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Time-dependent rheology of clay particle-stabilised emulsions
CLAY-02950; No of Pages 4 Applied Clay Science xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Time-dependent rheology of clay particle-stabilised emulsions Catherine P. Whitby ⁎, Paula Corbi Garcia Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, 5095, Australia
a r t i c l e
i n f o
Article history: Received 31 October 2013 Accepted 3 March 2014 Available online xxxx Keywords: Pickering emulsion Viscoelastic Breakdown Recovery
a b s t r a c t We have studied the thixotropic behaviour of clay particle-stabilised emulsions using a combination of rheology and confocal fluorescence microscopy. Oil drops were stabilised by laponite particles attached to their surfaces and incorporated into a three dimensional super-aggregate structure of laponite particles that formed in the water. The self-supporting, three-dimensional network of drops and laponite particles gives the emulsions pronounced gel-like properties. The transient flow behaviour of the emulsions depends on the mechanical properties of the laponite dispersions and the oil drop volume fraction. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Clay particles are added to emulsions to modify and control their flow behaviour (Viseras et al., 2007). The time-dependent rheology of emulsions is important in food products, cosmetics, paints and concrete (Barnes, 1997; Mewis and Wagner, 2009). Recently it was shown that the thixotropic response of surfactant-stabilised emulsions can be controlled by loading the emulsions with clay particles (Ragouilliaux et al., 2007). The addition of colloidal particles confers gel-like properties which disappear on shaking, but reappear on allowing the emulsions to rest. In this work, we instead investigate the thixotropic behaviour of emulsions stabilised by particles alone (Binks, 2002; Dickinson, 2010; Leal-Calderon and Schmitt, 2008). The aim is to understand how the time-dependent rheology of clay dispersions is incorporated into particle-stabilised (or Pickering) emulsions. In particular, the focus is to relate the time scales of structure breakdown and recovery in the emulsion and particle dispersions. A clay dispersion is defined as thixotropic (de Kretser et al., 1998) if, starting from rest, its viscosity decreases with time when a constant shear is applied (breakdown). When the shear ceases, the dispersion gradually recovers its consistency and the structure it had at rest (recovery). For example, laponite particles, a synthetic hectorite, cluster reversibly into dense micrometre-sized aggregates (Pignon et al., 1998; van Duijneveldt et al., 2005; Willenbacher, 1996). Dispersion flow behaviour is determined by the competition between the spontaneous build-up of the laponite aggregates at rest and their breakdown under shear. Typically, the evolution of the linear viscoelastic response (the storage and loss moduli) with time is used as a measure of the rebuilding structure in thixotropic materials. ⁎ Corresponding author. E-mail address: [email protected] (C.P. Whitby).
The literature describing the structure and rheology of emulsions containing clay particles can be divided into studies of mixtures of surfactant-stabilised emulsions and clay (suspoemulsions) and studies of clay-stabilised emulsions. Pal et al. (1992) showed that mixtures of water drops and kaolinite particles in oil flow like bimodal dispersions. Lagaly et al. (1999) found that emulsions stabilised by montmorillonites showed slightly pseudoplastic behaviour, with shear stresses and viscosities higher than those of the dispersions, due to the presence of the oil drops. The viscoelasticity of clay-particle stabilised emulsions (Binks et al., 2005; Torres et al., 2007) and the elastic properties of clay particle-coated interfaces (Nciri et al., 2010; Tan et al., 2012) have been linked to the kinetic stability of the emulsions. Examination of emulsion structures revealed that significant fractions of the particles remain in the continuous phase (Cui et al., 2011; Guillot et al., 2009; Thieme et al., 1999; Zhang et al., 2008). Abend et al. (1998) found that paraffin-in-water emulsions stabilised by mixtures of a layered double hydroxide particle and montmorillonite were highly elastic. Abend et al. (1998) argued that the positively charged hydroxide particles form a network with the negatively charged montmorillonite particles in the water that enhances drop stability. Few studies have considered the time-dependent rheology of emulsions containing clay (Abend et al., 1998). This work examines the thixotropy of emulsions of oil drops in laponite gels. The emulsion microstructure was investigated using confocal fluorescence microscopy. Transient rheology was used to probe the time scales of emulsion breakdown and recovery. This combination of techniques shows how particle aggregation causes thixotropy in Pickering emulsions. We show that incorporating oil drops into the laponite gels forms a stress-bearing, interconnected network. The emulsions behave like jammed solids at small stresses. Emulsion breakdown and recovery are determined by the flow behaviour of the laponite gel.
http://dx.doi.org/10.1016/j.clay.2014.03.005 0169-1317/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Whitby, C.P., Garcia, P.C., Time-dependent rheology of clay particle-stabilised emulsions, Appl. Clay Sci. (2014), http:// dx.doi.org/10.1016/j.clay.2014.03.005
2
C.P. Whitby, P.C. Garcia / Applied Clay Science xxx (2014) xxx–xxx
2. Experimental section
3. Results and discussion
2.1. Dispersion and emulsion preparation
Laponite-stabilised emulsions are solid-like materials that resist flowing, as shown in Fig. 1a. Coagulation of the laponite particles leads to complex structures in the emulsions as the interactions between the particles cause rapid growth of large particle clusters. The oil droplets are trapped within these super-aggregate particulate structures. The resulting structure is a network of interconnected drops and particles that spans the whole emulsion volume (Fig. 1b). This drop-particle heteroaggregate structure has a critical influence on the emulsion breakdown and recovery behaviour. The dynamic rheological behaviour of the laponite dispersions and emulsions in the linear viscoelastic regime was used to characterize the dispersions and emulsions at rest. The gelled laponite dispersions exhibit a yield stress and an elastic storage modulus, G′, which is much higher than their viscous loss modulus, G″. G′ shows limited dependence on the frequency of the oscillatory stress being applied, as shown in Fig. 2 (ϕo = 0, black symbols). A low-frequency plateau in G′ indicates that the particle network structure can bear stress (Trappe and Weitz, 2000). The emulsions exhibit enhanced viscosity and yield stresses compared to the clay dispersions alone. Fig. 2 shows that the emulsion elasticity increases with ϕo. The elastic storage modulus is independent of
Laponite RD particles (Rockwood Additives Limited) are disc shaped, 1 nm thick and 30 nm in diameter. Dispersions of laponite in solutions of sodium chloride (Chem Supply, 99%) in water (resistivity ≥ 18.2 MΩ cm) were prepared using a rotor–stator mixer (Ingenieurbüro CAT X1030D, M. Zipperer GmbH) with a 19 mm head operated at 11,000 revolutions per minute for 2 min to form transparent gels. The pH value of the dispersions was adjusted to 9.5 with concentrated solutions of NaOH. Emulsions were prepared by homogenising 1-bromohexadecane (Sigma Aldrich, 99%, passed through chromatographic alumina twice to remove polar impurities) with the aqueous laponite dispersions using the rotor–stator mixer operated at 13,000 rpm. Emulsification times were set to obtain similar drop size distributions (number average radius ~ 20 μm). The mechanical state of the laponite dispersions used to stabilise the emulsions was fixed by adjusting the laponite concentration relative to that required for gelation (Mourchid et al., 1998). This parameter depends only on the salt concentration in the dispersion. Typically the particle concentration was chosen to be close to that required for gelation. Emulsions prepared at oil volume fractions, ϕo ≤ 0.4, and laponite concentrations in the aqueous phase, Cp N 1.0 wt.%, did not coalesce for several weeks. The emulsions were diluted at constant particle and salt concentration to investigate the effect of oil volume fraction on the elasticity.
2.2. Dispersion and emulsion rheology Viscous flow and oscillatory measurements were made using a Rheometric Scientific Dynamic Stress Rheometer SR2000 in the controlled rate mode with a cone and plate geometry at a fixed temperature of 25 ± 0.1 °C. Geometries with either smooth hydrophobic surfaces or roughened (by sand blasting) steel surfaces were used to minimise wall slip effects. A solvent trap was used to prevent evaporation. Samples were typically stirred by hand for 30 s, loaded into the rheometer and then left to rest for 180 s prior to measurements. The viscous flow and oscillatory response of the laponite dispersions are reproducible after about 24 h, indicating that the fast ageing processes in the dispersions were completed. Slower ageing processes occur over a time period of several weeks (Bhatia et al., 2003). All the results shown here were obtained within 72 h of preparation to avoid any effect of the slow ageing dynamics of the laponite dispersions. Breakdown tests involved measuring the stress response while applying a jump in the shear rate from an initial value (typically 0 s−1) to a final value (typically 5 s− 1). Recovery tests involved applying a fixed small sinusoidal strain (corresponding to the linear viscoelastic region) after breakdown. The change in the viscoelastic moduli over time was measured.
2.3. Confocal fluorescence microscopy Confocal fluorescence microscopy (CFM, Leica SP5 spectra scanning confocal microscope) was used to visualise both dispersions and emulsions containing Rhodamine B (Sigma, 97%). At the low concentration used (~ 0.3 μM) the dye adsorbs completely onto the laponite particles. The samples were excited at a wavelength of 514 nm and the fluorescence emission intensity was collected over 555 to 655 nm. The particles appeared bright, while the aqueous and oil phases were black. The mean droplet size was calculated from at least 50 individual measurements of drop diameters in the confocal images.
Fig. 1. (a) Photo of an emulsion in an inverted tube. The emulsion is highly viscous and does not flow readily. It has a drop volume fraction of 20 vol.% and is stabilised by 2 wt.% particles in 0.01 M NaCl. (b) Confocal fluorescence image of the interconnected network of drops and particles in the emulsion. Shearing disrupts this structure, causing the emulsion viscoelasticity to decrease. (c) Recovery of the elastic storage moduli (G′) of emulsions with time (t) after shearing is halted. Data is shown for emulsions formed at different drop volume fractions (ϕo). The emulsions were stabilised by 2 wt.% particles and the salt concentration in the aqueous phase was 0.002 M NaCl.
Please cite this article as: Whitby, C.P., Garcia, P.C., Time-dependent rheology of clay particle-stabilised emulsions, Appl. Clay Sci. (2014), http:// dx.doi.org/10.1016/j.clay.2014.03.005
C.P. Whitby, P.C. Garcia / Applied Clay Science xxx (2014) xxx–xxx
Fig. 2. Frequency (ω) dependence of the elastic storage moduli (G′) of emulsions at different drop volume fractions (ϕo). The emulsions were stabilised by 2 wt.% particles and the salt concentration in the aqueous phase was 0.005 M NaCl.
the oscillation frequency (Fig. 2) (ϕo = 0.1 and 0.3, red and pink symbols). This is a characteristic feature of jammed solids. The oil drops and laponite particles have aggregated into a stress-bearing, interconnected network. Previously, emulsions with pronounced gel-like properties were stabilised by synergistic interactions between proteins and clay (Reger et al., 2011). These results show that self-supporting emulsions can be formed using gelled dispersions of laponite alone. The thixotropic behaviour of the emulsions is illustrated by two kinds of results: destruction of the network by shear and its recovery after cessation of the shear. Fig. 3 shows the transient response of the emulsions when a constant shear rate is applied. The emulsions (ϕo = 0.2 and 0.4, blue and green symbols) follow a curve that is characteristic of the dispersions (c.f. ϕo = 0, black symbols) used to stabilise the emulsions. At short times the stress increases linearly with time, indicating that the emulsion deformation is elastic. The maximum stress reached is related to the emulsion yield stress. At longer times, the emulsions start to flow and the stress relaxes until a steady flow regime is reached. The decrease corresponds to breakdown of the network (Pignon et al., 1998). The maximum stress and the steady state stress reached increase with the drop volume fraction. The time-dependent changes in stress reveal two periods with distinct time scales in the breakdown process, situated on either side
3
of the maximum stress and corresponding critical strain. The first period corresponds to elastic strain of the material and orientation of the structure. The data can be fitted with an exponential function (σ = σ0 exp(−t/τ1)) with a characteristic time, τ1 ~ 0.5 s. The second period corresponds to flow and disaggregation of the structure. Fitting the data with a stretched exponential function (σ = σ(1 − exp(t/τ2)m)) gives a characteristic time τ2 ~ 200 s. Relaxation is slow, consistent with the large scale structures in the emulsions (Fig. 1b). These time scales do not change with ϕo (Fig. 3). The influence of shear rate on the characteristic times of breakdown was investigated by measuring the time-dependent changes in stress for various shear rates. The kinetics of breakdown differs depending on the shear rate applied, however, the ratio of the time scales remains about the same. This means that the relaxation behaviour of laponitestabilised emulsions is determined primarily by the dispersions used to stabilise the emulsions. After breakdown, the emulsions were subjected to a low-strain harmonic shearing so the kinetics of recovery could be monitored. Fig. 1c compares the transient response of sheared emulsions to the response of the clay dispersion alone. The elastic storage moduli show an exponential increase with time (t). Typically there are two periods in the recovery curves. The first period lasts up to a few minutes and the rate of increase in G′ is several times faster than the second period. The levels reached for G′ (and G″) increase with the drop volume fraction (Fig. 1c). This indicates the increase in strength of the laponite network due to the incorporation of particle-coated oil drops. The evolution in G′ with time followed similar trends to that observed for the particle dispersions alone (ϕo = 0, black symbols). The maximum elasticity reached (t → ∞) is consistent with the values measured in dynamic oscillatory tests prior to shearing (Garcia and Whitby, 2012). The exponential recovery of the elastic storage moduli can be fitted (Khandal and Tadros, 1988) to estimate the gelation time (tg) of the networks using . 0 0 t G ðt Þ ¼ Gt→∞ 1− exp − : tg
ð1Þ
Similar gelation times (tg ~ 1000 s) were estimated for the emulsions and the particle dispersion. This means that the recovery behaviour of laponite-stabilised emulsions is determined primarily by the laponite particles. 4. Conclusions Thixotropic emulsions were formed by stabilizing oil-in-water emulsions with clay particles alone. The stress required to breakdown the thixotropic laponite networks increases with the fraction of drops incorporated into the particle dispersions. The network reforms once the shear stress is removed. The time taken for the strength of the interconnected network structure of drops and laponite aggregates to be recovered is determined by the gelation time of the particles. Acknowledgements Laponite particles were kindly supplied by Rockwood Additives Limited. CPW acknowledges receipt of an Australian Research Council Future Fellowship. PGC was funded by the University of South Australia. References
Fig. 3. Time (t) dependence of the emulsion stress (σ) during shearing. Data is shown for emulsions at different drop volume fractions (ϕo). The emulsions were stabilised by 2 wt.% particles and the salt concentration in the aqueous phase was 0.002 M NaCl.
Abend, S., Bonnke, N., Gutschner, U., Lagaly, G., 1998. Stabilization of emulsions by heterocoagulation of clay minerals and layered double hydroxides. Colloid Polym. Sci. 276 (8), 730–737. Barnes, H.A., 1997. Thixotropy — a review. J. Non-Newtonian Fluid Mech. 70 (1–2), 1–33. Bhatia, S., Barker, J., Mourchid, A., 2003. Scattering of disklike particle suspensions: evidence for repulsive interactions and large length scale structure from static light scattering and ultra-small-angle neutron scattering. Langmuir 19 (3), 532–535.
Please cite this article as: Whitby, C.P., Garcia, P.C., Time-dependent rheology of clay particle-stabilised emulsions, Appl. Clay Sci. (2014), http:// dx.doi.org/10.1016/j.clay.2014.03.005
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Binks, B.P., 2002. Particles as surfactants — similarities and differences. Curr. Opin. Colloid Interface Sci. 7 (1–2), 21–41. Binks, B.P., Clint, J.H., Whitby, C.P., 2005. Rheological behavior of water-in-oil emulsions stabilized by hydrophobic bentonite particles. Langmuir 21 (12), 5307–5316. Cui, Y., Threlfall, M., van Duijneveldt, J.S., 2011. Optimizing organoclay stabilized Pickering emulsions. J. Colloid Interface Sci. 356 (2), 665–671. de Kretser, R.G., Scales, P.J., Boger, D.V., 1998. Surface chemistry–rheology inter-relationships in clay suspensions. Colloids Surf. A Physicochem. Eng. Asp. 137 (1–3), 307–318. Dickinson, E., 2010. Food emulsions and foams: stabilization by particles. Curr. Opin. Colloid Interface Sci. 15 (1–2), 40–49. Garcia, P.C., Whitby, C.P., 2012. Laponite-stabilised oil-in-water emulsions: viscoelasticity and thixotropy. Soft Matter 8 (5), 1609–1615. Guillot, S., Bergaya, F., de Azevedo, C., Warmont, F., Tranchant, J.-F., 2009. Internally structured Pickering emulsions stabilized by clay mineral particles. J. Colloid Interface Sci. 333 (2), 563. Khandal, R.K., Tadros, T.F., 1988. Application of viscoelastic measurements to the investigation of the swelling of sodium montmorillonite suspensions. J. Colloid Interface Sci. 125 (1), 122–128. Lagaly, G., Reese, M., Abend, S., 1999. Smectites as colloidal stabilizers of emulsions II. Rheological properties of smectite-laden emulsions. Appl. Clay Sci. 14 (5–6), 279–298. Leal-Calderon, F., Schmitt, V., 2008. Solid-stabilized emulsions. Curr. Opin. Colloid Interface Sci. 13 (4), 217–227. Mewis, J., Wagner, N.J., 2009. Thixotropy. Adv. Colloid Interface Sci. 147–48, 214–227. Mourchid, A., Lecolier, E., Van Damme, H., Levitz, P., 1998. On viscoelastic, birefringent, and swelling properties of Laponite clay suspensions: revisited phase diagram. Langmuir 14 (17), 4718–4723. Nciri, H., Huang, N., Rosilio, V., Trabelsi-Ayadi, M., Benna-Zayani, M., Grossiord, J.-L., 2010. Rheological studies in the bulk and at the interface of Pickering oil/water emulsions. Rheol. Acta 49 (9), 961–969.
Pal, R., Yan, Y., Masliyah, J.H., 1992. Rheology of clay-in-oil suspensions with added water droplets. Chem. Eng. Sci. 47, 967–970. Pignon, F., Magnin, A., Piau, J.M., 1998. Thixotropic behavior of clay dispersions: combinations of scattering and rheometric techniques. J. Rheol. 42 (6), 1349–1373. Ragouilliaux, A., Ovarlez, G., Shahidzadeh-Bonn, N., Herzhaft, B., Palermo, T., Coussot, P., 2007. Transition from a simple yield-stress fluid to a thixotropic material. Phys. Rev. E. 76 (5), 051408. Reger, M., Sekine, T., Okamoto, T., Watanabe, K., Hoffmann, H., 2011. Pickering emulsions stabilized by novel clay–hydrophobin synergism. Soft Matter 7 (22), 11021–11030. Tan, S.-Y., Tabor, R.F., Ong, L., Stevens, G.W., Dagastine, R.R., 2012. Nanomechanical properties of clay-armoured emulsion droplets. Soft Matter 8 (11), 3112–3121. Thieme, J., Abend, S., Lagaly, G., 1999. Aggregation in Pickering emulsions. Colloid Polym. Sci. 277 (2–3), 257–260. Torres, L.G., Iturbe, R., Snowden, M.J., Chowdhry, B.Z., Leharne, S.A., 2007. Preparation of o/w emulsions stabilized by solid particles and their characterization by oscillatory rheology. Colloids Surf. A Physicochem. Eng. Asp. 302 (1–3), 439–448. Trappe, V., Weitz, D.A., 2000. Scaling of the viscoelasticity of weakly attractive particles. Phys. Rev. Lett. 85, 449. van Duijneveldt, J.S., Klein, S., Leach, E., Pizzey, C., Richardson, R.M., 2005. Large scale structures in liquid crystal/clay colloids. J. Phys. Condens. Matter 17 (15), 2255. Viseras, C., Aguzzi, C., Cerezo, P., Lopez-Galindo, A., 2007. Uses of clay minerals in semisolid health care and therapeutic products. Appl. Clay Sci. 36 (1–3), 37–50. Willenbacher, N., 1996. Unusual thixotropic properties of aqueous dispersions of laponite RD. J. Colloid Interface Sci. 182 (2), 501–510. Zhang, S., Sun, D., Dong, X., Li, C., Xu, J., 2008. Aqueous foams stabilized with particles and nonionic surfactants. Colloids Surf. A 324 (1–3), 1–8.
Please cite this article as: Whitby, C.P., Garcia, P.C., Time-dependent rheology of clay particle-stabilised emulsions, Appl. Clay Sci. (2014), http:// dx.doi.org/10.1016/j.clay.2014.03.005
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Time-dependent rheology of clay particle-stabilised emulsions Catherine P. Whitby ⁎, Paula Corbi Garcia Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, 5095, Australia
a r t i c l e
i n f o
Article history: Received 31 October 2013 Accepted 3 March 2014 Available online xxxx Keywords: Pickering emulsion Viscoelastic Breakdown Recovery
a b s t r a c t We have studied the thixotropic behaviour of clay particle-stabilised emulsions using a combination of rheology and confocal fluorescence microscopy. Oil drops were stabilised by laponite particles attached to their surfaces and incorporated into a three dimensional super-aggregate structure of laponite particles that formed in the water. The self-supporting, three-dimensional network of drops and laponite particles gives the emulsions pronounced gel-like properties. The transient flow behaviour of the emulsions depends on the mechanical properties of the laponite dispersions and the oil drop volume fraction. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Clay particles are added to emulsions to modify and control their flow behaviour (Viseras et al., 2007). The time-dependent rheology of emulsions is important in food products, cosmetics, paints and concrete (Barnes, 1997; Mewis and Wagner, 2009). Recently it was shown that the thixotropic response of surfactant-stabilised emulsions can be controlled by loading the emulsions with clay particles (Ragouilliaux et al., 2007). The addition of colloidal particles confers gel-like properties which disappear on shaking, but reappear on allowing the emulsions to rest. In this work, we instead investigate the thixotropic behaviour of emulsions stabilised by particles alone (Binks, 2002; Dickinson, 2010; Leal-Calderon and Schmitt, 2008). The aim is to understand how the time-dependent rheology of clay dispersions is incorporated into particle-stabilised (or Pickering) emulsions. In particular, the focus is to relate the time scales of structure breakdown and recovery in the emulsion and particle dispersions. A clay dispersion is defined as thixotropic (de Kretser et al., 1998) if, starting from rest, its viscosity decreases with time when a constant shear is applied (breakdown). When the shear ceases, the dispersion gradually recovers its consistency and the structure it had at rest (recovery). For example, laponite particles, a synthetic hectorite, cluster reversibly into dense micrometre-sized aggregates (Pignon et al., 1998; van Duijneveldt et al., 2005; Willenbacher, 1996). Dispersion flow behaviour is determined by the competition between the spontaneous build-up of the laponite aggregates at rest and their breakdown under shear. Typically, the evolution of the linear viscoelastic response (the storage and loss moduli) with time is used as a measure of the rebuilding structure in thixotropic materials. ⁎ Corresponding author. E-mail address: [email protected] (C.P. Whitby).
The literature describing the structure and rheology of emulsions containing clay particles can be divided into studies of mixtures of surfactant-stabilised emulsions and clay (suspoemulsions) and studies of clay-stabilised emulsions. Pal et al. (1992) showed that mixtures of water drops and kaolinite particles in oil flow like bimodal dispersions. Lagaly et al. (1999) found that emulsions stabilised by montmorillonites showed slightly pseudoplastic behaviour, with shear stresses and viscosities higher than those of the dispersions, due to the presence of the oil drops. The viscoelasticity of clay-particle stabilised emulsions (Binks et al., 2005; Torres et al., 2007) and the elastic properties of clay particle-coated interfaces (Nciri et al., 2010; Tan et al., 2012) have been linked to the kinetic stability of the emulsions. Examination of emulsion structures revealed that significant fractions of the particles remain in the continuous phase (Cui et al., 2011; Guillot et al., 2009; Thieme et al., 1999; Zhang et al., 2008). Abend et al. (1998) found that paraffin-in-water emulsions stabilised by mixtures of a layered double hydroxide particle and montmorillonite were highly elastic. Abend et al. (1998) argued that the positively charged hydroxide particles form a network with the negatively charged montmorillonite particles in the water that enhances drop stability. Few studies have considered the time-dependent rheology of emulsions containing clay (Abend et al., 1998). This work examines the thixotropy of emulsions of oil drops in laponite gels. The emulsion microstructure was investigated using confocal fluorescence microscopy. Transient rheology was used to probe the time scales of emulsion breakdown and recovery. This combination of techniques shows how particle aggregation causes thixotropy in Pickering emulsions. We show that incorporating oil drops into the laponite gels forms a stress-bearing, interconnected network. The emulsions behave like jammed solids at small stresses. Emulsion breakdown and recovery are determined by the flow behaviour of the laponite gel.
http://dx.doi.org/10.1016/j.clay.2014.03.005 0169-1317/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Whitby, C.P., Garcia, P.C., Time-dependent rheology of clay particle-stabilised emulsions, Appl. Clay Sci. (2014), http:// dx.doi.org/10.1016/j.clay.2014.03.005
2
C.P. Whitby, P.C. Garcia / Applied Clay Science xxx (2014) xxx–xxx
2. Experimental section
3. Results and discussion
2.1. Dispersion and emulsion preparation
Laponite-stabilised emulsions are solid-like materials that resist flowing, as shown in Fig. 1a. Coagulation of the laponite particles leads to complex structures in the emulsions as the interactions between the particles cause rapid growth of large particle clusters. The oil droplets are trapped within these super-aggregate particulate structures. The resulting structure is a network of interconnected drops and particles that spans the whole emulsion volume (Fig. 1b). This drop-particle heteroaggregate structure has a critical influence on the emulsion breakdown and recovery behaviour. The dynamic rheological behaviour of the laponite dispersions and emulsions in the linear viscoelastic regime was used to characterize the dispersions and emulsions at rest. The gelled laponite dispersions exhibit a yield stress and an elastic storage modulus, G′, which is much higher than their viscous loss modulus, G″. G′ shows limited dependence on the frequency of the oscillatory stress being applied, as shown in Fig. 2 (ϕo = 0, black symbols). A low-frequency plateau in G′ indicates that the particle network structure can bear stress (Trappe and Weitz, 2000). The emulsions exhibit enhanced viscosity and yield stresses compared to the clay dispersions alone. Fig. 2 shows that the emulsion elasticity increases with ϕo. The elastic storage modulus is independent of
Laponite RD particles (Rockwood Additives Limited) are disc shaped, 1 nm thick and 30 nm in diameter. Dispersions of laponite in solutions of sodium chloride (Chem Supply, 99%) in water (resistivity ≥ 18.2 MΩ cm) were prepared using a rotor–stator mixer (Ingenieurbüro CAT X1030D, M. Zipperer GmbH) with a 19 mm head operated at 11,000 revolutions per minute for 2 min to form transparent gels. The pH value of the dispersions was adjusted to 9.5 with concentrated solutions of NaOH. Emulsions were prepared by homogenising 1-bromohexadecane (Sigma Aldrich, 99%, passed through chromatographic alumina twice to remove polar impurities) with the aqueous laponite dispersions using the rotor–stator mixer operated at 13,000 rpm. Emulsification times were set to obtain similar drop size distributions (number average radius ~ 20 μm). The mechanical state of the laponite dispersions used to stabilise the emulsions was fixed by adjusting the laponite concentration relative to that required for gelation (Mourchid et al., 1998). This parameter depends only on the salt concentration in the dispersion. Typically the particle concentration was chosen to be close to that required for gelation. Emulsions prepared at oil volume fractions, ϕo ≤ 0.4, and laponite concentrations in the aqueous phase, Cp N 1.0 wt.%, did not coalesce for several weeks. The emulsions were diluted at constant particle and salt concentration to investigate the effect of oil volume fraction on the elasticity.
2.2. Dispersion and emulsion rheology Viscous flow and oscillatory measurements were made using a Rheometric Scientific Dynamic Stress Rheometer SR2000 in the controlled rate mode with a cone and plate geometry at a fixed temperature of 25 ± 0.1 °C. Geometries with either smooth hydrophobic surfaces or roughened (by sand blasting) steel surfaces were used to minimise wall slip effects. A solvent trap was used to prevent evaporation. Samples were typically stirred by hand for 30 s, loaded into the rheometer and then left to rest for 180 s prior to measurements. The viscous flow and oscillatory response of the laponite dispersions are reproducible after about 24 h, indicating that the fast ageing processes in the dispersions were completed. Slower ageing processes occur over a time period of several weeks (Bhatia et al., 2003). All the results shown here were obtained within 72 h of preparation to avoid any effect of the slow ageing dynamics of the laponite dispersions. Breakdown tests involved measuring the stress response while applying a jump in the shear rate from an initial value (typically 0 s−1) to a final value (typically 5 s− 1). Recovery tests involved applying a fixed small sinusoidal strain (corresponding to the linear viscoelastic region) after breakdown. The change in the viscoelastic moduli over time was measured.
2.3. Confocal fluorescence microscopy Confocal fluorescence microscopy (CFM, Leica SP5 spectra scanning confocal microscope) was used to visualise both dispersions and emulsions containing Rhodamine B (Sigma, 97%). At the low concentration used (~ 0.3 μM) the dye adsorbs completely onto the laponite particles. The samples were excited at a wavelength of 514 nm and the fluorescence emission intensity was collected over 555 to 655 nm. The particles appeared bright, while the aqueous and oil phases were black. The mean droplet size was calculated from at least 50 individual measurements of drop diameters in the confocal images.
Fig. 1. (a) Photo of an emulsion in an inverted tube. The emulsion is highly viscous and does not flow readily. It has a drop volume fraction of 20 vol.% and is stabilised by 2 wt.% particles in 0.01 M NaCl. (b) Confocal fluorescence image of the interconnected network of drops and particles in the emulsion. Shearing disrupts this structure, causing the emulsion viscoelasticity to decrease. (c) Recovery of the elastic storage moduli (G′) of emulsions with time (t) after shearing is halted. Data is shown for emulsions formed at different drop volume fractions (ϕo). The emulsions were stabilised by 2 wt.% particles and the salt concentration in the aqueous phase was 0.002 M NaCl.
Please cite this article as: Whitby, C.P., Garcia, P.C., Time-dependent rheology of clay particle-stabilised emulsions, Appl. Clay Sci. (2014), http:// dx.doi.org/10.1016/j.clay.2014.03.005
C.P. Whitby, P.C. Garcia / Applied Clay Science xxx (2014) xxx–xxx
Fig. 2. Frequency (ω) dependence of the elastic storage moduli (G′) of emulsions at different drop volume fractions (ϕo). The emulsions were stabilised by 2 wt.% particles and the salt concentration in the aqueous phase was 0.005 M NaCl.
the oscillation frequency (Fig. 2) (ϕo = 0.1 and 0.3, red and pink symbols). This is a characteristic feature of jammed solids. The oil drops and laponite particles have aggregated into a stress-bearing, interconnected network. Previously, emulsions with pronounced gel-like properties were stabilised by synergistic interactions between proteins and clay (Reger et al., 2011). These results show that self-supporting emulsions can be formed using gelled dispersions of laponite alone. The thixotropic behaviour of the emulsions is illustrated by two kinds of results: destruction of the network by shear and its recovery after cessation of the shear. Fig. 3 shows the transient response of the emulsions when a constant shear rate is applied. The emulsions (ϕo = 0.2 and 0.4, blue and green symbols) follow a curve that is characteristic of the dispersions (c.f. ϕo = 0, black symbols) used to stabilise the emulsions. At short times the stress increases linearly with time, indicating that the emulsion deformation is elastic. The maximum stress reached is related to the emulsion yield stress. At longer times, the emulsions start to flow and the stress relaxes until a steady flow regime is reached. The decrease corresponds to breakdown of the network (Pignon et al., 1998). The maximum stress and the steady state stress reached increase with the drop volume fraction. The time-dependent changes in stress reveal two periods with distinct time scales in the breakdown process, situated on either side
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of the maximum stress and corresponding critical strain. The first period corresponds to elastic strain of the material and orientation of the structure. The data can be fitted with an exponential function (σ = σ0 exp(−t/τ1)) with a characteristic time, τ1 ~ 0.5 s. The second period corresponds to flow and disaggregation of the structure. Fitting the data with a stretched exponential function (σ = σ(1 − exp(t/τ2)m)) gives a characteristic time τ2 ~ 200 s. Relaxation is slow, consistent with the large scale structures in the emulsions (Fig. 1b). These time scales do not change with ϕo (Fig. 3). The influence of shear rate on the characteristic times of breakdown was investigated by measuring the time-dependent changes in stress for various shear rates. The kinetics of breakdown differs depending on the shear rate applied, however, the ratio of the time scales remains about the same. This means that the relaxation behaviour of laponitestabilised emulsions is determined primarily by the dispersions used to stabilise the emulsions. After breakdown, the emulsions were subjected to a low-strain harmonic shearing so the kinetics of recovery could be monitored. Fig. 1c compares the transient response of sheared emulsions to the response of the clay dispersion alone. The elastic storage moduli show an exponential increase with time (t). Typically there are two periods in the recovery curves. The first period lasts up to a few minutes and the rate of increase in G′ is several times faster than the second period. The levels reached for G′ (and G″) increase with the drop volume fraction (Fig. 1c). This indicates the increase in strength of the laponite network due to the incorporation of particle-coated oil drops. The evolution in G′ with time followed similar trends to that observed for the particle dispersions alone (ϕo = 0, black symbols). The maximum elasticity reached (t → ∞) is consistent with the values measured in dynamic oscillatory tests prior to shearing (Garcia and Whitby, 2012). The exponential recovery of the elastic storage moduli can be fitted (Khandal and Tadros, 1988) to estimate the gelation time (tg) of the networks using . 0 0 t G ðt Þ ¼ Gt→∞ 1− exp − : tg
ð1Þ
Similar gelation times (tg ~ 1000 s) were estimated for the emulsions and the particle dispersion. This means that the recovery behaviour of laponite-stabilised emulsions is determined primarily by the laponite particles. 4. Conclusions Thixotropic emulsions were formed by stabilizing oil-in-water emulsions with clay particles alone. The stress required to breakdown the thixotropic laponite networks increases with the fraction of drops incorporated into the particle dispersions. The network reforms once the shear stress is removed. The time taken for the strength of the interconnected network structure of drops and laponite aggregates to be recovered is determined by the gelation time of the particles. Acknowledgements Laponite particles were kindly supplied by Rockwood Additives Limited. CPW acknowledges receipt of an Australian Research Council Future Fellowship. PGC was funded by the University of South Australia. References
Fig. 3. Time (t) dependence of the emulsion stress (σ) during shearing. Data is shown for emulsions at different drop volume fractions (ϕo). The emulsions were stabilised by 2 wt.% particles and the salt concentration in the aqueous phase was 0.002 M NaCl.
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Please cite this article as: Whitby, C.P., Garcia, P.C., Time-dependent rheology of clay particle-stabilised emulsions, Appl. Clay Sci. (2014), http:// dx.doi.org/10.1016/j.clay.2014.03.005