimizing the applicability of MR fluids to the analogue simulation of geological processes. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Analogue modelling includes well-established laboratory techniques providing support and inspiration to structural geology, tectonics and geodynamics interpretations (e.g., Bonini et al., 2012; Brun and Fort, 2011; Cadell, 1890; Corti, 2012; Dooley and Schreurs, 2012; Graveleau et al., 2012). Applicability of analogue models to nature requires appropriate scaling of material properties, stress fields, and kinematic pathways (Hubbert, 1937; Ramberg, 1981). Scaling is a function of both experimental technique and analogue materials. In normal gravity experiments simulating crustal deformations, loose sand and sandsilicone multilayers are by far the most used materials (Brun, 2002; Davis et al., 1984; Davy and Cobbold, 1988; Rosenau et al., 2009). Glass microspheres (Schellart, 2000), hollow aluminum microspheres (Rossi and Storti, 2003), and muscovite interlayers (McClay, 1990), are some of the solutions that have been used to vary the frictional properties of granular matter. Wet clay (Cooke and van der Elst, 2012; Withjack and William, 1986) and plaster (Fossen and Gabrielsen, 1996) have also been used as analogue materials, as well as rock slices (Chester et al., 1991). An alternative experimental technique developed to overcome possible scaling problems is centrifuge modelling, which is able to produce gravity fields up to hundreds g, thus allowing the use of cohesive materials like plasticine (Corti, 2004; Dixon and Summers, 1985; Ramberg, 1981).
⁎ Corresponding author. E-mail address: [email protected] (C. Cavozzi).
A common feature of all techniques listed above is the intrinsic difficulty to vary the rheological properties of analogue materials during experimental runs. A possibility may be provided by thermomechanical modelling, where temperature is used to control the rheology of paraffin vax (Cobbold and Jackson, 1992; Grujic and Mancktelow, 1998). In this technique, however, a temperature gradient is used to impose the vertical variability of the rheological behavior of the analogue material and remains typically unchanged during model runs (Rossetti et al., 2000). A technical solution that allows the effective frictional properties of décollement materials to be varied during deformation involves the use of pressurized air injected at the bottom of experimental multilayers undergoing deformation (Cobbold et al., 2001; Mourgues et al., 2009). In this work, we present the preliminary results of the attempt to use magnetorheological (MR) fluids as analogues of natural materials affected by significant rheological changes during deformation. MR fluids are suspensions of micron-sized magnetic particles in carrier oil, which can exhibit dramatic changes in its rheological properties after the application of a magnetic field (Fig. 1). They switch instantaneously from a fluid state (viscous regime) to a solid state (frictional regime) without changing chemical properties by a process that is reversible in the order of milliseconds (Rosenfeld et al., 2002). We explored the design of a basic experimental apparatus suitable to exploit the potential of using MR fluids for simulating mobile upper crustal rocks in analogue modelling experiments under natural gravity conditions. To ensure effective testing of the suitability of MR fluids as analogue materials in physical experiments, we replicated: a) gravitational gliding in a sloping sandpack, following
http://dx.doi.org/10.1016/j.tecto.2014.05.019 0040-1951/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
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C. Cavozzi et al. / Tectonophysics xxx (2014) xxx–xxx
Fig. 2. Graph of the shear stress versus apparent shear rate for the MR fluid Basonetic® 2040 at different intensities of the magnetic field B (after BASF, 2010). The inset shows the computed viscosity of the MR Fluid for intensities of B ranging from 0 to 350 mT.
Fig. 1. Cartoon illustrating the effects of a magnetorheological fluid (black) at the base of a sand multilayer (gray), in the presence or absence of a magnetic field, respectively. a) The multilayer is gently inclined under a magnetic field: no deformation occurs. b) When the magnetic field is removed, the sand suddenly undergoes gravitational gliding. c) Remagnetization halts deformation also when the multilayer tilt is increased. d) Removal of the magnetic field causes instantaneous resuming of gravitational gliding.
the work of Mourgues and Cobbold (2003), who used pressurized air to produce a basal décollement layer; b) the experimental strategy in model 2 of Mourgues et al. (2009), who used a forelandward migrating area of pressurized air at the base of the sand multilayer to simulate the effects of delta progradation above shales, which is commonly assumed to behave as a viscoplastic Bingham material (e.g., Bingham, 1922; Ings and Beaumont, 2010) Comparison of experimental results obtained by the two different techniques is briefly discussed.
(Fig. 3a). When necessary, the mobile lateral walls can be connected to computer-controlled stepping motors to impose contractional, extensional, or transcurrent motions. Model surface topography evolution is recorded by a structured-light 3D scanner and by timelapse photographing. The simplest experimental multilayer consists of a basal layer of MR fluid overlain by a multicolored sand pack (Fig. 3; Table 1).
3. Type 1 experiments: gravitational gliding In this experimental program, the Plexiglas sandbox was located above an array of 18 removable magnetic stripes. A 3 mm-thick basal layer of MR fluid was deposited above the Plexiglas in the absence of the magnetic field. After magnetization, three 5-mm-thick prekinematic sand layers were sieved on top of the MR fluid. The sand was not in contact with the side walls to prevent lateral friction. The
2. Materials and experimental apparatus We tested the magnetorheological fluid Basonetic® 2040, produced by BASF Chemical Company, which contains 24% in volume of carbonyl iron powder as magnetizable material, and poly-α-olefin as base fluid. The density is 2.47 g/cm3 at 25 °C, and the operating temperature range is from -40 °C to 120 °C, respectively. The producer provides details of the physical properties of this MR fluid and of its rheological behavior as a function of temperature and of the absence or presence of a magnetic field (BASF, 2010). The relations between shear stress and strain rate at room temperature and different intensities of the magnetic field are illustrated in Fig. 2, as well as the associated changes in viscosity. The graphs show that in the absence of a magnetic field, the strain rate increases non-linearly with the shear stress. This dependence decreases with increasing the magnetic field intensity and becomes almost negligible for values higher than 300 mT. Accordingly, for high values of the magnetic field the shear stress value is high and remains quasi-constant regardless of the applied shear strain. The experimental apparatus consists of a Plexiglas box with an array of removable magnets located below the rigid basal plate
Fig. 3. a) Conceptual sketch of the experimental apparatus designed for the use of magnetorheological fluids. b) Map view of the basal magnetic array used in the Type 1 experiments, after construction of the experimental multilayer and before tilting the sandbox. The central area from which the magnets were removed is indicated in orange. See text for details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
C. Cavozzi et al. / Tectonophysics xxx (2014) xxx–xxx
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Table 1 Physical parameters of the experimental materials. Materials
Density (g/cm3)
Grain size (μm)
Angle of internal friction ϕ
Cohesion (Pa)
Dynamic shear viscosity η (Pa s)
Quartz sand MR fluid (Basonetic® 2040)
1.5 2.47
60–250 –
34.1 –
50–100 –
– 3 × 10−1 ⁎2.5 × 102
⁎ Under 70 mT magnetic field.
linked by two lateral tear faults (Fig. 4). A thin layer of syn-kinematic sand was deposited in the newly forming basin to simulate deformation in a natural environment. Analysis of the 3D model topography at the end of the experiment shows a well-developed near symmetrical graben formed by conjugate extensional fault pairs. This is clearly visible on the cross-section cut in the central part of the model, where a strong reduction of the MR fluid thickness can be observed below the area that underwent extension. Upslope extensional displacement was accommodated downslope by a thrust-related anticline that formed a prominent relief in the model topography. Cross-sectional geometry indicates that the MR fluid underwent significant thinning in the downslope boundary region before failure of the overlying sandpack. Formation of a thrust ramp in the anticlinal core allowed the MR fluid to be passively dragged up along with the hangingwall (Fig. 4). This behavior resembles that of silicone putty in sandbox analogue models of fold-and-thrust belts developed above viscous evaporitic décollements (Costa and Vendeville, 2002). Fig. 4. 3D view of model topography at the end of the described Type 1 gravitational gliding experiment, and representative central cross section. Note the capability of the MR fluid to flow downslope, from the extensional domain to the contractional one, and to inject along the basal frontal ramp.
4. Type 2 experiments: sediment progradation 4.1. Experiment design
central part of the 12 central magnetic stripes was removed at the end of multilayer construction to create a 22 cm long × 14 cm wide nonmagnetic region at the base of the sandpack (Fig. 3b). In the three models that we ran with this experimental setup, progressive tilting of the sandbox up to ~ 14° eventually triggered gravitational gliding of the non-magnetic sector of the sandpack, which resulted in extensional deformations upslope and contractional ones downslope, kinematically
In this experimental program, the sandbox was 50 cm long and 12 cm wide, and was located on top of an array made up of 20 magnetic stripes. A 4 mm-thick basal layer of MR fluid was deposited on the floor of the sandbox in the absence of the magnetic field. Side walls were treated with antistatic fluid to minimize friction-related bias (Souloumiac et al., 2012). After magnetization, two 1-mm-thick prekinematic sand layers were sieved on top of the MR fluid. During
Fig. 5. MR 3, Type 2 experiment. a) Cross-sectional view of the undeformed topography in the five steps of forelandward sediment progradation above the MR fluid basal layer. The five bars indicate the width of the sector that was demagnetized during each deformation step. b) Cross-sectional view of model topography after each of the five deformation steps. See text for details.
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
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model runs, five cycles of synkinematic sedimentation in an overall forelandward progradation system where simulated. Each cycle consisted of: (i) sedimentation of two, 2 mm-thick horizontal sand layers above a sector of the pre-kinematics strata, with a frontal slope given by the angle of repose; (ii) 14° forelandward tilting of the sandbox; (iii) demagnetization of the MR fluid in the frontal portion of the prograding strata, from the outer tip of the foreset, hinterlandward; (iv) back rotation to the horizontal of the sandbox at the end of deformation and re-magnetization (Fig. 5a). After completion of the experiments, post-kinematic white sand was sieved on the top to preserve surface features during wetting and cutting of cross-sections along the sediment progradation direction. For the purpose of this works, the same model evolution was replicated three times to verify its reproducibility. Only model MR3 is described here as representative of the experimental program. 4.2. Results The first sediment progradation involved about 1/4 of the model (Figs. 5a, 6a). Forelandward tilting and demagnetization of sector 1 caused formation of a linear graben at the inner edge of the instantaneously created viscous domain. Such an amount of extension was compensated by contractional deformation at the outer edge of the viscous domain (Figs. 5b, 6b). The following four cycles of remagnetization, backtilting, sedimentation, forelandward tilting, and demagnetization of sectors 2 to 5, systematically produced two paired belts of extensional and contractional deformations at the inner and outer boundaries between frictional an viscous domains, respectively (Figs. 5, 6). By increasing the number of cycles, an increase in structural complexity in both the contractional and extensional domains was observed. Five cross sections were cut at the end of the experiment to analyze the geometry of deformation structures, including the behavior of the MR fluid. The first-order deformation pattern is preserved in all sections: the inner and central sectors of the model are dominated by foreland- and hinterland-dipping extensional fault zones while the frontal sector is characterized by the presence of three thrust-related anticlines having MR fluid diapirs in the core (Fig. 7). A fourth diapir is preserved in the center of the sections as a remnant of the contractional domain that formed in that position during stage 2. Overall, the forelandward migration of the MR fluid and its passive dragging up along the thrusts are clearly visible (Fig. 8). 5. Discussion and conclusions Despite oversimplification and lack of accurate MR fluid rheological calibration, the two experimental programs illustrated in this paper successfully reproduced the results previously obtained from pressurizedair sandbox models. In particular, results from Type 1 experiments closely resemble those described in Mourgues and Cobbold (2003), both in map view and cross-section views (Fig. 9a). Application of our experimental technique to shale tectonics in passive margins, where sedimentary successions are asymmetrically loaded by progradational deltas and display complex arrays of regional and counter-regional extensional fault systems linked with contractional folds and thrust systems at the basin-ward pinch-out of shales (Briggs et al., 2006), provided results very similar to those obtained by Mourgues et al. (2009) in their model 2 (Fig. 9b) and to natural deformational architectures described in the Niger delta (Fig. 9c). This evidence contributes to support the effectiveness of MR fluids as analogue materials for simulating décollement layers that can change their rheology during deformation and reproduce also diapiric structures i.e., the behavior of thick décollement layers. We are well aware that this work does not provide an exhaustive study of MR fluids as analogue materials for sandbox experiments. A
Fig. 6. Top view of model topography during model evolution. a) First progradation in the magnetized state. b) Model deformation after de-magnetization. c) Model deformation after re-magnetization, second step of sediment progradation, and de-magnetization. d) Model deformation after re-magnetization, third step of sediment progradation, and de-magnetization. e) Model deformation after re-magnetization, fourth step of sediment progradation, and de-magnetization. f) Model deformation after re-magnetization, fifth step of sediment progradation, and de-magnetization. The position of the five crosssections in Fig. 6 is provided. The white rectangle outside maps (b) to (f) indicates the cross-sectional width of the temporary demagnetized area, which is progressively shifted outward during model progression.
comprehensive rheological study of the Basonetic 40 MR fluid, as well as of other similar products, would be required before any systematic use of these materials in physical experiments can start. For example, the 14° basal slope angle that was necessary to trigger gravity gliding
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
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Fig. 7. Photographs of cross-sections S1 to S5. See Fig. 6 for precise location and text for details.
in our experiments has no equivalent in nature, where such angles are much smaller. This indicates that significant progresses have to be made on this experimental technique for obtaining rigorous scaling of material properties and force balance. The evidence that the use of pressurized-air in sandbox models followed a similar trend (11° of basal slope in Mourgues and Cobbold, 2003; 11 to 15° of basal slope in Mourgues and Cobbold, 2006, then reduced to 6–7° in Mourgues et al., 2009) is encouraging us to continue investigating on this experimental technique. Indeed, we offer this preliminary contribution as a starting point for future, more developed research on MR fluids physical properties, such as density and viscosity, and their tuning for improving the applicability to geological processes.
Acknowledgments The present study was performed using tools and methodologies implemented within an Eni R&D Project. We are grateful to Eni S.p.A. Exploration and Production Division, for releasing this material for publication. Initial experiments were carried out as part of a research project designed and implemented by Elisabetta Costa, passed away on November 24th 2009. We warmly acknowledge BASF Chemical Company for providing a free sample of the MR fluid BASONETIC®2040, and Midland Valley Exploration Ltd. for providing an academic license of MOVE™. Discussions with C. Magistroni, and G. Spadini (Eni-GEBA) were very useful during the completion of this research. We gratefully
Fig. 8. Three-dimensional view (a) and corresponding line drawing (b) of model geometry at the end of the experiment. See text for details.
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
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Fig. 9. a) Schematic line drawing in map view and cross section of the gliding experiment described by Mourgues and Cobbold, (2003), redrawn from their Fig. 9a and b. Note the similarity with our experimental results illustrated in Fig. 4. b) Cross section of model 2 in Mourgues et al. (2009), redrawn from their Fig. 8a. Comparison with cross sections in Figs. 7 and 8 indicates the similarity of the experimental results obtained from the two very different techniques. c) Schematic cross-sectional architecture of the south-central Niger Delta (after Hooper et al., 2002).
acknowledge constructive reviews from B. Maillot and B. Vendeville, and editorial handling by L. Jolivet. References BASF, 2010. Technical Data Sheet of BASONETIC(r) 2040. http://www.monomers.basf.com/ cm/internet/en/content/Microsite/Basonetic/Product_and_Technology/Basonetic_2040. Bingham, E.C., 1922. Fluidity and Plasticity. McGraw-Hill, New York,. Bonini, M., Sani, F., Antonielli, B., 2012. Basin inversion and contractional reactivation of inherited normal faults: a review based on previous and new experimental models. Tectonophysics 522–523, 55–88. Briggs, S.E., Davies, R.J., Cartwright, J.A., Morgan, R., 2006. Multiple detachment levels and their control on fold styles in the compressional domain of the deepwater west Niger Delta. Basin Res. 18, 435–450. Brun, J.-P., 2002. Deformation of the continental lithosphere: insights from brittle–ductile models. In: de Meer, S., Drury, M.R., de Bresser, J.H.P., Pennock, G.M. (Eds.), Deformation Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives. Geological Society, London, Special Publications, 200, pp. 355–370. Brun, J.P., Fort, X., 2011. Salt tectonics at passive margins: geology versus models. Mar. Pet. Geol. 28 (6), 1123–1145. Cadell, H.M., 1890. Experimental researches in mountain building. Trans. Roy. Soc. Edinb. 35, 337–357. Chester, J.S., Logan, J.M., Spang, H.J., 1991. Influence of layering and boundary condition on fault-bend and fault-propagation folding. Bull. Geol. Soc. Am. 103, 1059–1072. Cobbold, P.R., Jackson, M.P.A., 1992. Gum rosin (colophony): a suitable material for thermomechanical modelling of the lithosphere. Tectonophysics 210, 255–271. Cobbold, P.R., Durand, S., Mourgues, R., 2001. Sandbox modelling of thrust wedges with fluid-assisted detachments. Tectonophysics 334 (3–4), 245–258. Cooke, M.L., van der Elst, N.J., 2012. Rheologic testing of wet kaolin reveals frictional and bi-viscous behavior of crustal materials. Geophys. Res. Lett. 39, L01308. http://dx.doi. org/10.1029/2011GL050186. Corti, G., 2004. Centrifuge modelling of the influence of crustal fabrics on the development of transfer zones: insights into the mechanics of continental rifting architecture. Tectonophysics 384, 191–208. Corti, G., 2012. Evolution and characteristics of continental rifting: analogue modellinginspired view and comparison with examples from the East African Rift System. Tectonophysics 522–523, 1–33. Costa, E., Vendeville, B.C., 2002. Experimental insights on the geometry and kinematics of fold-and-thrust-belts above weak, viscous evaporitic décollement. J. Struct. Geol. 24, 1729–1739. Davis, D., Suppe, J., Dahlen, F.A., 1984. Mechanics of fold-and-thrust belts and accretionary wedges: cohesive Coulomb theory. J. Geophys. Res. 89, 10087–10101. Davy, Ph., Cobbold, P.R., 1988. Indentation tectonics in nature and experiments. 1. Experiments scaled for gravity. Bull. Geol. Inst. Univ. Upps. 14, 129–141.
Dixon, J.M., Summers, J.M., 1985. Recent developments in centrifuge modelling of tectonic processes: equipment, model construction techniques and rheology of model materials. J. Struct. Geol. 7, 83–102. Dooley, T.P., Schreurs, G., 2012. Analogue modelling of intraplate strike–slip tectonics: a review and new experimental results. Tectonophysics 574–575, 1–71. Fossen, H., Gabrielsen, R.H., 1996. Experimental modeling of extensional fault systems by use of plaster. J. Struct. Geol. 18 (5), 673–687. Graveleau, F., Malavieille, J., Dominguez, S., 2012. Experimental modelling of orogenic wedges: a review. Tectonophysics 538–540, 1–66. Grujic, D., Mancktelow, N.S., 1998. Melt-bearing shear zones: analogue experiments and comparison with examples from southern Madagascar. J. Struct. Geol. 20, 673–680. Hooper, R.J., Fitzsimmons, R.J., Grant, N., Vendeville, B.C., 2002. The role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa. J. Struct. Geol. 24, 847–859. Hubbert, M.K., 1937. Theory of scale models as applied to the study of geologic structures. Geol. Soc. Am. Bull. 48, 1459–1520. Ings, S.J., Beaumont, C., 2010. Continental margin shale tectonics: preliminary results from coupled fluid-mechanical models of large-scale delta instability. J. Geol. Soc. Lond. 167, 571–582. McClay, K.R., 1990. Deformation mechanics in analogue models of extensional fault systems. Geol. Soc. Lond. Spec. Publ. 54, 445–453. Mourgues, R., Cobbold, P.R., 2003. Some tectonic consequences of fluid overpressures and seepage forces as demonstrated by sandbox modelling. Tectonophysics 376, 75–97. Mourgues, R., Cobbold, P.R., 2006. Sandbox experiments on gravitational spreading and gliding in the presence of fluid overpressures. J. Struct. Geol. 28, 887–901. Mourgues, R., Lecomte, E., Vendeville, B., Raillard, S., 2009. An experimental investigation of gravity-driven shale tectonics in progradational delta. Tectonophysics 474, 643–656. Ramberg, H., 1981. Gravity, Deformation and the Earth's Crust. Academic Press, New York,. Rosenau, M., Lohrmann, J., Oncken, O., 2009. Shocks in a box: an analogue model of subduction earthquake cycles with application to seismotectonic forearc evolution. J. Geophys. Res. 114, B01409. Rosenfeld, N., Wereley, N.M., Radakrishnan, R., Sudarshan, T.S., 2002. Behavior of magnetorheological fluids utilizing nanopowder iron. Int. J. Mod. Phys. B16, 2392–2398. Rossetti, F., Faccenna, C., Ranalli, G., Storti, F., 2000. Convergence rate-dependent growth of experimental viscous orogenic wedges. Earth Planet. Sci. Lett. 178 (3–4), 367–372 (30). Rossi, D., Storti, F., 2003. New artificial granular materials for analogue laboratory experiments: aluminium and siliceous microspheres. J. Struct. Geol. 25 (11), 1893–1899. Schellart, W.P., 2000. Shear test results for cohesion and friction coefficients for different granular materials: scaling implications for their usage in analogue modelling. Tectonophysics 324, 1–16. Souloumiac, P., Maillot, B., Leroy, Y.M., 2012. Bias due to side wall friction in sand box experiments. Journal of Structural Geology Volume 35 (Issues), 90–101. Withjack, M.O., William, J.R., 1986. Deformation produced by oblique rifting. Tectonophysics 126, 99–124.
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
Contents lists available at ScienceDirect
Tectonophysics journal homepage: www.elsevier.com/locate/tecto
New materials for analogue experiments: Preliminary tests of magnetorheological fluids C. Cavozzi a,⁎, F. Storti a, Y. Nestola a, F. Salvi b, G. Davoli b a b
NEXT — Natural and Experimental Tectonics Research Group, Dipartimento di Fisica e Scienze della Terra “Macedonio Melloni”, Università degli Studi di Parma, Italy Eni E&P, Via Emilia 1, I-20097 San Donato Milanese, MI, Italy
a r t i c l e
i n f o
Article history: Received 11 January 2014 Received in revised form 7 May 2014 Accepted 12 May 2014 Available online xxxx Keywords: Analogue modelling MR fluid Shale tectonics Decollement layer
a b s t r a c t New materials and related apparatuses are welcome to advance analogue modelling techniques. In this contribution, we report on a first attempt to use magnetorheological (MR) fluids as analogue materials for simulating the mechanical behavior of mobile décollement layers that change their mechanical properties during deformation. For this purpose, a specific sandbox was designed to include the possibility of quickly applying and removing a magnetic field below a MR fluid layer, in order to induce an instantaneous change from a frictional to a viscous behavior in the basal décollement material. The simulation of gravitational gliding and sediment progradation above a basal mobile shale layer provided results that compare well with analogue models produced with other experimental techniques, and with natural structures like those developed in the Niger delta region. This pilot study thus encourages further research for optimizing the applicability of MR fluids to the analogue simulation of geological processes. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Analogue modelling includes well-established laboratory techniques providing support and inspiration to structural geology, tectonics and geodynamics interpretations (e.g., Bonini et al., 2012; Brun and Fort, 2011; Cadell, 1890; Corti, 2012; Dooley and Schreurs, 2012; Graveleau et al., 2012). Applicability of analogue models to nature requires appropriate scaling of material properties, stress fields, and kinematic pathways (Hubbert, 1937; Ramberg, 1981). Scaling is a function of both experimental technique and analogue materials. In normal gravity experiments simulating crustal deformations, loose sand and sandsilicone multilayers are by far the most used materials (Brun, 2002; Davis et al., 1984; Davy and Cobbold, 1988; Rosenau et al., 2009). Glass microspheres (Schellart, 2000), hollow aluminum microspheres (Rossi and Storti, 2003), and muscovite interlayers (McClay, 1990), are some of the solutions that have been used to vary the frictional properties of granular matter. Wet clay (Cooke and van der Elst, 2012; Withjack and William, 1986) and plaster (Fossen and Gabrielsen, 1996) have also been used as analogue materials, as well as rock slices (Chester et al., 1991). An alternative experimental technique developed to overcome possible scaling problems is centrifuge modelling, which is able to produce gravity fields up to hundreds g, thus allowing the use of cohesive materials like plasticine (Corti, 2004; Dixon and Summers, 1985; Ramberg, 1981).
⁎ Corresponding author. E-mail address: [email protected] (C. Cavozzi).
A common feature of all techniques listed above is the intrinsic difficulty to vary the rheological properties of analogue materials during experimental runs. A possibility may be provided by thermomechanical modelling, where temperature is used to control the rheology of paraffin vax (Cobbold and Jackson, 1992; Grujic and Mancktelow, 1998). In this technique, however, a temperature gradient is used to impose the vertical variability of the rheological behavior of the analogue material and remains typically unchanged during model runs (Rossetti et al., 2000). A technical solution that allows the effective frictional properties of décollement materials to be varied during deformation involves the use of pressurized air injected at the bottom of experimental multilayers undergoing deformation (Cobbold et al., 2001; Mourgues et al., 2009). In this work, we present the preliminary results of the attempt to use magnetorheological (MR) fluids as analogues of natural materials affected by significant rheological changes during deformation. MR fluids are suspensions of micron-sized magnetic particles in carrier oil, which can exhibit dramatic changes in its rheological properties after the application of a magnetic field (Fig. 1). They switch instantaneously from a fluid state (viscous regime) to a solid state (frictional regime) without changing chemical properties by a process that is reversible in the order of milliseconds (Rosenfeld et al., 2002). We explored the design of a basic experimental apparatus suitable to exploit the potential of using MR fluids for simulating mobile upper crustal rocks in analogue modelling experiments under natural gravity conditions. To ensure effective testing of the suitability of MR fluids as analogue materials in physical experiments, we replicated: a) gravitational gliding in a sloping sandpack, following
http://dx.doi.org/10.1016/j.tecto.2014.05.019 0040-1951/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
2
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Fig. 2. Graph of the shear stress versus apparent shear rate for the MR fluid Basonetic® 2040 at different intensities of the magnetic field B (after BASF, 2010). The inset shows the computed viscosity of the MR Fluid for intensities of B ranging from 0 to 350 mT.
Fig. 1. Cartoon illustrating the effects of a magnetorheological fluid (black) at the base of a sand multilayer (gray), in the presence or absence of a magnetic field, respectively. a) The multilayer is gently inclined under a magnetic field: no deformation occurs. b) When the magnetic field is removed, the sand suddenly undergoes gravitational gliding. c) Remagnetization halts deformation also when the multilayer tilt is increased. d) Removal of the magnetic field causes instantaneous resuming of gravitational gliding.
the work of Mourgues and Cobbold (2003), who used pressurized air to produce a basal décollement layer; b) the experimental strategy in model 2 of Mourgues et al. (2009), who used a forelandward migrating area of pressurized air at the base of the sand multilayer to simulate the effects of delta progradation above shales, which is commonly assumed to behave as a viscoplastic Bingham material (e.g., Bingham, 1922; Ings and Beaumont, 2010) Comparison of experimental results obtained by the two different techniques is briefly discussed.
(Fig. 3a). When necessary, the mobile lateral walls can be connected to computer-controlled stepping motors to impose contractional, extensional, or transcurrent motions. Model surface topography evolution is recorded by a structured-light 3D scanner and by timelapse photographing. The simplest experimental multilayer consists of a basal layer of MR fluid overlain by a multicolored sand pack (Fig. 3; Table 1).
3. Type 1 experiments: gravitational gliding In this experimental program, the Plexiglas sandbox was located above an array of 18 removable magnetic stripes. A 3 mm-thick basal layer of MR fluid was deposited above the Plexiglas in the absence of the magnetic field. After magnetization, three 5-mm-thick prekinematic sand layers were sieved on top of the MR fluid. The sand was not in contact with the side walls to prevent lateral friction. The
2. Materials and experimental apparatus We tested the magnetorheological fluid Basonetic® 2040, produced by BASF Chemical Company, which contains 24% in volume of carbonyl iron powder as magnetizable material, and poly-α-olefin as base fluid. The density is 2.47 g/cm3 at 25 °C, and the operating temperature range is from -40 °C to 120 °C, respectively. The producer provides details of the physical properties of this MR fluid and of its rheological behavior as a function of temperature and of the absence or presence of a magnetic field (BASF, 2010). The relations between shear stress and strain rate at room temperature and different intensities of the magnetic field are illustrated in Fig. 2, as well as the associated changes in viscosity. The graphs show that in the absence of a magnetic field, the strain rate increases non-linearly with the shear stress. This dependence decreases with increasing the magnetic field intensity and becomes almost negligible for values higher than 300 mT. Accordingly, for high values of the magnetic field the shear stress value is high and remains quasi-constant regardless of the applied shear strain. The experimental apparatus consists of a Plexiglas box with an array of removable magnets located below the rigid basal plate
Fig. 3. a) Conceptual sketch of the experimental apparatus designed for the use of magnetorheological fluids. b) Map view of the basal magnetic array used in the Type 1 experiments, after construction of the experimental multilayer and before tilting the sandbox. The central area from which the magnets were removed is indicated in orange. See text for details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Physical parameters of the experimental materials. Materials
Density (g/cm3)
Grain size (μm)
Angle of internal friction ϕ
Cohesion (Pa)
Dynamic shear viscosity η (Pa s)
Quartz sand MR fluid (Basonetic® 2040)
1.5 2.47
60–250 –
34.1 –
50–100 –
– 3 × 10−1 ⁎2.5 × 102
⁎ Under 70 mT magnetic field.
linked by two lateral tear faults (Fig. 4). A thin layer of syn-kinematic sand was deposited in the newly forming basin to simulate deformation in a natural environment. Analysis of the 3D model topography at the end of the experiment shows a well-developed near symmetrical graben formed by conjugate extensional fault pairs. This is clearly visible on the cross-section cut in the central part of the model, where a strong reduction of the MR fluid thickness can be observed below the area that underwent extension. Upslope extensional displacement was accommodated downslope by a thrust-related anticline that formed a prominent relief in the model topography. Cross-sectional geometry indicates that the MR fluid underwent significant thinning in the downslope boundary region before failure of the overlying sandpack. Formation of a thrust ramp in the anticlinal core allowed the MR fluid to be passively dragged up along with the hangingwall (Fig. 4). This behavior resembles that of silicone putty in sandbox analogue models of fold-and-thrust belts developed above viscous evaporitic décollements (Costa and Vendeville, 2002). Fig. 4. 3D view of model topography at the end of the described Type 1 gravitational gliding experiment, and representative central cross section. Note the capability of the MR fluid to flow downslope, from the extensional domain to the contractional one, and to inject along the basal frontal ramp.
4. Type 2 experiments: sediment progradation 4.1. Experiment design
central part of the 12 central magnetic stripes was removed at the end of multilayer construction to create a 22 cm long × 14 cm wide nonmagnetic region at the base of the sandpack (Fig. 3b). In the three models that we ran with this experimental setup, progressive tilting of the sandbox up to ~ 14° eventually triggered gravitational gliding of the non-magnetic sector of the sandpack, which resulted in extensional deformations upslope and contractional ones downslope, kinematically
In this experimental program, the sandbox was 50 cm long and 12 cm wide, and was located on top of an array made up of 20 magnetic stripes. A 4 mm-thick basal layer of MR fluid was deposited on the floor of the sandbox in the absence of the magnetic field. Side walls were treated with antistatic fluid to minimize friction-related bias (Souloumiac et al., 2012). After magnetization, two 1-mm-thick prekinematic sand layers were sieved on top of the MR fluid. During
Fig. 5. MR 3, Type 2 experiment. a) Cross-sectional view of the undeformed topography in the five steps of forelandward sediment progradation above the MR fluid basal layer. The five bars indicate the width of the sector that was demagnetized during each deformation step. b) Cross-sectional view of model topography after each of the five deformation steps. See text for details.
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model runs, five cycles of synkinematic sedimentation in an overall forelandward progradation system where simulated. Each cycle consisted of: (i) sedimentation of two, 2 mm-thick horizontal sand layers above a sector of the pre-kinematics strata, with a frontal slope given by the angle of repose; (ii) 14° forelandward tilting of the sandbox; (iii) demagnetization of the MR fluid in the frontal portion of the prograding strata, from the outer tip of the foreset, hinterlandward; (iv) back rotation to the horizontal of the sandbox at the end of deformation and re-magnetization (Fig. 5a). After completion of the experiments, post-kinematic white sand was sieved on the top to preserve surface features during wetting and cutting of cross-sections along the sediment progradation direction. For the purpose of this works, the same model evolution was replicated three times to verify its reproducibility. Only model MR3 is described here as representative of the experimental program. 4.2. Results The first sediment progradation involved about 1/4 of the model (Figs. 5a, 6a). Forelandward tilting and demagnetization of sector 1 caused formation of a linear graben at the inner edge of the instantaneously created viscous domain. Such an amount of extension was compensated by contractional deformation at the outer edge of the viscous domain (Figs. 5b, 6b). The following four cycles of remagnetization, backtilting, sedimentation, forelandward tilting, and demagnetization of sectors 2 to 5, systematically produced two paired belts of extensional and contractional deformations at the inner and outer boundaries between frictional an viscous domains, respectively (Figs. 5, 6). By increasing the number of cycles, an increase in structural complexity in both the contractional and extensional domains was observed. Five cross sections were cut at the end of the experiment to analyze the geometry of deformation structures, including the behavior of the MR fluid. The first-order deformation pattern is preserved in all sections: the inner and central sectors of the model are dominated by foreland- and hinterland-dipping extensional fault zones while the frontal sector is characterized by the presence of three thrust-related anticlines having MR fluid diapirs in the core (Fig. 7). A fourth diapir is preserved in the center of the sections as a remnant of the contractional domain that formed in that position during stage 2. Overall, the forelandward migration of the MR fluid and its passive dragging up along the thrusts are clearly visible (Fig. 8). 5. Discussion and conclusions Despite oversimplification and lack of accurate MR fluid rheological calibration, the two experimental programs illustrated in this paper successfully reproduced the results previously obtained from pressurizedair sandbox models. In particular, results from Type 1 experiments closely resemble those described in Mourgues and Cobbold (2003), both in map view and cross-section views (Fig. 9a). Application of our experimental technique to shale tectonics in passive margins, where sedimentary successions are asymmetrically loaded by progradational deltas and display complex arrays of regional and counter-regional extensional fault systems linked with contractional folds and thrust systems at the basin-ward pinch-out of shales (Briggs et al., 2006), provided results very similar to those obtained by Mourgues et al. (2009) in their model 2 (Fig. 9b) and to natural deformational architectures described in the Niger delta (Fig. 9c). This evidence contributes to support the effectiveness of MR fluids as analogue materials for simulating décollement layers that can change their rheology during deformation and reproduce also diapiric structures i.e., the behavior of thick décollement layers. We are well aware that this work does not provide an exhaustive study of MR fluids as analogue materials for sandbox experiments. A
Fig. 6. Top view of model topography during model evolution. a) First progradation in the magnetized state. b) Model deformation after de-magnetization. c) Model deformation after re-magnetization, second step of sediment progradation, and de-magnetization. d) Model deformation after re-magnetization, third step of sediment progradation, and de-magnetization. e) Model deformation after re-magnetization, fourth step of sediment progradation, and de-magnetization. f) Model deformation after re-magnetization, fifth step of sediment progradation, and de-magnetization. The position of the five crosssections in Fig. 6 is provided. The white rectangle outside maps (b) to (f) indicates the cross-sectional width of the temporary demagnetized area, which is progressively shifted outward during model progression.
comprehensive rheological study of the Basonetic 40 MR fluid, as well as of other similar products, would be required before any systematic use of these materials in physical experiments can start. For example, the 14° basal slope angle that was necessary to trigger gravity gliding
Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019
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Fig. 7. Photographs of cross-sections S1 to S5. See Fig. 6 for precise location and text for details.
in our experiments has no equivalent in nature, where such angles are much smaller. This indicates that significant progresses have to be made on this experimental technique for obtaining rigorous scaling of material properties and force balance. The evidence that the use of pressurized-air in sandbox models followed a similar trend (11° of basal slope in Mourgues and Cobbold, 2003; 11 to 15° of basal slope in Mourgues and Cobbold, 2006, then reduced to 6–7° in Mourgues et al., 2009) is encouraging us to continue investigating on this experimental technique. Indeed, we offer this preliminary contribution as a starting point for future, more developed research on MR fluids physical properties, such as density and viscosity, and their tuning for improving the applicability to geological processes.
Acknowledgments The present study was performed using tools and methodologies implemented within an Eni R&D Project. We are grateful to Eni S.p.A. Exploration and Production Division, for releasing this material for publication. Initial experiments were carried out as part of a research project designed and implemented by Elisabetta Costa, passed away on November 24th 2009. We warmly acknowledge BASF Chemical Company for providing a free sample of the MR fluid BASONETIC®2040, and Midland Valley Exploration Ltd. for providing an academic license of MOVE™. Discussions with C. Magistroni, and G. Spadini (Eni-GEBA) were very useful during the completion of this research. We gratefully
Fig. 8. Three-dimensional view (a) and corresponding line drawing (b) of model geometry at the end of the experiment. See text for details.
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Fig. 9. a) Schematic line drawing in map view and cross section of the gliding experiment described by Mourgues and Cobbold, (2003), redrawn from their Fig. 9a and b. Note the similarity with our experimental results illustrated in Fig. 4. b) Cross section of model 2 in Mourgues et al. (2009), redrawn from their Fig. 8a. Comparison with cross sections in Figs. 7 and 8 indicates the similarity of the experimental results obtained from the two very different techniques. c) Schematic cross-sectional architecture of the south-central Niger Delta (after Hooper et al., 2002).
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Please cite this article as: Cavozzi, C., et al., New materials for analogue experiments: Preliminary tests of magnetorheological fluids, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.019